Power output of tube amps compared to solid states

Trelja, thanks for the kind words.

Unsound, are you referring to the Threshold S/500? If so, there must be some inconsistencies around what is meant by "double its rated power for several minutes" . . . because I remember this amplifier having rather conventional headroom characteristics (but it's been at least 15 years since I've worked on one, so I may be wrong). To clarify, what I'm terming "dynamic headroom" will generally manifest itself as the ability to generate additional unclipped short-term voltage and current beyond the steady-state clipping power.

If "short term" means several minutes . . . then the limitation that keeps short-term capacity from being long-term capacity is almost surely thermal dissapation, not energy storage or output device current-limiting. The only SS amps that I can think of that exhibit this characteristic are what I would call "asymmetrical class H" operation, like the old NAD "power envelope" design. This works by having the amplifier operate from lower-voltage rails most of the time, but for large-signal peaks there is another set of commutating transistors (like conventional class G or H) that pulls the voltage up to a higher rail, and then keeps it there for a fairly long time-constant. The reason why it doesn't operate at the higher rail all the time is purely thermal . . . so I would actually consider it this design a higher-powered amp with thermal limitations, rather than a lower-powered amp with lots of headroom. But that's purely terminological I guess . . .

Do tubes have the same "advantage" in preamplifiers?

No. Tube preamp stages almost universally operate in Class A, so they have a steady-steady current draw regardless of the output level. Consumer preamps also usually operate into high-impedance loads (little current required), and virtually never have output transformers (to perform the power-supply impedance transformation).

In a headroom sense, tube preamps do operate from much higher-voltage rails . . . and even though they tend to have more linearity problems as they approach their voltage limits, there's still usually a certain headroom advantage. But this is likely to only be an issue for something like a professional mic preamp, seeing the raw feed from a large-diaphragm dynamic mic that's three inches from a kick-drum head.

Atmasphere wrote:

I have an additional comment about distortion that Joe mentioned- that of, shall we say, 'dynamic distortion'. Its my opinion that we need some sort of distortion test that uses a non-repeating waveform similar to what you see in real music.

I take it you're familiar with Matti Otala's 1972 AES paper that started the whole TIM measurement thing, and the huge number of papers and followup over several years in response? Do you feel that there are aspects of this issue that need further research?

BTW Norman Crowhurst pointed this out 50 years ago in his writings about negative feedback.

I take it you're referring to Crowhurst's two AES papers from 1957 and 1969. There are several historical things to keep in mind with how amplifiers were rated in those days, and most of what Crowhurst seems to have been concerned with in the 1957 paper is practical problems with the use of negative feedback as a method for reducing costs. He also makes limited to "regenerative distortion" (feedback making distortion of the distortion), but much of the theory here is very vague.

If we look at the perception of simply how powerful the amplifier is . . . I think it breaks down over tube/solid-state lines much more than global-NFB/no-global-NFB lines, and I personally don't feel that negative feedback has a whole lot to do with it.

Now for subjective sound quality and measured performance, feedback is a huge topic, and I'd love to get some discussion on these papers and their theory!! But let's start a new thread for it . . .

@Ralph:

Found you comment about the MBL101e's interesting. Years ago, Michael Gindi had the original MBL, and the best sound I've ever heard from the MBL101. It was I think in part a large function of the size and dimensions of his room blending in with an omnidirectional speaker.

Now Michael had used several tube amps quite happily in those days with the MBLs including Ken Stevens original tube amps as well as the Jadis JA-500. Neither sounded shrill but one could still hear the sonic siggie of each amp.

Also, the impedance of estats drops too--as with my Martin Logans--to around 1 ohm or so in the upper octaves. Yet, the cj tube amps have always worked quite well with the ML (and I've tried many other ss and tube amps with them) despite their obviously not being able to increase power as the impedance drops (and there's the case of Quad 63 that sounded great with Futterman OTL because their impedance went up).

Cheers,

Myles

Myles, the mbl has an impedance peak of about 8-9 ohms in the midrange driver. The designer is expecting the amp to reduce power by 3 db through this range. An amplifier with feedback will do it, one without will not. A good number of tube amps tend to sound shrill on this speaker even though otherwise they have plenty of power. So Michael was probably using an amp with a lot of feedback.

The problem here is that IMO, amps with feedback sound somewhat shrill out of the box, IOW its my opinion that a speaker that requires this will never sound like real music.

Kirkus, I know about the TIM articles but obviously amps designed to overcome that 'issue' were horrendous.

IMO the issue with feedback boils down to open loop propagation delay in the amplifier- IOW its a timing issue. The feedback signal simply does not arrive back at the input in time to make the correction. With a steady-state signal, the amp locks in pretty well over a few iterations, but with a constantly-changing waveform the amp will be chaotic. This is an interesting subject and I agree- a topic for another thread.

What a incredible collection of very high-tech posts! You guys lost me at the first page of the string.I guess all the music I have been listening to while trying to follow along has put me into a brain overload.
I own Acoustat 2s with a really old tube preamp and a old ss power amp.This seems to be a pretty good combination, though I have to say I look forward trying out a tube mono block setup.
I will say that my experience is that tubes sound more alive and real to me.IMHO goes without saying,s
ince I have no where near the knowledge of you guys.I bow to your expertise.

Ralph: Will your 60watt OTLs work (with zeros) or do I need to save up more for your 140watt monsters?

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