Dual MAX98357 mono amps grounding issue

[aaron94]

Hi everyone,

I was wondering if I could get some assistance with an issue I’m experiencing with an audio amplifier raspberry pi hat I’m trying to get working.

Basically it uses two MAX98357 mono amp ics to give me two seperate +/- right and left audio channels to drive two small speakers, which is what I wanted. And this works fine.

But I also wanted to wire up a 3.5mm audio jack for headphones, and they all use a common ground, wired to the sleeve of the jack.

So not really knowing anything about the hat I purchased for the pi (this is for a quick and dirty internet radio I needed to make and never wanted to put too much effort into it to begin with) I ran the left channel positive and the right channel positive to the 3.5mm jack and then I only wired up the left channel negative to the sleeve of the jack. I just assumed the two channels already shared a common ground without checking.

The left channel sounds fine on the earphones but the right channel, the channel that I ignored the negative side of, had a nasty digital hiss on it. 🐍

So I tried joining both grounds and this made both channels hiss.

At this point I had no idea it was even a dual mono amp.

I’ve found a few articles online about others experiencing this issue and people have said it simply cannot be done, the amp hat is only capable of driving two seperate isolated speakers.

I’m wondering if this can be done, but I’m not sure how? I don’t really have much AC skills. Could I use two isolation transformers and somehow get two channel outputs with a common ground?

Thanks a lot! I attached a quick schematic.

[tooki]

The key here is to understand that OUTN is not ground.

There are ways to bridge two channels via a resistor network, but the 3 pins of the headphone jack must remain floating with respect to ground. (Note that I can’t guarantee it works when using two discrete amp chips, rather than one stereo chip: stereo chips usually stagger the left and right output pulses; two mono chips wouldn’t know to do that.)

You’re better off getting a different hat that has outputs that are more useful to you.