Active crossover by using a class D power amp itself as an opamp

[GTMo4]

I have a Class D 60W amplifier by Allo and a tweeter that needs to crossed over with a 2nd order filter at 2500kHz.

Normally to do this, one might use an active filter in the input of the amp like this :

But what if we use the class D amp itself as the opamp to create the filter? like this:

Would this work fine? has anyone done this? What could possibly be the cons of doing this?

 

[b_force]

Would this work fine? has anyone done this? What could possibly be the cons of doing this?

In theory; yes

In practice; no or maybe.

Mostly has to do with stability of the power amplifier.
The vast majority of power amplifiers don't like unity gain (gain of or around 1).

That being said, your mileage may vary and work out as long as you keep the gain of the amplifier right.
Still you have to keep an eye on oscillations as soon as you feed something back to the input.

With most Class-D amplifiers this is even more tricky, because they have quite the inductance in series as well "HF" residu.

Practically speaking a lot of Class-D amplifiers work with a single-rail voltage, meaning there is 1/2 Vcc at both outputs.
Although in this case that should be blocked by the capacitors.
A lot of these kind of Class-D amplifiers have a bridged output, which is basically just a differential output.
Which means that you have to feedback not only the positive but also the negative output as well.

To make things worse, if I am not mistaken (but have to check again), quite a few of those Class-D IC's have a inverting input.

 

[rayma]

This has been done with linear amplifiers, but not class D, at least AFAIK.
Doubt it would be successful.

 

[adason]

Most of the classD amps have floating ground, not galvanically connected with audio input ground. So feedback filter would be problematic. Same way classD amp do not like speakers grounds connected between left and right channel.