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I’m currently working on making a lavalier microphone for use with my phone and gopro camera. I know these types of microphones are readily available and can be purchased fairly cheap, but where would be the fun in that? I’ve purchased lavalier mics in the past and dissected and realized that they connect the electret microphone straight to the trrs jack using the 2-3v supplied by the mobile device or camera to power the microphone. I’m trying to do the same but between the ECM and the TRRS I want to add a high pass filter. I attempted to try this on breadboard and there is no signal coming through with a high pass filter of 97 Hz. Using a 170 ohm resister and 10uf capacitor. I have done some research and I cannot find anything that pertains to my exact predicament.

What I did,

Electret mic + to 10uf cap then to trrs but between the Cap and TRRS I put a 170 ohm resister to the ground wire.

The - side of mic I connected straight to ground on trrs with the 170 ohm resistor connecting to it.

Here is a horribly drawn illustration of how I wired the mic.

enter image description here

EDIT!!!!!!!

Guys take a look at this circuit now and see if it’s better.

edited circuit to include 9v dc bias before the highpass filter

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  • \$\begingroup\$ Your post does not contain an illustration. You can edit your post and use the built-in schematic editor. This would greatly help us to help you. \$\endgroup\$ – evildemonic Feb 20 at 17:21
  • \$\begingroup\$ The capacitor is blocking the needed bias voltage to the electret, you will not be able to high-pass like this. \$\endgroup\$ – evildemonic Feb 20 at 17:48
  • \$\begingroup\$ Indeed, you need to either add a battery to supply the microphone element, use an active filter, or backtrack the connection to a point where the bias supply and audio input are distinct, which is probably not external to the product being fed. Unfortunately you can't do what you could at RF and use a choke to supply power, because for the audio frequencies you want to reject it would be physically huge. Practically speaking, consider or develop recording software which can do the filtering in software. \$\endgroup\$ – Chris Stratton Feb 20 at 17:52
  • \$\begingroup\$ What would you recommend I do? I’m new to this but my goal is to filter as mentioned before while maintain a small circuit since this microphone will be eventually placed inside a motorcycle helmet. Should I add a 9v dc bias to power the electret and then use the high pass filter to block it from the output? \$\endgroup\$ – Florida Stunters Feb 20 at 17:55
  • \$\begingroup\$ For that you probably want to start with a bluetooth link. Using a radio module changes the whole basis of the problem. You'll have a battery of course then. A custom Android ROM on carefully chosen models (or maybe even "rooting" them) may make software filtering of call audio user configurable even for a wired connection. What you really want however may be active noise cancellation, which is more of a DSP thing. \$\endgroup\$ – Chris Stratton Feb 20 at 17:57
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You cannot put a capacitor in series because it blocks the DC supply. Your resistor in addition pulls the DC onto its knees, because the typical resistor between mic+ input and system's supply voltage (3 or 5 volts) is 2...3 kOhm.

The only electrical circuit which works is a full filtering circuit which provides the needed supply voltage to your mic capsule. I bet you want something maximally as complex as your own attempt due the limited space and before all, due the wanted simplicity. I guess you want to avoid external battery.

If you can afford the inconvenience of a dedicated battery, the obvious solution is simple (assuming RC high-pass filtering is enough):

ADD: seemingly you have already figured this principle by yourself when this was under construction.

enter image description here

The voltage 3V is a guess. Your mic can be happy even with 1,5V and it can stand much more than 3V.

R1 is not a part of the filtering circuit. It's DC path to mic and it surely has some recommendations in mic's datasheet as the voltage. Too high resistance causes distortion. Too low resistance degrades the sensitivity but I'm sure it's not critical, maybe just opposite, you want to make it less sensitive, I guess. Even as low as 270 ohm can be good for this purpose. I would start with 2,7kOhm.

C1 is essential for the filtering. It breaks the DC path, so the included battery is a must. The resistor of the filter is in your camera, it's the DC supply circuit. If that has too high resistance you can insert a resistor in parallel with the camera mic input. I would try at first C1 = 0,47uF . If it still is too bassy, reduce the capacitance or add a resistor in parallel with the camera mic input.

Some metal shielding can be needed against electrical disturbances like in mic cables. Connect it to the minus pole of the camera mic input.

If simple RC filtering isn't enough (=idiotically gritty speech sound, but still too much low bass noises) you need 2nd or higher order active filter which needs an operational amplifier. Electronics application notes, web and books contain thousands of schematics for active filters as well as calculation receipes.

Alternatively you can do it acoustically. Put the capsule inside a rigid small bottle with thick walls. Let the bottle have small hole near the sensitive end of the capsule. If the mic is near enough user's mouth, it can still get enough sound.

Another idea is to punch a hole which lets the sound enter to the back side of the membrane of the mic. When user's mouth is remarkably closer the front side than the hole, the mic can well still produce enough signal. Here's the common mic capsule opened:

https://www.personal-view.com/talks/discussion/comment/225394

Finally the ultimate solution: Try a dynamic mic. It needs no DC supply. A half of in-ear headphones goes well. You need only a series capacitor to get the highpass filtering, you already have the electret mic supply circuit as the resistor. Try series capacitor = 0,47uF at first.

A general note for all mic types: You must have some soft material which is sparse enough to let the sound go through, but prevents all wind turbulence near the mic. You need some space between the mic and this windshield. 1cm, even less can be enough.

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  • \$\begingroup\$ Thank you guys! So here’s a question, since I already have a crap ton of parts, I wanna go the complex route just to make it happen, where can I find a full filtering circuit like you said? \$\endgroup\$ – Florida Stunters Feb 20 at 21:53
  • \$\begingroup\$ look at my original post at the bottom is the new circuit I drew up. Also tested and got it working like I wanted it to. Thank you though. \$\endgroup\$ – Florida Stunters Feb 21 at 13:44
  • \$\begingroup\$ @FloridaStunters I have seen it and fixed my answer to contain that fact. \$\endgroup\$ – user287001 Feb 21 at 13:46
  • \$\begingroup\$ Shoot, sorry I didn’t realize where you had added the circuit. Thank you! I might try that as well. \$\endgroup\$ – Florida Stunters Feb 21 at 13:49
  • \$\begingroup\$ Is there a way of adding videos on here? I’d like to show you guys how my project is coming along \$\endgroup\$ – Florida Stunters Feb 22 at 4:16
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Because electret microphones need bias voltage, they are hooked up (generally) like this:

schematic

simulate this circuit – Schematic created using CircuitLab

If you have access to the part of the schematic labeled 'OUTPUT' or the capacitor you can add a high-pass or change the properties of the existing high-pass. If you only have access to nodes A and B, you are going to have a hard time. This will require a much more sophisticated circuit and most likely a second power source.

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Why do you need a highpass filter? The phone and camera probably already have it. Simply plugin your electret mic and use it.

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  • \$\begingroup\$ Can you guys look at the edited circuit I added a picture of? Tell me what you think of it. \$\endgroup\$ – Florida Stunters Feb 21 at 4:08

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