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MondyT 18th September 2009 08:49 AM

Quiet Audio mute switch?
Can you guys and girls help me on this one?

I am looking to put together an audio mute switch that when inserted in a mono signal path will quietly mute the audio with out any pops, clicks or bangs and when switched out, will restore the audio without any noise also. I preferably would like the unit to be stand alone and so would need to run from say a 9v battery.

I checked this FET circuit here as it seemed ideal….

I tried to model it in LTSpice and it seems to work but with a distortion figure well over 1% so I guess I may have down wrong. I am not sure if the circuit given is meant for a symmetrical power supply and therefore may not be of much use to me. I am also not 100% sure of the component values given (m,M etc) or how to config the circuit for a 9v supply. As I see it, no power is required unless the signal is muted, but I may be wrong!

Does anyone know of a suitable circuit for a mute switch. The main features would need to be…

1. Low distortion with line level signals up to around 2v pk
2. Quiet mute and restore
3. Low power consumption from a single 9v battery
4. Low impedance (signal to handle is in the range of a few hundred ohms)


AndrewT 18th September 2009 09:04 AM

you may want to look at using the FET as a shunt for the mute function.
That way the FET when near infinite impedance has least effect on the signal passing through. It's just it's capacitance that will affect the signal.

You can use a series FET and a shunt FET. that operate in opposite directions.

Find out what happens to a FET, whether in the series position or in the shunt position, when the AC signal is positive with respect to signal ground and when it is negative with respect to signal ground.

MondyT 18th September 2009 09:23 AM

Hi Andy

Many thanks for your reply my man, appreciated. I was up your neck of the woods for the Oil & Gas exhibition in Aberdeen last week. Lovely part of the world Scotland!

Shunt made the best sense to me as you stated, this way the circuit should have minimum impact on the non muted signal. I tried this circuit here first …

But no joy! I could not get enough attenuation on a low impedance signal. It states 10K to round 100K impedance signals, so this could be the reason. Again I could have implemented the circuit wrong in LTSpice for use with a 9v battery etc (bit of a dimmo I’m afraid!) It is a shame coz I liked the LED indication.

My knowledge of transistor basics is too sketchy to put to test with circuit configs and so that is why I have turned to the experts for help!!


AndrewT 18th September 2009 09:39 AM

to allow the shunt mute to attenuate, the source impedance must not be near zero.
If your source has a low impedance, say around 200r then the attenuation you will get in shunt mode will be low, i.e. a high volume will leak through.
You will need to add a series resistor between the input DC blocking cap and the FET. Try a 1k0 as a starter.

richie00boy 18th September 2009 10:05 AM

This could be done completely passive using just a resistor and DPDT switch.

MondyT 18th September 2009 10:22 AM


I will try and add some series resistance and see what happens, I do not want to raise the signal impedance too much however!!


Thanks bud. Can something this simple not be prone to clicks and bangs etc?


Geek 18th September 2009 10:27 AM

Bipolars have insanely low "on" resistances compared to jFET's.



richie00boy 18th September 2009 10:31 AM

With the addition of one or two resistors yes I do believe that you could achieve clean switching.

MondyT 18th September 2009 10:55 AM


I tried the shunt circuit above with a 2.2K resister in series with the audio and it reduced the signal from 2.5 Vpk to 50 mV pk but the distortion was around 6% and over 2% with the signal at 1.0 Vpk. I am no expert at measuring distortion in LTSpice however and I just used the .FOUR 1k V(output) directive and read what was in the error log. Is this the right way to go about things?

Geek, do you have any circuits for using a Bi-Polar transistor for the mute as I *never* seem to have much luck with FET circuits with signals as high as 2.5v pk and using 9v batteries


MondyT 18th September 2009 10:56 AM

Thanks Richie

Could you sketch what switching arrangement you propose?


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