In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...


I don't think we should take this DF=1 as a norm. It is just something
that Andre has mentioned to me, in connected with amplifiers without
NFB. There are several on this, and other groups who use SET amps, so
maybe they can provide us with a more reliable figure.

We would need a figure for the amp in question. If specific is unavailable
we could only work on the basis that - from what I have seen in magazines -
values of the order of 1 Ohm seem common. A DF of 1 strikes me as a
remarkably low value even for SET.

Yes indeed. I am still intrigued to know how an amplifier with such
a
poor test bench specification can sound so good. I have no axe to
grind here. I do not own an SET amp.

From what you say, maybe you preferred the resulting frequency
response which would have been *very* different than when used with an
amp with a nominally flat response and a low o/p impedance.

There are still factors in this equation which we have to mark as
unknown. I can hardly go to the gentleman's house and ask to take his
amp to bits to look for the presence of a feedback loop. It would be a
bit like asking the owner of a vintage Bentley for permission to strip
the engine to measure the crankshaft:-))

That is fair comment w.r.t to being polite. :-) However it means that
without the required info then it may be the case that a large portion of
the 'sound' you heard was due to marked changes in the frequency response.

A complication is that the 57 can provoke oscillations in amps that
are not unconditionally stable. The low and reactive speaker
impedance can also aggravate 'd.c. ducking' effects that can generate
off LF effects as well with musical dynamics. No idea if these have
any relevance in this case, though.

We have now established that the speakers are not type 57 but a later
improved export model known as type 63 Mk II.

So far as I know, there was no "Mk 2" as such. There were various issues of
circuit boards, which have some changes of components, etc. So the
electrical properties did alter a bit over the period of the production
runs. But these were mostly slight IIUC. I am not sure if the ones I have
were issue 1 or issue 2, but again IIRC I think these mainly differred in
terms of the LF behaviour at low powers.

I would need to check to be sure, but I think the early versions all show
dips down to around 3 Ohms or less at LF and around 10kHz. With an amp o/p
of 1 Ohm this implies response changes of the order of 2.5 dB. With an o/p
impedance of 8 Ohms (i.e. DF=1) this becomes 11 dB. In both cases I'd
expect this change to be noticable. Indeed, I'd expect that with a lot of
types of music 10dB changes would be quite marked! The results would be
further complicated as the speaker is also fairly reactive. The upshot is
that I would expect the results to sound distinctly different to using an
amp with a low o/p impedance.


At lowish listening levels, the SET distortion was not even faintly
audible. I daresay that at higher levels it might have been so. I
was not invited to touch the level controls, and did not do so.

OK. At low powers I would expect the distortions to fall and hence
what you say seems entirely plausible to me.

Andy tells me that the 211 can deliver 20W with lowish THD. So, if our
listen level was 3-5W, this would explain why there was no audible
distortion.

That seems quite plausible to me in the absence of more specific
information.

Slainte,

Jim

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