In article , Serge Auckland
wrote:



As has been previously suggested, the Sugden's higher output impedance
could well result in an altered frequency response into an ESL load
compared with the 303. It's that altered frequency response that's
audible, not any intrinsic difference between amplifiers.

I don't think it is quite a simple as the old A21 having a higher output
impedance.

The complications are that the A21 has a different size output cap, and no
output series inductor. Also note where the A21 (original) has the feedback
from the *output* side of the coupling capacitor. As with the old Armstrong
600 amp, taking the feedback from that side has a significant effect on the
output impedance at low frequency, as well as on the LF distortion to some
extent. (The 600 could actually have a *negative* output impedance in an LF
band due to this and its dual feedback arrangement. Although this varied in
production as I altered some of the component values, etc.)

FWIW I'd expect the A21 to tend to have a *lower* output impedance at LF
and HF than the 303 - although this depends on the open loop gain to some
extent so I can't be sure. I don't have sufficiently detailed info on the
A21 as yet, but hope to soon. :-)

Slainte,

Jim

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