Audio output muting circuit

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
Hello everybody.
i wanted to ask you an advice for a muting circuit that i need since after i remove power to my device, i get embarrasing chirps noises at the output because the opamps.
I wanted to get rid of this with a muting circuit of some sort but i really can't find good informations about it around. some say don't use relays, some say don't use Jfet..

My application is stereo and the idea was to tie a Spdt Relay to the psitive rail so that when the power goes off, the switch will connect each output to ground via a 100k resistor.
Shouldn't i do it or are there better solutions?
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
beside my ''embarrassing'' joke, i need to mute the output. when i switch the power off, the voltage slowly decrease at the - and + pin of the opamp, increasing sound distorsion untill silence.
 

crutschow

Joined Mar 14, 2008
27,229
My application is stereo and the idea was to tie a Spdt Relay to the psitive rail so that when the power goes off, the switch will connect each output to ground via a 100k resistor.
A 100k resistor will likely not mute the signal, you would need more like 100Ω.
For that you would then use a DPDT relay, one pole for each channel.

What is the supply voltage for the relay?
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
A 100k resistor will likely not mute the signal, you would need more like 100Ω.
For that you would then use a DPDT relay, one pole for each channel.

What is the supply voltage for the relay?
i still need to choose a solution so i still need to buy the relays as well.
the device is powered from +-12V so i guess something like this
https://www.digikey.de/product-detail/en/panasonic-electric-works/JW2ASN-DC9V/255-3673-ND/648822

9vdc coil ( i will need to drop slightly less then 3V with a resistor in series with the coil.
 

Yaakov

Joined Jan 27, 2019
3,161

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
i thought that if the spply voltage is 11.90V for instance, that could create an unwanted issue. with a 9VDC i would be safe! is it wrong thinking?
 

crutschow

Joined Mar 14, 2008
27,229
You are also interested in the release voltage for the relay to make sure the relay releases to mute the output before the noise starts.
Do you have an oscilloscope to determine at what voltage the noise occurs>
 

hrs

Joined Jun 13, 2014
321
thanks a lot! this is interesting. Did you use the Figure 4 - Transformer Based MOSFET Driver?
Yes. I was interested in the solid state relay idea, but it seems to be most common to use a normal relay as has been suggested by crutschow and Yaakov. In any case the esp articles are a good read with pros and cons to the various options.
 

DickCappels

Joined Aug 21, 2008
7,442
Just a note
1616333921710.png
The 2SC2878 gives a very low resistance when biased on. The reverse breakdown on the emitter is very high so it will not clip the audio signal. KTD1302 is also designed for this kind of use. You can use other transistors such as the 2N2222 just be careful not to exceed the emitter-base breakdown voltage.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
tried a relay solution and N Jfet solutions and both have pros and cons.
N channel jfet gives a bit of sound distorsion when the voltage slowly descend towards 0.
Relay completely cut away but can hear the transient as a sort of click (negligible).
i wonder if a comparator could be used feed a J108 (i used a J109) so that as soon as power is off, the comparator cuts instantly its output (to jFet gate).
 

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crutschow

Joined Mar 14, 2008
27,229
I wonder if a comparator could be used feed a J108 (i used a J109) so that as soon as power is off,
Yes, you could use a comparator such as the LM339/393 to mute at any desired voltage level.
You would also need a voltage reference, such as a Zener, standard diode or LED, or TL431, for the comparator.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
41
Hello again! at the end i decided for a Jfet Nchannel mute Switch (picture attached) and is working out but it acts still a bit slow and i would like him to be faster and I'm trying to understand how!

i tried to halve the capacitor but still a bit slow. then i tried to feed the gate to something closer to the Cutoff voltage (in this case it was -4V) and was much faster.

Do you recommend something else instead of feeding that Voltage to the gate?
i have looked around for a Jfet with higher Cutoff Voltage but strangely they look uncommon or very expensive (that's why I think i can be mistaking somewhere)
Thanks a lot!
J
 

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AnalogKid

Joined Aug 1, 2013
9,257
1 M - 100 nF is a time constant of 100 ms. That's noticeable, but not particularly slow. What do you mean by "slow", and how fast is fast enough.

The simple thing to try is to reduce R127 to 100 K, and then to 10 K, and see if that is ok.

ak
 
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