Hi there

Sorry for a repeated thread but I have read the other posts and tried their solutions with no luck so far. I don't have much experience with analogue circuits so please bear with me and I apologise if it cannot be done.

The background is that I have created a circuit that adds a PTT switch inline of a Dell headset.
It is simply a SPST PTM (NO) switch in series on the Mic + line. When pressed this is obviously inducing a 'Pop' which causes Windows to adjust the microphone level.
This can be stopped in the settings but the product will be used in a military environment, meaning the end user will not have access to these settings.

The input to the PC is a standard 4 pin audio jack going through a splitter, the 4 lines are:
Left speaker
Right speaker
Mic +
Gnd

Please see the basic schematic below.


I have no access to power so cannot use an Op Amp.
Any ideas on how I could remove the 'Pop' noise?

Many thanks for any answers in advance
Charlie

 

I assume this is an electret microphone. Electrets require a bias voltage to operate, and the source supply is of necessity higher than the bias voltage. When you open the switch you dump a huge positive spike into the mic preamp, close your switch and you dump a huge negative spike (pop) into the mic preamp. You might try to change to a SPDT switch set up to select either the microphone or a resistor that drops the exact same voltage as the microphone. Hitting "exact" could be problematic. The switch won't transfer perfectly, so that will still allow some pop. FWIW you are attempting a simplistic solution to your goal, without understanding the limitations and tradeoffs. You may not like the tradeoffs, but you appear to have no choice but to live with them.

 

You could use a circuit as below this makes sure that there is no DC voltage either side of the switch so no pop when it closes or opens. I applied this to the mic switch on a university radio station back in the days of yore. The capacitors should be big enough to pass the lowest frequencies (depends on the impedance of the circuit. The resistors can be quite big just so the leakage of the capacitors drops no significant voltage across the resistors.

 

You could use a circuit as below this makes sure that there is no DC voltage either side of the switch so no pop when it closes or opens.

View attachment 221543

This will work fine for a dynamic mic, but not for a common electret condensor mic, which requires DC power.

 

Do it like this:

 

Do it like this:

In that configuration the mic is normally ON until the switch is pressed.

 

In that configuration the mic is normally ON until the switch is pressed.

That's the beauty of it. Use a NC switch and your PTT is effectively a "push to unmute" switch with no pop.

 

In that configuration the mic is normally ON until the switch is pressed.

The switch needs to be normally closed, then PTT works as expected. There may also be some tradeoffs with low frequency rejection depending on the 100uF vs mic amp low frequency response.

 

A SPDT switch can be either Press To talk or Press To Mute depending on how you connect it.

 

Thank you for your replies.
It doesn't seem to be an electret microphone because it does not have a voltage supply?
I will try each circuit to see if I can remove the DC spike and will post the results.
Thanks

 

You could use a circuit as below this makes sure that there is no DC voltage either side of the switch so no pop when it closes or opens. I applied this to the mic switch on a university radio station back in the days of yore. The capacitors should be big enough to pass the lowest frequencies (depends on the impedance of the circuit. The resistors can be quite big just so the leakage of the capacitors drops no significant voltage across the resistors.

View attachment 221543

Hi
Thanks for the reply. Do the capacitors need to be polarised and do you have a rough estimate on what size they would need to be?
Thanks

 

This will work fine for a dynamic mic, but not for a common electret condensor mic, which requires DC power.

It will be fine if the resistor supplying power to the electret is before the coupling capacitor.

 

Hi
Thanks for the reply. Do the capacitors need to be polarised and do you have a rough estimate on what size they would need to be?
Thanks

What kind of microphone is it?

 

What kind of microphone is it?

It seems to be a dynamic microphone, it is used in the Dell UC350 headset. I'm afraid I cannot find a datasheet for it.

 

For a dynamic mic I would suggest the capacitors should be 22uF and, at least for the microphone side, should not be polarised (as there is no voltage to polarise it) so likely the cheapest will be ceramic. Try 47k for the resistors - this is not critical.

e.g. https://www.mouser.co.uk/Passive-Co...tors-MLCC-SMD-SMT/_/N-bkrdv?P=1z0wrkiZ1yx4atv

 

Looks like it has a built in noise cancelling circuit, that uses two mics and inverts the background noise and cancels it out, .

 

If the mic is a dynamic type then why does it POP when it is shorted during muting?
Maybe your computer or whatever the headset is used with, assumes that it has an electret mic and sends power out to the mic and your PTT switch passes the audio and unfortunately also passes the DC. Then your PTT switch needs to be moved and maybe a coupling capacitor must be added to pass the audio but not affect the DC.

 

I'm afraid I cannot find a datasheet for it.

The Dell UC350 headset plugs into a USB controller which has a mic mute switch. Is this the part you called a "splitter"?
If so why do you need to add a switch?

 

For a dynamic mic I would suggest the capacitors should be 22uF and, at least for the microphone side, should not be polarised (as there is no voltage to polarise it) so likely the cheapest will be ceramic. Try 47k for the resistors - this is not critical.

e.g. https://www.mouser.co.uk/Passive-Co...tors-MLCC-SMD-SMT/_/N-bkrdv?P=1z0wrkiZ1yx4atv

Thanks for you help. I tried the circuit using this cap GRM31CR61E226KE15L and I am unable to get any sound out on the PC now. Any ideas?

 

Do it like this:

Hi, I tested this circuit and it does work but the mute can be overcome by talking louder through the microphone. Do you think that could be stopped by altering the values?