How Do Capacitor Value and Voltage Rating Affect Electret Microphone Performance?

I am new to tinkering with electrets.
According to most the information I find, it is stated that your power capacitor on the electret can be any value.

But surely it's not irrelevant.
I am simply asking what is the relevance of voltage and farad ratings to the performance of the electret. My first build used a 10uF 16 volt. Sounds just fine. My second build uses a 33uF 100 volt. Sounds much better IMO or at least, higher output. The real testament to how good it sounds has of course been my second stage of filtering and amplification architecture.

I am of course assuming this cap is merely a power cap so higher farad and voltage ratings will increase output or at least attain the electret's full potential?

Not sure what you mean by the "power capacitor". Do you mean the coupling capacitor [cap]? If so, then it's low frequency passage that makes the difference. A larger cap (higher microfarad value) will pass lower frequencies. Look up capacitive reactance:

*http://en.wikipedia.org/wiki/Electrical_reactance
*http://www.allaboutcircuits.com/vol_2/chpt_4/2.html
*http://www.kusashi.com/reactance-c.php

The voltage rating of a cap is merely the maximum voltage it is speced to take before the insulation between the plates will fail. So, choose a voltage rating that is around 50% higher than the highest voltage that will be encountered by the cap in circuit (also consider possible spikes and surges in that assessment).

AH thanks. I 'm well beyond the ABC's of caps but the information regarding electrets is all over the place. One author tells you they're FET devices meaning they are actually IC's, another tells you that's incorrect.

I think the truly important lesson here is, most the basic schematics for electrets waste parts. A low pass filter is far more crucial than a coupling cap considering the overwhelming high frequency response of these devices. My first build used a 10uf only to be followed by a 10K resistor and 2.2uf to bleed off the whispy S'ing highs. My 2nd build incorporated a 2 band EQ and second gain stage and by the 3rd, I eliminated the bias resistor since my electrets can handle 9 volts and beyond.

There are so many sites where guys are SO dictating about using these parts and how you will destroy things if you do not but I suppose they have no idea what they are talking about and are just really good at sounding like they do lol. When your electret handles 16 volts there's no need for a bias resistor on a 9 volt supply unless you are concerned about saving electricity.

Thanks again and anyone who stumbles across this thread, hope it's of some benefit.

Mr. Death: A 'FET' is a type of transistor. They _are_ used in ICs, but they can also be discrete devices. In the case of the Electret, it's a single JFET transistor.

Not sure how you can "eliminated the 'bias'† resistor". There needs to be a closed current path of some kind in the drain circuit, otherwise how are you accessing the signal? The resistor on the drain of the JFET is to create a voltage divider so the current signal from the JFET can be converted to a voltage. But, it isn't necessary to convert that current to a voltage to access the signal, if you use the current directly--say, by feeding it to an OP-Amp or running it through the base of a transistor. But, more than likely there will be a resistor somewhere, especially if 9 volts is involved.

You could apply 9 volts directly to the drain (on the mic) without damaging the device (unless it's possible to generate a sound loud enough to run the JFET beyond it's maximum drain current handling ability), but, how would you access the signal (unless the JFET is some how modulating the battery voltage, on say, a battery with a sufficiently high internal resistance -- in which case you're probably going to run the battery down rather quickly [and how is the JFET surviving this--just doesn't sound likely])?

Here's an excellent article on the inner workings of the Electret microphone:

* http://www.openmusiclabs.com/learning/sensors/electret-microphones/

† I would call it the "drain resistor" or "load resistor" -- because the JFET is 'biased' by the variable capacitor at it's gate.

I referred to it as an integrated circuit because as simple as it is, a variable cap and transistor, that does by definition of the words make it part of an integrated circuit. They've been integrated into a capsule. I suppose demanding it to be labeled a discrete device is splitting hairs but I won't argue with an entire industry.

I also don't see the electret biasing since it's voltage output is dependent on SPL. The drain resistor is determining a fixed output on the devices potential output so more or less, it's biasing the circuit in that sense but it's unnecessary since the voltage output of the electret will never exceed the 9 volt battery. The electret won't "bias" the circuit but it will indeed control output. I'm unsure of what you mean by convert a current to a voltage as one does not exist without the other nor does one become the other that I have ever heard of.
I can make no sense of successfully eliminating the drain resistor to divide the voltage for accessing the output from the electret but it indeed works. I let it sitting on a rechargable 9 volt for at least a week without any noticeable loss of mah capacitance. With that in mind even without mass power consumption I would certainly expect excessive noise but again, not an issue.

I built 3 versions of a maturing circuit. The most basic running the electret without the drain resistor and it worked fine though very S'ing. The second incorporated a low pass filter so no more S'ing. The third finally adding a 2 band EQ and second gain stage with up to 200X gain. The mystery of the coupling, and truly useless, capacitor solved. How I got away with eliminating that resistor is beyond me so I wish you luck figuring it out lol

Well, then, I guess you put me in my place.

Why not simply study a selection of circuit diagrams and then check the coupling capacitor or decoupling capacitor, however you whish to term this and then consider what bandwidth you whish to cover typically for human voice this is btw 100hz to 15khz

I don’t quite know what application you intend to use this for if its ultra Sonics then obviously an electrolet would not be the answer

Mark www.harrington.force9.co.uk

Once Steve linked that accurate electret mic diagram, it was all incredibly clear to me. The number of bad diagrams I found from claimed engineers or technicians from sites commonly linked on numerous forums was nearly a dozen high traffic pages.
I literally had a laugh when I saw it's just a transistor with base current coming from an electret diaphram. A couple sites referred to the cap as a "charge bucket" implying it was a power cap. So yeah, it's just there to block DC and eliminate low frequency hum.

These will be used for vocal and instrument mics. I have some expensive condensers from MXL(Marshall) and found a well designed preamp for a 25 cent electret mic is on par with them. Not surprising though.

That wouldn’t surprise me either Yes I do laugh as well at some of this as I’m sure many other decent engineers do too , Still not to worry hardly surprising I guess when you consider some of this more super info from the so called importers and manufacturers who all swear blind that have designed this fantastic equipment themselves " Ho hum !! " Yes another huge great walloping fibber

Indeed I say gazing into microscopic diagrams , no voltages , no pcb layouts , illegible , loads of modifications just to make it work and uh also no mention anywhere of these mod’s that have had to be done " Not to worry I suppose

Where were you trained!! aaaah Not the cowboy ranch in England , sadly yes this is the norm for companies in the UK Still I am pleased to say that its not just the electronics Industry with Britain’s fab , super bright wonderfully impressive Yop schemes that don’t teach anything either but also would you believe something as simple as a wooden shed for the garden which I went to erect for someone yesterday

This would also make you die laughing how easy for this country to manufacture Oh yah they exclaim B and Q Yah we design this on PC you know

Super dooper , is that right I say , Wait till you see this peace of rubbish they have put together in Kit form for the so called DIY person and Yah its only a mere “£100.00 “

What a load f twaffle this is I could do better myself at making these than this Gigantasdourious outlet could do in a million years by bloody hand as well

Hope you have had a laugh at this as I have

Best of luck with your future generation Im not impressed at all