Audio amplifier with hum only when subwoofer is active

Question

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

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.