How to power an electret microphone in a +/- 6V circuit? - Page 1
Forum section: electronics / beginners

author: wobbly
author_rank: Regular Contributor
timestamp: September 12, 2024, 06:57:02 pm
content: I'm learning about purely analog electronics and my project at present is growing organically and more or less at random.So far I'm using a 12V battery as the power source and I'm using a uA741 op-amp to split that into +/- 6V with a ground in the middle. This works beautifully so far. So the next thing I thought I'd try is to make a very basic microphone circuit, using an electret condenser microphone (ECM).The ECM I have is a discrete, bare, two-terminal one, like this...So I started trying to find example circuits in the usual places and I was not able to find any examples of an electret mic being used in a split-rail circuit. The Art of Electronics (editions 2 and 3) doesn't seem to cover electret mics in any much detail as far as I can see. All of the online resources I can find (turorials, videos, ancient HAM websites) are single-rail (e.g. 9V and GND) circuits.So, fair enough, I figure that an ECM's output is a very small AC signal with a DC offset centred at the more negative terminal (in most cases GND).But if my circuit is split +6V / GND / -6V, does that mean that I only need to connect the ECM to the +6V and GND rails?I don't really know how to search for more information about this because I'm ignorant of the terminology involved. I know I'll need a FET-based preamplifier to get usable results from the ECM, I've found plenty of examples for that side of things.It's just the question of how to power the ECM itself in a split-rail circuit that puzzles me.Advice, sarcasm invited.

author: Phil1977
author_rank: Frequent Contributor
timestamp: September 12, 2024, 07:25:58 pm
content: In the two-terminal microphone capsules is the first amplification fet already included.That means you need a few volt DC bias to operate that thing. You can e.g. connect the pin that is usually connected to the housing to your GND and the other pin to +6V via a 2kOhm resistor. The signal is gathered by a cap from the + pin of the microphone.AFAIK the values I told are typical - I´ve never seen any electret condenser microphone with other parameters.

author: ledtester
author_rank: Super Contributor
timestamp: September 12, 2024, 07:53:46 pm
content: You normally capacitive couple the output of the electret to the next stage of your circuit and that allows you to change the DC bias to whatever you want, e.g. in this circuit:the 2.2uF and the 10k resistors change the DC bias of the electrec output from whatever it was (something dependent on the electret itself) to 2.5V.

author: wobbly
author_rank: Regular Contributor
timestamp: September 13, 2024, 05:31:39 pm
content: Thank you @Phil1977 and @ledtester,I had to trawl my parts bin for some suitable passives. I don't have many MLCCs to hand, so I had to use a 4.7uF electrolytic as the AC coupler on the positive terminal of the mic. And I experimented with a few different resistors to go from +6V to the positive terminal. I settled on 3.7Kas it seems to give the best sensitivity (without even amplifying it yet).In a quiet room, I'm getting about 30mVpp of noise. Oddly enough, the noiseincreasesvery slightly when I turn off power to the circuit! Don't know what's going on there, but I'll take itI grabbed my phone and played some music on loudspeaker right into the mic and my scope showed a nice audio waveform! Drum hits showing as sharp spikes of about 150 mv and instrumentals as a broad shape about half of that size. Very cool. For fun, I measured the consistency of the drummer's blast-beat timing. The guy is a beast!All that and the circuit is taking no more than 600 uA (including the 12V to +6V/-6V splitter opamp!). Amazing. I think I'm getting the analog electronics bug, finally!Thank you both again. It's little things like this that really help me