Help! LM386 interference

Discussion in 'The Projects Forum' started by electricsheep, Aug 2, 2009.

  1. electricsheep

    Thread Starter New Member

    Aug 2, 2009
    I am building a lm-386 based stereo for my ipod, and while the circuit works great when connected to a battery, it has a low frequency hum when I connect it to the wall current.

    The circuit I am using to adapt the wall current into 12 V is a transformer connected to a rectifier, and a relay to switch between the power cable and battery output. I have measured the output of the adaptor circuit with a cheep multimeter at 12 v, and the indicator light, (parallel to the amplifier circuit) lights up. The problem is that when the speakers are connected, instead of the signal, I get a hum, probably 60hz, and I can't figure out if it is a shielding problem, or if it is an issue with an inconsistent voltage. Any insight into this problem that can be provided would be very appreciated, and as soon as the kinks are worked out I will be sure to post the schematics.

    also i forgot to add, the AC-DC converter that i built is just an off the shelf 120v-12v transformer, and a diode bridge rectifier, without a smoothing capacitor, all the grounds are connected together with a wire nut, and every board/component is screwed into the back of the cabinet
    Last edited: Aug 2, 2009
  2. bertus


    Apr 5, 2008
  3. electricsheep

    Thread Starter New Member

    Aug 2, 2009
    For the record, I found that a 2200µf, 35v capactor worked perfectly
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    Just for safety check the DC voltage at the filter cap. The voltage over this cap will be (transformer output-rectifier drop)*.1.4 Volt. The LM386 can be used up to 18 volt so I think you are safe
  5. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    If still you get a hum after filtering the supply, earth the cabinet.