[Scott Dorsey]

gareth magennis wrote:

This Pultech clone has "catastophic ringing" with a 1kHz square wave, almost identical to the example posted.

Assuming that you are doing this with the EQ out of circuit (controls set
flat), then the output transformer is garbage.

A 600 ohm resistor alone reduces level by almost half, but the ringing remains at the same proportion of the square wave amplitude.

This is how a normal Pultec would be expected to operate. 600 ohm source,
600 ohm load.

A 100ohm resistor in series with 10nF reduces the catastophing ringing initial spikes hugely, but nowhere near to "mild ringing", and has a slight rolloff on a sine wave at 10 and 20 kHz but not 1kHz.
A 100ohm resistor with 68 nF has way too much hf rolloff, but the ringing is eliminated entirely with a sloping squarewave wavefront showing the hf cut.
A 100 ohm resistor with 4.4nF to 1nF causes full amplitude oscillation, very bad.

The 10nF seems to be the most useful. Anything higher does eventually elliminate the ringing, but with unacceptable rolloff.

A well-designed transformer might ring a little bit, but it won't ring as
severely as in that picture. More importantly, it will ring somewhere in
the 100kc-200kc range so that you can add a zobel network to notch it out
without affecting the response in the audible range.

It sounds to me like your transformer is ringing close to or even within
the audible range, and so the only way to get it to stop is with a very
low frequency network. This will have audible consequences.

Out of curiosity, who makes their output transformer? I want to make sure
I never use one of those.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."