300b lovers

@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? 

Ouch.

None.

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!