Using transistor to switch audio?

[mth5044]

I've gotten down switching relays using a micro controller to send 5V to the base of a 2N3904 so connect a relay (or LED, etc) to ground as in this schematic:



However, instead of using a transistor as purely a power switching, could it be used as a SPST switch for audio purposes? I have a feeling I'm missing something completely fundamental 

[PRR]

Audio swings both ways. Metal contacts don't care. Active devices mostly DO care: in general you get only part of the audio wave.

Also there's only 3 legs on that thing. We need 4 legs to get complete isolation between the control circuit and the signal circuit.

THE paper for you: (120KB PDF)

Page 2, stuff in blue-dash box is a "mute", "LPF" is where the sound comes from, Line Out is where it goes (or doesn't go).

This is WIDELY used to mute the output of CD players when they are not making a proper output (while scanning, while the controller is booting-up, etc).


Problems:

When muted, it is a heavy load on the source (just Rs)

For signals more than a few tenths-Volt, the negative tips break-through unless you have a negative pull-down (-VA)

Signals over 7V are likely to be clipped

The full signal is slightly distorted (hardly-any with the 200 ohm Rs, probably more with the larger Rs values we might like in stompbox work)

The mute signal is also slightly distorted

The mute is not DEAD silence, only 40dB-50dB down (pretty quiet, easily masked by another signal, but a bit disappointing)

As in any switching, stray DC means pop

It _is_ fast and (for reasonable Rs) flat.

[artifus]

wow. i feel like i just learned a thing or two. thanks, prr!

[mth5044]

That was great, thanks PRR

[Kesh]

lots of pedals use jfets for switching audio