Pre-amp for interface of Balanced/MC mic to 2-wire electret mic input

Feb 25, 2011 9 Replies

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Tom 15 years ago

Hi group,

I must put together a single channel low-voltage (~2v) balanced mic compatible preamplifier in order to interface a dynamic/balanced/moving coil mic to a host circuit that was designed to operate with for a 2-wire 'phantom' powered electret capsule (ECM)

My circuit must operate on the small power that the host would otherwise provide to the ECM. I've measured this and ascertained that it supplies circa 1.5 - 2.1v via 1k8 so my dynamic mic pre-amp will need to be quite low power.

So, my plan is presently to use one of National Semi's electret pre-amps (Ref1) in a modified configuration to amplify my dynamic mic, single-ended but interface with a 2-Wire ECM powering configuration.

Has anybody come upon any suitable circuits that might save me some design time?

Tom

Ref1:

National Semi LMV1032

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National Semi LMV1012

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Phil Allison 15 years ago

"Tom"

** From the specs, those IC are very noisy - plus the pack is extremely small.

I would use a single j-fet preamp, like electret capsules normally do.

.... Phil

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Mark 15 years ago

and maybe a step up transformer.. Mark

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miso 15 years ago

If you are starved for power and want low noise, I'd 2nd the step up transformer. It may not be the whole solution, i.e. some active circuitry might still be required, but at least the transformer get the input referred noise of the active circuitry to a reasonable level given power requirements.

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Phil Allison 15 years ago

"Mark the Jerk"

and maybe a step up transformer..

** The j-fet would provide voltage gain similar to an input tranny.

Plus be about 20dB less noisy than the Natsemi ICs.

Plus be about 20 dB cheaper too.

.... Phil

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Tom 15 years ago

Thanks all,

I had been considering a transformer initially but, quite simply, I haven't found a supplier of good quality audio transformers that will be small enough to fit in my sub-matchbox-sized housing. Any pointers to suitable xformer manufacturers welcome!

My use of 2-wire electret amp may not be optimal noise-wise but I think it will be acceptable and it gives me some welcome gain with almost no external components.

National and Texas do common parts (LMV1012 / TLV1012), I am exploring Analog Devices too - any other sources?. I hoping that in my search I'll stumble upon a balanced MC mic app circuit and save myself some time.

Tom

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Jeroen 15 years ago

So that makes 1/100th the price then, because money is power. ;-)

Jeroen Belleman

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Phil Allison 15 years ago

"Tom"

** So would a single j-fet - you imbecile.

** Watch where you are treading - pal.

Ocs fools like you stumble over all sorts of little things.

.... Phil

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Tom 15 years ago

Oh dear, bad day?

I think most on this group will know how to use a jFet - they have their disadvantages, certainly in this app.

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Phil Allison 15 years ago

"Tom theTosser "

** But not a total fool like you.

** What app is that ?

You have not revealed one relevant detail of you fictitious "app".

Wot a piss head you are.

.... Phil

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