Auditronics 501 interstage cap values?

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nielsk

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Nov 12, 2004
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Megapopulas, Florida
I'm making major headway on the project of turning this into a properly working console, so far I have made all the mic pre's into 312's w/ transformer out, fully balanced line & tape inputs, added a true input flip, changed the MTR routing from mono only to stereo, got rid of the crazy "monitor" idea and made this into a stereo console with 60 inputs in mix..... here is my current question:
Most of the interstage caps are 10 uF (tants) but the final one to the transformer from the transistor pair is always 330 uF. Why so high a value? What effect would changing it to 10, or 100 uF, or somewhere in the range where it seems that most other gear uses? Am I correct in thinking that the higher the value, the higher the corner of the filter frequency, so the low end could suffer from too high a value?
 
personally, I would put 22uf or 47uf in place of the 10 uf caps, for thee interstage coupling caps.

often larger coupling caps are used to feed busses that are fed by a large number of channels,
as some sort of compensation for the impedance of so many channels tying together. 
Less phase issues, I think.

Audio schools talk about the seven characteristcs of a waveform.  How ''bout #8?  The period of a wave.

The period of a wave is how long a cycle lasts.  lower freq. waves will have a longer period.
the lower the frequency, the longer the cap needs to charge.  the larger value the cap (microfarads),
the bigger the "tank", so to speak.  A small cap "fills up" sooner, a larger cap can charge up longer,
therefore the larger cap can deal with the longer period low freq waves.  A small cap can charge up for so long, then it has charged up to it's maximum, it is "full".  If the frequency is low enough that the wave is still not done, then we get capacitor distortion.

That half-assed explanation should get someone who really knows what they are talking about to respond.

Also, not a big deal but larger caps generally have less ESR... less is better.
But smaller caps have less leakage, also better.
 
Interesting, thanks for the reply
so maybe they knew what they were doing... although 300 uF caps are on the main outs (Multitrack, mix, monitor), after any busses have been summed.. the last thing on the way to the output transformer.
Your explanation is the best help I have had yet in wrapping me head around the trade offs in interstage cap value
 
an inductive load like a transformer can have a series resonance with the coupling cap.  10uf might be enough to pass full bandwidth, but you could also get a big bump in your low frequency response.  if you increase the value of the cap, you move the resonant bump lower.  make the cap big enough and the upper edge of the resonance will be mostly below the audio band.

ed
 

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