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Old 19th Jul 2019, 7:33 pm   #1
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Default Balanced audio O/P from 3-terminal electret capsule.

Backstory: I have an old "Shure" microphone whose insert has gone open-circuit.

I want to replace this with a modern electret capsule-mic.

The input circuit to the modulator is a balanced stage, floating above earth.

The electret capsule I have has 3 terminals: Bias, Earth, and Output.

How to wire this? I'm wondering if I ignored the 'output' terminal and then used two equal-value resistors to connect the battery + and - to the 'bias' and 'earth' terminals, could I then use 2 capacitors from the electret's terminals to the modulator's balanced input?

[Essentially this would be building a 'split load' for the electret in the same style as a popular phase-splitter design]
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Old 19th Jul 2019, 10:14 pm   #2
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Default Re: Balanced audio O/P from 3-terminal electret capsule.

The new insert may well be one with a FET wired as a source follower in it.
The original will most likely be a "posh" one for balanced feed.
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Old 19th Jul 2019, 10:22 pm   #3
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Default Re: Balanced audio O/P from 3-terminal electret capsule.

Could you connect the unbalanced output of the electret module to a small audio transformer (LT700 sort of thing) and then use the secondary winding to provide your true balanced output, maybe with the centre tap earthed? Not sure about audio quality though. You may need to play about with equalisation depending on your requirements.
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Old 20th Jul 2019, 11:44 am   #4
Alistair D
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Default Re: Balanced audio O/P from 3-terminal electret capsule.

When I've done this kind of thing in the past I simply connected the output of the unbalanced mic to the 2 signal wires from the XLR plug and used the cable screen to earth any exposed metalwork. In this case you would also need a coupling cap to block the DC from the insert. So far I have never needed any ground reference resistors. That said I have never tried connecting an AC coupled source before so I not 100% sure if it will need resistors.

I won't tell you how I discovered that.
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Old 20th Jul 2019, 1:05 pm   #5
Join Date: Mar 2009
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Default Re: Balanced audio O/P from 3-terminal electret capsule.

Here's a thing I made earlier that runs off the phantom power from a mixing desk etc. Any old low noise transistor will do - the microphone capsule will have of course to be able to withstand the phantom power - not many will take the regular 48V - this design ran on a PP3.

Attached Files
File Type: pdf TL3509 Iss1.PDF (21.4 KB, 43 views)
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Old 20th Jul 2019, 4:20 pm   #6
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Default Re: Balanced audio O/P from 3-terminal electret capsule.

The usual type of electret mic insert uses a FET as an internal preamp, in a circuit that certainly does not look hugely different from a phase splitter if you connect a resistor in series with the source and drain ..... It's got to be worth a try! You might need a high-value capacitor from one end of the battery to the outer shielding of the mic cable.
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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