I hope somebody can help me with this. I have built an audio amplifier inspired by a youtube video. In my case I had already available a PAM8610 which has a stereo input (but they are not independent as they share the ground.)

As perhaps the schematic is not the best, here is a brief description of what it does: It mixes the audio from left and right channels into the left input of the audio amplifier, and the left output goes directly to the main speakers. Also from the left output I take the signal through a 1:1 audio transformer and I filter with a low-pass filter. In addition I add a potentiometer to regulate the volume of the subwoofer. The signal from the potentiometer then goes to the right input of the audio amplifier and from the right output goes directly to the subwoofer.

The problem is that the output has a lot of hum noise, and the only way to get rid of it is decreasing totally the volume on the subwoofer side. I optimized as much as I could the build, but it seems that there is something wrong with the grounds, perhaps it is bad practice to join together two grounds as I did.

Is there any way to address the hum noise in my amplifier, or there is a better way to build the circuit in a better way to get the desired outcome?

Edit: I changed the sketch as is wrong in the output section

Audio aplifier sketch

  • \$\begingroup\$ How you route power for the PAM8610 and how the ground power connection might share some unwanted prolonged connectivity with the ground signal wire is vitally important to avoid this situation. \$\endgroup\$
    – Andy aka
    Mar 22 at 13:10
  • \$\begingroup\$ Anyway, I can see no sensible reason why the output to the mid-range speaker is tapped off and fed back to the other channel. Why not filter at the input side and spur off the filtered signal to the input of the spare channel for the bass speaker? This would get rid of the transformer (a bulky item for bass signals). However, I still suspect the hum is due to poor input wiring. \$\endgroup\$
    – Andy aka
    Mar 22 at 13:20

1 Answer 1


While feeding a single-ended signal to differential-input amplifier you should AC couple the inverting input amplifier to the ground i.e. through a series cap, as opposed to direct connection to ground as shown in your sketch:


simulate this circuit – Schematic created using CircuitLab

As for the subwoofer output: What you are doing is, sampling the output signal via 1:1 transformer and filtering then feeding to the other input, nonsense to me. No offense. The common practice is to filter the input signal at input stage before feeding to the power amp (and get rid of the transformer, of course):


simulate this circuit

Some notes:

  1. Generally, I can't see a good reason of driving a speaker and an audio transformer from the same output.
  2. I don't know what type of audio transformer (i.e. line drive, mic, etc.) you are using, but all I can say is that it's not that simple to drive and terminate an audio transformer.
  3. Running long cables from transformer/filter to RIN, if you are doing so, is to ask for trouble.
  • \$\begingroup\$ Hi,You are right. As per the coment of Justme and yours, I removed the transformer and I am filteting at the input, now it works like a charm. Thanks a lot \$\endgroup\$
    – Manuel SD
    Mar 22 at 16:42
  • \$\begingroup\$ @ManuelSD good to hear. So the solution worked, and I think you can accept my answer so that the question does not remain open. \$\endgroup\$ Mar 27 at 7:52
  • \$\begingroup\$ I am sorry but I don't know how I accept the answer to close the question \$\endgroup\$
    – Manuel SD
    Mar 28 at 8:08
  • \$\begingroup\$ @ManuelSD you can click on the tick symbol under the points counter of this answer. \$\endgroup\$ Mar 28 at 8:10

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