300b lovers

I certainly confirmed that the 300b monos will drive the Dyn's to the full measure of their sound in my 13'X26'X9' room.  Lynn points out the need for speakers of a certain design, as he listed examples off, to have lots of power (current) to those speakers with amps which have a high damping factor.   My main speakers are Cube Audio Jazzon single driver speakers which by design, require an amp with a very low damping factor, such as most tube amps I can imagine and ss amps like the Enleum 23R and most of the Pass Labs First Watt amps.  

I am very happy that these 300b monos drive both my Cube speakers as well as the Dynaudio C-1's so I conclude that these monos are much more compatible with a larger variety than probably typical ~8wpc 300b amps would drive. 

I started this thread with reference to my Willsenton r300 300b tube amp with 8 wpc.  I swapped that into the rack to try to drive the Dyn's and it ran out of clean power pretty darn quickly, while the 300b monos cruised along easily and cleanly at very high SPLs.   It is a very unfair comparison, but here it is... a 300b tube amp that will drive a lot of speakers that many others likely would not.  This fact dramatically broadens the selection of speakers one could pair with these fantastic amps because having a tube amp that only pairs with high efficiency speakers is awfully limiting as one's choice of speakers today might not be the same speakers you have down the road.  

No, the comparison isn't really fair.  The little Chinese amp is a nice value for the money, but it is a single ended design made of, shall we say, "cost effective" parts.  It probably makes 7 or 8 watts per channel.   The mono 300b amps are built with cost no object parts and custom wound transformers, with state of the art power supplies, and they really are about 27 watts/ch.   They cost multiple times what the Willsenton does.   The Willsenton has a chance on efficient speakers.  It doesn't have a prayer on a speaker presenting an inefficient and difficult load.

Reflecting on a recent phone conversation with Don (he’s in BC Canada and I’m in Colorado, a bit north of Denver), I suggested that SET amps are kind of like a paint-box, and a much more fun way of tuning a system than messing with cables. If you are DIY’ing, there are many ways of changing the tone color ... which coupling caps, what kind of passive power supply, which rectifiers ... the options are endless. And a lot of fun if the amp is on a breadboard and you can solder in new parts in a few minutes.

The characteristic SET sound works in your favor, giving a lot of leeway with parts selection. And the amp is fundamentally simple and easy to understand, a godsend when you are tuning with many variables. Like I said, a paintbox. When you learn painting, you learn color harmony and the art of mixing. Nothing teaches faster what XYZ cap sounds like than heating up the soldering iron and swapping parts.

A balanced amp is a harsher taskmaster. Yes, more transparent, potentially by 20 to 30 dB, but not nearly as forgiving. Colorations can sound pretty ugly if the wrong part is in the wrong place. And there is no feedback to tidy up the mess. Maybe more like working with an airbrush, or transparent watercolors, instead of pigments. There’s still balancing to be done, but the high level of transparency, and lack of feedback, exposes everything. I found this out the hard way with the original Amity amplifier back in the Nineties.

And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising. The SET experience can be a very rough guide telling you which parts sound really awful, but it will not tell you which sound the best.

This mirrors working with speakers. As transparency goes up, tolerance for coloration goes down. In the absolute sense, this is wonderful, because now you’re really hearing the music. In a way, I’m not surprised the simplest topology won ... less to go wrong, and with the most efficient plate-to-grid coupling.

I have explored all sorts of variants of this basic circuit for about two years now, and have learned what I like and understood why.   This is a deceptively simple circuit, except that there is subtle complexity.   Often, we get caught up in the conventional way of doing things, and then spend all our time refining them to try and hide all of the problems and colorations.   Sometimes we need to think outside the box.   I left traditional power supply design for tube amps probably 5 or 6 years ago.  There are huge threads on the sonics of various capacitors in speaker crossovers and amplifiers.  Trust me, I have heard many of the top caps.  The best cap is no cap.  It took time to arrive at that notion.  Time that was wasted auditioning top capacitors instead of thinking outside that box.  As we discussed earlier in the thread, your alternatives are to either directly couple tubes, or use transformer coupling.  Both have advantages in different sorts of circuits.  In this circuit, transformer coupling is best because of the balance at all stages, which is the key to this amp.  Think of it as constantly canceling distortion and balancing itself.   But you cannot just buy off the shelf transformers from company L or H or E or whomever.  For this circuit you need interstage transformers that are optimized for the loads they see and can give flat response from below 20 Hz to above 20 KHz with little or no phase shift.  That is no easy task, and you have to work with a very experienced winder and it takes some prototyping and testing.  Well, over a year of prototyping and testing.   It is not an accident that the best of the vintage tube amps had killer transformers.  So, as Lynn elegantly stated above, this circuit is totally revealing and even though you have relatively few parts in the signal path, they have to be really good parts, and some of them have to be custom designed to get the best performance.  The audio circuit looks trivial, except that it isn't trivial to get that hand full of parts to work really well together.

Two years ago I would never have built a cathode biased amp, and I resisted that notion at first.  Then I thought outside that box and understood what this circuit was doing and why the cathode biasing was best.  I had the notion that all cathode biased amps sound slow and syrupy and deliver far less power than a fixed bias alternative.  Well, in a conventional circuit that is correct, but not in this one.  

The power supply is also highly optimized and we use some tricks to further isolate it from the audio circuit, and ways to isolate the input tube from the driver circuit power supply.  Of course the 300b supply is separate from the driver and input supply.   The filament supplies also are regulated and isolated from each other.   So there is a fairly complex, yet very conservatively rated and reliable power supply that drives this subtly complex, but fairly simple circuit.   If you change one thing you instantly hear it.  So, in this amp there was quite a bit of tuning, again, to remove coloration.  The result is a transparency I have not heard in any other amp to date, plus the ability to drive quite an array of speakers.  Again, don't let the 27 watts fool you.

Is this a perfect amplifier?  Of course not.  There is no perfect amplifier.  Again, if I were to magically create a straight wire with gain and play it for 100 folks, half of them would love it, and half would probably hate it.  This amp's hallmark is utter transparency and tonal correctness.  The piano is in the room with you and you can easily tell a Steinway from a Bosendorfer.  I realize that most everything I have heard to date is quite colored, or if fairly neutral, lacks the resolution of this circuit built this way with these parts.  This circuit is uncolored, transparent, and highly resolving, and has a boatload of driving ability.  It packs serious punch.  Unlike a single ended amp, where the idea is to tune the coloration inherent in the design to suit your taste, this project was about removing the coloration so the circuit could really shine.  Trust me, it doesn't sound cold and clinical.  It sounds like music with all over the overtones in the instruments, the inflections of the voices, etc...  It is not sterile sounding at all, but rather it invites you into the music.  

To each their own, but this amp is wonderful to my ear and ready for production and this thread has been an insight into our design choices, and the journey.  Others make other choices and that is as it should be.  I have no desire to build a 200+ watt amp to drive a very difficult speaker, but I do want to drive most speakers in most rooms.  

And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising.

SETs produce a quadratic non-linearity, which in turn makes for a fairly prodigious 2nd harmonic. If set up properly (if you see what I did there) the succeeding harmonics will fall off on an exponential curve.

A fully balanced amp will have even ordered harmonic cancellation, so the resulting non-linearity is cubic in nature. So the 3rd harmonic will be dominant, but at an amplitude slightly less than the 3rd is when seen in an SET. Succeeding harmonics should also fall off on an exponential curve, but it will be one with a different exponent- the harmonics will decrease in amplitude faster as the order of the harmonic is increased. The reason for this is distortion is compounded less from stage to stage throughout the circuit. Since distortion obscures detail (in addition to altering the tone colors of instruments) this makes for a more detailed presentation, with less harshness and brightness than an SET can manage, which is saying something. In either case of SET or fully differential, the lower ordered harmonics will mask the higher orders.

The ear treats the 2nd and 3rd in much the same way- in that it finds them innocuous. FWIW, a properly functioning tape recorder will produce a 3rd harmonic as its primary distortion component also, so we have a pretty good indication on that alone that the 3rd isn’t a problem.

Its also worth mentioning that a fully balanced circuit, running zero feedback, will produce a greater percentage of usable power- close to or exceeding 95% of full power, while an SET is doing well to make 25% usable power.

Of course setting the correct operating point is critical in either circuit. In a balanced differential circuit, the best operating bias point will usually be just above the maximum gain that the differential circuit can do, with symmetrical clipping. If this rule of thumb is followed, there will be no unwelcome ’left over residue’. To achieve this, a proper Constant Current Source circuit should be used- a simple resistor to B- is inadequate owing to the rather low mu that most tubes have. It will be found that the current sensing resistor that is tied to B- is quite critical. I usually set it slightly high to allow for variations in the tubes themselves.

A good quality CCS cannot be made using a single tube or transistor- you’ll need at least two devices. Semiconductor CCS circuits can work exceptionally well and offer the benefit of no likelihood of tube damage as the tube warms up due to cathode filament arc-over concerns. If you work with differential circuits you find out quickly how important the CCS really is. In most solid state amps I’ve seen the CCS leaves performance on the table. If it is not optimized, the differential circuit will not achieve its best gain, distortion or Common Mode Rejection. So its important to get this bit right.