For those reading this and trying to understand what is going on, the TPA3116 modules that I have seen seem to generally have their own boost power converter and can work off 12 to 24 V DC, internally converting whatever it receives to 24 V DC to power itself. This means that they produce 24 V maximum on the audio output. Unrelated, but they also usually have reverse polarity protection.
*edit* I forgot that this is a 40W TX requiring 48V peak mod, and have not yet found an COTS audio amp IC that will run at that supply voltage.
That same particular vendor that I linked to has some Sure amps that can do as much as 60 V into low impedances:
https://www.parts-express.com/pedocs/manuals/320-3328--sure-electronics-aa-bk31394-1x1000w-audio-amplifier-board-manual.pdfThe drive capability is overkill for the Stretchy boxes but hey, it's not too expensive, not physically large and of course, includes protection mechanisms along with the audio LPF, of course.
I considered ditching the transformer and going straight PWM for my "cannibalization" of the Stretchy box but decided to keep the transformer. The cannibalization is already enough disruption for right now (IMO) and I felt like minimizing the amount of messing around. The messing around will come later.
Sure appears to make some that can do up to 120 V. (Scroll down on the linked datasheet.)
Might be time to build a simple PWM modulator?
** WARNING: Thread drift.
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Oh, I agree that this can be done too. I have some things in the works.
With the number of little Class-D audio chips on the market for the portable consumer electronics applications, there are a number of options. In the portable application arena, they do not use big LPF inductors on the speaker output of the Class-D amp; instead they rely on the speaker and speaker wire inductance to do the filtering, and it's usually good enough that most consumers don't notice or live with the artifacts.
My thought was that since these chips put out a PWM signal but at a voltage and drive current level that isn't going to modulate a transmitter well, simply add a follower/level-shifter after the PWM chip which will do the heavy switching and let that modulate the transmitter thought appropriate LPF filtering. As a simple feasibility check, I obtained an Adafruit PAM8302 module and put that output into the gate of a IRL510 which I had in my supply and chosen because of the low threshold voltage. It worked well enough that I am going to put together a test board with the PAM8302 driving a real FET driver and then a good FET and some filtering to drive a TX B+ input.
I can't be the first person to propose doing this.
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