Can I use a Class D amplifier as a summing amplifier?

Question

Context: I'm creating a mono, battery powered speaker. I'd like to sum the left and right channels of a stereo input signal into a mono output signal, which I feed into a Class-D Mono Audio Amplifier to drive the speaker.

The stereo-to-mono solution I've decided on is to create a summing amplifier with unity gain, and I'd like to keep my circuit simple by using the Class D amp in place of the op amp.

Will this work? Or do Class D amplifiers work too fundamentally differently from a regular op amp?

^{simulate this circuit – Schematic created using CircuitLab}

EDIT:

Here's my revised circuit diagram, based on what I learned from the answers below. I've opted not to DC-decouple for the moment, but I haven't yet tested this.

^{simulate this circuit}

Answer 1

Technically, yes, but you're doing it wrong :)

Assuming your class D amplifier is an operational amplifier, that is,

• practically no current flowing into the inputs, and
• output is a (high) multiple of the difference between the + and - input.

The fact that it's a class D amplifier doesn't really matter here – idealized, after the output filter stages, it should work as such an amplifier after all, within its bandwidth.

A small thing you've got to realize: you might not have overly linear phase in your amp's bode plot – especially towards the higher end of its bandwidth. So, there might be instability for signals that are very high in frequency – make sure your input signals are already properly band-limited.

Note that for most signal applications, 200 Ω does sound a little low as resistance between the two inputs. But that completely depends on what you drive the amplifier with.

Also note that audio amplifiers are typically not operational in the sense that their inputs are not designed to be high-Z (zero current flowing into the amp), but properly terminated to achieve high power transfer into the amplifier to avoid noise/oscillations.

Furthermore, you'll find that, at least logically, audio amplifiers typically already have the feedback that you've drawn – that's how they do reliable gain, after all.

So, even the second aspect of op-amps, namely the "very high gain" isn't properly going to work out: after all, a typical amplifier has maybe a gain of 10V/V to 50V/V – that's far less than a typical opamp itself has, and that will limit how well your summing amplifier works.

So, as FakeMoustache said: not like this. Just sum up the inputs with two larger resistors. You can even DC-decouple them with caps, if you like (the classical RC highpass formulas apply).

Answer 2

Yes you can but not like that !

You treat the Class D amplifier as if it simply is a linear opamp. It is not.

Look in the datasheet of the TPA3116 and you will see that the application diagram does not show feedback configuration like in your schematic. Do you understand what a class-D amplifier actually is ?

I'm also ignoring the fact that the TPA3116 has a balanced output for the loudspeaker, it is not (and cannot be) grounded on one side.

These "Tripath" amplifiers also have a sophisticated Analog to PWM thingy going on inside there, also ignoring that.

The fact that TI just bought Tripath instead of designing their own also means these amps are complex beasts.

But you're making this way too complex you can just combine the two audio input signals at the input of the power amplifier, feed each signal to the power amplifier's input via a 1 kohm resistor. There is no need to make a proper virtual ground summing point. The combined audio signal will simply be an average of the two and whenever I used that, that simply works.

So forget about feedback and especially feedback in combination with class D amplifiers.