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Topic: get better frequency response from an electret mic? (Read 1 time) previous topic - next topic

winner10920

I have a general electret -> opamp preamp circuit feeding a msgeq7 which is used for dimming lights according to frequency, but I get a large peak around the middle frequencies and decent high range, but nearly no low range
is there a way to make it a little more equalized simply in the circuit? I may be able to compensate in software at the cost of noise, but it will have multiple sources and I don't want to screw up those inputs when used since they don't have a problem since they are an actual audio source
Is it even possible with the electret mic or do I need to get a better mic too?

winner10920

Also how much of a pulldown should I have after the ac coupling? I put a 750k and it keeps it from being constantly high but I noticed on my oscope that I was actually getting the full swing amplified as if it was biased above ground
this is with a lm328n

James C4S

What about scaling the outputs of the MSEQ7 instead of messing with its input?
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winner10920

Its already scaled pretty well when it gets a solid input from a radio or an aux cable,  just my mic setup doesn't respond to bass as well
Unfortunetly I mostly care about the high and the low more than the middle, and while the mid range is very responsive it doesn't get a very good effect as when otherwise, I also am planning on having it automatically scale the microphone gain as to not max the msgeq7 out when the music is played loud

dc42

Post a schematic of your preamp, including component values. The usual cause of poor low frequency response is using capacitor values that are too low for the circuit resistance at that point.
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DVDdoug

Quote
I may be able to compensate in software at the cost of noise
If you compensate with an analog circuit, you will have exactly the same noise issues.   Hopefully, it's a capacitor issue...

Quote
I may be able to compensate in software at the cost of noise, but it will have multiple sources and I don't want to screw up those inputs when used since they don't have a problem since they are an actual audio source...

... I also am planning on having it automatically scale the microphone gain as to not max the msgeq7 out when the music is played loud
If you scale the 7 channels independently, that might solve most of your problems.

winner10920

I do have them scaled differently already for a good effect when the source is good quality, im on my phone so can't post a shematic but I have aan op amp with ~25x gain, and on the non inverting input a .1uf cap as ac coupling, and a 750kohm pulldown
on the other side of the cap I have a 10k pullup and one terminal of the electret mic, other terminal is grounded
with this setup and positive supply at 10v the mid range is good, others are lacking, and a higher supply @15v puts everything too noisy and not very responsive to the actual music
But since its such a high impedance input a .1uf cap should be plenty right?

dc42


I do have them scaled differently already for a good effect when the source is good quality, im on my phone so can't post a shematic but I have an op amp with ~25x gain, and on the non inverting input a .1uf cap as ac coupling, and a 750kohm pulldown... But since its such a high impedance input a .1uf cap should be plenty right?


Yes, by my calculation, 0.1uF and 750K gives you a -3db point of about 2Hz.

Do you have the resistor from the inverting input connected directly to ground, or through a capacitor?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

winner10920

Directly to ground, should it be through a cap?
also I have a cap on the output, at first I didn't and it doesn't affect the response but I wanted to make sure no dc went out when I noticed that I was not only getting half the sound but the whole thing off center

Docedison

try increasing the coupling capacitors by a factor of 10 this will drop the low frequency 3Db point by a factor of 10 and a factor of 20 increase in value will drop the corner frequency by half again to 100 Hz.

Doc
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MarkT

By electret do you mean an actual microphone or an electret mic insert?  The latter won't have much bass response without the right acoustic housing.
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winner10920

Just an element, what kind of housing do u mean?

MarkT

An actual microphone body - this is where the cleverness resides, not just the transducer - like an ear the microphone collects and focuses sound onto the transducer, has a frequency response and has a directional sensitivity.  I think this is quite a specialised area of acoustics (good microphones are extremely expensive, which is a hint).

In theory an electret is supposed to just respond to the air pressure in front of the diaphram (and the thin diaphram makes its mass less important than that of the air), but in practice sound waves are refracted and deflected by the housing and resonate at frequencies determined by the geometry of the sensor.  Also the same sound pressure tends (at low frequencies) to move the whole microphone, not just the diaphram - this is why good microphones are heavy, extra mass is added to prevent this.
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