Adapter p48 balanced to single ended 2.5V behind 2.7kOhm

I tried searching, found nothing.

Does anyone have a circuit to connect a single-ended electret measurement mike requiring 2.5V behind 2.7kOhm to a p48 phantom powering microphone XLR input?

Second photo, bottom half:

http://www.audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html

Also http://audioimprov.com/AudioImprov/Mics/Entries/2013/12/10_The_Minimic.html.

RPC360, that is a cute idea! Clever in its utter simplicity, and if all you need is a microphone for recording some sound täit is perfection.

However: a measurement microphone is calibrated for a particular feed layout, in this case 2.5V behind 2.7kΩ.
The resistance value is critical, the voltage not so much.

Also, the cicuit is not impedance balanced, so long leads may be a problem.

http://sound.whsites.net/project93.htm is an interesting page, particularly fig:s 6 & 7.
And the method of creating the circuit power supply using the phantom power supply is very neat in fig 7.

Note that there are minor mistakes in the text, but it is obvious what should have been written.

Got a datasheet for that "measurement microphone" (capsule)?

I'd argue the idle current is more critical than the voltage or resistance value. But what do i know? It's not like Ohm's law is worth anything, right? ;D

And out of sheer curiosity, what are some of those "minor mistakes" on the ESP project page (or did you mean on the AudioImprov page)?

Texas Instruments has an opamp (they sell them) based circuit:

http://www.ti.com/lit/an/sboa320a/sboa320a.pdf

I like the biasing scheme.

I've found something like this works well ......you would need to select R14 to suit the volts you want to appear across R1.....

(Error in attached schematic - ZD1 should be 9v1 - Sorry about that - I can't seem to change the attached file)

That looks 90% identical to the circuitry inside the BM700 / BM800 mics.

I'm still not quite sure what's with that 2.7k as the only possible drain resistor there, but maybe i'm just weird like that...

rogs said:

I've found something like this works well ......you would need to select R14 to suit the volts you want to appear across R1.....

Click to expand...

Khron said:

Got a datasheet for that "measurement microphone" (capsule)?

I'd argue the idle current is more critical than the voltage or resistance value. But what do i know? It's not like Ohm's law is worth anything, right? ;D

And out of sheer curiosity, what are some of those "minor mistakes" on the ESP project page (or did you mean on the AudioImprov page)?

Click to expand...

He says one should make a resistor 1k for unity gain but it should be made 10k. R8, I think.

It helps if you read the entire text, in context.

http://sound.whsites.net/p93-f7.gif

"Although U1A is shown with a gain of 2, this can be reduced to unity by removing R8 and shorting R9 so that U1A is a unity gain buffer."

n3mmr said:

He says one should make a resistor 1k for unity gain but it should be made 10k. R8, I think.

Click to expand...

Khron said:

It helps if you read the entire text, in context.

http://sound.whsites.net/p93-f7.gif

"Although U1A is shown with a gain of 2, this can be reduced to unity by removing R8 and shorting R9 so that U1A is a unity gain buffer."

Click to expand...

You're right, of course.
My mistake. Can't think why I thought so, probably got my wires crossed.

Khron said:

That looks 90% identical to the circuitry inside the BM700 / BM800 mics.

I'm still not quite sure what's with that 2.7k as the only possible drain resistor there, but maybe i'm just weird like that...

Click to expand...

I've borrowed most of the idea from the Schoeps circuit - although with the orignall JFET phase splitter replaced by a low noise BJT.
 
The BM700/800 circuitry varies from version to version (I've come across 3 versions so far) so I'm not sure which version you mean? ....but I guess you could say that some versions of that follow Schoeps to some extent.
The lack of a low pass filter applied to the zener output on the BM700/800  tend to make them noisy, so I tend to avoid their circuit where possible.

I only included R1 as 2K7 because that value seems important to the OP?....

I've tried several versions, the most successful with the excellent Primo EM204 capsules.
For that I  fit R1 as 4k7 , and do not fit the second low pass filter (R14/C9).
I showed that here because the OP seems keen to derive 2.5V across his 2k7 capsule feed resistor.
You need higher DC supply volts for the BC549 phase splitter .