# Decoupling sound-card bias.

#### Olddumandnew

Joined Jun 6, 2019
23
I have an audio circuit with it's own supply. I intend to put this into the mic input of a USB soundcard.
I had intended to decouple the sound card bias by popping in a cap (non-polarised). I just came accross a video of someone physically removing the bias from a sound card for this very purpose (they just removed the limiting resistor on the bias thus breaking that circuit). Am I being a tad obtuse by thinking my simple cap method is fine?
Also, since the bias is +ve & -ve why aren't decoupling caps required on the -ve rail as well? (that last Q is more a curiosity - I would have thought the -ve bias rail also potentially interferes with the alternating -ve output from an audio circuit). Ignore that bit. I was being stupid and realised it just after posting

(last Q but I might just google this one - why did Ge diodes go extinct in terms of manufacture? Is it an economics issue (as in Ge is simply vastly more expensive that Si)? They are very useful little things with their low Vf)

Cheers all.

Last edited:

#### KeithWalker

Joined Jul 10, 2017
1,213
The bias on the input of a computer sound card is to power a condenser microphone. It would make sense to connect the output of your circuit through a coupling capacitor. The input of the sound card is high impedance so you can use a fairly small capacitor (0.1 - 1 uF). Use a value that is high enough to not attenuate the low frequencies.

Germanium small signal diodes are still available: 1N34A, NTE109, 1N276, etc.

Regards,
Keith

#### Olddumandnew

Joined Jun 6, 2019
23
The bias on the input of a computer sound card is to power a condenser microphone. It would make sense to connect the output of your circuit through a coupling capacitor. The input of the sound card is high impedance so you can use a fairly small capacitor (0.1 - 1 uF). Use a value that is high enough to not attenuate the low frequencies.

Germanium small signal diodes are still available: 1N34A, NTE109, 1N276, etc.

Regards,
Keith
Many thanks. I was going to stick in a 470nF. (I don't mind attenuating some low frequency - especially around the 50Hz area ;-) )
I can get Ge diodes - it just seems they're not manufactured any more (try getting any from the likes of RS)