Balanced shunt pot in front of a differential receiver

[00940]

I don't really know in what forum to ask...

Can I put a simple shunt volume control in front of differential line receiver ? Just as attached, but without the two opamps buffering the inputs of the receiver. Any problems I might run into ?

It's to sum the outputs of a pcm1794 (after I/V conversion).

 

[richie00boy]

You would have to use low value resistors and a pot infront of the existing diff stage (inc resistors). This may mean that when the volume is attenuated a lot you place too much load on the driving stage.

Think of it as the same impednace problem with any passive pre-amp.

 

[00940]

What I had in mind:

4.7K resistors for the fixed resistors of the shunt and also 4.7K for the input resistor of the receiver. 22k resistors for the feedback around the opamp. A 10k stepped attenuator as shunt.

I can tailor the values of the attenuator to get true logarithmic attenuation. I'm not overly concerned by the current drawn by the shunt as the I/V stage is buffered by class A buffers iddling at 20ma. The whole thing is of course in the same box, no cables involved here.

 

[richie00boy]

Doesn't sound too bad. There may be some funny curve characteristic due to interaction between the shunt and input resistors, but other than that OK I think.

Is there a reason you can't just use one side of the converter with a conventional unbalanced level control? As you seem to be ending up with an unbalanced output anyway.