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Using $1 stereo amp in mono mode

Discussion in 'Audio' started by SammyT, Feb 4, 2015.

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  1. Audioguru


    Sep 24, 2016
    The PAM8403 datasheet says the speaker can be 4 ohms or 8 ohms. Yours is 3 ohms which is close to a dead short. The amplifier protects itself from a shorted output by disconnecting its output which might happen with your 3 ohm speaker. Or the amplifier might overheat.

    Connect the two wires (audio and ground) of the mono output from your sound system to one channel input on the PAM8403 and connect your subwoofer speaker to the output of that channel. Do not bridge anything, you cannot increase its small output.
  2. SammyT


    Feb 4, 2015
    Just a follow up to the original project here: I've been using the small amp/speaker with my MacBook pretty much every day for a few years now and it works great. I'm a graphic artist and while I'm working on my Photoshop projects I watch YouTube videos, usually researching people I've heard about, bands from the 60's, stuff like that. the amp works great for that.
    As far as using it for a subwoofer amp... it's way too small to do much. But since it costs only a buck, I'd go ahead and try all the things you've asked about to see what happens... it's not a bad way to learn.
    I've had a few pa system power stereo amps that you could use like you suggested (feeding a mono input to both inputs then powering the speaker from the L negative and R positive wires) and I don't understand why that wouldn't give you double the power output with this little amp like it did with those pa amps. I don't need more power from my little amp but I would risk $1 to see what happens - for the same reason that when I was 10 I hooked a 25 ft garden hose up to my snorkel to see if you could stay at the bottom of the pool (you can't). You usually do most of your learning by being wrong. If you do try it please post the results.
  3. MichaelZ


    Dec 23, 2017
    Thanks both - Audioguru and SammyT - will try tonight to solder it all together. Will post here what came out of it..
  4. Audioguru


    Sep 24, 2016
    Usually an amplifier output is one wire with the signal and the other wire is ground.
    But this little amplifier has bridged outputs so both speaker wires have signal but they are opposing phase so the output voltage swing is doubled which results in almost 4 times the power of a single output with ground.

    If the amplifier maximum output is 10V p-p then its + wire goes to +5V and its - wire goes to -5V.
    1) When the signal is mono then the + wire on each channel both go to +5V and the - wire on each channel go to -5V. If you connect a speaker to R+ and L- then you get the same power as for one channel, not doubled.
    2) If there is a signal on only the left channel then the L- goes to 5V and the R+ is nothing so the left channel voltage is half of what is should be which is 1/4 the power.
    3) If there is a signal on only the right channel then the R+ goes to +5V and the L- is nothing so the right channel voltage is half of what it should be which is 1/4 the power.
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