Electret Microphone Amplifier Input/Output

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ChuckMcG
 
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Electret Microphone Amplifier Input/Output

Post by ChuckMcG »

I'm intending to replace the supplied microphone with another electret microphone.(Knowles BT-1759).

What would be the effect on the input operation using an electret with a nominal 3500 ohm output impedance* (apart from the fact the replacement microphone will see about 1.5 V with my intended 2x AAA battery supply)?..

*Note: The specified impedance is actually a range from 2000 to 6000 ohms, but likely close to nominal.

What is the output impedance of the amplifier? This will be feeding a step down transformer (-18 dB) with an input Z between 75k and 600k, depending on the equipment I'm using. However, the amplifier's output impedance will affect the output impedance of the transformer, which I need to know for my circuit design..

The reason for the microphone change is three fold. The replacement is simply a much better microphone.. It has a lower sensitivity. I need this because the levels of the acoustic input will be well above what the stock microphone/amplifier can handle (calculated as 113 dB SPL peak). .The replacement will handle about 130 dB SPL peak which is at the very least 6 dB above what is needed. Lastly, it has 1/20th the current draw: 25 uA (typical) versus some 500 uA for the stock microphone. This is a significant difference for batter life.

Thank you for any information you can provide about all this....

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adafruit_support_mike
 
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Re: Electret Microphone Amplifier Input/Output

Post by adafruit_support_mike »

Which of the electret amplifiers were you planning to use?

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ChuckMcG
 
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Re: Electret Microphone Amplifier Input/Output

Post by ChuckMcG »

I should have specified it. Sorry about that!

Electret Microphone Amplifier - MAX4466 with Adjustable Gain. PRODUCT ID: 1063

The reason I asked about the effect on input operation was another post that noted the stock microphone was part of a voltage divider but was "more complicated" than this simple explanation...
Last edited by ChuckMcG on Sat Aug 22, 2015 10:57 am, edited 1 time in total.

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ChuckMcG
 
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Re: Electret Microphone Amplifier Input/Output

Post by ChuckMcG »

By the way, the Knowles BT-1759 is the microphone capsule used in the original version of the Crown PZM microphone. It is (or was) also the element used in the astronaut's headset microphones for NASA (a clue to its reliability). Specified for a maximum of 140 dB SPL, Crown wanted Knowles to increase that to 150 dB SPL (think usage with kick drums). Considering the small quantities of the product that Crown bought, Knowles refused to either modify the microphone or simply increase their specification. Crown opted to find a different supplier for a similar capsule. However, the PZM with the Knowles element is still considered the "gold" standard, sort of like pre- and post- CBS Fender guitars.

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adafruit_support_mike
 
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Re: Electret Microphone Amplifier Input/Output

Post by adafruit_support_mike »

The MAX4466's electret bias is 2k, so swapping in a mic whose nominal impedance is 3.5k will shift the output from VCC*1/2 to about VCC*3/4, and will make the output swing about 50% farther.

The mic's output is AC-coupled to the amplifier, so a DC offset won't hurt anything, and the automatic gain control will compensate for the extra voltage swing.

The only risk would be a combination of shift and voltage swing that would cause clipping. The breakout's gain ranges from 25 to 125 though, so largest design voltage swing would be about +/-40mV around 1.65v. Increasing the mic's impedance to 3.5k would take the swing to about +/-70mV around 2.1v, which still has plenty of headroom.

The worst case would be a mic impedance of 6k, which would take the output to +/-120mV around 2.5v. That should still work.

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