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Trying to identify what these are

maker_oc2k2llq_1661757028

Aug 29, 2022
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I got a bunch of these in a jameco grab bag. They are microphones (I'm 90% sure lol) but I can't find a part number or anything....what type of microphones are these?
 

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hevans1944

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Jun 21, 2012
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You might want to look at the information at this link. Many electret microphones use a permanently polarized dielectric and will therefore produce an audio output using just two wires. However, the output impedance of a bare electret is very high, so (although the signal level is quite large) the active element must be buffered, usually with a JFET (Junction Field Effect Transistor). Often the JFET is integrated into the microphone case, in which instance it must have a DC power supply (usually called a "phantom" supply) to operate. Sometimes the "phantom" power is applied via a third wire in addition to the two signal wires. Other times, it is applied to one wire of the signal pair and the audio signal is decoupled from the DC with a blocking capacitor. All this is described in the link.
 

maker_oc2k2llq_1661757028

Aug 29, 2022
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How can I tell if there is an integrated JFET? I have like 10 of these thinking of making a microphone array....I'm wondering what the ideal circuit for these would be..hence I was hoping for some specs on it..
 

AnalogKid

Jun 10, 2015
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Almost all small, low-cost electret microphone cartridges have a FET buffer built in. Two wires means you need an external bias current resistor between the audio out connection point and a low DC voltage. What is the operating voltage of the system you want to build?

ak
 

hevans1944

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How can I tell if there is an integrated JFET? I have like 10 of these thinking of making a microphone array....I'm wondering what the ideal circuit for these would be..hence I was hoping for some specs on it..
Go to the link and look at the two schematics on the first page. The circuit on the left provides some amplification and uses a +9V DC supply. The circuit on the right performs an impedance transformation (from the high impedance of the electret to the low impedance of a 8.2 kΩ ohm resistor) and uses a +5V DC supply. These voltages are typical, not cast in stone. Try operating the microphone without the DC supply (leave it disconnected) to see if you need the DC supply. If you get no output when speaking into the microphone, use the circuit on the right to feed an audio amplifier. Five volts is probably safe enough, but you could start with just one AA cell (1.5V DC) and add more in series up to about nine volts or six cells connected in series. I recommend that you use a battery for the DC supply because it minimizes noise.

As @AnalogKid stated, most electret mikes have a built-in JFET buffer. The circuit on the right has no amplification (it is a source-follower with, virtually, a gain of unity), so if your mike isn't sensitive enough, try the circuit on the left.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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That link is awesum....No batteries for me...
The point of suggesting that you use AA cells was to allow you to try different voltages while minimizing noise pickup. An adjustable bench power supply is okay of course, unless you experience noise problems. Power supply switching noise can be a common problem for some applications that use a switch-mode power supply, and there can be power-line frequencies that are unintentionally coupled through wiring in the walls to the electret element and/or the connecting wiring to your audio signal processor, whether that be an audio amplifier or an audio input to a computer. Just be aware of the possible pitfalls before blaming your "grab bag" of microphones.

BTW, I have been using and experimenting with electrets (not just microphones) since the 1970s. The first electret device that I had an opportunity to "play" with was an electret infrared-sensing element. The device responded only to a change in its temperature, so I had to interpose an optical "chopper" synchronized to a lock-in amplifier (or boxcar integrator) to see any significant output. I used a very long time constant (100 seconds IIRC) with the lock-in amplifier, but without any lenses or optics of any kind, I was able to detect the presence of infrared radiation from a human body standing in an open doorway at a distance of about fifty feet... after allowing a suitable amount of time for the signal to build up above the background noise level.

I wish I had spent a lot more time playing around with my sample device, but this occurred during the height of the Cold War with the USSR, and we were more interested in developing infrared laser weapon systems to (hopefully) defeat intercontinental ballistic missiles than playing with novel room-temperature-operating infrared sensors... turns out that if you were to take two of my devices, mount them side-by-side with a matrix of Fresnel lenses to image slightly different "views" on each sensor, and then measure the differential voltage produced by the sensors connected in series opposition...voila! You have built a Passive InfraRed motion sensor or PIR.

There is a critical mass of intelligent people among the seven or eight billion people in the world today. A few years after my experiments PIR motion sensors were everywhere, so I garnered neither fame nor fortune for pairing just one with a lock-in amplifier. That was just totally impractical for a mass market back then, as was the requirement to periodically interrupt the radiation path to the sensor to allow it to "cool off" during the path blockage. Perhaps I could make a single-sensor PIR today with modern electronics and some form of vibrating reed chopper, but pairing two sensors with a Fresnel lens illuminating both, combined with differential detection, was sheer genius in my opinion. Sure wish I had thought of that!

Electrets are extremely simple to make. You place a slab of the right kind of material between two parallel plates, thus forming a capacitor. Then you heat the material above a certain temperature while applying a voltage across the plates. This causes the dipolar molecules in the dielectric to rotate into alignment with the electrical field between the plates, aided in this rotation by the elevated temperature. After a few minutes you slowly lower the temperature back down to ambient while maintaining the electrical field. After reaching room temperature you can, depending on the dielectric chosen, remove the voltage from the plates and you then have a permanently polarized dielectric. This can then have metal electrodes and a thin diaphragm attached to replace the plates, thereby creating an electret microphone. Actually, unless I am mistaken, I believe electret microphones are made from thin films of electret dielectric onto which metal electrodes are evaporated. In any case, they are inexpensive to make and most have a very wide audio frequency range. Electret microphones have revolutionized the audio and broadcast recording industry, replacing virtually every other microphone type, such as ribbon, dynamic, and capacitor microphones.
 
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bertus

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Nov 8, 2019
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Hello,

Here is some more info on those electret microphones.
An article on Electret Microphones - Powering & Uses:
And a project for Recording and Measurement Microphones:

Bertus
 
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