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Dear mic experts,

I'm trying to design a discrete amplifier for an electret capsule and have quite a technical question that is puzzling me. The device is a PUI AOM-5024L.

I encountered the possibility of driving the capsule with a (common-emitter) preamp that has an input impedance much lower than the capsule itself. Say the electret has 2.2k out-Z, while the preamp would have 850 in-Z. I know this will result in huge attenuation, but this is not a problem for me but rather an advantage. It means lower resistance (noise) on the preamp's first stage bias and the possibility of bypassing later stages for capturing loud environments–which is problematic with this very sensitive capsule. However, I don't know if loading the electret with a much lower impedance will affect ie. the capsule's frequency response (flatness) or its maximum headroom. Since there is a JFET buffer in between inside the cartridge, I tend to think that the JFET would absorb the load or separate and only attenuation would be affected, but I'm not sure about it.

Has anyone for instance seen or done frequency and/or headroom measurements on (JFET) electrets at different loading impedances? Or have any theoretical clues about this? I've read opinions that overloaded mics tend to sound bassy, lack highs or sound 'obscure' whatever that means, but might be that those comments refer to passive mics.
All opinions are more than welcome.

Best regards,
Domingo

Le 24/11/2023 à 00:25, chumariesco@... a écrit?:

Dear mic experts,

I'm trying to design a discrete amplifier for an electret capsule and have quite a technical question that is puzzling me. The device is a PUI AOM-5024L.

I encountered the possibility of driving the capsule with a (common-emitter) preamp that has an input impedance much lower than the capsule itself. Say the electret has 2.2k out-Z, while the preamp would have 850 in-Z. I know this will result in huge attenuation, but this is not a problem for me but rather an advantage.

The quoted impedance of 2.2k is actually that of the supply resistor.

The FET in the capsule operates in teh so-called "pentode region", which means that its actual impedance is very high, several 10's of kilohms.

It means that, in "normal" operation? output voltage of teh capsule is already constrained by the load.
By loading the capsule with a lower impedance, the result is just a level decrease.

It means lower resistance (noise) on the preamp's first stage bias and the possibility of bypassing later stages for capturing loud environments–which is problematic with this very sensitive capsule.

The max spl capability is not limited by voltage, but rather by teh current capability of the built-in FET. When this capability is exceeded, there is clipping, almost irrelevant of the actual load.

However, I don't know if loading the electret with a much lower impedance will affect ie. the capsule's frequency response (flatness) or its maximum headroom.

Basically, no.

Since there is a JFET buffer in between inside the cartridge, I tend to think that the JFET would absorb the load or separate and only attenuation would be affected, but I'm not sure about it.

You're correct, of course within limits. Pushed too far, reducing the load impedance may result in weak level, which will make the preamp noise dominant.

For your particular application, a 3-wire capsule may be more suitable.

Has anyone for instance seen or done frequency and/or headroom measurements on (JFET) electrets at different loading impedances?

I have and that's pretty well documented.

Or have any theoretical clues about this?

Any textbook about FET's will answer that.

I've read opinions

The problem with opinions is they're based on subjectivity.
Assessments are based on objective experiments.

that overloaded mics tend to sound bassy, lack highs or sound 'obscure' whatever that means, but might be that those comments refer to passive mics.

Certainly.

Le 24/11/2023 à 00:25, chumariesco@... a écrit?:

I've read opinions that overloaded mics tend to sound bassy, lack highs or sound 'obscure' whatever that means, but might be that those comments refer to passive mics.

I may not have been clear in my former answer.
It's certainly true for passive dynamic and ribbon mics. Microphone designers design their products to operate within the context of existing equipment, where the input impedance is pretty much standardized at about 1.5-2kohms.
In most cases, a too low impedance results in loss of treble, because of the inductive nature of their own impedance.
This is also partially true for active mics that have a transformer output.

Thanks a lot Jerry. That answers my question.
I was pretty convinced that using a tight coupling capacitor on the output (with cut-off at 30-40Hz) was helping the mic clip less. Deep low sounds like doors smashing would make the capsule clip and stop working for a few seconds, until coming back to life. This improved after dramatically lowering the cap value (2uF). But maybe it was the preamp or something else. From what you explain, not even the output capacitor should affect the capsule‘s behavior internally, right?
I have to dig into 3-terminal ECMs like most reasonable people. I know.

Regards!
Domingo

Le 24/11/2023 à 23:10, Domingo Riesco a écrit?:

Thanks a lot Jerry. That answers my question.
I was pretty convinced that using a tight coupling capacitor on the output (with cut-off at 30-40Hz) was helping the mic clip less. Deep low sounds like doors smashing would make the capsule clip and stop working for a few seconds, until coming back to life. This improved after dramatically lowering the cap value (2uF). But maybe it was the preamp or something else.

That's probable.

From what you explain, not even the output capacitor should affect the capsule‘s behavior internally, right?

Correct.

I have to dig into 3-terminal ECMs like most reasonable people. I know.

Regards!
Domingo