daveshel wrote:I understand that sometimes preamps have some DC in their output, and that the fix for this is to add some extra coupling caps to the output of the preamp or the input of the amp. I am unclear as to what kind of symptoms this will cause. I'm wondering if it's better to add them to the preamp or the amp, and if there's any downside of using them when they're not needed.
Background: I have a Van Alstine Super PAS Three that's 20+ years old. I used it with my Van Alstine U-70 until I built my ST-35 w/EFB (and dumped that U-70 as fast as I could). Later I got ahold of a pair of MK-IIIs that are essentially stock but with a lot of upgraded esoteric parts, and one of them makes some farting and popping sounds for a few minutes until it warms up. At the same time, I'm working on a rebuild of an old stock PAS2, on which I've been told I'll need to add some coupling caps after I remove the tone controls. So I'm feeling the need to get a better understanding of this area as to when this is needed - and whether I should add it to the preamps or the amps.
It ought to be a "rule" that may be expressed as :
"Be liberal with what to receive, assume bad things and do not be harmed with it, at the same
time be careful with what you transmit, never transmit anything that can harm or is out of spec"
Interpreted on an amp would be : "protect inputs from DC, transients and signals outside the spectra, on outputs make sure no DC , no turn off/on transients, and no disturbing high frequency "
An amp like the ST-70 breaks this, it has no protecting cap on input. The result is that
if feeded with a signal with DC component the 7199 will have wring bias on the first tube, this will affect sound. PAS 3 before "X" also breaks this as it emits a DC component which
might/will upset the power amp.
The fact that dynaco who made many well designed amps got away with this is that
the effects are minor in this case ( it will affect working point of 7199 but not fatally. A slip
in an other brilliant series of amps.