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5532 1st order bandpass for electret mic preamp

I found the capacitors that give the desired response.  The mic runs straight into the non-inverting, with R1 at 50k and R2 at 10k in the negative feeback loop.  It works perfectly but when I attempt to increase the gain by increasing the value of R1 but does not gain as it should, even at 100k.

I had tried the awesome SSM2219 and had no problems with the one-resistor gain adjustment.  I was able to pick up dogs barking two block away with just the window in the room open. So does the resistor combination have to change for extended gain ranges or are the caps influencing the feedback?

The band is about 500-3500Hz, first order.  The application is a speech recognizer which requires the substance of 300Hz-10khz.  However, the electret's frequency response is not flat, and begins to rise substantially after 3500Hz.  The lows are being attenuated to reduce background noise, and also the highs to some extent, but this particular electret takes a dive after 10kHz anyway.

Any pointers will be appreciated.

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  • I have some questions:

    Are you able to use a 3-wire connection to the mic, or does your design require a 2-wire? The WM-60A is now obsolete because of RoHS non-compliance, I believe that the WM-61A is the compliant replacement. I have not run into the issues that you raise in your Figure A note, that the mic distorts at moderate SPL. This is the recommended circuit in the datasheet, and creating the differential signal that I showed above should not be stretching the application; I have seen this topology in a few customer designs. Your Figures B and C both show merit, do you have any results from testing? I have not tried either one myself.

    Are you planning on using the SSM2019? You could use a standard op amp as your gain stage as well, even use it in a differential configuration. Providing a gain adjustment for the user would be much more complicated, however. Given your available power supply, I would look at the AD8672 or OP275. These are both quiet, unity gain stable and are plenty fast for this type of audio application.

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  • I have some questions:

    Are you able to use a 3-wire connection to the mic, or does your design require a 2-wire? The WM-60A is now obsolete because of RoHS non-compliance, I believe that the WM-61A is the compliant replacement. I have not run into the issues that you raise in your Figure A note, that the mic distorts at moderate SPL. This is the recommended circuit in the datasheet, and creating the differential signal that I showed above should not be stretching the application; I have seen this topology in a few customer designs. Your Figures B and C both show merit, do you have any results from testing? I have not tried either one myself.

    Are you planning on using the SSM2019? You could use a standard op amp as your gain stage as well, even use it in a differential configuration. Providing a gain adjustment for the user would be much more complicated, however. Given your available power supply, I would look at the AD8672 or OP275. These are both quiet, unity gain stable and are plenty fast for this type of audio application.

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