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I'm using an LM386 for voice amplification. The transmitter transmits the signal from the tank circuit on the correct frequency but without voice.

I'm a beginner and a hobbyist and I'm still trying to understand the process.

  • I tried using a power supply from 3,3 V up to 9V and it made no diffetence.
  • When I touch the microphone the signal modulates a bit.
  • The circuit is tested currently on a breadboard and soldered on a perfboard.
  • I have an SDR, an osciloscope and a signal generator.

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Perf board:

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  • \$\begingroup\$ 100uF for the coupling capacitor seems a bit large. You're probably upsetting the oscillator conditions such that it is turning on and off - OOK modulation along with some FM. \$\endgroup\$
    – Kartman
    Mar 6 at 15:00
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    \$\begingroup\$ The LM386 design breaks many rules found in the datasheet when the gain is set to 200. Also the mic has no supply bias so most likely there is no audio output from the mic. \$\endgroup\$
    – Justme
    Mar 6 at 15:22
  • \$\begingroup\$ Agree with above, the LM386 probably isn't doing what you think it is doing. Your circuit might work better without it. See: envirementalb.com/fm-transmitter as an example. Biasing the mic and eliminating the LM386 probably would help a lot. \$\endgroup\$
    – KD9PDP
    Mar 6 at 15:30
  • \$\begingroup\$ Where did you find this design? It has too many problems (like 1.8pF and no emitter c and Cce too large, would take much longer than the answers so far to explain in addition to what has been revealed already and you won't understand YET yet similar to good ones, \$\endgroup\$ Mar 6 at 16:51
  • \$\begingroup\$ It actually works without the LM386.What is wrong with the 1.8 pF ? Isn't the C2 the CCE? \$\endgroup\$
    – titpoettt
    Mar 7 at 7:59
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Test each part individually.

Feed the LM386 output to an audio detector (high impedance earphone, oscilloscope, "line in" on a PC, whatever). Can you hear it and does it get louder when you turn up POT1?

No?

Try biasing the mic capsule properly (like a 2K resistor to +3V3) and see if that helps... You might want to clean up the 3V3 supply for the purpose...

schematic

simulate this circuit – Schematic created using CircuitLab

The purpose of C3 is to keep DC bias away from the pot, which would otherwise cause crackling noise as the pot was turned.

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If you check the datasheet for LM386 it suggests this input configuration:

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One problem with your design is that the bias current which inherently must flow in input pins 2 and 3 have no place to go - there is no dc path to gnd or to supply. These inputs are merely the bases of transistors and thus always need a current flowing when active. Again - there is a diagram in the datasheet. In figure 12 this is solved by putting pin 2 directly to gnd and pin 3 through the potmeter. The reason your microphone is sensitive to "touching" is precisely that it is floating and has no reference to gnd.

You dont specify the type of microphone but as a starter put it between Vin and gnd on figure 12. Maybe that is all what is needed. look at pin 5 with an oscilloscope to check if it amplifies your sound properly.

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