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Old 15th June 2008, 01:52 AM   #1
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Default high-quality single-rail mic preamp?

I'm building a standalone audio processing box using an ARM chip running embedded linux, and I'd like to put a good mic pre on the front end. I was planning on using a regulated pregnant-snake style power supply, so that I wouldn't have to worry about shielding it in the smallish box, but none of them seem to be available with dual +/- supply rails, which all preamp designs I've found seem to need(usually 15V or 18V).

I'm looking at op-amp based designs, and in past designs I've done single-supply opamp circuits by just running the input signal through a big cap and then using a voltage divider to bias the input up to Vcc/2.

I'd imagine that the inductance of electrolytics degrades the signal somewhat, but are there other caps that can be found that are large enough so as not to kill bass response? Other considerations?

I want the signal to be clean all the way through the device, so I'm planning on using nice A/D and D/A chips, but it's not going to matter if the analog portion of the circuit sounds like crap.

thanks!
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Old 15th June 2008, 08:37 AM   #2
sek is offline sek  Germany
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Take a look at i.e. the M headphone amp schematic, intended for use with battery supplies.
It employs a TLE2426 rail splitter. The resulting +/-7V or +/-9V should suffice in a mic preamp intended for an ADC drive application.

Another way would be converting your favorite mic preamp circuit to singly supply, i.e. by applying the usual design techniques from (e.g.) TI's Opamps for Everyone, chapters 4, 12 and 18 as well as appendices A and B. Maybe you find a way to circumvent electrolytics at the input (because you usually only need a couple of microfarad due to large input impedance levels).

Just a thought,
Sebastian.
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Old 17th June 2008, 03:48 PM   #3
Minion is offline Minion  Canada
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I believe you could use a INA217 on a single supply...Though this chip is not meant for a single supply I believe it could be easilly made into a single supply curcuit...

If you look at the INA217 Datashhet and look at the Mic preamp schematic and connect R4 and R5 to VR (Voltage referance/Virtual ground) which should Bias the inputs to 1/2 supply ,and use a Output cap to Block Output DC.....



Cheers
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