[moved] Electret microphone amplifier

[helmi_mjd]

Hi guys,
I'm current working on electret microphone amplifier. I found two circuits from other thread that are almost similar but with some differences as highlighted in the attached figure. Could anybody explain the differences referring to the figure? Awaiting your valuable response. Thanks.

 

[BradtheRad]

Re: Electret microphone amplifier

The 1k and 47uF create an extremely smooth, low-ripple power supply for the mic. The lefthand schematic includes the 100k-100k network, to give it a smooth supply as well. However if it were to cause oscillations, then maybe the righthand schematic should be used instead.

The output components are not critical values. They can be adjusted to affect frequency response. The 47 ohm resistor limits current, maybe as a safeguard against drawing too much current.

 

[Warpspeed]

The changes are so very minor, I doubt if you will see any difference in actual operation.
As Brad says, the component values are non critical.

 

[helmi_mjd]

Re: Electret microphone amplifier

Thank you for your reply. You are right. I have simulate the output of both circuits (discard the output components) as in the figure. There are not much different between them. I'm not really good in analog...so I'm not sure the actual response. Is this response is what should we expected? Why when I try to simulate with output capacitor 0.33uF (Left circuit) and 0.1uF (right circuit) it gives me error?

 

[Warpspeed]

The value of that output capacitor will only have an effect at very low frequencies.

Your simulation appears to be at around 25 Khz which is pretty high for a circuit like this.

 

[Audioguru]

Both circuits should have the 47 ohm series output resistor. It isolates the capacitance of a shielded output cable from causing phase shift which causes the opamp to oscillate at a high frequency.
The left circuit has a maximum voltage gain of 1+ (470k/2.2k)= 215 times and it is 1+ (100k/3.9k)= 26.6 times in the right circuit.

Your simulation program probably needs to have a load on the output capacitors, try 20k ohms to ground. Then the output capacitor feeding this resistor forms a highpass filter.

 

[helmi_mjd]

Here is the update according to Audioguru suggestion. I think it is much better now. Am I right?

 

[Audioguru]

It looks like you picked values at random without calculating them.
Don't you want wideband audio response (20Hz to 20kHz) for high fidelity? Do you want adult male voices to have all low frequencies cut and sound like chipmunks? Do you want to record very low earthquake rumbling frequencies?

Your 100nF input coupling capacitor cuts all low frequencies. Your 22uF capacitors pass earthquake frequencies. Your 100nF output coupling capacitor cuts low frequencies.
You selected only 10k for the trimpot P1 so the maximum voltage gain is now much too low and maybe if you scream very loudly into the microphone then you might be heard faintly.
The output level from an electret microphone is about 10mV (0.01V) when you speak at a normal conversation level about 10cm away. But your signal generator is blasting about 200mV.

 

[helmi_mjd]

Actually, i try to understand the circuit as I'm not good in analog system design. This amp will be used to amplify hydrophone (underwater mic) signal for frequency range between 20kHz to 50 kHz. The actual trimpot value is 470k. Basically where should I start?

 

[Audioguru]

The wideband hydrophones on the internet have a very low output level so they need a lot of amplification. But the datasheet for the TL071 opamp shows that its maximum gain at 50kHz is only 50 to 80 times which is not enough. So a few opamps (maybe in a quad TL074) must be used for enough gain but then their noise is also amplified. Probably a better opamp must be used.

 

[helmi_mjd]

Thank you for the information. Now, it getting more challenging for me. How to measure the output of the hydrophone since it requires bias power?

 

[Audioguru]

I do not know why you started this thread about a preamp for common audio electret mics and now it uses an rare ultrasonic hydrophone and you did not attach its datasheet.
Please post the datasheet for the hydrophone and say what it is used for.

 

[helmi_mjd]

Thank you for your time. Ok let me make everything clearer. I bought low cost hydrophones (electret based) as in datasheet (as in the link below) for about USD159 . The manufacturer told me, that the hydrophone works with common electret microphone amplifier. My purpose is to use four hydrophones to localize 3D position (x,y,z) of the underwater acoustic pinger (as in the link). The pinger is assume to be static. The hydrophone frequency range is between 10Hz to 100kHz while pinger frequency is between 20kHz to 50 kHz. To simplify the problem, maybe i will choose only one pinger frequency to simplify the design. I really appreciate your opinion.

Hydrophone:
https://www.aquarianaudio.com/h2c-hydrophone.html

Pinger (Mid-Frequency):
https://www.jwfishers.com/pingers.htm

 

[Audioguru]

The hydrophone has a very poor frequency response (+/- 4dB 20Hz-4KHz) then at 100kHz its response is down -40dB. Its sensitivity is extremely low compared to a microphone but maybe your pingers are loud and close by.

 

[helmi_mjd]

Ok Audioguru, thanks for your help. I will try to make it by using this hydrophones and if not not work I will have to search for a better hydrophone.