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I want to test a condenser microphone pre-amplifier using the signal from a function generator.

Normally I would connect the condenser microphone via balanced XLR to the preamplifier.

For my test setup, I want to replace the microphone with a signal generator, connecting it to the preamplifier in order to determine the gain and other properties by measuring the output.

The input of the preamp looks something like this:

preamp input circuit

What circuit do I need to fake the electrical characteristics of the microphone as closely as possible when connecting the signal generator to the preamp?

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    \$\begingroup\$ Not entirely sure what you are asking. Do you want to feed a signal in place of the microphone capsule to test the internal preamp in the mic before it outputs XLR, or do you want to test a separate preamp that takes in XLR from microphone? \$\endgroup\$
    – Justme
    Sep 11 at 23:43
  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Sep 12 at 4:37
  • \$\begingroup\$ @Justme Your second explanation is exactly what I want! I will edit the question to clarify... \$\endgroup\$
    – helixfoo
    Sep 12 at 11:02
  • \$\begingroup\$ dpamicrophones.com/mic-university/… . As stated in this site, characteristics and specs are defined within "manufacturers" ... So, it may be a bit difficult to give an general "answer". \$\endgroup\$
    – Antonio51
    Sep 12 at 11:58
  • \$\begingroup\$ does the sig gen have a balanced output? Also, what parameters of the micamp do you need to test? \$\endgroup\$ Sep 12 at 13:41
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You can pass the output of a function generator through a small 1:1 audio signal transformer. that will make the signal common voltage adapt to the inamp range and decouple the signal from the generator ground.

If the inamp common mode range ( see diamond plot in datasheet) includes the generated signal you can even connect the generator directly to the In+/- terminals.

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  • \$\begingroup\$ Thank you! A transformer is a great idea, however I guess it has to be a good one, so the signal gets not distorted... However in a mic, signal is generated by variable capacitance, not inductance, is this a problem for propper testing? \$\endgroup\$
    – helixfoo
    Sep 12 at 14:33
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    \$\begingroup\$ every mic has different output impedance properties. while a generator would easily drive the capacitors in your circuit, a mic might be loaded by them in a significant way. for a better test, you would have to model the output impedance of the mic in question in a reasonable way. maybe its best to use the actual mic, place it infront of a speaker and play a test tone / noise or something like this. \$\endgroup\$
    – tobalt
    Sep 12 at 19:00

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