300b lovers

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.