Audio op-amp issue. LM386 only amplifies noise.

Discussion in 'The Projects Forum' started by NoOneYouExpect, Feb 4, 2013.

  1. NoOneYouExpect

    Thread Starter New Member

    Feb 4, 2013
    My professor is at a loss… can you help?
    I’m required to use an LM386 in an audio amplifier project with an 8 ohm speaker. The thing is it doesn’t seem to work. I set up the opamp as seen in the first figure on page 5 of the data sheet ( which should give me a gain of 20. Two interesting things happened. First, I only get the signal I input back out if I use the oscilloscope probe on AC 50 ohm, if I change it AC 1M ohm the signal becomes unrecognizable. Second, when I can actually see the signal coming back out it has a gain of 1. If I remove the input however the amplifier does in fact amplify the noise. If I put in a .1Vpp signal I get out a .1Vpp signal but the noise is amplified so much that it looks like a square wave bouncing between the rails.

    So… can anyone explain to me why my signal would be destroyed by changing from 50 ohm to 1M ohm?

    And if you are familiar with the LM386 can you please let me know if you have encountered this issue before? I have to use this part and I’m really not sure what to do about the behavior I’m getting.

    I appreciate any help I can get.
  2. MrChips


    Oct 2, 2009
    Did you put a 47μF electrolytic capacitor across the power supply pins?
  3. Audioguru

    New Member

    Dec 20, 2007
    Did you make the amplifier with a tangle of long wires on a breadboard?
    Of course it needs a supply bypass capacitor that is not shown on the datasheet because it assumes you have one.

    The datasheet for the LM386 shows that it has plenty of gain at 1MHz and higher.
    Then without a supply bypass capacitor near it and/or when made on a breadboard it oscillates at a high frequency.
  4. radio_davio

    New Member

    Mar 1, 2013
    The 47uF cap across the power inputs stopped the hiss in my circuit, and I'm using a 9 volts battery. Thanks!!
  5. recklessrog


    May 23, 2013
    I just posted a reply to a similar post, over the last few weeks I have been working with the 386 and have found them to be very prone to instability unless certain precautions are taken. first, all wiring must be as short as possible, all decoupling must be very close to the ic, breadboarding capacitive coupling between tracks can turn it into a very powerful oscillator so if using stripboard keep the racks short.
    another problem is the way the input level control is connected. dont fed pin 3 from the slider, wire the control between pin 3 and 4 (-ve) and
    feed the input signal via a capacitor into the slider.
    Pins 1 and 8 control the gain and you can INCREASE the gain by putting a capacitor & series resistor between these pins. I already had too much gain and to REDUCE it, connect a 1-47mfd capacitor in series with a 100k pre-set between pins 1 and 4 (-ve) and adjust for gain required then replace with a fixed resistor. If you don't use them, do decouple pins 1 and 8 with 1n capacitors to -ve.
    output leads must be tightly twisted. I did find I needed to put a 47ohm resistor in series with pin 5 (output) and the speaker coupling capacitor when using low impedance speakers to reduce distortion.
  6. recklessrog


    May 23, 2013
    P.S, once you have adjusted the preset to give the required gain, measure its resistance and replace with a close value fixed resistor, the capacitor +ve end goes to pin 1