[sdiy] reducing crosstalk

[Neil Johnson]

Hi,

Looking at that circuit, it is not a voltage divider.  All it is a  
10k load across the +12V supply, and a 10k load resistor for the  
phototransistor.  You might want to swap the connections to +12V and  
the BPW40 to fix that if you really want to drive the BPW40 from 6V.   
As Alan suggests, a low-impedance source, like a voltage regulator or  
another op-amp would sort out that.

Why do have two stages of unity gain?  Surely for this application it  
doesn't matter?  A single stage would be sufficient.

I'd also put a DC blocking capacitor on the output, plus a series  
resistor of, say, 100R.

For each module I'd put some filtering on the incoming supply, to  
help isolate each module.

Put 100n decoupling caps close to the supply pins on the TL074.

There is nothing of high gain here so shielding cans are probably not  
needed.

Neil

On 14 Feb 2008, at 17:16, Needham, Alan wrote:

>
>
>> -----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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