The thd is less than 0.1% at up to 10 watts into any load between 4 and
12 ohms, which means that at 1 watt its down around 0.03%, even though
it is an SE amp.

That is quite good in itself. Does it maintain that into reactive loads, or
in 'intermod' situation where both LF and HF are present?

Its no worse than any other amplifier which is class A,
and which measures with such low thd.


I'd also be interested to know the (complex) o/p impedance as a function of
frequency and perhaps power level....

Ro = 0.5 ohms at 1 kHz, with about 7 uH inductance in series.
The leakage inductance in a tube amp is similar to the LR
zobel network fitted to the output of an SS amp to stop
the transistors ****ting themselves with a capacitive load.
But with the tube amp the leakage L is included in the FB loop, wheras the
L on the SS amp isn't, and many SS amps give a worse peaked
response into capacitive loads.

However, its rare that a pure C load is encountered; most ESL have some series
R in series with an equivalant amount of C, and a parallel R across the C,
and its enough to damp the peak in the response that would otherwise occur
at between 15 kHz and 35 kHz, depending on the C value.





[snip]

But an SS amp needs to have 10 times lower thd at the same levels of the
tube amp, say 0.003% because they mainly operate in the middle of the
switching region of the output transistors, and although the thd is low,

I note the "mainly" in your statement. :-)

Well, they do.

Many transitor amps have declining thd with output voltage,
and its difficult to see any thd in the signal at 3vo of output.

I recently built a tranny amp with ten MJL21193/94 devices per channel,
and after some effort, I got thd down to 0.01% at 157 watts,
mainly 3H, and then at 10 watts it was 0.004%, and little more than the thd in
my test signal,
and it fell towards zero as po was reduced.

But the thd was modulated by the rail swings from the mains jitter,
so the thd level changed dynamically..
Under normal conditions, class B amps have somewhat large swings
on their supply rails and these modulate all other frequencies.
Unless one regulates the rails or uses 100,000 uF caps, the thd is far worse
than what one
measures with a sine wave.

Class A tube amps have the advanatage ofr common mode rejection in the
CT OPT output stage.
SE class A tube designs have a continuous drain of power, and in any case huge
supply
caps are used, say 1,000 uF.

All class B amps which include nearly all SS amps used today
really need all the NFB they can muster because of the
nature of their intermodulation production.
Usually, because SS amps measure 10 times less thd than a tube amp,
they often cannot be distinguished from a decent tube amp which measures well
enough.
But I think the dynamic distortion mechanism within SS class B amps is 10 times

worse than the mainly class A tube amp, so the ten times greater NFB
amount leaves the two genres somewhat similar sounding to many people.
But not to all ppl, and some hear a lot more though a decent tube amp, and it
ain't got
much to do with measurements if the buyers of SET 300B amps are witnessed.

Regulated rails are one solution for class B amps.

But I have heard SS amps which ran from lead acid batteries,
and I heard nothing different.

Theives wouldn't ever steal that power supply!

Patrick Turner.