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I am using an LM386 to generate white noise. The circuit also has a second 386 for an on-board audio amplifier. Both chips are powered by the same 9V battery.

The audio amp won't pass the white noise signal and I can't figure out why, is there an impedance-matching issue? I have tried different value capacitors to buffer between the two ICs, resistors, resistors-to-ground, and an op-amp with 1:1 ratio.

When I turn the noise generator on and off, I can hear like a DC 'pop' on the speaker, but I can't hear the noise. When I tap the signal at C3 (between the two 386's) and plug that into a commercial amp, I can hear the signal fine. There's just something about that second amp that won't pass the white noise.

schematic https://www.circuitlab.com/circuit/g85pxv/white-noise-and-lm386-amp/

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    How are you mixing the signals together - do you have a circuit to show us? – Andy aka Apr 19 '13 at 18:03
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    @pinhead, a schematic of your actual circuit would be very helpful. It would be much easier to read a complete schematic than to try to work out what is in it from your comments and text. – The Photon Apr 19 '13 at 19:05
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    where is the blocking cap between the two LM386? – markrages Apr 19 '13 at 20:08
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    Listen dude, you've been repeatedly asked to show the full circuit and you still haven't. This is getting very frustrating and until we see what your circuit is I really don't think anyone is going to be able to answer this. FULL circuit please – Andy aka Apr 19 '13 at 20:51
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    @MattYoung There are other sound sources being mixed at the input of the audio amp (square waves generated with a 40106 Schmitt Trigger Oscillator). The other sound sources pass through the amp fine to the speaker. With those components removed (until the circuit is as drawn above), I still have the problem. – pinhead Apr 19 '13 at 21:41
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Pin 3 (final stage) might need a resistor to ground - try 1kohm. All the application circuits I've seen do have a lowish value.

Also I have reservations about it working correctly without a low impedance on the 1st stage speaker output (after the capacitor) so try a resistor on pin 3 (2nd stage) that is more like 10 ohms.

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protected by W5VO♦ Apr 20 '13 at 1:39

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