ACA amp with premium parts

Yes, a pair of the 0.56 Ω resistors in the R3 and R4 positions will be close enough to my original combined resistance to work. Recall that I originally added a 2.0 Ω resistor in parallel with the existing pair of 0.68 Ω resistors.


If you can stay within 3% of my combined resistance of 0.291 Ω, then you should be good to go.
 
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Yes, a pair of the 0.56 Ω resistors in the R3 and R4 positions will be close enough to my original combined resistance to work. Recall that I originally added a 2.0 Ω resistor in parallel with the existing pair of 0.68 Ω resistors.


If you can stay within 3% of my combined resistance of 0.291 Ω, then you should be good to go.
Would you recommend your mods with some 114db horns? I will be running in balanced mode as my amps will be 75' from the preamp. I will be running maybe 1 watt max, and my horns are not a bright...K402 with EV DH1A drivers. The horns will only be being used from 400hz on up with the ACA monos. I am a beginning solderer so I want to do this right the first time to the best of my ability. I have 3 of the newest parts kits on the way with the 24v power supplies.
 
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Yes, I have found the mods to improve the performance and sound of the amp in several ways, inclusive of using a single stereo amp, or different ways of bridging a pair of amps. It sounds like you will be using a pair of ACAmps, with each one running in balanced-input bridged mode. The performance should be similar to the other form of bridging this amp, but with better cancellation of noise and distortion picked by the long cables. The mods increase the input impedance of the amp, which will help reduce the effect of using long interconnect cables. Using a pair of bridged monoblocks really helps with soundstage presentation by having separate power supplies for each channel.

The illustrated assembly guide has good instructions for how to wire the XLR input connector for balanced input. (It is one of the standard ways of using the ACA.) The modifications I have shown are almost all simple part substitutions that easily replace the parts in the kit. The exception is C101, the 10 pF Mica capacitor installed in parallel with R12. C101 may be considered optional if you do not feel comfortable soldering one part on top of another.

I have not yet had the opportunity to try mine in the balanced-input bridged configuration, as I don't have a preamp with balanced outputs yet. The high efficiency of your speakers should make for a very dynamic sound, even with a relatively low volume knob setting on your preamp. It would be fun to hear about your impression of the amps, after you have had a chance to listen to them for a while. Give them about 20 hours of playing time to break-in.
 
Thank you very much for the info. I am sure i will be posting up more in here once i start to search for the needed mod parts, and def once i get it up and running. i am very much looking forward to building 3 of these in bridged mono as a first solder project. i have a friend that has built kits, and said he will guide me through the first one or 2. If he does it will def make my life easier.
 
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A reader of these forums writes:
(It) turns out that those Nichicon parts were not available in Germany. Shipping from the US is prohibitively expensive.

There are plenty of 1000 uF, 16V & 100 uF, 35V capacitors available from different Nichicon series or other vendors like Panasonic. Didn't find any Mundorfs or Jansens that would fit into C2 & C4 - either the proper values are not available or they are way to large for that space.

When picking alternatives do you have a hint what to look for: ripple, type of electrolyte?

Can I go higher with the voltage? If so there might be some options.
Capacitors C2 and C4 play a couple important supporting roles in the ACA design. Selection of C2, as noted by the Mighty ZM, may include good quality industrial parts. I chose an Aluminum Organic Polymer device for its low ESR and relatively high ripple capacity. There are a number of other parts that would be quite suitable for this position, including 25V parts in the Nichicon KA or KW series. Panasonie and Kemet also offer Organic Polymer devices that will fit. The footprint on the PCB has a 5mm lead spacing, so that may help narrow the selection. (My suggested part actually has a LS of 3.5mm, which I consider acceptable, if non-ideal.)

My 'upgrade' of C4 was intended to address any possible switching artifacts from from the SMPS brick. I wanted to increase the capacitance while maintaining low ESR, high ripple and a 3.5mm lead spacing. It also should have a voltage rating of 35V. This area of the board would not be a good place to install oversize components with flying leads. Again, Panasonic and Kemet may have suitable substitutes in their Organic Polymer series.

Why Organic Polymer? In low voltage applications, they provide good characteristics in compact form factors, and have made their way into a number of modern industrial designs. Also, for what it is worth, I believe the Organic electrolyte may be the next closest thing to the 'silk' substrate used by the famed Elna Silmic II series.
 
Possible Discrete Regulator for ACA

If I were to build a discrete regulator for one channel of an ACA, it would look like this:

I built four of these boards for my DIY version of a dual HiCap DR. (They are serious overkill for that application, in keeping with Naim standard practice for their separate power supplies.) The heatsinks for each of the output MOSFETs are 50mm tall. The rectifier diodes are MUR820, which I would probably replace with a chassis mount 35A, 200V bridge rectifier, as is best practice for the DIY First Watt amps.
 

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Still very much in the assembly and R&D phases.
Max voltage in this chassis is probably about 32V, with a bias current of 1.6 Amps or so.

Transformer look larger are you using a 600 or 800 mA transformer ?.
Sense I and many other started building these as mono block we run into the dilemma of buying to transformer for two power supplies I just recently found out by calling the manufacturer or just a few dollars more I can have a custom wound 800 mA toroidal Single tap transformer for each monoblock rated at 800 mA or whatever you’re choosing. I’m a newbie but correct me if I’m wrong in all my reading about transformers and recommendation if you need a 500 mA transformer that rating it’s for a two tap transformer meaning each tab is 250 mA and if you were building mono power supplies and you needed 500 mA and you only used one tap on each 500 mA transformer you would actually be only delivering 250 mA . And for audio usage to keep the heat down and vibration from the Hom that is possible from a transformer I was told to have my 800 mA transformer wound around a 1000 mA core . Correct me if there’s any errors in my understanding a power transformers I’m still learning .
 
In this case, the transformer is a 300 VA model, the Antek AS-3222. It has a pair of 22V secondaries, each rated at 6.8 Amps. Since the ACA is a single rail design, the transformer can be used in a quasi dual-mono configuration. For the first iteration, I'm using a CRCRC filter and no regulation.
The first stage capacitor is 22,000 uF, 40V from Kemet. By adjusting the first resistor after that, I can get final rail voltages from 24V to 28V.
 
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It Lives!

A few years ago, when I was working on my first set of Hafler DH-220 mods, I ran across a tidbit from Papa about the possibility of modifying the DH-200/220 circuit to run at Class A on lower voltage rails. Sadly, this idea never came to fruition, as the standard power transformer was only wired with a single primary and didn't offer an easy way to halve the voltage supplied to the driver boards. If anyone ever tried this idea with a substitute transformer, I never saw the write-up on the Diy forums.

Well, here is another solution – simply gut a Hafler chassis and drop in a shiny new ACA with a linear power supply! ;)
For a new set of PCBs to make this possible, I have forum member Rudi to thank. He was eager to try the "Premium ACA" circuit in a dedicated PCB with support for couple new features. We spent some time collaborating on the layout – that is, he did all the work as I kibbitzed from the sideline. We now have a small set of prototype PCBs that I will use for exploring various ideas over the next couple months.

I spent a few hours with it last night, and am very pleased with the results. First of all, there is no turn-on or turn off thump or other noises. The amp is completely silent until music starts playing. (So silent, that I wondered if it was working when it was powered up with speakers for the first time.)
And the amp sounds like the ACA that I have come to know and love. This build benefits from a dual-mono power supply, and presents the same three-dimensional sound stage that I've been enjoying from my parallel-bridged amps. While it is still breaking in, the absolute blackness of the background is doing a wonderful job of letting each recording come through with great detail. It is also simply a pleasure to listen to, as are the original ACAs.

My short term plans call for a simple voltage boost to the power rails. They are currently running at 23.7 Volts, with quiescent bias of 1.57 Amps and voltage bias of 12.2V. The first R of the CRCRC filter is dropping about 3.5V, so there is some headroom to work with. Even with a total of 37 Watts per side, the heatsinks are just nice and warm. I have another Hafler donor chassis waiting for a second build, and also have two of the Modushop mini 3U Dissipante chassis on order. Further experimentation will involve higher voltage transformers as well as a couple different sets of enclosed SMPS power supplies followed by linear regulation.

I'm looking forward to much quality time with this wonderful amp. Thank you, Papa.
 

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