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I have a small thump on power off from my ACAs as well. It is much smaller in magnitude that actually driving the speakers with some higher volume music. I've been using 9 channels of ACAs for just about a decade now - no problems have ever resulted. Unless you have super sensitive speakers, I'd say just leave well enough alone.
I have better suited amplifiers and I listen to when I really want to critically listen. I used the ACAs for background noise/light listening. I don’t like putting hours and hours on tubes and other expensive gear listening to the radio at 40db
Arrange for a pair of switch contacts (for each channel) to short across the speakers when power is turned off to your amps.
I have a speaker selector switch so I can unselect this amplifier before I turn it on or off. Just wondering if there was a more graceful Solution.
There's a fair amount of discussion of the turn off thump and potential mitigations in the main ACA thread. I'll admit to having not read the entire thread so I don't know if consensus on how to eliminate it was ever reached 
https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/post-4844068
https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/post-4844068
The most "graceful solution" for ACA Redux, would be to keep the same form factor and the same Hedgehog heatsink, but incorporate anti-thump protection in the circuit itself. Which means laying out a new board with new components and a new circuit for thumpus-minimus , yet still fitting to the same ACA Redux heatsink.
Fortunately this is quite possible. The version-zero ACA Redux was 120mm in diameter, and there was enough empty space on that PCB, for diyAudio members to add a two stage SMPS filter (RLRC - RLRC) with eight new components, and still fit on a 120mm PCB on the same heatsink. Today's version-one ACA Redux is now 140mm in diameter without the SMPS filter; surely the bigger board (with 36% more area) will have plenty of room for enterprising hobbyists to do a re-layout which incorporates anti-thump protection circuitry.
The only potential fly in the ointment might be component height: since the PCB is mounted upside down, all electronic components need to be less tall than the ACA Redux's support legs. This might be an issue with some electromechanical relays, so you may have to either search for shortie-boy relays, or lay them over on their side, or implement a non-relay circuit solution.
Edit: remember that it is totally up to you, how many ohms you put in parallel with the 4 ohm loudspeaker during thump-mitigation. If you decide to put 1.0 ohms in parallel with the 4 ohm loudspeaker, then the speaker gets (1/5th) of the total thump current, and the 1 ohm resistor gets (4/5ths) of the total thump current. Thump power delivered to the loudspeaker is (1/5)^2 = (1/25) as large as the no-thump-preventer original circuit. Different values of this resistor will of course give different attenuations; AND they will also give different values of peak current through the relay contacts -- which you may or may not find significant when hunting shortie boy relays.
Fortunately this is quite possible. The version-zero ACA Redux was 120mm in diameter, and there was enough empty space on that PCB, for diyAudio members to add a two stage SMPS filter (RLRC - RLRC) with eight new components, and still fit on a 120mm PCB on the same heatsink. Today's version-one ACA Redux is now 140mm in diameter without the SMPS filter; surely the bigger board (with 36% more area) will have plenty of room for enterprising hobbyists to do a re-layout which incorporates anti-thump protection circuitry.
The only potential fly in the ointment might be component height: since the PCB is mounted upside down, all electronic components need to be less tall than the ACA Redux's support legs. This might be an issue with some electromechanical relays, so you may have to either search for shortie-boy relays, or lay them over on their side, or implement a non-relay circuit solution.
Edit: remember that it is totally up to you, how many ohms you put in parallel with the 4 ohm loudspeaker during thump-mitigation. If you decide to put 1.0 ohms in parallel with the 4 ohm loudspeaker, then the speaker gets (1/5th) of the total thump current, and the 1 ohm resistor gets (4/5ths) of the total thump current. Thump power delivered to the loudspeaker is (1/5)^2 = (1/25) as large as the no-thump-preventer original circuit. Different values of this resistor will of course give different attenuations; AND they will also give different values of peak current through the relay contacts -- which you may or may not find significant when hunting shortie boy relays.
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—SOLD—
Pair of ACA Redux, nicely built, work perfectly. All proceeds will go to Burning Amp Fest.
More photos can be found here - https://guides.diyaudio.com/Guide/ACA+Redux/22?lang=en
SOLD $220+shipping SOLD
Pair of ACA Redux, nicely built, work perfectly. All proceeds will go to Burning Amp Fest.
More photos can be found here - https://guides.diyaudio.com/Guide/ACA+Redux/22?lang=en
SOLD $220+shipping SOLD
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Hello! A new batch of these kits is now available, see the details over in the sales thread.
Since I don't want to just double post, here are a few "behind the scenes" pics of what I've been working with the past several weeks
Since I don't want to just double post, here are a few "behind the scenes" pics of what I've been working with the past several weeks
Attachments
@N Brock
I read on the F5 redux page about SMPS filtering you've included.
I'm assembling a pair of balanced ACA Redux so I have 4 power supplies one for each stereo half. have you any thoughts about filtering etc for these? (I may have missed this in the threads somewhere)
Also, if I am only wiring them up balanced with XLRs do I need that "extra" 39k resister?
I read on the F5 redux page about SMPS filtering you've included.
I'm assembling a pair of balanced ACA Redux so I have 4 power supplies one for each stereo half. have you any thoughts about filtering etc for these? (I may have missed this in the threads somewhere)
Also, if I am only wiring them up balanced with XLRs do I need that "extra" 39k resister?
AmyAlice would work beautifully for this.
https://www.diyaudio.com/community/...apacitors-smd-assembly-max-3a-max-48v.405099/
https://www.diyaudio.com/community/...apacitors-smd-assembly-max-3a-max-48v.405099/
If you're balanced input only then no resistor needed.
FWIW, the P089ZB filter would also work and is sold on the store, though it has a bit less filtering performance compared to AmyAlice.
FWIW, the P089ZB filter would also work and is sold on the store, though it has a bit less filtering performance compared to AmyAlice.