Increase output cap when doing a direct out mod ???

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jwhmca

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Jan 28, 2009
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I'm doing a direct out mod for a Soundcraft series 200. The cap that I'm coupling off is 47u. I was reading about the pm-1000 and how it is recommend to increase the size of that cap... what is the reason? should I do it to this circuit? I won't be driving a transformer.
 
HPF @ 3.7Hz worst case (all Aux1-4 dialed in full CW and all assigned pre- or post-EQ, external Rin assumed 10K) doesn't look that bad. Double up C15 (pre-EQ out) and C25 (post-EQ-out) to 100uF to get this HPF an octave down if it helps better sleep. How well the TL072 (IC2 and IC4) behaves is another subject, when driving this worst case ~900 ohms load (R21,R24,R49,R50,R51R52,VR8,VR9,VR10,VR11 + the assumed 10K external load all in parallel).
I wouldn't call the connection of Con1-pin8 (insert send) to another jack or multipin connector a mod, but YMMV.
 
Thanks for the input... but what are you talking about?

"Double up c15" where is that?

IC4 ???

"I wouldn't call the connection of Con1-pin8 (insert send) to another jack or multipin connector a mod, but YMMV."

Our we talking about the same mixer?
 
yeah... this is series 200 not the series 200B

anyways, my question is a generic one about output caps... could you explain your answer in light of that?
 
DC blocking cap in series with signal, followed by shunting resistor(s) to reference voltage (usually 0V) form a 1st. order (-6dB slope) HPF with -3dB cutoff in Hz @ 1/(2*PI()*R*C) with R in ohms and C in Farad.
FI a perfect (there is no such thing) 47uF series cap followed by a single 10K shunt would set this HPF @ 1/(2*3.14*10000R*0.000047F)=0.3386Hz.
 
Right, I understand all that... well at least I get it... But, what I was asking about was "why does it seem standard to increase the "C37" on the PM-1000? Is it just a freq response thing? Because they are driving a 600ohm transformer?
 
From your PM-1000 question C37 is 33uF. The following shunt is 4*50K pots, 4*5.6K law benders and 4*33K resistors into the buses, connecting to a virtual ground node, all resistors in parallel with pots dialed in full CW, so total shunt is 1092R, giving a [email protected].
When you assume the audiorange of interest maybe between 20Hz up to 20kHz, this 4.4Hz will have a phase response of -12.45° and level will be 0.2dB down when tested at 20Hz, so this is a frequency response thing that might be done better. YMMV.
Substituting this 33uF cap with a 100uF cap will set this HPF down to 1.46Hz with phase response of -4.17° and level will be next to nothing 0.02dB down when tested at 20Hz.
No 600ohm transformer to drive here.
 

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