I want to receive a stereo balanced audio signal and feed it into the stereo microphone inputs via my motherboard's header pins.

enter image description here

I added about a 30 dB reduction in gain which looks good on the scope and audio sounds great from headphones/speakers.

But here is my issue:

If the opamp is not powered, I will be sending the hot signal into my L/R mic inputs without gain reduction and I am afraid that will cause damage to the motherboard. So I need to attenuate or mute the signal if no power is applied.

My two ideas are:

  1. Use a JFET to mute the circuit like shown here: https://sound-au.com/articles/muting.html#s2


  1. Use a unity gained design for the differential amplifier then just have a voltage divider at the input of my circuit to reduce the level.

My question is: Do you see issues with these approaches and is there an alternative approach you would suggest?


Here is my modified circuit where I removed the regulator and just use the output from the 7660 charge pump. I also added parallel JFET's with gates tied to -5v rail so they will attenuate any signal headed to the sink when there is no power applied to the opamp:

enter image description here

  • \$\begingroup\$ I'd go for JFET. \$\endgroup\$
    – winny
    Commented Mar 21, 2023 at 16:43
  • \$\begingroup\$ I'd go for 2 because it's easier and is clearly going to work. Having only a -5 volt rail to turn off the JFETs may not work. \$\endgroup\$
    – Andy aka
    Commented Mar 21, 2023 at 16:49
  • \$\begingroup\$ I vote for 2, for simplicity. \$\endgroup\$ Commented Mar 21, 2023 at 16:55
  • 2
    \$\begingroup\$ Am concerned with the -5V DC supply: doesn't that 7660 put out -5V on pin 5? The 7905 regulator that follows requires at least -7V on its input pin. Do you really have a regulated -5VDC at 7905 output pin? \$\endgroup\$
    – glen_geek
    Commented Mar 21, 2023 at 16:59
  • 1
    \$\begingroup\$ You could just use a couple of small audio transformers. I very much doubt you'd damage the mic inputs on your motherboard anyway. \$\endgroup\$
    – Finbarr
    Commented Mar 21, 2023 at 17:07


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