[sdiy] Op Amp AC Coupling/DC Biasing (was Blocking audio at power off

[Justin Owen]

-----Original Message-----
From: Florian Teply [usenet at teply.info]

> As Fabio already pointed out, a non-inverting configuration would be better in that case.

> I put a very basic example to http://www.teply.info/noninverting.png in order to show what i mean.

Many thanks to you, Fabio (and everyone else) for the input on this - it's helped loads and is much appreciated.

I ended up using the schematic that Fabio provided (with a voltage divider/bias on the non-inverting input rather than just a resistor to ground).

I messed around with the resistor values a bit - but it looks like the inputs want to be at VCC/4 (2.34 Volts from a supply that is sitting at about 9.38 Volts) and the output then sits at about VCC/2 (about 4.68 Volts)?

Even with my fragile grasp of maths that makes sense. ;) I also tested the circuit using a battery that was putting out about 7+ volts and the signal back into the soundcard was exactly the same as with 9+ volts.

At power off there is virtually no signal leakage at all (If I turn the input of the soundcard up to 11 - I get a slight trickle at about -48dB) - a massive improvement.

Not sure yet if this is THE solution - but it's been fun and educational.

Thanks again.

Justin