chOPA EM700 (op amp based charge amplifier preamp)
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I wanted to use a t.bone (which I believe uses a 797 AUDIO 16mm electret capsule) as a boom mic into a XLR transmitter, but it¡¯s sensitivity of about 8 mV/Pa was a bit low for my needs (and the XLR transmitter doesn¡¯t have massive gain I can play with).
I wanted to get at least 20 mV/Pa out of it.?
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So I wanted to replace its preamp with one where it's easy to add gain, which usually is a charge amp, but without the complications of a transformer. Here is the topology I arrived at, which I¡¯d like to call chOPA (pronounced cho-pah, of course).
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Because of the battery powered XLR transmitter, I used one OPA1641 and one ADA4084 to save 1.15 mA of the P48 current budget vs. using one OPA1642. OPA1692 doesn¡¯t come in singles, I would¡¯ve liked that instead of the ADA4084.
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The mic sounds fine and has the sensitivity I wanted, with no perceivable SNR degradation vs. the original circuit (although that¡¯s something I need to characterize more rigorously). Of course it will have a lower max. SPL, especially when considering the input impedance of the XLR transmitter.
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The original mic had pad and low-cut switches, I covered the holes with conductive tape (there's a layer of proper copper tape under the aluminium).
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Feedback and criticism are welcome.
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