Electret Mic decoupling capacitor polarity.?

Thread Starter

tarts

Joined May 27, 2009
20
So the question is which way should a unipolar decoupling capacitor(normal aluminium electrolytic) be mounted for electret microphone with bias supply voltage?

positive on the opamp side:
https://www.electronics-lab.com/project/low-noise-mini-electret-microphone-preamplifier/
http://www.minidisc.org/mic_preamp/Simple Stereo Electret Mic Preamp.htm
Positive on the mic side:
https://www.eleccircuit.com/pre-mic-audio-amplifier-with-ic-lm386/
https://lowvoltage.wordpress.com/2011/05/15/lm386-mic-amp/
https://www.epanorama.net/circuits/microphone_powering.html

It amazes me that there are schematics on the internet with the decoupling capacitor mounted either way. What is the justification of those schematics? My quess is there there is no justification and a bipolar capacitor should always be used. Why? well lets consider two scenarios:
1. The microphone is disconnected or its power consumption is very low. The voltage on the microphone side of the cap is equal to supply voltage and the voltage on opamp side is half the supply voltage- So positive terminal should be on Mic side right?
2. The microphone consumes a lot of bias supply current, this means the voltage drop on bias supply resistor (i.e R1 here) is high and the voltage on the Mic positive terminal is fairly low (could be 1/4 of supply voltage). The voltage on opamp side is again about half the supply voltage so the positive terminal should be on the opamp side right?

So either case can be true and if you have detachable microphone, each of these cases can occur during normal use and thus damage the capacitor.
Any thoughts?
 
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Sensacell

Joined Jun 19, 2012
2,852
It depends on the polarity of the applied DC. Period.

If the opamp is referenced to 0 volts, the mic input is usually more positive, so the '+' faces the mic.
Things are different if the opamp is referenced to some other voltage, like when you use a single supply opamp with a virtual ground at 1/2 Vcc- then the gound might be higher than the mic voltage. Figure out the circuit voltages and align the cap to the resulting voltage difference.
 

Ian0

Joined Aug 7, 2020
2,689
Electret condensor microphones are often used on a single supply, with an op-amp. The op-amp is generally biassed to mid-supply. The electret condensor has a bias resistor to V+. The spread of microphone standing current is quite wide, so the voltage on the microphone can vary quite a lot. It's a good place for a bipolar electrolytic, or a film capacitor.
The output of the microphone is a JFET drain, so is pretty much a constant current source, so that the impedance is defined by the pullup resistor.
A 1uF capacitor will give a bass rolloff of 15Hz, 470nF will be plenty good enough for most applications, so a film capacitor is a good choice. Don't use a X7R type ceramic, as it might be more microphonic than the microphone.
Many of the microphones inserts roll off at 100Hz, so a capacitor as low as 220nF will do the job.

On this link https://www.eleccircuit.com/pre-mic-audio-amplifier-with-ic-lm386/ the amplifier is a bipolar transistor, biassed to about 0.6V. That will be lower than the microphone voltage, so the circuit is correct.
On these links https://www.electronics-lab.com/project/low-noise-mini-electret-microphone-preamplifier/
http://www.minidisc.org/mic_preamp/Simple Stereo Electret Mic Preamp.htm
a normal 250uA microphone will produce 2.5V across the resistor. The microphone voltage will be 6.5V. The capacitor is the wrong way round (unless it is some sort of microphone that uses much more than 250uA, in which case, it may be correct)
On this link https://lowvoltage.wordpress.com/2011/05/15/lm386-mic-amp/ the amplifier is an LM386 which has a PNP emitter follower on the input and can have its inputs biassed at 0V. The circuit is correct.
The final link appears to be about phantom-powering a condensor, not making a preamp.
 
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AnalogKid

Joined Aug 1, 2013
9,292
It amazes me that there are schematics on the internet with the decoupling capacitor mounted either way.
It shouldn't. There is a lot of sloppy junk on the innergoogle. However, in this case you have found five correct links.
My quess is there there is no justification
Incorrect. All five circuits are correct. The coupling (not decoupling) capacitor orientation depends on the steady-state (DC component of the microphone output signal) voltages at each end. For the single transistor and LM386 circuits, this is lower than a typical mic cartridge bias of 2-3 V, and the capacitor orientation reflects this. The other three circuits are powered by 9 V, so the amp input is around 4.5 V, higher than the mic bias point, and the capacitor orientation reflects this.
and a bipolar capacitor should always be used.
It could make life easier, but there are other considerations. A bipolar electrolytic always is more expensive that a unipolar one because of internal construction and lower popularity. Depending on the amplifier circuit topology, input impedance, and target market, good low frequency response might require a value too large to be met with a ceramic, and a film cap will be much larger and more expensive.
1. The microphone is disconnected
This is a valid point. None of the circuits address this directly, although two are naturally immune; all are designed as if the mic always is attached, and for three of them a disconnected mic is a problem.
2. The microphone consumes a lot of bias supply current,
Nope.

ak
 

Ian0

Joined Aug 7, 2020
2,689
All five circuits are correct
I had a quick look at what is available and they are generally 500uA types, so you are correct.
If they were the 250uA types that I used to use, then two of them would be back-to-front, with the bias resistors shown.
I seem to remember quite a large spread on bias current, so it's definitely a "maybe".
and a film cap will be much larger and more expensive.
Really not much difference in size between a 470nF film capacitor and an electrolytic.
Polyphenylene sulphide caps are available in 1206 sizes, but I agree they are pricey.

If space-saving is important, then surely the answer is the MEMS microphone. Some of them even have I2S outputs - no preamp, no codec required.
 

Audioguru again

Joined Oct 21, 2019
3,319
And, in case you didn't know, the reason is that the electret microphone is basically a capacitor, which doesn't consume current.
Of course the Jfet in an electret mic consumes current.
An electret mic is a condenser mic which has the required 48V stored in its electret material. Its impedance is extremely high so it has a Jfet impedance converter in it that draws 0.5mA max. I normally power the Jfet with a 10k resistor (its drain resistor) fed from filtered 8V or 9V.
 

Thread Starter

tarts

Joined May 27, 2009
20
2. The microphone consumes a lot of bias supply current
As AnalogKid noted, it doesn't. And, in case you didn't know, the reason is that the electret microphone is basically a capacitor, which doesn't consume current.
Well a lot of current is subjective. The bias voltage is clearly there to provide supply current. What I mean is the current is enough to drop the voltage under the typical 1/2VCC level of opamp input. So for example with 9V supply through 10k, 0.5mA is enough to drive the mic line below 1/2VCC.

I should have made my statement more clear:
You should always use bipolar capacitor if you design a universal mic input (i.e 3.5mm mic jack where you can theoretically plug(or unplug) whatever microphone)
Integrated preamp is a different story of course.

So in light of that atleast the second link is problematic because it does not specify microphone model or whether their mic is integrated or detachable. If I would just copy that design I would have no idea to check for these issues.

And lets thow in one more:
https://www.digikey.com/en/maker/pr...-using-opamp/19c75ab5e6824e858044b80f791515da
 
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Ian0

Joined Aug 7, 2020
2,689
Or use a proper microphone with a proper XLR connector and a proper 48V phantom power (if it needs it), and leave the condensor insert for telephone handsets where it belongs, or let it go into peaceful retirement being replaced by the MEMS microphone which has a low impedance push-pull output and needs no pullup resistor.
The condensor insert wasn't a particularly reliable component, depending on the crimping of the can to make sure that the jfet gate was in contact with the condensor element.
 

Audioguru again

Joined Oct 21, 2019
3,319
I have used many non-inverting TL071 low noise opamps as a preamp for an electret mic. But I do not make the mistakes in the last one shown:
1) The 10k resistor powering the Jfet in the mic is not directly fed the noise on the power supply. Instead it is fed through an RC noise filter.
2) The coupling capacitor does not need to be a polarized electrolytic and the voltage divider does not needs to be as low as 47k resistors that load down the mic signal. The massive 4.7uF capacitor feeding the two 47k resistors pass earthquake frequencies down to 1.5Hz!!
I use a voltage divider of two 100k resistors and a small 330nF non-polarized and inexpensive film capacitor that passes frequencies down to 10Hz.
3) A gain of 1000 is like having the distance from the mic to the person who is talking down the street.
4) The 10uF capacitor in series with a 150 ohms resistor when the gain is 1000 cuts frequencies below 107Hz.

Name Brand electret mics from Digikey are much more reliable than junk mics from you know where. Most of the mics are not expensive at Digikey.
 
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