OPA1612 as microphone pramplifier

[RuudNL]

In the updated schematic, the gain of the first OpAmp stage can be extremely high, with a feedback resistor of 1 M.ohm. (Depending on the drain impedance of the FET.)
The termination of the microphone capsule is low, with a resistor of 1 M.ohm.
I would think about a value of for example 100 M.ohm.

 

[abbey road d enfer]

@abbey road d enfer
I thought I could use DC-DC converter to isolate PC's supply noise and buzz.

I believe cleaning a power rail can be done without resorting to a DC/DC converter.

To everyone: I had to leave C1=50pF, when I increase it into 1n I got more input noise:

OPA1612 is a bipolar opamp, it is not optimized for such high resistance. Ideally R1 should be about 5-20 kohms.

+1 for increasing R0 to much larger. Typically 1Gohm is found here.

 

[Devicet]

Thank you for reminding about R1.

Am I right, that I could leave second stage like this?



Nothing wrong gonna happen when P1 = 0 ohm.
I would connect C2 straight into 3.5mm jack and short R and L chanel. I did it before and had no issues. After searching web I'm taking into consideration to use two 10k resistors. Important info - Rinput is just model of my sound card input impedance.

 

[abbey road d enfer]

Am I right, that I could leave second stage like this?

More or less.

View attachment 138996

Nothing wrong gonna happen when P1 = 0 ohm.

Except that the output of the opamp is loaded by 440 ohms, which may impair headroom and increase distortion.

I would connect C2 straight into 3.5mm jack and short R and L chanel. I did it before and had no issues.

It would work.

After searching web I'm taking into consideration to use two 10k resistors.

It would also work, with little less headroom.

 

[Devicet]

Thanks for all help. I would leave it like that:

 

[Cqwet Dbdfte]

How much gain is really needed? 37dB?
This OP amp has 100dB of open loop gain at 1kHz.
Is 5V going to be enough for head room. The part can take 36V.
Application examples by TI has +/-15V for a DAC output.

 

[ccaudle]

Is 5V going to be enough for head room.

The earlier posts indicated it was driving a computer line input with very low nominal input level.

 

[ntch]

There are many, many quality electret capsules with no built-in FET. In fact most of the best ones do not.

Ok, this is news to me .-) evidently I'm a user of low-quality electrets

 

[ntch]

In that case, maybe this video can be helpful: