5
\$\begingroup\$

I built a simple microphone mute circuit that appears to be pretty common. It mutes the mic without making a popping sound by shorting the connection through a capacitor.

It works pretty well, but if I blow into the mic or tap it hard, it is able to pick that up. I first noticed this when watching audio level meter on my computer. The actual audio that it picks up sounds like high pitch feedback.

Is there any way to eliminate this? I'm planning to control the switch with an arduino, so I will have additional power + logic available if there is a more clever solution.

I am testing with two PC headset microphones. I measured their resistance at ~800ohm and ~1100ohm.

Currently I'm using a 470uf 16v cap and a 10k resistor. I've also tried swapping the cap with: 330uf, 100uf and swapping the 10k resistor with a 100k resistor.

Here is the circuit I am using - just the left diagram (taken from 5v Electret microphone to PC mute switch pop help): Circuit

\$\endgroup\$
  • 1
    \$\begingroup\$ What type of cap are you using? \$\endgroup\$ – BobT Nov 30 '16 at 3:56
  • \$\begingroup\$ @BobT Radial aluminum electrolytic. No good? \$\endgroup\$ – kr4sh Nov 30 '16 at 5:18
  • \$\begingroup\$ Polarized aluminium electrolytic? \$\endgroup\$ – winny Dec 5 '16 at 10:21
  • \$\begingroup\$ @winny Yes polarized. \$\endgroup\$ – kr4sh Dec 8 '16 at 4:12
  • \$\begingroup\$ If you have enough voltage rating on it, it will behave OK in reverse at low voltage levels. Going higher, it will "clamp" the voltage. It's probably not the problem here but please use unpolarized capacitors for your audio/AC circuits to avoid potential issues. \$\endgroup\$ – winny Dec 8 '16 at 9:04
2
\$\begingroup\$

Check to see if C2 is microphonic by listening while tapping directly on C2. If this is the problem, it can be fixed by replacing C2 with a Mylar capacitor. Ceramic capacitors are often microphonic. I don't expect the aluminum electrolytic to be microphonic, but it is worth checking.

It could be that the effective series resistance (ESR) of C1 is too high. Check for this by adding another 100uF capacitor in parallel with it. If this improves the problem by about 6dB, ESR is probably the root cause.

Good aluminum electrolytic capacitors have a specification for ESR. Typical 100uF capacitor ESR is in the 0.3 to 2 Ohm range, with extremes from 0.07 to 14 Ohms. Choose a 100uF part with an ESR of about 0.08 Ohms to make a better short.

\$\endgroup\$
  • \$\begingroup\$ I don't have control over C2 because that is the device that I am plugging the microphone into (my PC in this case). Connecting two 100uf caps in parallel seems to reduce the issue a little. Connecting a single 470uf cap reduces it even more. I have a generic set of caps that I ordered off of amazon a while ago, are they just junky (I can't find the ESR value)? amazon.com/gp/product/B018SBOTDG/… \$\endgroup\$ – kr4sh Dec 8 '16 at 4:39
  • \$\begingroup\$ Your caps probably have too high ESR. I suggest a better part such as a ESY107M050AG3AA 100µF 50V Radial Aluminum Capacitors 74 mOhm @ 100kHz 3000 Hrs @ 105°C. The higher voltage rating is not needed, but to get lower ESR, use a higher voltage part. This part will wear out eventually, because electrolytic capacitors should operate above 20% of their rated voltage. But, I think low ESR will provide a big improvement. If you can afford it, use a large, expensive 100uF plastic film capacitor, which provides an even better 0.0025 Ohm ESR. These are great parts if you have $20 to $50 to spend. \$\endgroup\$ – Tom Anderson Dec 8 '16 at 6:26
  • \$\begingroup\$ Is there a cable between the switch and the 0.1uF capacitor? This would help if the click is caused by the microphonics of the 0.1uF capacitor. \$\endgroup\$ – Tom Anderson Dec 8 '16 at 6:30
  • \$\begingroup\$ Yeah I have like a 6 foot cable between the switch and where it plugs into the PC. I just ordered some low ESR caps. I will see how it impacts the circuit. Thanks for the info. I'll report back after I have tried the new parts. \$\endgroup\$ – kr4sh Dec 8 '16 at 16:19
  • \$\begingroup\$ I tried a low ESR cap. It definitely works better but unfortunately yelling into the mic or blowing on it still makes it through as a ping noise. I think this is probably the limit of this circuit since it is sort of a hack according to another commenter. \$\endgroup\$ – kr4sh Dec 11 '16 at 1:40
2
\$\begingroup\$

You could also try adding a smaller-value capacitor across (in parallel with) the large 100uF capacitor. That would better attenuate high frequency sound that was still leaking through.

It is impossible to imagine that "blow into the mic or tap it hard" could "sound like high pitch feedback". You may not be using the terminology in the common meanings. An audio sample is worth 1000 words.

\$\endgroup\$
  • \$\begingroup\$ I tried adding a 22uf cap in parallel with the 100uf cap, but it didn't seem to make a difference. If I use a 470uf cap instead of the 100uf it seems to perform a little better, but it still picks up some audio if I blow into the mic. Feedback is probably the wrong terminology. When I configure my PC to redirect the mic audio directly to my headphones, it sounds like a "ping" sound. When I tried to record it, it ended up sounding like a faint click. Regardless of the actual sound, I wouldn't expect anything to get picked up when in the muted state. Thoughts? \$\endgroup\$ – kr4sh Dec 8 '16 at 4:25
  • \$\begingroup\$ By "small value" I meant something 0.1uF ~ 1uF. i.e. something to handle those high-frequency remaining transients. To be honest you may never get "perfect" silent muting because your scheme is somewhat of a kludge since you are trying to mute a millivolt AC signal int the presence of several volts of DC (the "plug-in power"). Proper muting would take place AFTER the power and audio signal are "separated". (i.e. AFTER C2) \$\endgroup\$ – Richard Crowley Dec 8 '16 at 4:36
  • \$\begingroup\$ Ah got it. Sorry. I just tried .1uF and 1uF in parallel with the 100uF. No difference. Dumb question: would it be unreasonable to proxy this by separating the power/audio signal and then recombine them? I'm guessing that is probably beyond my skill level.. just curious. \$\endgroup\$ – kr4sh Dec 8 '16 at 4:46
  • \$\begingroup\$ The audio and power are separated by C2. If you want to do anything BEFORE C2 then you are in kludge territory with combined audio and power. Is completely noiseless muting REALLY that important??? \$\endgroup\$ – Richard Crowley Dec 8 '16 at 4:54
  • \$\begingroup\$ I guess I just find it irritating that it doesn't completely mute. I found this circuit referenced in several places, but didn't realize that it was a hack until you said so. Is it unreasonable to move the switch to after C2? Would I then need to build an amplifier circuit to output to a device such as a PC? \$\endgroup\$ – kr4sh Dec 8 '16 at 5:13
2
\$\begingroup\$

The problem with your circuit is that your mic is still connected when you activate the SPST switch.
The circuit you copied this from has a slider DPDT switch that disconnects the mic at the same time that it connects the cap and resistor across the input to the amp.

Use a DPDT switch and connect the common contacts to the plug, the NO to the cap and resistor, and the NC to the mic. The 100uf and 100k should work fine.

\$\endgroup\$
  • \$\begingroup\$ This doesn't seem to work properly. Breaking the connection to the mic creates a high pitched ping sound. I tested both parts to this independently before using both together. When the switch is thrown, it shorts the circuit through the cap (this alone is silent). It simultaneously disconnects the mic (this generates a ping sound). Are you 100% sure this should work? \$\endgroup\$ – kr4sh Dec 8 '16 at 4:10
0
\$\begingroup\$

If you build it into a small box with two Xlr chassis connectors one input the other output. Use a momentary switch for PTT Try Using a BT47 type relay capable of switching audio & telecommunications equipment. Wire a 12v supply to the switch to control the BT47 ..take the Hot wire & Cold wire of your Mic connections coming from your mic via the Xlr input wire the Hot directly to the Relay on one leg and Cold to the other side. Now take the switched output of your Relay wired to your Xlr output. When you push the button you are only switching 12v so should achieve a fast switching PTT without popping or noise. Then the caps etc will really quieten the switch however I found it worked without a cap. 👻

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.