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Old 15th August 2012, 01:00 AM   #7771
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Originally Posted by Lynn Olson View Post
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True Class A operation for transistor amplifiers involves large heatsinks (the size of a tower PC), fan cooling, or both. For vacuum-tube amplifiers, which, by design, must operate hot, it's just a modest 2:1 power derating, rather than a complete redesign.

Class A transistor amplifiers and Class A tube amplifiers are very different animals; the first is quite exotic, large, and very low-powered by the standards of the transistor world, while the latter is a rebiased, perfectionist version of a conventional PP tube amplifier. My Amity and Karna amplifiers take advantage of the very linear behavior of Class A PP direct-heated triodes to avoid feedback altogether; with less linear devices, the topology is probably not the best choice.

Moving to the JBL Synthesis, well, the horn is pretty different. The AH425 combined with a large-format Radian 745Neo or Altec/GPA 288 requires no in-band EQ at all; I'm seeing 2-meter on-axis response curves flat to +/- 1dB, and an impulse response that is 0.5 mSec or less. I believe the JBL constant-directivity horn requires a fair amount of in-band EQ, but I have little knowledge of the JBL crossover, and have no idea of the impulse response of the overall speaker system.

I'm seeing figures around 99~100 dB/meter/watt for the new speaker, which is short of the all-horn-system figures of more than 103 dB/meter/watt, but not bad for a direct-radiator, and well-suited for moderate-power vacuum-tube amplifiers. I don't really think single-ended 45 or 2A3 amplifiers have enough power, but then, I tend to think of 1~2 watt amps as headphone amplifiers. Remember, even Paul Klipsch toured the USA using a Brook 12-watt amplifier to drive his demo Klipschorn, and Klipschorns are about the most efficient hi-fi speaker ever manufactured.
Thanks Lynn

I don't mean to imply that Susan's amp could be better than a Karna or SET driving the new speaker. I doubt it will. No challenge here.

The point I'd like to now make is that some of us are not ready to work with potentially lethal voltages; but at the same time, we don't want feedback of any kind.

In my case, the amp is already built. The amp has heatsinks the size of tower PCs. At idle, it burns 110 Watts per monoblock, each push pull device burning 55 Watts. Let's call this a hot-rodded version of Susan's amp. (Sorry I didn't mention this before.)

There's an opportunity here with the Beyond the Ariel. At 100 db /W/M a hot-rodded version of Susan's amp barely leaves the Class A operating point at all. If I have time this weekend, or if you ask, I'll post current waveforms and transconductance curves. Otherwise, trust me, with Beyond the Ariel, the hot-rodded amp operates so far away from class AB that it can't be called AB; and so close to idle that it can only be called class A.

So with that as an adjustment to my original post, would you care to speculate on the potential compatibility of the hot-rodded version of the amp and the Beyond the Ariel speaker?

Thank You
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Old 15th August 2012, 10:28 AM   #7772
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Well, to (indirectly) answer the question, the Ariels sound pretty gruesome with low-power Class A transistor amplifiers, if my experience with the Nelson Pass Class A amps is anything to go by.

The Ariels were designed with low-power tube amps in mind, and I can't think of any good technical reason why they shouldn't sound good with low-power Class A transistor amps. The response of the loudspeaker is flat, the crossover and bass alignment are designed to accept a wide range of source impedances, impulse response is good, diffraction is low, etc. etc.

But in practice - they sound mediocre at best on transistor amps, and are frankly not the right choice for a transistor-amp owner who intends to stay with transistors. I genuinely do not know the reason for this, because on paper, it should work just fine. The measurements are there, but the sound isn't. Set aside the transistor amp, turn on any half-decent tube amp, and sound just glows, getting better as the amp gets better (direct-heated triodes, etc.)

I haven't auditioned the new speaker with a transistor amp, but I suspect this predisposition towards tube amps will be even more pronounced than the Ariels, which I've auditioned with thirty or more amplifiers, of all different topologies and power ranges.

I admit this is purely subjective territory here, and I've reached a situation where I basically don't care for the sound of transistor amps - the tone colors and dynamics are flattened, and stereo images are dried-out and paper-thin. Other folks - friends whose taste I respect - hear the same transistor amps and like them. So this is quite subjective.

When I speak of "tone color", I'm not talking about gross colorations added by the amplifier itself, like the 1950's jukebox sound of badly-designed tube amps, or the gritty sound of home-theater receivers. There's a subtler sort of tone color, the vividness in the music itself - small dynamic shadings, noticing what the body of the instrument sounds like, emotional inflections by the singer, all those kinds of things. Once we get past the gross colorations of mass-market products, there's the deeper issue of whether the system can transmit what's going on with the music. Many high-end amps flatten out the tone colors, in the same way that MP3 compression flattens the music - yes, the tune is there, but the vividness is gone.

Since I design equipment for my own tastes, not that of a boss or a magazine reviewer, I have no idea what other folks experience when they listen to my designs. Most audiophiles seem to listen in a completely different way than I do - I've even met folks who use a 20-item checklist. Not me. It either sounds good or it doesn't, without much middle ground.

I twiddle with the design until I am satisfied, then stop. I am not a tweaker. I stop at "good enough to make me happy", which usually takes six months to several years, depending on how ambitious the design. The new speaker was/is far outside my comfort zone, so it took a lot more time to research and find good collaborators in areas where I knew nothing.

Circling back to the original question - if you're a committed transistor-amp guy, look elsewhere, maybe a dipole, which seem to work well with moderate-to-high power transistor amps. I've yet to hear a successful combination of transistors and horn speakers, but that's just me. For parties, dance clubs or heavy metal, it might be just the ticket.

Last edited by Lynn Olson; 15th August 2012 at 10:51 AM.
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Old 15th August 2012, 12:00 PM   #7773
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Originally Posted by Lynn Olson View Post
Well, to (indirectly) answer the question, the Ariels sound pretty gruesome with low-power Class A transistor amplifiers, if my experience with the Nelson Pass Class A amps is anything to go by.
Leaping in the manner of fools I will say that of small range of transistor amps I've tried, a J.L. Hood is the most pleasant with my early model Ariels.

I say "like a fool" as I've not got a lot of valve amps to compare with. But I've not been able to find a quote on the topic from Mr Olson I would like to ask him the question: have you tried a JLH? And if so where, on the spectrum from truely aweful to merely not good, does it sit, in your experience?

Yours, probably foolishly, etc
(and yes, I've got the bits to build a IT coupled 2a3 PP amp slowly collecting)
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Old 15th August 2012, 01:10 PM   #7774
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Originally Posted by Lynn Olson View Post
Well, to (indirectly) answer the question, the Ariels sound pretty gruesome with low-power Class A transistor amplifiers, if my experience with the Nelson Pass Class A amps is anything to go by.
Thanks Lynn. Well, Nelson's Class A MOSFET amps take the output from the drains. Per Susan, drains are a bad place from which to take output because 1) gain is involved and transistor gain colors sound, and 2) the drain capacitance is large (amplified by Miller) and non-linear if the voltage swings are large (which adds a kind of "jitter" so to speak). Whereas with Susan's design, the sources just follow the input from the IT, no transistor gain, and the drain's capacitor is small (not amplified by Miller) and largely not in the audio path; furthermore, the input capacitance is bootstrapped (sees very little change in V). But even better, the controlling capacitance (CRSS) is not only small, it is effectively constant when driving a 100db speaker (none of that virtual jitter). In short, the principal of operation is fundamentally different from any Pass design.

Quote:
Originally Posted by Lynn Olson View Post
Circling back to the original question - if you're a committed transistor-amp guy, look elsewhere, maybe a dipole, which seem to work well with moderate-to-high power transistor amps. I've yet to hear a successful combination of transistors and horn speakers, but that's just me. For parties, dance clubs or heavy metal, it might be just the ticket.
With the output transformer wired in 4:1 mode, the output power into a 4 ohm load is only 27 fast Watts. Damping factor is 29. That's the mode I was thinking mated well with this new speaker.

In 2:1 mode, output power is 74 Watts @4R with a damping factor of 4. This could be the mode for the dipole, except maybe for the low damping factor.

I realize that it is difficult to speculate on these kinds of things and with an amp you've never heard. Still, I greatly appreciate your thoughts (and anyone else's).
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Old 15th August 2012, 06:15 PM   #7775
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Originally Posted by charliemb View Post
Thanks Lynn. Well, Nelson's Class A MOSFET amps take the output from the drains. Per Susan, drains are a bad place from which to take output because 1) gain is involved and transistor gain colors sound, and 2) the drain capacitance is large (amplified by Miller) and non-linear if the voltage swings are large (which adds a kind of "jitter" so to speak). Whereas with Susan's design, the sources just follow the input from the IT, no transistor gain, and the drain's capacitor is small (not amplified by Miller) and largely not in the audio path; furthermore, the input capacitance is bootstrapped (sees very little change in V). But even better, the controlling capacitance (CRSS) is not only small, it is effectively constant when driving a 100db speaker (none of that virtual jitter). In short, the principal of operation is fundamentally different from any Pass design.
I've always wondered why the Pass amps sound they way they do. I found the commercial Pass amp that I auditioned with the Ariels to be extremely sensitive to choice of speaker wire, which to me is a bad sign (about the amplifier). MOSFET capacitance is notoriously nonlinear and also very high - in the range of 500~1000pF if memory serves.

For reasons that are not clear to me, the Ariels are very revealing of amplifier design decisions. Change the current in the driver 30%, and oh yes, you hear it. Usually in the first few seconds. Decisions that affect the slew rate of the amplifier appear to be the most audible of all - fast amplifiers with high-current drivers pretty much always sound better. The new speaker appears to be even more so - not surprising, since IM distortion is probably several times lower than audiophile (87~90 dB/meter) direct-radiator speakers.

That's part of the reason the Karna amplifiers have such overdesigned drivers; if you keep increasing the input level to offset transformer losses, the Karna can put out nice-looking sine waves at 500 kHz - at full power. Trying that stunt with most transistor amplifiers would be asking for complete destruction of the output stage.

At any rate, the Ariels and the new speaker like high speed amplifiers with very linear high-current drivers. If the amp looks good at more than 200 kHz at full power, remains unconditionally stable with output load gizmos removed, and has a deep Class A region (at least several watts) chances are that the Ariels and the new speaker will like it.

Big slow transistor amps with small Class A regions and marginal stability do not sound good with the Ariels. The sound is either murky and dull, or gritty and harsh, depending on how the amp screws up as it approaches slewing.

Last edited by Lynn Olson; 15th August 2012 at 06:41 PM.
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Old 15th August 2012, 07:19 PM   #7776
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MOSFET capacitance is notoriously nonlinear and also very high - in the range of 500~1000pF if memory serves.
Almost certainly a vote in favor of Susan's investigations.

Over the past two years I have been involved with a hybrid amplifier, designed by Rene' Jaeger. Uses the same PP OPT as you have in your Karna Lynn and the newest solid state devices available. I cannot ever remember just which one's, but Rene' (think Berkley Audio DAC) does obsess over them. The amp uses a large and famous power tube as it's driver stage and the SS portion just adds gain, without interrupting the deeply colorful gradient structures in tones and transients known to come from using this tube. The result is an amp without the somewhat typical noise and distortion artifacts of SE tube amps and without the paper thin rendering of space and texture typical of most SS amps. Interestingly, the sound also has an elusive character to it, that I can only describe as integrity, the same quality I find in the Berkley DAC.

I have heard this same change in SS sound quality in a guitar amp made by another friend, one designed to switch from direct drive to transformer coupled drive with the flick of a switch. The difference here is stupendous. So, I am of the opinion that sand really does love iron/copper and I strongly suggest that Susan and Rene' are on to something quite important. And yes, I do realize that many will be offended by this addition of an "unnecessary" component to a pure SS design, but I think it's worth trying out, I know you will be surprised.

Bud
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Last edited by BudP; 15th August 2012 at 07:47 PM.
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Old 15th August 2012, 07:29 PM   #7777
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Every time I read one of Lynn's posts on amplifiers, I feel so lucky that I'm not him.
Everyone is sensitive to different issues, and its a learned thing.

Susan's amp has some design elements that are appealing.
No global feedback, thumbs up.
Voltage gain by transformers has the potential to be very musical.
Transformer output is a big plus for constant output Z.
Class AB operation; not so much. Especially 100 mA bias. Your modification sounds sensible at the much higher bias.
Followers. With 100% local feedback, do followers sound good. I am not sure.

Bottom line, its your ears.

Doug
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Old 15th August 2012, 09:49 PM   #7778
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Originally Posted by DougL View Post
Every time I read one of Lynn's posts on amplifiers, I feel so lucky that I'm not him.
Everyone is sensitive to different issues, and its a learned thing.

Doug
That's why I ended up designing my own amplifiers; after auditioning what seemed like a metric ton of amplifiers on the Ariels, it dawned on me I had to design my own. At the time (1997~98), there was practically nobody in the USA doing IT-coupled amps, and the NY Triode Mafia told me I was nuts to pursue that direction. But there was something there to be discovered, so off I went ...

Actually, DougL, that sounds like a pretty good list. At a wild guess, solid-state followers sound better than the tube versions. One difference might be nonlinear Miller capacitance; this is a big deal for transistors, and basically hardly happens at all for vacuum tubes. One of the nice things about followers and cascodes is they pretty much wipe out Miller capacitance, so transistors would benefit the most.

Last edited by Lynn Olson; 15th August 2012 at 09:56 PM.
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Old 15th August 2012, 10:27 PM   #7779
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Originally Posted by DougL View Post
Every time I read one of Lynn's posts on amplifiers, I feel so lucky that I'm not him.
Bottom line, its your ears.

Doug
Amen :-)
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Old 16th August 2012, 03:13 AM   #7780
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Originally Posted by BudP View Post
Almost certainly a vote in favor of Susan's investigations.

Over the past two years I have been involved with a hybrid amplifier, designed by Rene' Jaeger. Uses the same PP OPT as you have in your Karna Lynn and the newest solid state devices available. I cannot ever remember just which one's, but Rene' (think Berkley Audio DAC) does obsess over them. The amp uses a large and famous power tube as it's driver stage and the SS portion just adds gain, without interrupting the deeply colorful gradient structures in tones and transients known to come from using this tube. The result is an amp without the somewhat typical noise and distortion artifacts of SE tube amps and without the paper thin rendering of space and texture typical of most SS amps. Interestingly, the sound also has an elusive character to it, that I can only describe as integrity, the same quality I find in the Berkley DAC.

I have heard this same change in SS sound quality in a guitar amp made by another friend, one designed to switch from direct drive to transformer coupled drive with the flick of a switch. The difference here is stupendous. So, I am of the opinion that sand really does love iron/copper and I strongly suggest that Susan and Rene' are on to something quite important. And yes, I do realize that many will be offended by this addition of an "unnecessary" component to a pure SS design, but I think it's worth trying out, I know you will be surprised.

Bud
Along those lines, there's nothing to stop anyone - particularly in this intrepid forum - from combining the Susan final stage with the first stage of the Amity amplifier. In other words, 1:1 Input Transformer -> PP 5687, 7119, ECC99 -> 1:1 IT Transformer -> PP MOSFET followers -> Output Transformer.

The gain structure would avoid any need for step-up transformers - the input and IT could both be 1:1 - and distortion from the PP 5687, 7119, ECC99 drivers would be very low. The driver tubes would use a modest 120V to 150V B+ voltage, avoiding the more troublesome B+ supplies in the 450 to 550V range. And isolated supplies for the driver and output sections absolutely sounds better than shared supplies.

A more exotic version would use a 1:2 Input Transformer -> PP 45, 2A3 -> 1:2 IT Transformer -> PP MOSFET followers -> Output Transformer. This driver would have (much) lower distortion and more dynamic drive characteristics. The B+ would be raised to the 200~250V range for the best possible dynamic range and slew rate from the balanced pair of direct-heated triodes.

If the builder wants to go all the way with this design, I would recommend shunt regulation for the driver stage - fortunately, since the final stage is an ultrahigh-gain MOSFET follower, there's less need for regulation in the power stage (since the followers have inherently high power-supply rejection as well as inherent balance from using matched pairs). Gary Pimm's shunt regulators have extremely high PSRR, well under 1pF capacitance, as well as excellent reliability (they can withstand 1KV transients).

That's a complete hybrid amplifier; anyone interested?

Last edited by Lynn Olson; 16th August 2012 at 03:23 AM.
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