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Following this question I wanted to check if my amplifiers are faulty. I rebuilt the schematic in the datasheet (p.10 9.2.2) and slowly increased Vin. There was no change in output gain when increasing Vin or changing Rv-10k. The chip is supplied via an external power supply. Between Pin 6 and Vss I read 4.98V. Vin is generated on the second channel of the power supply over an load resistor of 20Ohm. On the output (pin5) are headphones and an oscilloscope. On the headphones I can not hear the sound induced in the MIC and the osci-signal shows also no change.

Shouldn't I detect something, even if my circuit is configured badly? The output varies between 0-0.09V, which might be just the error of my multimeter. Should the output be this low?

EDIT: As suggested the list of pin voltages (all against GND)

  • 1 -> -0.102V
  • 2 -> GND
  • 3 -> 0.00V
  • 4 -> GND
  • 5 -> varying. It counts down from 0.09V to 0.0V
  • 6 -> 4.74V
  • 7 -> 0.013V
  • 8 -> 0.0V

EDIT: The real circuit circuit

v3xX
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    (1) What are you using as an input signal? (2) What are you using as a loudspeaker? (3) Gain is not a function of voltage. Hit the edit link below your question and add in the details. – Transistor Feb 16 '20 at 13:30
  • (1): An external power supply ranging from 0-75V (capped at 5V). (2) Headphones and measuring Pin5-GND using an oscilloscope (gives 50mV) and no change. (3) How else would is a change in the mic detected? – v3xX Feb 16 '20 at 13:44
  • You still have an invalid output Vdc level. List all pin voltages – Tony Stewart EE75 Feb 16 '20 at 13:46
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    I suggested that you add the missing details into your question rather than bury them in the comments. – Transistor Feb 16 '20 at 13:52
  • what exactly is your question? – jsotola Feb 16 '20 at 15:28
  • Can you post a good sharp picture about the chip and the connections it has? – Justme Feb 16 '20 at 15:54

2 Answers2

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It seems that you are trying to use an LM386 as a DC amplifier. The LM386 is not an op-amp as explained in the comments to your previous question .This will not work because there is an input capacitor which will block DC. The circuit you're using is intended for audio amplification and audio is alternating voltage which will pass through the DC blocking capacitors.

Test the circuit with an audio signal from your mobile phone.

Transistor
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  • I applied sound direct to the microphone, increased the volume. No effect on the headphones or the osci – v3xX Feb 16 '20 at 15:24
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In this question @glen_geek supplied me with reference values of the chip. Since none of the values (especially the 2.5V bias voltage of the output) seem to match my values, the conclusion seems to be a bad batch of chips. I tested a total amount of 8 chips in various configuration (9.2.1,9.2.2 of the datasheet, config1,config2) all with the same result.

Using this design I was able to hear and measure input change of the MIC (crackling and high noise, but nevertheless).

Next steps:

  • LM368 from different vendor (?)
  • alternative audio amplifier
  • be satisfied with the performance of the PNP circuit
v3xX
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