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

[Tom]

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
http://www.national.com/mpf/LM/LMV1032.html#Overview

National Semi LMV1012
http://www.national.com/mpf/LM/LMV1012.html#Overview

 

[Phil Allison]

"Tom"

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?


Ref1:

National Semi LMV1032
http://www.national.com/mpf/LM/LMV1032.html#Overview

National Semi LMV1012
http://www.national.com/mpf/LM/LMV1012.html#Overview

** 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

 

[Phil Allison]

"Mark the Jerk"

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

.... Phil

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

 

[Tom]

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