Some time ago I started a discussion about problems I was facing with audio switching. I never ended up finding a satisfactory solution and now I have tried a slightly different approach which works better but I still feel there is something I'm missing out of.
Attached is a schematic of the switching portion of the system.
- Audio in is an audio signal. Usually it comes from another device. AC coupled.
- Audio out goes to another device or an amplifier, sees high Z (500k and above)
- Ctrl is either high or low (5V-0V), together with the cap. it enables driving a single coil bipolar latching relay using a single pin.
- U4 is an opto relay (TLP222G)
- The Ctrl signal is fed with an RC delay line in parallel into a XNOR gate which will create a pulse.
- The pulse length is variable using the RC values.
- The pulse drives an LED which in turn opens the mosfets and shorts the signal to ground faster than relay is switching (that's the idea anyway), the relay action should start after the signal is muted and finish before it is un-muted.
I have tested the opto relay on it's own and the switching action is silent. On the scope it's smooth with no sudden transient. I've tested the relay switching on it's own and it does cause a 'thump' noise when switching. I did try to bypass the relay only leaving it attached to the circuit and that did not cause a noise which switching. Together (relay + mosefts) does not mute the signal while switching and I can hear some noise. I am not sure of the source but it is not the opto relay and it is not the ctrl signal itself.