300b lovers

I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations.  I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is.  Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.  

Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence.  They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music.  They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding.  Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a  unique circuitry to mate with his 300b monos via XLR connections.  Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it. 

I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system.  300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm.  Of course, 300b amps favor efficient speakers so carefully component matching is paramount.

Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public.  Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC.  I will be there to hear what I expect to be an outstanding sonic presentation.  

To allay any questions about the cost of Don's 300b mono, I do not have an answer. 




Quick recap: actually, vacuum tubes are far from saturation when set to normal bias points. Look at a 300B, or any other power tube. Normal quiescent bias is set between 60 and 85 mA, if Class A operation is desired. If Class AB is desired, 35 to 40 mA is more typical. With 400 volts from cathode (or filament), that's a steady-state plate dissipation between 14 and 34 watts, well within the 40-watt rating.

But that's nowhere close to the peak current emission of the cathode. I've measured 250 mA from a generic 300B, and the exotic European 300B's can slam out nearly 500 mA (transient). The only time I've ever seen a 300B current-limit around 80 mA were some particularly weak Chinese tubes from the mid-Eighties ... they sounded and measured pretty bad, and were near-defective. Other vacuum tubes are similar; the recommended quiescent currents are set by plate dissipation limits, not cathode emission maximums (which are left unspecified). Transistors will melt the internal copper links, but damaging the cathode in a vacuum tube is really hard to do unless the tube is operated with no B+ present.

It's transistors that have Safe Operating Area (SOA) curves that are log-log in both current and voltage (with an additional time dimension), not tubes. The current saturation mechanisms are totally different and have nothing in common.

Unlike transistors, vacuum tubes have very large areas of peak current emission that are left untapped by most circuits. Of course, plate heating goes up when these areas are explored, but unlike transistors, tubes do not fail in milliseconds (this is shown in the SOA curves of transistors, and must be respected). It takes sustained abuse, over many seconds, before mechanical deformation dooms the plate.

I think Ralph will agree that Class AB operation is not "false". In Class AB, one device cuts off (goes to infinite impedance and conducts no current) while the opposing device goes to a large multiple of the quiescent current. In conventional Class AB transistor amps, the idling current is a tiny fraction of the peak current, and in Class AB tube amps, it's still a small fraction.

Let's look at what happens in pure differential circuit, either tube or transistor, with a current source setting the quiescent current. This circuit must always operate in Class A. Unless something fails, the current source will always deliver the programmed current ... that is a hard limit that cannot be exceeded under any condition.

The late Allen Wright actually built a PP 300B power amplifier that had a current source under the pair of VV52B's (massive Czech power tubes). He stayed at my house during one of the VSAC shows, and we compared his amp to my early version of the Karna (which has bypassed cathodes and can operate in Class A, Class A2, or Class AB, or even Class AB2, depending on current demand). Allen's output stage was true differential, and true Class A, with a powerful solid-state current source running around 160 mA (if memory serves ... this was in 2003 or so).

The two amps sounded completely different. That's when Allen, and I, realized that differential, and balanced, are not in fact the same. This is a common illusion, a hangover from the Fifties. The question is what happens when one device cuts off.

When this happens in a current-sourced differential circuit, the "ON" device can never pass more than the total current programmed in the current source (by definition). That's a hard limit. It is a brick wall. The circuit, as a whole, will always pass whatever the current source is programmed to do ... no more, no less, always the same. This is why this circuit is seen in the Mullard topology as a low-power, medium-voltage phase splitter. Allen, as a big fan of differential circuits in Tek scopes, took it all the way and used it in a power stage.

This is quite different than a Class AB, or conventional Class A, power stage. Whether cathode or fixed-bias, current flow through the output pair is dynamic. IF (a very big if here) the output tubes were distortionless, perfectly matched, AND never voltage-clipped or driven into Class AB, yes, it would behave the same as a current-sourced pure differential stage. Only then are they the same.

But we don't live in a world of Platonic ideals. Tubes are not actually the same as the tube models, they are not perfectly assembled in perfect factories by robots, loudspeakers have odd ideas when they want lots of current, and bass drivers in particular are notorious for nonlinearity and very long energy storage .., all of which affects output stages.

So a power amplifier must deal with speakers as they are, not as we want them to be. So peak current excursions can be accommodated when necessary, without the amplifier grossly departing from basic design assumptions. The loudspeaker conforms to Theile/Small equations most of the time, but both Neville Theile and Richard Small warn us that these are only small-signal approximations. They are not valid once the voice coils start to move significantly. Speakers are only linear on average, not all the time.

My goal with Class A output is to synthesize a fixed output impedance that remains constant with real-world loudspeakers, which I have been designing since 1975. I know how awful speakers are. Most power amps use 20 to 50 dB of feedback to synthesize a perfect voltage source, and they do a pretty decent job of it. With zero feedback, the best I can hope for is a fixed, moderate-value equivalent resistor, about 2 ohms or so, which a low-Q vented or closed box speaker can deal with. And an output stage that does not have a hard current limit, but soft-clips in both voltage and current, without requiring protection circuits.

The above, in a nutshell, is why tube amps behave very differently at clipping than SS amps. It is why, with the right speaker of course, that a 60 watt tube amp can sound like it has the drive of a 200 watt SS amp, and especially why these 300b monos with a mere 27 watts each can sound like a 200 watt SS amp. Those of you who heard them at the show when the system was cranked up could hear their drive capability on an open baffle speaker of approx. 88-89 dB efficiency with a stable 4 ohm that is very well behaved. You cannot clip the amps at any sane volume level, and they can deliver large amounts of instantaneous current, while maintaining their sound quality.

There are certainly speaker designs that require 200 watts of SS amp power to wake them up. These amps are not for those speakers. But any reasonably "tube amp friendly" speaker is no problem.   If you have a speaker that presents a difficult load for the amp, then certainly one of the class D amps may be a very good choice for you.  


You might want to read this article:

The Power Paradigm

Most zero feedback tube amplifiers are Power Paradigm devices.

Regarding some of your comments in your post above:

How an output section behaves was not the topic when you brought up this bit of conversation (balanced vs differential). I never said anything about an output section. FWIW its possible to build a differential circuit so a tube can saturate when the other half is in cutoff. Its all about operating points as you rightly pointed out.

FWIW the first differential amplifiers were single pentode circuits; the grid being one input and the cathode being the other. IOW all tubes behave differentially- they amplify what is different between the cathode and grid. On this account, you can see that setting the operating point is the crucial bit which may or may not allow the tube to swing from saturation to cutoff. Drive has a lot do do with it of course. My surmise is Allen simply didn’t set his operating point correctly in your anecdote.

We were building tube differential voltage amplifiers before Allen came on the scene- we were the first worldwide to offer them to the public in a audio product meant for home use. My recommendation is to spend more time working with them and see if you might arrive at a different conclusion.

The above, in a nutshell, is why tube amps behave very differently at clipping than SS amps. It is why, with the right speaker of course, that a 60 watt tube amp can sound like it has the drive of a 200 watt SS amp,

In case there’s any question about why this might seem so, its how the tube output section makes distortion at clipping. A zero feedback tube output section has a very gentle clipping character; at early onset you don’t hear the amp breaking up at all, but the distortion has skyrocketed and the higher ordered harmonics cause the amp to sound louder than it really is, despite no obvious breakup. Its an illusion.

It has led to the myth that tube power is more robust than transistor power. But in simple terms a Watt is a Watt; but how distortion interacts with our ears is a different thing altogether. A sound pressure meter will reveal the truth of the situation easily enough.

I’m always interested in boundary conditions ... what happens when the amp leaves its happy place and a surge of current or voltage is required. Does a circuit saturate and hit the wall? Does a transistor fail? Does it store charge and "stick" for a few milliseconds? How smooth is the transition in and out of the Bad Place?

I mention this because speakers are badly behaved much of the time. They store energy for tens to hundreds of milliseconds, then throw it back to the amplifier. The feedback network might, or might not, keep correcting this, but the error overshoots can be very large and can saturate an input section.

Many power amps do not accept boundary conditions gracefully. Not just the output section, but the driver as well. Driver transistors fail when SOA is exceeded by transient reactive loads (failure to accurately read a SOA graph almost bankrupted Audionics). In tube amps, drivers can’t push enough linear current into the Miller capacitance of the output tubes. The voltage-amp section of a transistor amp can’t charge the dominant-pole capacitor fast enough, resulting in slewing.

These are all boundary conditions, and they are audible not just when they reach 100% failure, but well before that, when nonlinearity just begins. The previous point about Class A operation in a differential stage still holds: what happens when more than 100% of the current programmed in a current source is exceeded?

This is a boundary condition problem. When current is exceeded, what next? What’s after that? Does anything fail? What does current clipping look like? Are there any energy storage mechanisms that result in "sticking", a well-known problem in solid-state power stages. If sticking happens, how long does it take before it gets unstuck? In milliseconds?

The approach in the Blackbird/Karna does not use current sources, nor differential stages. Each side is parallel, but in antiphase, and all phase splitting, and re-summing, is done by passive devices, which do not have slew limitations. The 1:1 interstage transformer makes sure that recovery from A2 grid-current events happen in microseconds, not milliseconds.

Overload happens in the tubes, mostly in the output section, and the overload condition is not affected by local or global feedback, so the overall boundary characteristic is that of a (very) fast-recovery limiter/compressor. There is no hard boundary between Class A, where it remains most of the time, and A2, AB, or AB2, depending on current or voltage demand.

During the development of the Karna amp, I was in a kind of perverse mood, so I was curious just how much abuse the circuit, and the tubes, could take. I set the oscillator level so the scope display was just below clipping, around 20 watts, and the 8-ohm test load was nice and warm. I increased the drive frequency beyond 20 kHz, and as transformer gain started to fall off beyond 50 kHz, I just increased the input level to keep the output at a steady, undistorted 20 watts. Because why not?

I finally lost my nerve at 500 kHz. The scope display was still an undistorted 20 watts, with no sign of triangle waves or flat-topping, but playing around with an AM-band transmitter (with 500 volts inside) was asking for trouble. I wasn’t trying to kill anything, but sooner or later some part was going to fail. (If any of you customers try this stunt, yes, we will void the warranty, so don’t do this. Ever.)

Not many transistor amps would survive full power at 500 kHz. Some would, some wouldn’t. It’s an absurd test, with no relation to audio use. But it’s interesting to know the development prototype survived it. No, I would never do this to the current production model, and don’t you guys try it, either.


Let me tell the story about how misreading a graph almost bankrupted a company. This is a true story.

Our chief engineer, a Tektronix veteran, designed a power amplifier called the Point Zero Three, or PZ3 (hey, I didn’t name it, OK?).

A 100-watt/channel transistor amp. It measured great, sounded so-so, but had a little fault ... it blew up without warning, and worse, took out all the power and driver transistors, scorching the circuit board as it went. Take out the circuit board, replace the power transistors (all of them), and put in a new one. Repeat as necessary.

There were days when more came back than were shipped out. Obviously, this couldn’t continue. Word was getting out, and a failure rate approaching 50% is unsustainable.

Our new engineer, Bob Sickler, looked more closely at the SOA curves for the driver transistors. Bob told me these curves are intentionally hard to read, and it usually escapes notice that both voltage and current axis are log scale. Not only that, a straight load-line is assumed by most engineers, but with real loads, that line opens into an ellipse. Once that ellipse touches the no-go line, and stays there for more than 10 milliseconds, boom!

The chief engineer, despite being an old Tek hand, had missed this tiny little detail. It turned out the driver transistors were undersized by a factor of three, so we had to parallel them in a 3-stack with individual emitter resistors to have a stable amplifier. The fix worked; but we had to recall every amplifier in the field and update the circuit board with the new driver stack. It wasn’t cheap, but it stopped them coming back, and they kept working.

This incident resulted in Bob Sickler, the new guy who saved the company, getting permission to design a new amp of his own, which became the Audionics CC-2, their most successful, reliable, and best-sounding product. They sold more than 1,000 with a failure rate of less than 0.3%, the lowest in the industry. The CC-2 was their most profitable product.

All this happened because one part, a driver transistor, had poorly understood behavior in a boundary condition. Ask for too much current for too long, go past the SOA boundary, and blooey! A scorched circuit board with mostly shorted transistors, all thanks to DC coupling propagating the single point of failure through the entire output section in less than a second. Engineers love DC coupling, but it can propagate failure very, very fast, in less time than it takes to jump across the room and turn it off.

That experience is why I am wary of dismissing boundary conditions. Poorly understood boundary conditions can destroy a product, destroy consumer trust, and take down a company. All from not reading a data sheet carefully enough.

Hi Guys,

I thought I’ve give an update on the raven (preamp). I’ve had it in my house for the last few days and have been putting it through the paces. My system setup is currently, the optical rendu from Sonore going into the Holo Audio May, with raven preamp and the Don Sachs 300b mono statements. All the electronics are plugged into the Puritan PSM156. Speakers are modded Spatial M4 Sapphires. The system is in a treated, dedicated room with dimensions 17W x 25L with a partial opening in the back. Overall the raven just terrific, it’s incredibly transparent it seems to just add some air and some tonal vividness. I’ve tried running the 300b amps directly from the May using AUDIRVĀNAs volume control, it’s very good but with the raven in place the soundstage grows larger, separation increases and theres more palpability to instruments. The timbre of interments just pops more. The best part is I’ve gained all of this with no loss of resolution. I’ve yet to experience a preamp that didn’t loose resolution to good digital volume control until now!!

One of the key features I was interested in is the headphone amp in the raven. And it didn’t disappoint either! Comparing it to my Burson Soloist SL MKII with Sennheiser HD650’s. The Raven is far less grainy sounding, the proverbial veil has been lifted from the music. That same wonderful vivid tone is there in spades, and instruments are more isolated sounding, especially when panning from left to right. The Burson just smears a little bit more. 

The Raven 🐦‍⬛ definitely is my favorite preamp I’ve had in. It excels with all the genres I’ve thrown at it. (I’m currently listening to Stevie Ray Vaughan and ZZ Top 🤘) I circle pop, rock, electronic, jazz, hip-hop, and singer songwriter/folk. And i don’t find myself preferring one genre over the other. It’s been terrific with all of it. 


Some standout music to listen try

Boys at school by Spellling

Simmer by Hayley Williams

Elektro Kardiogramm by Kraftwork

Twist of Rit by Lee Ritenour

Magma by Yello

Rich by Yard Act

Risky business by ZHU (if your system can do bass turn it up to 11 and smile)

Business time by flight of the Conchords (because it’s Wednesday) 

Glad you’re enjoying it! As I might have mentioned earlier, in person or in this forum, I’ve been listening to the Karna/Blackbird amps for twenty years. It’s just what an amp sounds like to me.

But the PAF is the first time I ever heard what my slightly older design, the Raven preamp, actually sounds like, and it was quite a revelation. Ultra-fast, vivid, and of course, dead silent. Many thanks to Don and the Spatial team for taking it to the next level, rolling in a lot of good ideas of your own. This has been a very enjoyable collaboration, and I look forward to more.

I should also thank Don for suggesting the best way for the preamp and amps to work together, as a unit. This is the special XLR direct-in mode for the Blackbird, bypassing and disconnecting the input transformer. This lets the Raven output transformer do the phase-splitting chores with its matched split secondary windings, one of the charms of custom iron, made for the purpose.

@atmasphere Please don't think I was implying that "tube watts" are somehow louder than "SS watts".  Just that I have heard both driving speakers and straining and I know which I would prefer to listen to when driven hard.   When I was building the 60 watt push pull Kootenay amps I had numerous customers sell 120-200 watt SS amps after getting one.  Many of them described the amp as easily playing as loud as their big SS amps on 87-88 dB speakers and sounding better to their ear.  That of course means nothing since obviously 60 watts was plenty to drive their speakers to levels they liked in their rooms, but they were shocked what a "mere" 60 watts could do.  I am not implying that my 60 watt tube amp was the world's greatest, just that a competent 60 watt tube amp can do a heck of lot more than many people think.

As an aside, I have an old customer who has the stereo prototype of the 300b mono project amps.  He hooked it up to a small pair of Maggies that is a difficult load and told me that was the first amp (27 watts/ch) that drove those speakers that well and he had tried some large SS amps before.   So it is quite amazing what a reasonably well made tube amp can actually drive.

The previous point about Class A operation in a differential stage still holds: what happens when more than 100% of the current programmed in a current source is exceeded?

It won't if the circuit is properly designed!

The real question is what happens when the drive to the differential gain stage exceeds the range of that gain stage. The answer is one of the devices saturates while the other goes into cutoff. Picking the right amount of current in the constant current source (if there is one, differential amplifiers do not need a CCS to work... the first circuits we built employed a bipolar power supply; the cathode resistor had the entire B- Voltage dropped across it; this limited current to the same extent that any cathode resistor might in any single-ended circuit) is the key to making sure that the design isn't limited by the CCS. Instead you want it limited by other parameters- the tubes themselves, the plate load, etc. The addition of a CCS increases differential effect- thereby increasing gain and decreasing distortion, as well as improving bandwidth, assuming that the CCS does not impose a bandwidth limit.

@donsachs I get it. I was trying to point out the difference between what sounds 'louder' and actual sound pressure; as you know from playing tube amps the two are not always the same. IMO this is one of the bigger failings of SETs with zero feedback since, more than any other kind of amplifier made, they tend to sound louder than they really are due to how they make distortion.

60 Watts isn't a whole lot less to our ears than 250 Watts is due to the logarithmic nature of our ears. So as long as the 60 Watts can adequately drive the load it can do quite well. This is the same reason we didn't try to build a super high powered class D amp. It was more important to get it right than it was to make a lot of power- as it is, it makes 200 Watts into 4 Ohms (250 at clipping). If your speaker really needs more than that kind of power to really fly, its borderline criminally inefficient, since to merely double the sense of volume to the ear, you need ten times the power. To my understanding there are no 2500 Watt amplifiers that sound like music.


Yes we agree.  If you cannot drive a loudspeaker in a normal room to adequate levels with 60 watts/ch, then you really need to reconsider your speaker choice!

The least efficient speaker I ever owned was a pair of Joseph Audio RM25si Mk2, which were 89 dB and a true 8 ohm easy load.   With the 60 watt amp I could play them to FAR higher levels than I would ever listen to in a very big living room.  Now my speakers are generally in the 95-97 dB range so I could do the same with 10 watts.   The 27 watt 300b monos hardly know the speakers are connected.....

There are so many just brilliant speaker designs out there that are 89-90 dB+ and easy loads.   Why on Earth people get huge 84 dB poorly behaved speakers that require 200 watts/ch or more is beyond me.....

@donsachs Yes- those Joseph speakers were nice.

I can see 88-89 for smaller speakers. I have a little 5 Watt tube amp  I designed for desktop or a bedroom system and I use a pair of Fritz Carbon 6s with it, which are 88dB and I never run the amp out of gas (but I never play it that loud either).

But if they are going to be large there's no reason they should be hard to drive. I keep telling people that if you want to get the most out of your amplifier dollar investment, its best served by a speaker that is higher impedance and easier to drive, on account of the simple fact that the harder the speaker is to drive and the lower the impedance it is, the more distortion the amp is going to make. IOW a simple way to make any amp sound smoother and more detailed is to have it drive a higher load impedance (if all other things were somehow equal, which they never are...).

Most audiophiles do not realize how stupendously inefficient speakers are. By way of reference, 92 dB/watt/meter is about 1% efficient, or put another way, 100 watts of electricity is converted to one acoustic watt (which is plenty loud).

So where does the other 99% of these pricey watts go? Voice coil heating, which isn't great considering how tiny voice coils are, and how poor thermal coupling to the outside world is. First the voice coil has radiate its heat to the magnet, which is the closest thermal sink, then the warmed magnet has to transfer its heat to the inside of the enclosure.

Since the goal is to create X amount of acoustic watts, not a clumsy form of room heating, even small gains in efficiency are worthwhile, since less voice coil heating is occurring for given acoustic output.

Aside from outright failure, another problem with VC heating is copper's change in resistance with temperature. The resistance goes up with temperature, which might be acceptable, excerpt the time constant is fairly slow, on the order of several seconds, This creates a dynamic slurring which is pretty audible.

I should add I am completing a large-format 2-way speaker this summer, a collaboration with Thom Mackris of Galibier Designs, and an entirely separate project from Don Sachs and the Spatial Audio team. It’s a culmination of the extremely long "Beyond the Ariel" thread over on DIYaudio.com, and the first version was built by Gary Dahl, of Silverdale, Washington.

The woofer is an Alnico-magnet 416 (15" midbass) from Great Plains Audio, the successor to Altec Lansing, using Altec staff and tooling. It’s in a low-diffraction (4" radius curved edge) 4.2 cubic foot closed box. My version will have Bubinga (African rosewood) veneer on all sides.

The high frequencies are from an Athos Audio Yuichi A290 wood horn, with a to-be-determined 1.4" exit monitor-class compression driver. Crossover will be around 700 Hz, most likely Altec-style 2nd-order. The RCF 850 and 18Sound drivers are candidates. I also have a pair of Altec/GPA 288’s in house as fallbacks.

Efficiency will be a true T/S value of 97 dB/meter/watt. With a 27 watt/channel amplifier, headroom should be, in the timeless words of Rolls-Royce, "adequate". Alternatively, sufficient for a studio monitor application.

A 20-watt amplifier and 97 dB/meter loudspeaker was pretty typical for a serious high-end system in the mid-Fifties, so it’s not as weird as it sounds. It’s only weird in the modern context of 200 to 500-watt Class D amplifiers and 85 to 87 dB/meter audiophile speakers.

@lynn_olson - First, let me state that I've really enjoyed your, Don's and Ralph's discussion of amplifier design and tradeoffs.

I've been intrigued by Alan Wright's designs (I'm currently building a line stage preamp inspired by the RTP3D). You've done a good job explaining the downsides of a differential output stage. Alan was a big proponent of this approach so he obviously felt it had advantages. Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp? 



As you might imagine, Allen was pretty shocked at the direct comparison, since his amp had much more powerful tubes than mine, which had generic Sovtek 300B’s, good and tough, but nowhere in the same league as Vaic’s finest. I mean, a quartet of top-of-the-line 300B’s ain’t cheap, so I never went down that road.

And Allen had just given a presentation at the VSAC, only hours before, on the power of this secret circuit, which he did not fully reveal. It was a very large current source with heat sinks and all. Yes, he could have cranked up the current even more, but the heat sinks and power transistors set an upper limit on the current. It was already close to max output.

He expected that I, an old Tek hand, would be thrilled with Tek-scope type circuit. But I disappointed him. Driving deflection plates (at very high speed) on a CRT is one thing, driving a loudspeaker is quite another. And I’d been designing speakers for Audionics several years before joining Tektronix in 1979.

Scopes are about speed, and the load is a very well-defined capacitance. Cascode differential circuits are the right answer for that problem ... they’re very fast, ideally suited for square waves, and linear enough for the purpose.

Speakers are orders of magnitude slower and are inherently vile loads. The best speakers are the worst loads ... the ones that have near-resistive loads are planar-magnetics with very low BL product (which is magnetic coupling). As you raise BL product, efficiency goes up, they get snappier sounding as the coupling gets better, and ... they also get more reactive, for the simple reason the amplifier is in more intimate contact with the big, sloppy, electromechanical system. Few amplifier designers are aware of this ugly reality. They keep hoping for speakers that can never exist.

The worst thing speakers do is insert speaker colorations (through back-EMFs) into the feedback loop, where they do not belong. Feedback is great at correcting amplifier nonlinearities ... it’s fast and responds in microseconds, just what you want. Speakers have inherent high-Q resonances that are an inescapable part of an electromechanical device. The better the magnetic coupling, the worse it is for the amplifier, which has dirty spurious currents injected into the output node by the speaker.

My approach is to brickwall-isolate these back-EMFs to the final output stage, and not expose the rest of the amplifier to them. I think of the speaker load like attaching a vacuum cleaner motor to the output section ... a source of noise and garbage, nothing good about it. The amp has to ignore this racket and continue to do its job. Feedback amps can get into trouble when the error voltages get very large; this can saturate the input section, and induce additional distortion.

In a more conventional application, like a long-tail Mullard phase splitter, differential circuits have a subtle imbalance that is not obvious at first glance. On the top, or front, side of the circuit, there is the expected Miller capacitance, as per expectation. This is the inverting side ... grid goes down, plate goes up, just like you expect.

The non-inverting side can be drawn (and is better understood) as a cathode follower driving a grounded-grid stage. Rotate the other tube by ninety degrees and it becomes more obvious. This side of the circuit has very little Miller capacitance, making it ten to twenty times faster than the other side. The beautiful symmetry falls apart at (very) high frequencies. As mentioned earlier, it can never enter Class AB drive when one side cuts off, although this is not a problem if the diff-pair is not used as a driver. In a scope, you see clever bootstrap circuits and cascodes to give that extra push at high frequencies.

This is Nelson Pass’ speciality; high speed cascode differential circuits. If that’s your thing, he has an amp or preamp just for you. If you’re using transistors, this is an attractive path.

I should mention that Allen Wright liked a very different sound than I do; he liked fast, snappy, and what sounded to me like thin bass. I like a big, lush, spectacular, CinemaScope sound, the sound I heard in 70mm theaters when I was growing up. (Which had Altec Voice of the Theater speakers behind the screen, along with Altec amplifiers.)

The same applies to my brief encounters with Nelson Pass. He likes it a lot thinner than I do, but with a different tuning than Allen Wright. Kind of hard to describe, actually, since this was all a long time ago. Allen liked the sound he was getting, and he liked his own amp, even at that meeting all those years ago. What I thought was a disaster seemed OK to him. In all honesty, it was a split decision.

I mean, I didn’t like it, nor did Gary Pimm, but we were on a different wavelength than Allen Wright. His designs, like mine, are tuned to his own tastes, and we found out they were surprisingly different. Similarly, I was surprised at Nelson Pass’ tunings, very different than my own.

As it is, Don and I have a bit different preferences, but at least we are still on the same planet, so we get along. From what I heard of Allen’s designs, no way, they are too different, no good way to reconcile the two approaches. But he was a really fun houseguest, and Gary Pimm and I had great discussions with him about everything under the sun.

I miss him very much. He was really funny and one sassy dude with total disrespect for the high and mighty poo-bahs in the industry, which I very much shared.

There was a funny incident a few years back at the Dallas Audio Show. Back then, it was a little bitty thing, just a few exhibitors, but very much a home-town thing where everyone knew other. New to me, of course, as a former West Coast guy fairly new to Colorado. Never been to Texas before.

I wander aimlessly down the halls, no real goal in mind, looking for interesting tube gear. I walked into one room, and whoa, that’s Nelson Pass! Now people joke about me being Mr. Natural, but Nelson really looks the part. You can’t miss him. Me, my only trademark at a show are the Hawaiian shirts I like to wear.

Nelson had actually built an open-baffle speaker around a Lowther and a 12" guitar speaker called the Tone Tubby that I had written about some time ago. Well, that’s different, but why not? As I turned towards the door, Nelson blocked the exit.  how do I get in these situations? Me and my big mouth.

It turned out the two drivers were bi-amped with a simple low-level crossover. Oh, now I get it. Four knobs, two for level, two for the crossover frequency. Nelson wanted me to tune the thing ... by ear.

Now I really want to escape, but Nelson is still in the way. Fine, anything to get out. Twiddle, twiddle. Too little bass. Mo’ bass, man. Turn that knob up. A bit less Lowther, but not too dull. Mess with the crossover overlap some, so that mellow hemp cone transitions into the characteristic hard paper Lowther cone. A few minutes later, sounds OK, as good as I can get it right now. (Did not sound OK when I walked in.)

Escape permitted. Afterward, Nelson allowed as to how he saw that article I wrote about the charms of the Tone Tubby and wanted to build a simple open baffle around it, with a Lowther on top. So he figured if anyone could tune it on the fly, it would be me. Well, he had me there, but I allowed that he might have different preferences than I did, so feel free to mess with the knobs, although he might want to mark the current positions before changing anything.

These weird things happen to me at shows. That’s how I met Nelson Pass.

Yes we all have different tastes.  I would say the Lynn and I overlap considerably.  When we discussed our favourite rooms at the PAF we were in general agreement.  I stand by what I said a million pages back in this thread.  If I were to magically create a straight wire with gain that could drive any speaker load with infinite power and had no sonic signature at all, my guess is that half the people wouldn't like it.   Heck, maybe I wouldn't like it.   If I were to characterize what I favour it would be lush, not mush, an ethereal high end where cymbals and triangles hang in the air and decay, and tons of detail without brightness.  I hate artificial, in-your-face detail created by bright sounding systems.   I am an imaging freak as well.  If I build an amp or preamp and the sound stage doesn't extend at least a few feet outside the speaker boundary and the vocal doesn't appear at the floor to ceiling interface, then something is wrong.   I was a little dismayed at the show when our system could give great depth, but we had a shoe box of a room and the sound stage was constricted in height and width by the room boundaries.  Same system in my room threw the sound stage I just described.  I voice things with acoustic instruments and vocals.  If you can get a piano right you can pretty much play anything else with correct tonality.   If you can follow individual voices in a choir or instruments in an ensemble of some type, then your system can handle complex passages without breaking down.

Oh, and I love a good horn system.  Not the cheap ones that are popular, built with junk parts and cabinets, but a good horn system.  

So that is what I like.  Lynn and I agree more than we disagree!

Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp?

@jaytor Since Lynn isn’t going there I’ll take this one. The advantage is the differential effect reduces distortion in the output section and makes the output section easier to drive since it will have a bit more gain.

There are a lot of differential output sections in well known tube amps- such as the Dynaco ST-70. What makes it differential is the use of a common cathode resistor. A Constant Current Source (CCS) can help performance but isn’t needed for the gain stage (whether an output section or not) to be considered differential.

Despite Lynn’s remonstrations, if designed properly a CCS in the output section of an amplifier will not limit current right up to the full power of the amplifier; in fact if the output section isn’t differential, using a pair of cathode resistors rather than a common one, the output power is unchanged or even reduced. I’ve seen applications where the use of the CCS actually increased the output power by a few Watts since the distortion was held in check to a higher power level.

What might not be obvious WRT an output section is you can set up the cathode circuit regulator to sense B+ variation and adjust the cathode voltage in response, which reduces distortion and increases tube life. This eliminates the benefit of a regulated B+ which would otherwise be a hefty lift in terms of execution and cost. IMO Lynn is missing a bet on this one and leaving performance on the table.


@lynn_olson @donsachs @atmasphere - Thanks for your responses. My system is bi-amped with a SS amp driving the lower three octaves, so I'm primarily concerned about upper bass on up. I'm pretty happy with my current DIY 300B PSET monoblocks, but I like to tinker.

I've primarily been building electronics based on other's designs (at least at the module level), but I've recently been trying my own designs (heavily influenced by others). I think I want to build a PP amp as my next amp, using either 300Bs or 2A3s. My main speakers are pretty sensitive (98db/w line arrays) so I don't need a ton of power. I'm still thinking about what approach I want to take. 

I have a question about the first 6sn7 stage in my 300B SET. What is the best way to make load and coupling?
1. The drawback of the interstage transformers is a narrow bandwidth in combination with 6sn7 that has high internal impedance. But low distortions.
2. RC - coupling has wide bandwidth but higher distortions.
3. LC - coupling looks good, but what is the optimal inductance for 6sn7? The issue can be not deep enough low frequency bandwidth and high LC resonance Q.
4. What are the drawbacks of SRPP with C or direct coupling? What is for and against it?


your choice of 6sn7 as driving tube is not good (high output Z, low max plate current...)

Now, your choice 1 of implementation is the better one, only your concern is invalid.

High output Z does not automatically leads to narrow bandwidth.  Bad Interstage transformer causes it.  High Z tube can't drive 300B into deep A2 so you never get 8W  class A output from 300B that way. However, you can use Western electric's classic Ultra path design pattern, combining a CCS with a 0 DC current nickel 1:0.75 interstage transformer and achieve pretty good result if you bi-amp the speaker.


@alexberger The 6SN7 can support fairly high current which is why it can make a good driver tube if used properly. It can also make an excellent voltage amplifier (1st stage of gain) since it is quite linear if used correctly.

SETs usually do not need much gain since the speaker used with the amplifier should be high sensitivity (if a 300b power tube, +100dB is a good value). The 6SN7 will allow plenty of gain for this. One section can be used as the voltage amplifier and the other section the driver.

You should be able to get plenty of bandwidth using a 6SN7 and an interstage transformer! The issue will come down to the quality of the transformer itself.


I use 1/2 of 6sn7 in the input stage. The driver stage is the 6f6 tube drives the 300b though interstage transformer. The coupling between 6sn7 and 6f6 is RC with Vcup Cutf capacitors.

Ralph, I sincerely invite you to build your own 300B amplifier. (The name of the thread is "300B lovers", after all.) You’re smart enough to bring Allen Wright’s topology back to life, and give it unique improvements of your own. Seriously, if anyone can do that, it would be you, not me. The only way to explore the 300B sound is design your own amp around it, and I have no doubt it would turn out well. I think you would be quite pleased with the result.

Don, myself, and the Spatial team are developing the Raven/Karna/Blackbird architecture discussed in this thread, with really good suggestions from the whole team. Much appreciated, I can’t say that enough. You guys have taken it way beyond the 2003 thought experiment that Gary Pimm and I built.

What folks heard at the 2023 PAF show was a good preview of the full-on production model, which Don is charging ahead with as we speak. (Well, actually, I was just on the phone with Don, and he was at the beach, but tomorrow, OK?)

Alex Berger, what you have sounds fine to me. The only refinement would be separate power supplies for the input+driver and the output section. And maybe a dedicated filament transformer for the 300B, whether AC or DC powered.

Hi Lynn, 
Reading your previous post encouraged me to order a power transformer and a pair of 15H chokes for input and driver tube. It will give me not just PS separation but also reduces the load current to the 5u4g rectifier that I use for all tubes in this stereo amplifier (180mA total). It also gives me a freedom to increase drivers idle current and probably I will try 6v6 instead of 6f6. 6v6 needs more current.
Now I use very big capacitors for B+ - 2000uf for 300B and 1000uf for drivers. What is the advantage of separate power supply for drivers over big capacitors? How do behave big capacitors during the clipping of 300B?

@alexberger Consider using the tube rectifier for the front half of the amp and good diodes for the 300b.   Do a C-L-C for each and use a good film cap as the last one after the choke.   Separate power supplies allow the input and driver tubes to basically not know the 300b is clipping.  When you push the 300b the power supply for the whole amp sags a bit.   With separate supplies the input and driver sections are isolated from the 300b.   You will hear a certain ease and clarity to the sound that you don't have now.  The Blackbird uses a similar approach, but much more sophisticated power supplies, which I will not get into.

I second what Don just said. Isolation is the key. You can get away with a power transformer that has dual isolated secondaries, but this is a specialty item so don’t bother tracking it down. At the DIY level, just get another B+ transformer that runs at a voltage suitable for the IT-connected driver tube, and take it from there. In terms of rectifiers, damper diodes are the quietest of all, but they consume a lot of heater current at 6.3 volts.

The dual power supply approach is surprisingly rare in consumer equipment, even at extremely high price levels. It is the single biggest improvement you can make to any tube gear, from push-pull 6L6, EL34, and KT88 to SET amplifiers of any type.

Extremely large banks of electrolytics are popular over in transistor land, but they are frankly mediocre sounding caps, compared to good film caps of more moderate values from 50 to 200 uF. In the Karna, I used banks of industrial-type motor-run caps from ASC and GE.

These are precision parts designed for extremely severe duty outdoors. I prefer them to audiophile parts in that application. A minor audiophile tweak is to bypass the industrial array with a single 0.1uF cap of very high quality, such as copper foil. (Also use copper foil for the RC or LC coupling of the 6SN7, but be aware that wax caps are not suitable for under-chassis use.)

Location is important. Keep the wiring, especially in the cathode circuit, as short as possible. This is more important than the type of wiring, although if you want to go nuts, use industrial Litz wire for the critical audio path. Litz does require a solder pot to get rid of the enamel coating, a minor annoyance when working with it. A close second choice is tinned stranded, which is super easy to work with.

This approach, if done right, will take your SET performance to the mid to upper tier of Audio Note, in the $20,000 to $50,000 price range.

Here’s a picture of Gary Dahl (seated) and Gary Pimm, shortly after Gary delivered the newly built Karna amps to our living room in Silverdale, Washington. 2003.

Silverdale, 2003

Gary Pimm and Gary Dahl look under the chassis

New Karna amps, 2003

Ariel speakers and Karna amps in Silverdale, 2003

Full schematic, 2006

The dual B+ power supply chassis is external, connected by the aviation-grade Amphenol connectors at the rear of the chassis. The glowing VR tubes are at the front of the chassis. The EL34 on the left side is part of the high voltage current source that feeds the VR tubes. The audio tubes are 5687 input, 45 drivers, and 300B power tubes. Interstage transformers are under the chassis.

This picture gives an idea of what Don accomplished over the past year, reducing this behemoth four-chassis prototype to something that could be practically built, and then exhibited at the Pacific Audio Festival.

The very first one, the Amity, designed by yours truly, and built by Matt Kamna on an open breadboard-style chassis. Matt has since gone on to co-found Whammerdyne, a company that makes 2A3-based SET amplifiers, and exhibited in the Songer Audio room at the PAF show this year.

Amity at night, 1996, Aloha, Oregon

We’ve gone a long way since then!

Ralph, I sincerely invite you to build your own 300B amplifier.

@lynn_olson I did just exactly that some years ago and played the amplifier at a CES in the late 1990s. The 300bs were driven by a cathode follower direct coupled to the grids of the power tubes so the bias was obtained from the driver tube which in turn ran fixed bias. In this manner the capacitance of the grids was a non-issue.

How would you compare the sonics of your 300B amp to the the Class D amp you make now? You’re the creator of both, so you’re in the best position to evaluate and compare. I only spent a half-hour of casual listening to the Purifi at the show (and Audio Group of Denmark), so I’m hardly an expert on the subject.

The thread is called "300B lovers", so tell us what you think about a 300B (of your own creation) vs your latest Class D amp. At the risk of thread derailment (I ask the forgiveness of the Audiogon moderator), and then I’ll return to the walk down Memory Lane.

P.S. Karna and I actually lived in a house on Memory Lane when we were in SW Portland, up in the West Hills, just off Sunset Highway. It's a real place.

This most genial, thoughtful, educational, respectful and generous thread of all time deserves to be a "sticky". Not only is it full of wonderful history, a person can learn a lot from it.

A little ways up @donsachs used the expression "nutshell" in reference to one of Lynn’s posts. Hah! I think it is more like an advanced panel discussion between Don, Ralph and Lynn and some of our other technical gray-beards. Or maybe it can be likened to a graduate seminar in esoteric Amp design.

Wonderful. My nomination for thread of the year, 2023.

Alex, I wish you every success with your revised 300B amplifier. An isolated power supply for the input+driver, and replacing all the electrolytics with arrays of 440VAC (630VDC) industrial motor-run caps, will make a difference that will astonish you. Not joking here. It will definitely take it into the top class of SETs.

Be prepared for a pretty large and heavy chassis for an 8-watt amplifier. 18" x 18" and 50 lbs or more would not be out of line.

As for cap-value tuning, set the RC (or LC) frequencies between 3 and 4 Hz. The beat of most music is between 1.3 and 2 Hz, and you do NOT want any of the RC networks interfering with that. Anything slower than the beat of the music will give a kind of seasick, unsteady feeling, so don’t go there.

Leave the banks of electrolytics to the DC-coupled transistor guys. A lot of their tuning (and amplifier sonics) comes down to the brand of the electrolytic. (Oops, did I let the cat out of the bag? Sorry, guys.)

Don accomplished a miracle of miniaturization for the show, but we’re dialing it back a bit for the production models (to simplify assembly). We’re expecting 18" wide, the exact same width as the preamp, and maybe 16" to 18" deep. Weight ... yeah, maybe 50 lbs or so. Depends on what the transformers weigh. Specs and overall performance will be the same as the show models, so if you like what you heard, that’s what you’ll be getting.



Actually, I have already improved the preamp considerably with a few subtle changes, and the power amps will get improvements to the power supply with the larger case, as well as higher quality and physically larger film caps in what we now know are 3 really important places.  I could not fit some things in the previous case that Lynn and I agree should be added.  I would expect the whole system to sound noticeably better than what was at the show.  Same tonality, but a bit faster, with even better imaging and separation of elements in the sound stage, and tighter and deeper bass.  Experiments are already showing these improvements.  Production by the end of the year is the goal.  When we have production models there will be photos posted on the Spatial Audio Lab site.   I really appreciated meeting people at the show and getting their opinions!

OK, I didn’t know how much Don was keeping under wraps, so I was a little vague about our continued progress.

The new full-size chassis of the Blackbird (compared to the models at the show) gives us the freedom to "open up" the Blackbird ... ultra performance caps in critical locations, a bit of Raven tech in the front end, and more rigorous isolation between the high-voltage power supply and the audio circuitry. All of these need more room, which is why the production chassis will be 18" wide. Sonically ... well, I haven’t heard it yet, but they’re all good things that move in the same direction as the past year of collaboration.

One the things about the Raven/Karna/Blackbird that is frustrating, but also very gratifying, is the circuit is extremely transparent and revealing. The frustrating part is that parts quality is revealed in a relentless glare, at least in the critical nodes of the circuit. This is the downside of any zero-feedback design; there is no clean-up crew of servo feedback to tidy up afterward. You hear things as they are.

But the transparency is also a gift, because "minor" substitutions are immediately apparent in the first minute of listening. I think Don, Cloud, and the rest of the Spatial team will agree on that point. I feel we are fairly close to the upper bound of what the circuit can do, but I keep being surprised.

The Raven, which I had never heard before, took me aback ... that was not what I was expecting. It is super fast and resolved, with sounds flying out of dead-black space. You can practically see the shapes of the notes as they fly by. No exaggeration, no tipped-up HiFi sound, no artificial edge sharpening, but boy, it’s all there. If it was on the recording, you will hear it.

I find it kind of shocking that a late-Twenties Bell Labs/Western Electric telephone repeater, built with modern ultra-wideband parts and quiet MOSFET cascode power supplies, sounds like that. There ain’t nuthin’ retro about the sound at all.

These pictures give me a bit of a chill. It isn’t a Raven, not quite, but it’s pretty dog-gone close. Both date from the Twenties, and they come out of Bell Labs. In the first, note the archaic nomenclature for the direct-heated tubes and the weird little capacitor ... and what the heck is it doing, dragging the lower grid off-center?

Western Electric 7A Repeater Amplifier

This might look familiar - the WE 42A amplifier

Build either with modern parts, and it sounds like it’s from outer space. It’s like discovering a working spaceship under a long-forgotten tomb.

On a more technical level, here’s the relevant discussion, a deep dive into audio archeology:

Western Electric, the Rosetta Stone of Audio

Oh, that wasn’t technical enough for you? Well, chew on this, the way I got my first job at Audionics, by inventing this little gizmo:

Shadow Vector Quadraphonic Decoder

Sadly, only one prototype was ever built, but it did get demonstrated at EMI and the BBC in 1975. After that, loudspeakers, Tektronix, and various magazines.

Believe it or not, there’s a programmer in the UK who actually built this in software a couple of years ago. Now, that’s impressive. The 1975 hardware prototype took nine circuit boards plugged into a backplane ... the UK programming genius took it to the next level, and made it into an eight-band decoder, all working in parallel, thanks to the wonder of modern DSP.

Yes, it is quite surprising how sensitive these two circuits are to parts choices.  Much more so than any I have ever worked with.  I am used to coupling cap differences, and I have my favourite types, but there are several film caps in here that make as much or more difference.  Also, I have been through several versions of anode loading, and it is clear in this circuit that a custom designed interstage transformer walked all over the other choices in the amps.  We developed a new twist on the regulated supplies for the preamp, and now we will extend that idea to the power amp and then we are done.  As Lynn said, I think we are reaching the limits of what the circuits can do.  Also, I am trying the excellent Monolith Magnetics transformers as soon as they arrive.  I will see if this pushes the amps over the top!  As I believe I said earlier in the thread, this is pretty much a cost no object project to see what is possible with these circuits.   They sound unlike anything I have ever heard or worked with.   The better you make them, the less they sound like anything at all, which is about the highest compliment I can give an amp or preamp.

@bcurtis1 I looked that piece up.  It has absolutely no tubes of any sort in it at all.  Can you enlighten me?  Did I miss something?

I tried to paste my screen shot showing the 300 B line stage in the chassis. Hybrid so not a purist design like yours. Being a biased owner  with other Amplifier ownership over some years it is a worthwhile audition if convenient.

Best Regards 

Much appreciated.  I will investigate further.  Vinnie is a well respected designer of note.




Since this is (mostly) a 300B thread, I’m still curious about the general circuit of Ralph’s 300B amp. He’s been doing this a long time, so his design choices are of considerable interest.

I have my own way of doing things, and that was strongly influenced by my research when I was writing for Glass Audio and Vacuum Tube Valley. John Atwood, in particular, showed me the Bell Labs archives and other primary sources. Charlie Kittleson, the magazine’s founder, had a treasure trove of working 1930’s electronics, which sounded very different than anything I’d heard before ... not like the Fifties sound at all. They clearly had different priorities back then.

John Atwood and I have a lively interest in early technology, particularly early monochrome and color TV in the USA, the UK, France, Germany, and Russia. Developments in color TV filter technology went on to influence Neville Theile in Australia, modern crossover design (Laurie Fincham in the UK), and the time-switching technology used in the GE/Zenith FM Stereo multiplex system.

Hi Lynn and Don,
What do you think? Is it a good idea to use driver-input power transformer taps to feed 300B filament? This power transformer should be less pushed then the 300B power transformer.
I don't use only electrolytics capacitors for B+. My DIY friend taught me to use a mix of electrolytes, polypropylene and vintage industrial oil capacitors (Siemens MKV, Tesla, KBG-MN and new Obbligato). All this capacitor bank is bypassed by 0.1-0.22uF PIO and 0.001-0.01 uf Soviet silver mica. This bunch of capacitors doesn't smare sound. The only drawback I can hear is the relatively long warm up time (at least 2-3 hours). Probably a 200+uF set of good quality vintage industrial oil capacitors will sound better. But it will be more expensive.

The 300B filament circuit is a very delicate circuit node. The high voltage windings of the B+ transformers see switching pulses of hundreds of volts with very steep rise times. It only takes a few pF of winding-to-winding capacitance to transfer that 120 Hz switching noise straight into that expensive 300B. Unless you know that the low-voltage winding is electrostatically screened (with copper foil), don’t do it. Use a separate transformer just for the 300B alone.

You wonder where low-level buzz comes from? Winding to winding stray capacitance. Whenever you hear buzz instead of low-frequency hum, that’s a capacitive coupling, not magnetic. The spectrum gives it away.

I would NEVER use vintage caps in a power supply. Never never never. Use modern parts. They’re not that expensive, and used in air conditioners all over the world. Vintage is OK in a crossover, where failure is no big deal. In an amp, just say no.

Not sure I see the merit of mixing films and electrolytics. If you have the space, use the industrial parts, and bypass caps to personal taste. In terms of location, the cap bank can be several inches away from the tube socket, but the little caps (0.1 uF or less) need to be close by, an inch or less.

You can do a lot of sleuthing just by listening to the spectra of noise. Magnetic induction is going to be pure 50/60 Hz and fairly hard to hear. Capacitive coupling is high frequency only, and will sound like buzz, usually harmonics of 100/120 Hz switch noise from the rectifiers and transformer secondaries.

Ground loop noise can be isolated by shorting the input plugs of your preamp or power amp. If the input is shorted, and the noise persists, it is inside the component itself, and is usually a design or layout error.

If the noise is the result of two components connected together, that is a ground loop. This can be confirmed by disconnecting the interconnects between them, turning both on (with volume down), and using a DVM to measure the AC voltage potential between the two chassis. Scrape through the paint or anodize if you need to, then measure.

The AC potential between the two should be less than 1 or 2 volts. If it is more, then you have a ground loop. This is caused by capacitive leakage from the power transformer to the chassis. It can cured by reversing the AC polarity going into the power transformer on ONE of the components, but this is not a DIY job.

What causes this is that consumer AC power is not balanced; instead, there is neutral, which is only 1 or 2 volts away from safety ground, and hot, which is 120 volts in North America and 220 to 240 volts elsewhere. Power transformers are not symmetrically wound; one side has lower capacitance to ground than the other, but unfortunately, the leads are not marked, so they can randomly assembled in production. Ideally, the low-capacitance side of the primary should go to HOT, and the high-capacitance side of the primary to NEUTRAL.

If all your components were assembled this way, you would never have ground loops. Unfortunately, the phasing of the power transformers is random. The capacitive leakage from primary to transformer case will let the chassis float to a high value relative to safety ground, which is the true ground. The only real solution are medical-grade power transformers, which have extremely small leakage to chassis.

Short of that, you can hire a skilled technician to wire all of the power transformers in your system for minimum AC HOT to chassis leakage ... which is a good idea from a safety perspective anyway. No more little shocks when you touch a component (which should never happen in equipment built to code). Safety code requires that the fuse, then the power switch (in that order), always be on the HOT side of the line.

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Hi Lynn,

I found online only one filament transformer for 5V - Hammond 546-166MS. This transformer has only one 5v 3A tap. So I need 2 such transformers. I use Hammond in my DIY phono stage. But IMHO Hammond transformers are built cheap compared to Lundahl, Hashimoto, AN or even James Audio. 
Is these Hammond good enough for this task?