Update, 14 July 2022: Had a number of good responses, but no clear solution. Staying with my work-around for now. Thanks, everyone! (Not sure what the protocol is with respect to post-deletion; I'll leave this post as is.)
I'm looking for a hands-on analogue designer equipped with, and well-versed in, at least an oscilloscope. Freelancer sites are filled with EEs without lab equipment, and that won't do for this task; on-circuit tests and measurements are pre-requisites. And, their ideas I already knew anyway!
The task is non-dramatic enough for a competent analogue designer: A micro-controller controls two sources which feed the same speaker, one amplified, the other not; there must be no interference of one on the other, and no adverse effects on the larger circuitry of the PCB. The scale is 3-12v and up to 20W. The two sources are off, for most of the time, and never operate simultaneously.
One approach may be to combine the two sources, using the micro-controller as a single source, with the amplification-level controlled by it. For this, you'll need to be conversant with typical amplifier circuits. (Off-the-shelf dynamic-gain amplifiers are, nowadays, expensive and out of stock!) ... Frankly, this is the approach I am seeking now, as complete isolation has failed, in practice!
You'll need to set up your proposed circuit, to test. You will later test it on the larger PCB, to verify that current-draw does not affect the larger functionality. (I test the system-level functionality, which may or may not immediately reveal this circuit's underlying symptoms.) Even a working circuit will need to be tweaked, once integrated into the PCB. And, yes, as in all work nowadays, we may need to change your proposed components, subject to stock availability!
I emphasize again: This is not an academic exercise, so don't come proposing a generic diode, relay, transistor, XOR etc!
A flat rate would be preferred, with respect to pay, and availability is expected to be immediate and substantive. Propose your payment mechanism, which can even be a freelancer site, as a de facto escrow.
You may post generic questions _here_, but details will be provided via only email. Thanks for reading. (And Thank you, Dave, for the forum and all your videos!)
This seems kind of cool and I love analog stuff. I don't really have time right now. But if you haven't found someone I'd love to get the details and if I have the time you can pay for a finished design (Assuming you don't find someone in the mean time) as opposed to hours or something in advance. If this were a few months ago I'd be able to whip something up nice and quick but I only get my 15 minutes of forum browsing a day with no spare time for another design.
Big ones being what the target BOM is as well as the unit count for the final design. That and the actual goal of the setup is. Cause presumably you want a DAC in there and you're not just driving the speakers with powered PWM signals.
So yeah if you can email me any needed details that'd be great.