Electret mic question

[Sceadwian]

I asked about this in another unrelated thread but got no replies. Hoping someone could shed some light on it. Electret mics are really compound devices, they're the actual microphone element and a simple Jfet amplifier stage. The - lead gets grounded and the + lead gets fed a couple of volts through a resistor and the signal is drawn off after the resistor. This makes it a polar device, thing is no matter which way I hook them up to my sound cards mic in jack they work. The ground is normal and the tip is getting a pinch over 2 volts. I'm just trying to understand how it works regardless of polarity?

 

[kchriste]

Some JFETs work the same when the source and drain terminals are reversed. Depending on how the internal circuitry is wired, this could be the reason. The only doubt I have about that, is that the input to the FET also swaps making it a source follower. Just speculating at this point. Do you get the same sensitivity with both polarities?

 

[audioguru]

I have used hundreds of two-wire electret mics but I have never tried one that is connected with backwards polarity.

It doesn't make sense to connect an electret mic backwards because the metal case is supposed to be grounded as a shield.

 

[Mikebits]

Is it possible the mics are not electret?

 

[Sceadwian]

No, it's definitely an electret. 1mm X 6mm. The rubber housing it's in is bigger than it is. I did a continuity test, one of the leads is connected to the case although there's no sign of it on the external pad. I never tested for dynamic response, basically just a tap test to see if I got anything so I might not be getting any gain when it's reverse biased, but definitely getting something. The bias voltage of the sound card is only 2 volts and I've seen circuits on the net that suggest biasing much higher, like 9+ volts to get decent response from them, so even properly biased it might not be getting much gain.
I've heard of basic JFets working in both directions before, because they don't have the body diode like a MosFet does right?

 

[audioguru]

My first microphone was a horrible-sounding crystal type. My second was a very good sounding dynamic type. My 3rd was a horrinle-sounding piezo type and my last mics are perfect-sounding electret type.

 

[Sceadwian]

I have quiet a few electrets floating around, that's why I'm asking, I've always been curious about acoustical spacial dynamics and electrets have a flat response over the useful human hearing band. I'm assuming if I make a circuit that has a better bias voltage and buffer amp that I should get some pretty decent input signals. Then I can play around with putting them in various array configurations.

 

[Hero999]

The fact that most JUGFETS work the other way round shouldn't make any difference given the internal schematic of an electret microphone. If I'm understanding this correctly, if the power is reversed the gate will be forward biased by the electric field causing the JUGFET to turn on and short circuit the bias resistor.

I got the same schematic from two sources just to make sure.

**broken link removed**
**broken link removed**

 

[Blueteeth]

Sceadwian,

You may be interested in the 'linkwitz mod' for two capsule electret mics. I have used this myself and it does exactly what it says on the tin - higher SPL levels at a reduced gain.

https://www.linkwitzlab.com/sys_test.htm#Mic

It seems to play about with polarity, although I guess all it does is change the way in which the FET operates.

I too have noticed that the electret mics work more or less regardless of polarity. Dunno why, but I gave up using my soundcard input a while ago as its pretty poor (cheapo on-board soundcard, mic input is a bit noisy).

Sorry if this is a little off topic.

Blueteeth

 

[prprog]

The fact that most JUGFETS work the other way round shouldn't make any difference given the internal schematic of an electret microphone. If I'm understanding this correctly, if the power is reversed the gate will be forward biased by the electric field causing the JUGFET to turn on and short circuit the bias resistor.

I got the same schematic from two sources just to make sure.

Is a JUGFET different than a JFET? What will be the typical values for the resistor and the V+?

 

[Hero999]

They are the same thing.

The the value of the resistor depends on the power supply.

 

[mneary]

If I'm understanding this correctly, if the power is reversed the gate will be forward biased by the electric field causing the JUGFET to turn on and short circuit the bias resistor.

This would be true if it were a MOSFET. But this is a Junction FET, and it is already on (it's a depletion mode device). The load resistor is chosen to work well with the FET's ON resistance. Since the electret element is a capacitor with minimal leakage, it doesn't bias that diode no matter which way it is hooked up.

 

[prprog]

This would be true if it were a MOSFET. But this is a Junction FET, and it is already on (it's a depletion mode device). The load resistor is chosen to work well with the FET's ON resistance. Since the electret element is a capacitor with minimal leakage, it doesn't bias that diode no matter which way it is hooked up.

As it is can the electrec/FET be used as a variable resistance device?

 

[mneary]

As it is can the electrec/FET be used as a variable resistance device?

You could probably use the JFET as a JFET.

 

[prprog]

You could probably use the JFET as a JFET.

What voltages and resistance values can I used to kept the FET inside the electrec in the ohmic region?

 

[Hero999]

https://www.epanorama.net/circuits/microphone_powering.html

 

[audioguru]

The datasheet for most electret mics spec's a current in the jFET of 0.5mA. Then a standard 2-wire electret mic uses a 10k resistor feeding it from 6V to 9V.
The total impedance is about 3k ohms so it should have a load of at least 30k ohms.