Modifying Speaker Design to Use Mono Inputs

Discussion in 'The Projects Forum' started by dvetdaniels, Jun 21, 2013.

  1. dvetdaniels

    Thread Starter New Member

    Jun 21, 2013
    I'm preparing to teach an engineering survey course to high school students this summer. The students requested a soldering project and since I have limited electrical engineering experience I am having them build a simple speaker. I am using a design posted on the Make Magazine site (

    If I understand, this design takes a signal from a stereo audio jack and shorts the two terminals and uses one signal. I'm trying to reduce the cost of the project since I'm making 60 speakers. Can I wire a mono cable with a 3.5mm plug directly to IC socket? I have a bunch of dual plug cables I want to just cut in half. If so, can I eliminate R1?

    I've attached a pdf of the circuit diagram.
  2. #12


    Nov 30, 2010
    You can just ignore one of the signal wires, but you shouldn't short them together. The amplifiers in the MP3 player (or other stereo source) might fight if you short them together. You can't leave out the resistor because it carries the bias current of the 386 chip.

    You can buy resistors really cheap at online vendors like, Digikey, Jameco. Like, 8 cents instead of 2 for a dollar at Radio Shack.
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    If you want to modify the circuit with three additional resistors, you can connect both channels to the amplifier.

    Bridging two channels has been done many times in the past.
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Two resistors should be enough. I would use two 4.7 ohm 1W resistors, each from the jack plug signal and both going to the 8 ohm speaker.

    Normally in stereo music etc the two channels are not that different in voltage apart from some low energy and temporary subtleties. They never run at full power 180 degrees out of phase for sustained periods.
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    Welcome to the forum.

    If you want to run through the LM386 amplifier instead of directly to a speaker, then 1k 1/4w or 1/2w resistors in series with each channel should be fine.