Finally I got around to figuring this out. Had to poke around a little bit.
Ran across this schematic in the land of diy forums. It’s for the G2 Rokit 6 amps, which had the notorious black goo of death problem, so totally different design (well, probably not that different… these budget companies never really redesign their stuff if they don’t have to, unless they can make it simpler/stupider), but it struck me that their limiter was using just one bipolar transistor right after the input buffer, so I went looking for lone transistors near the input, and lo and behold, there is only one (Q5, just above the right-most large ceramic capacitor).
Oscilloscope revealed bits of signal being leached off, so it seemed I had found the culprit. Removed Q5, but both amplifiers suddenly muted… hm.
Traced signal from top-most pin of Q5, found it running to the ‘mystery chip’ just above, U5. Looks like it’s some kind of gate circuit: when it receives enough input voltage at pin 5 (lower left pin), seems like it opens pin 4 (upper left) and passes 5V, which is what the amp circuits need to unmute and shut off standby (datasheet for TDA7296 actually specifies 3.5V minimum to disengage mute/standby function).
This reveals a couple of things about KRK’s design. First off, the limiter is running all the time. Even at relatively low levels it seems to be leaching signal, and globally for both treble and bass. Secondly, the mute circuit depends on the output of the limiter in order to engage, so the limiter needs to be operating rather aggressively in order to unmute the amplifiers. This is why, when I removed Q5, the mute circuit wasn’t seeing any input. I’ll say again: IF YOU HEAR ANY SOUND, THE LIMITER IS LIMITING, A LOT. 🤦♂️
So… remove the transistor (hot air is easiest), remove the IC, and short pins 3 and 4 as shown (I used a little piece of wire clipping).
Now the amp engages immediately, without any lag. There are electrolytic caps decoupling the input to the mute/standby pins in the amplifier, so there won’t be a big pop. Can see the voltage at pins 9/10 of TDA7296 rising and falling gently with on/off.
Things sound ‘snappier’, more precise––these are some real speakers now! This project has officially reached it’s zenith. EDIT: I’m shitting my pants at how amazing this sounds. All my previous mods really seem to have come to life now, the low end is incredibly clear and integrated (it’s like all the overtones just lock), I find I’m listening with the volume a bit lower, and both speakers sound more balanced (the right speaker always felt just a little softer, and they’re ‘matched’). My reference recordings sound uncanny. And just over a little transistor…
*PROCEED AT YER OWN RISK* I listen primarily to Classical stuff, I’m not pounding EDM or anything. I assume this circuit is here for a reason.
If you want someone to do this for you, check out this guy: https://www.danieldialatone.com/gallery. Used to be a head engineer for Black Lion Audio, studio monitor ‘enhancements’ are his specialty. I just use the TI SoundPlus series of opamps (which I think sound great, enough of an improvement), based on their functional equivalency to what’s on the board (FET/bipolar, comparing input impedance, etc) he actually hand selects different opamps for their unique flavor. If my amateurish experimentation got me this far, just think what a real pro can do!
Coming up: RME UFX mods, Presonus DP88 mods, Behringer ADA8200 (re-)mods, Yamaha TG77 mods