300b lovers

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.