Muting Circuit

[Tenson]

Hi,

I want to add a muting circuit to my line outputs. However I don't have any mute control, it just needs to do it automatically so I was thinking of a circuit like this one. The idea being that the PNP transistor will connect the signal line to ground when there is no voltage on the base and it will break the connection when there is voltage. There is an RC network on the base so that it comes up slowly when voltage is applied, but when the voltage is taken away it will de-charge very quickly through the diode.

However, I've no idea what parts and values to actually use. Can anyone help please? Also I'm not really confident that it will mute fast enough when the power is disconnected to stop switch-off thump. The output op-amps will be running on that same power line.

 

[AndrewT]

The signal line carries AC.
A PNP BJT cannot reliably pass the AC to Audio Ground.

I believe there are special BJT that can do this job.
Most use FETs, which can pass AC when switched ON.

 

[Tenson]

Thanks Andrew, I don't want it to pass when switched on though, I want it to pull to ground when there is no power. Apply power and it breaks the short to ground.

 

[Mooly]

Normally NPN devices are used, the 2SC2878 being a typical example (most devices would work but this is specially for the job).

A problem with your circuit is that +15 volts will exceed the E-B reverse breakdown voltage of the transistor... it will behave as a noisy zener of around 7 volts or so. So your control voltage needs to be limited.

 

[AndrewT]

2sc2878.
ultra low Vce sat at low Ic is a special feature.

The description states:

High reverse hFE: Reverse hFE = 150 (typ.) (VCE = −2 V, IC = −4 mA)

Is this the parameter that makes this an AC switch?

 

[Mooly]

I would assume so.

Seeing reverse hFE mentioned is unusual but seeing as bjt transistors can work either way around at low voltages, I suppose this figure means it can still turn on and mute effectively when the output signal reverses polarity and the transistor is reverse biased in the conventional sense.

 

[gmphadte]

This link might be of help.
Muting Transistor Attenuator Circuits and the 2SC2878

Gajanan Phadte

 

[Mooly]

Interesting link, particularly the ESD part ... thanks

 

[nnx]

Check this out.

Another mute circuit, I believe it is from a CD player.

 

[Mooly]

That's a typical kind of implementation with two transistors to get a good level of muting.

 

[Tenson]

Are any of these devices going to mute when the power is off, and un-mute when the power is on?

Really I'd have gone for a series relay so the signal only connects when the kit comes up to power and disconnects when power drops, but I don't have circuit space for it.

 

[Mooly]

They would only mute for as long as the mute voltage was present. So in the example nnx posted, the mute line would need to be at plus 5 volts (the 2k2 resistors would be transistor specific and might be much higher than that device depending) to mute and need to go to -5 volts to unmute. Other control voltages outside those limits wouldn't work.

Have you room to hard wire a miniature relay right at the sockets ? It would be arranged to short the output to ground (default unpowered relay state) at all times except when the unit was on.