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I have an audio signal running through a device that switches it over using a mechanical relay, as a result of the DC offsets and the parasitic capacitance on the output (which also connects to a high Z input) I can hear a 'pop'.

My original plan was to use a device called TLP222G which I had around to shunt the signal to ground quickly (before the relay action starts) and release it after a few ms, after the relay was set.

I had to use the same signal that feeds the relay's coil when that triggers I'm creating a pulse which should open the MOSFET, short the path to ground and keep it that was for about 0.2ms which is enough time for the relay to switch over.

The problem is that I can hear noise when the MOSFET opens or closes. Is there something else I should add to the circuit in order to keep the switching action completely silent?

Below you can see the simple circuit. The 360 Ohm resistor is fed from the 5V pulse that also activates the mechanical relay.

The LED inside the photocoupler has a max forward voltage of 1.3V so with the resistor it gets about 10mA.

enter image description here

This is a typical output stage of a device that is connected to the input of a high Z amplifier. The relay in the system shorts the input to the output so the entire device is bypassed. This creates a noise.

enter image description here

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    \$\begingroup\$ You need to get rid of the DC offsets or you'll never fix the problem. \$\endgroup\$ – Andy aka Aug 25 '17 at 10:51
  • \$\begingroup\$ @Andyaka - If the above device shorts the output path to ground (I am assuming it's a DC short) it should be taken care of, right? That's what I'm trying to do. Shorting the audio signal itself is almost a "bonus". \$\endgroup\$ – user34920 Aug 25 '17 at 10:55
  • \$\begingroup\$ No, you need to get rid of the offsets. Thought experiment - imagine no audio signal and just a dc offset - what will you hear? \$\endgroup\$ – Andy aka Aug 25 '17 at 10:57
  • \$\begingroup\$ @Andyaka - I'll add a typical input and output to better explain the situation. The offsets should be very small. \$\endgroup\$ – user34920 Aug 25 '17 at 10:59
  • \$\begingroup\$ You've added your schematic after I posted my answer. It's not clear wher "input" and "output" are on your schematic. Add in the relay contact that's causing you trouble. \$\endgroup\$ – Transistor Aug 25 '17 at 11:15
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The circuit you are looking for is called anti thump circuit. You might find various solutions googling that term.

Shorting the signal to ground will have no benefits (on the contrary, this could be dangerous).

The problem is the DC blocking capacitor, that will abruptly discharge, creating a strong click/thump.

To avoid such effect, I would add a series switch, that, instead of abruptly going from "Full OFF" to "Full ON", it goes gradually (slowly).

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  • \$\begingroup\$ Thanks for your reply, can you suggest such a series soft switching circuit? \$\endgroup\$ – user34920 Aug 25 '17 at 11:08
  • \$\begingroup\$ If you don't want to change your part number, just try this: use that optocoupler in series (instead of parallel) to your signal path, and slowly turn the LED of the optocoupler on/off (e.g. use a large parallel capacitor OR, if you don't want to use a large capacitor value, you can achieve it using a transistor and a smaller capacitor). \$\endgroup\$ – next-hack Aug 25 '17 at 11:12
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I've never tried this but it might work to solve the problem without removing the DC offset and with minimum modification to the circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) The (presumed) existing circuit. (b) The modified circuit.

Instead of shorting to ground you could short to the DC offset. R3 and C4 provide a low-pass filter and the voltage on C4 will be DC and, effectively, audio ground.

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  • \$\begingroup\$ I have tried your circuit and noticed two things. I manipulated the circuit so the pulse will be longer so I could hear (vs. see on scope) what goes on. I can hear that while the MOSFET is open there is a lot of noise pickup. Also, when the LED opens it is slow and almost no "thump" noise is present. When the LED turns off the noise is much louder. \$\endgroup\$ – user34920 Aug 25 '17 at 12:20
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You may use the CMOS analog switch CD4066 enter image description here

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