Wednesday, August 13, 2014

Experiment 12 (Chapter 12) -

Preamp and poweramp... both are found in the simple circuit for Experiment 12. I actually had an LM386 in my collection of parts, but it had a mangled pin. Thankfully this is a fairly common component, and Radio Shack sells them for $2.00... so no waiting.

The experiment does explain how to bump up the gain from the basic 20:1 to 200:1, but I'm going to stay with the default setting for now. If anyone attempts the upgrade and has a video, let me know and I'll be happy to share here with an update.

For my circuit, I did have to make just three modifications. Obtaining two matching 68k resistors was easy (for the middle voltage), but I didn't have a .68 microfarad... in goes a 105 or 1microfarad. I had to substitute a .1 for the .047 microfarad, and I took Charles' advice to add a very large capacitor between + and GND... a 1000microfarad... that helped cut the noise substantially!

I did have the 10microfarad capacitors (x2) and the 330microfarad. After adding in the electret and the 50ohm speaker, you can see my final circuit below.

Initial tests were horrible... lots of static and hissing. Only after replacing the wires to the speaker with a 3' length of braided wire did I get some great results as you'll see in the video. The troubleshooting section is valuable... if you're having static and popping, there's probably a fix.


  1. Great post about elektroonika.Thanks so much for your sharing

  2. When I built the circuit I didn't understand why the audio output from the LM386 was so noisy and distorted. I rechecked all my wiring, cut down leads so they were short and still a terrible hiss from the speaker and distorted tones. I couldn't believe that the LM386 performance was designed to be this crappy. It sounded a lot like your example in the video above. Essentially useless as an audio amplifier. Something else had to be wrong. I used a function generator to make a 1kHz sine wave tone and played it though a speaker into the microphone. Result, nasty crunchy noise from the amplifier. I used an oscilloscope to measure the output from the 741 alone without the LM386 in the circuit. Result.... clean smooth waveform at the 741 output. I checked the output from the LM386 connect into the cirucuit. Result...ugly, noisy crap. Then I put a 1000 uF electrolytic across the power supply buss from Vcc to GND as Mr Platt had suggested. Result....beautiful clean tone. I checked my bench top power supply and it seemed to be providing clean voltage. I don't know why the there was so much noise on the power supply buss lines on the breadboard but boy did using a big cap to filter it all out work wonders!