[sdiy] reducing crosstalk

[Needham, Alan]

>-----Original Message-----
>From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy->bounces at dropmix.xs4all.nl] On Behalf Of Derek Holzer
>Sent: 25 January 2008 20:01
>To: synth-diy at dropmix.xs4all.nl
>Subject: [sdiy] reducing crosstalk
<snip>
>There is crosstalk between all the channels, not just the channels
>Which share an IC.
<snip>........................................

Derek, could the above be the key here?
What is shared between all channels,
You say there is some 0.1u caps across the power rails, try adding a
couple of 10u or 100u caps as smoothing (I never did care for specific
values - whatever you have should prove a point). If the crosstalk
reduces then we may be onto something...
...BUT, more likely...
Also shared between all channels (as I read your diagram) is the two
resistor potential divider for the 6v feed to all optos. This seems to
be two 10k resistors, the input impedance to each inverting amp is also
10k (all values read from photos - so am I right?).
The impedance of this 6v rail seems to be too high for the 8 amps it is
expected to drive. Lash a 100u or 470u cap from 6v to 0v and see if that
helps, if it does, you could also try swapping the first amp stage's
input resistor and feedback resistor for 100k on all 8 channels, 16
resistors, or does this wreck the opto's operation???
Ideally, the 6v rail would be a regulator (78L05?) with bypass caps.

	Alan


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