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I have a setup similar to this one: amp, 100uF cap leading to headphones, nothing more. Powered from a Pi-Ion battery @3.9V.

It had worked fine with MAX4466, but after replacing that with MAX9814, I get VERY loud high-pitched noise. The mic is responsive, and the noise even dies for a fraction of a second when I am providing it with very loud sounds myself (like tapping the mic)

The noise appears to be feedback from the (very small) 32 ohm Philips headphone speaker: Philips headphoneMoving the speaker closer and further from the module changes the noise frequency (Doppler effect) Works even at 80cm (my cables don't reach further).
[edit] Tried different headphones, 140cm away - same.

I have tried 2 different MAX9814 modules, replacing caps, even ditching the breadboard and mounting on a piece of PCL plastic (insulator): soldered No change.

Dropping max gain from the default 60dB to 50dB makes is a little better (until the autogain gets there again) and at 40dB makes the noise cease at distances over 20cm. Similar with a pair of Panasonic headphones (not disassembled), but there, noise can disappear at over 35cm at 50dB. At 60dB, still appears at 140cm.

Granted, the module was purchased from ebay not from adafruit, but the board design appears to be the same.

The question, again. is whether all that is perfectly normal, or not.
The fact I have not read about it on the sites which decribe how to connect a headphone jack to the MAX9814, on the default gain setting, no less, leads me to expect it is not. The effect happens with the (Panasonic) headphones a full cable length away, after all.

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  • \$\begingroup\$ With the mic so close to the speaker, is this in any way unexpected? What is your actual question? \$\endgroup\$
    – user16324
    May 20, 2018 at 14:20
  • \$\begingroup\$ That's not the doppler effect you are hearing. The changes in the frequency come from the changed distance rather than motion - a particular distance will give the same frequency (height of the pitch.) If it were the doppler effect, it would go up in frequency while the microphone and speaker are moving towards each other, but drop to a lower frequency when stopped. \$\endgroup\$
    – JRE
    May 20, 2018 at 17:20
  • \$\begingroup\$ @JRE Then It IS the Doppler effect :) I hear a "IIIUUUIIIUUUIIIUUU" when quickly moving the speaker back and forth. Distance itself doesn't matter. \$\endgroup\$
    – kaay
    May 20, 2018 at 17:53
  • \$\begingroup\$ @BrianDrummond "is this in any way unexpected" is EXACTLY the question, asked by someone holding the modules for the first time in their life. Please consider this part especially: "Works even at 80cm". At the default (pin floating, 60dB) gain, I have extended the headphones 1.4m away (cable length) and the mic still caught it. If this were expected \$\endgroup\$
    – kaay
    May 20, 2018 at 17:57
  • \$\begingroup\$ I wonder if his module: MAX9814 can have a 30db gain fixed setting. The minimal i can get it set is 40db (gain and vcc shorted). \$\endgroup\$
    – Panamanian
    Jan 5 at 21:46

1 Answer 1

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The MAX9814 has an automatic gain control - it jacks up the gain to try to get a constant output level. It also has several selectable gain settings - the highest gain is 60dB

The MAX4466 doesn't have automatic gain. It has fixed (or manually variable) gain. It is a simple op-amp that can be used for other purposes.

What you are hearing is feedback - the microphone picks up a sound, the amplifier makes it louder, and sends out back out through the speaker. The microphone hears the output from the speaker and makes it louder. Round and round it goes, getting louder all the time. This feedback loop is same thing that makes an oscillator oscillate - and that's what your amplifier does. It oscillates at a frequency that depends on the time it takes the signal to make one round trip - and since that depends on the distance from microphone to speaker, you get a tone whose frequency gets lower as the distance from microphone to speaker grows.

This happens with the MAX9814 and not the MAX4466 because the circuit with the MAX9814 amplifies more (has a higher gain) than the circuit with the MAX4466.

To stop the feedback squeal, you can select a lower gain on the MAX9814. Download the datasheet, and have a look at it. It tells you how to set the gain. Try it. See how far you get. If you have trouble, come back and ask for help changing the gain (new question, reference this one if needed.)

This has nothing to do with RF or the doppler effect.

You have no transmitter or receiver, so no RF.

The sound changes with distance, not motion so it is not the doppler effect.

And finally:

Yes, this is what you should expect when connecting a microphone to a speaker through a high gain amplifier.

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  • \$\begingroup\$ Thanks. Please check comment above re: Doppler. True, it IS likely the sound that gives it, but why does it happen in the first place at the default gain, with a weak headphone speaker a meter away? I'd sort of expect that effect to have been mentioned somewhere the module + headphones were played with. I have already experimented with gain, as written in the question. \$\endgroup\$
    – kaay
    May 20, 2018 at 18:06
  • \$\begingroup\$ I disagree. 1. It is Doppler, though possibly acoustic not RF. You must have misread: tone is same regardless of distance. Changes only for the duration of motion. Please reevaluate with this in mind. 2. I do possibly have an RF receiver. All is needed is a short length of cable or circuit, and an amp. \$\endgroup\$
    – kaay
    May 20, 2018 at 18:28
  • \$\begingroup\$ I'm accepting the answer (or rather, just the final point). \$\endgroup\$
    – kaay
    May 25, 2018 at 9:57

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