preamp to amp

Discussion in 'The Projects Forum' started by crivoli, May 22, 2006.

  1. crivoli

    Thread Starter Member

    May 4, 2006
    Question 1:
    I built the circuit on the LM386 datasheet with the amp at a gain = 20. Everything works fine.
    I then adjusted the gain to 50 and it works correctly.
    When I adjust the gain to 200 the speaker just squeals horribly.
    I have built this circuit before and used a gain of 200 with no problems. Any idea of what I could be doing wrong?

    LM386 datasheet:

    Queation 2:
    I hooked up a preamp circuit using a LM741 and send the output to the amp I built with the LM386. Needless to say the output to the speaker is very bad quality.
    Is there anything special I need to do with the output of the preamp before it goes to the input of the audio amp?

    I have attached the LM741 preamp schematic

    Thanks all
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    Ok...stupid question ....

    When you say it squeals ... is the microphone still connected causing outrageous feedback?
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    Intresting problem, as no component values are given.

    In general, though, operational amplifiers have very limited drive capability, and are not used to drive significant loads. The OPA134PA op amp, which I am familiar with, is only warranted to drive loads of 600 ohms and above. Even if your speaker is an ear bud, the impedance is going to be something like 32 ohms.

    Why all the gain? The more-or-less standard line level is 1.25 volts RMS. You've got a single 9 volt source. Even a gain of 8 will cause your circuit to clip. That's why you just get a squeal with the gain of 200.
  4. crivoli

    Thread Starter Member

    May 4, 2006
    Thanks for the feedback. I have adjusted the gain lower and it all sounds fine now.