In article , Iain M Churches
wrote:


Hmm! Thanks for the info. These are pretty impressive figs. I have
access to a phase shift generator/meter combination (digital of
course:-) made by Feedback Electronics in the UK. That's why I was
interested to obtain some kind of a yardstick.

An initial investigation of my 50W amplifier which has the first and
second stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at
50kHz. So about the same as the Audiolab 8000P, and very close to the
Spectral? Not too shabby:-))

Are the above values with an 8 Ohm load from an 8 Ohm tap?

If you are concerned about this, I'd suggest you also check with reactive
and/or mismatched loads. FWIW Naim amps have tended in the past to have a
series resistor (about 0.2 Ohms IIRC) in their output, and then tell the
user to employ about 10 microH worth of special cable in series. (Which is
conveniently omitted from magazine measurements but is in place when using
the amp as recommended.)

Whereas many non-Naim SS amps either have direct output or have a small
series inductor. These can affect the phase response at 20kHz and mean that
values obtained into an 8 Ohm resistor are not always a good guide.
Similarly, if your valve amp has any reactance or an impedance much above
0.1 Ohms, this may interact with reactive loads and give a distinct change
in result.

How much this matters is a different matter, though. The figures you quoted
look - in themselves - perfectly respectable. Hence unless your amp has a
particularly high output impedance and/or is reactive at HF/LF I'd doubt
this matters compared to the way most speakers and rooms will affect the
phase of what reaches the listener. :-)

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Since you are doing this, I'd be interested to know the complex output
impedance you get across the audio band as this seems to be the kind of
measurement that rarely gets done and published for *any* amp these days...

Slainte,

Jim

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