Le 24/11/2023 ¨¤ 00:25, chumariesco@... a ¨¦crit?:
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¨Cwhich 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.
