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What happens if an electret microphone capsule (JFET in catridge) is connected to a preamp with considerably lower input impedance than the capsule's output impedance itself?

ECM output Z: 2.2K Preamp input Z: 680ohm

The immediate effect will be attenuation by overload obviously, which is not an issue in my case but rather desired–I'm trying to design a preamp with different gain stages and big range. But will the overload affect the capsule's frequency response, or produce predictable distortions?

I've read this cannot be done with ribbon microphones for example, or passive capsules in general. But since (these) ECM's are buffered with a JFET, I'm in doubt if only attenuation is effected.

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Your High-Z microphone may struggle to provide the current needed for the Low-Z preamp, which in turn results in a smaller signal (as you mentioned). Ideally, for good amplification you want a Low-Z output to a High-Z input (and matched impedances for best power transfer when hooking up a speaker at the final amp stage).

Example for reference:

schematic

simulate this circuit – Schematic created using CircuitLab

To answer your question, in short: Check the ECM's datasheet for its capabilities.

My personal opinion is that It shouldn't affect any frequency response characteristics.

If you want to be extra sure, you can always match impedances using a T or PI matching network, or have a simple common emitter amplifier before your lower impedance amplifier.

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  • \$\begingroup\$ Unfortunately the ECM datasheet (AOM5024-L) doesn't show an impedance-based bode plot. My concern is about transient distortion and headroom reduction, which seems like an issue to passive microphones. \$\endgroup\$
    – Domingo
    Commented Nov 22, 2023 at 0:37
  • \$\begingroup\$ Many years ago a speaker matched the output impedance of an old vacuum tubes amplifier. Modern solid state amplifiers have an extremely low output impedance that damps speaker resonances, then the impedances are not matched, then the attenuation caused by matching impedances does not occur. Your mic and preamp impedance problems will cause high attenuation. \$\endgroup\$
    – Audioguru
    Commented Nov 22, 2023 at 1:25
  • \$\begingroup\$ Only attenuation? Or also the way the capsule responds to certain frequencies? \$\endgroup\$
    – Domingo
    Commented Nov 22, 2023 at 11:36
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    \$\begingroup\$ @Domingo considering the construction of an electret mic I would say it only affects attenuation as the pressure sensitive capacitor (two pieces of metal film, with one being "loose") is connected to an internal FET preamp which acts as a buffer between the actual acoustic device (capacitor) and your circuit. The impedance mismatch will therefore not affect the capacitor or any possible deviations to the normal frequency response. \$\endgroup\$
    – Pxl
    Commented Nov 22, 2023 at 16:31
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    \$\begingroup\$ A JFET uses voltage at its gate to control current flow between drain to source. Think of a JFET as a sort of voltage controlled resistor. The input impedance of a JFET is usually very high, or infinite in ideal cases, meaning nothing on the output side of the JFET can really affect the input (unless you overvolt it, or let the magic smoke out). The gate of a JFET can be really sensitive to small changes in voltage, which is why it is commonly used in electret mics, radio front-end amplifiers, etc. \$\endgroup\$
    – Pxl
    Commented Nov 23, 2023 at 22:58

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