Cooked my MAX98306 trying to combind audio signals

[akschu]

I'm using the MAX98306 because I want to use the differential inputs to combine some audio signals. The trouble is that one signal is mono and I would like to hear it in both ears of my headset, and the other signal is stereo. To make things further complex I want to use a headset which has a common ground.

I built the circuit in the attached drawing, and of course it shorted something out and my Max98306 got super hot. I disconnected it quickly, but I think it's toast as applying power to it with nothing else connected causes it to draw .5amp and start getting hot again.

So, I'll need to order another one, but the question is how do I accomplish this? According to the datasheat both sides of the amplifier are independent, so I'm not sure where the short is. Would it make more sense to use two discrete mono amps? Perhaps a voltage divider on the mono input with the source in the middle to limit current between everything?

Help much appreciated.

Matt

Untitled Sketch_bb.png (73.1 KiB) Viewed 148 times

[adafruit_support_bill]

As noted on the connections page, the outputs are 'bridge-tied'. : https://learn.adafruit.com/stereo-3-7w- ... nd-outputs

The outputs of each channel are "Bridge-Tied" with no connection to ground. This means that for each of the two channels, the + and - alternate polarity to create a single channel amplifier with twice the available power.
However, that means you cannot bridge R and L together, so don't try to connect ROUT to LOUT, it will damage the amp! If you only need one speaker, just connect to either ROUT or LOUT and leave the other output set alone

That effectively means that internally there are 4 amplifiers, two of which are bridged internally for each of the left and right channels. So R- and L- are not ground connections and connecting them together will cause problems. See page 11 of the datasheet for a diagram: https://cdn-shop.adafruit.com/datasheets/MAX98306.pdf

The problem is that most stereo headphones are wired for single-ended inputs with a common connection on the sleeve of a TRS phono plug. "Balanced" headphones have a different plug (typically dual XLR or TRRS) with entirely separate wiring for left and right drivers.

[akschu]

Okay, that makes sense. If I'm picking up what you are laying down this amplifier cannot drive a common headphone due to the common ground and the fact that it's switching (class D). I'm assuming that this is also the case with any other class-D amp that adafruit carries. Correct?

I thought about an isolation transformer of sorts, but since the class D is amp is designed the speakers directly, that probably wouldn't work.

I'm guessing that leaves me with a class-b amp like a LM386. Did I get that right?

[akschu]

Or this?

https://www.ti.com/lit/ds/symlink/tpa6021a4.pdf

[adafruit_support_bill]

I'm not sure if all class D amps work that way. But it is a common design in the smaller amp-chips because it allows twice as much effective voltage swing on the output with a relatively low voltage supply.

The TI chip you linked looks like it should work.