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Old 20th January 2019, 08:33 PM   #61
anotherJoachim is offline anotherJoachim  Germany
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Quote:
Originally Posted by TungstenAudio View Post
If I were to build a discrete regulator for one channel of an ACA, it would look like this:
can this little Sigma11 drive half a ACA?
As far as i remember it is rated around 1A
Maybe a bigger radiator will help, but....

regards Joachim
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Old 20th January 2019, 11:58 PM   #62
TungstenAudio is offline TungstenAudio  United States
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Join Date: Jan 2014
Location: Kenmore, WA
Quote:
Originally Posted by anotherJoachim View Post
can this little Sigma11 drive half a ACA?
As far as i remember it is rated around 1A
Maybe a bigger radiator will help, but....

regards Joachim
I would say yes. There is no inherent current limiting in the Sigma11 circuit; one simply needs to observe the power dissipation of the ouput MOSFETs and the diode bridge. Since the output devices tend to have about 10 Volts across them, power dissipation will be roughly 10 Watts a piece. The rectifier diodes might also need to be moved off the board so they can be attached to a heat sink of some sort.


My latest ACA build is drawing 1.6 Amps of quiescent current at 24.5 Volts (after a minor tweak). That's on the high side for an ACA, as the stock circuit is rated for about 1.45 Amps.
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Old 21st January 2019, 03:36 PM   #63
anotherJoachim is offline anotherJoachim  Germany
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thanks

regards Joachim
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Old 21st January 2019, 04:01 PM   #64
Mazeppa is offline Mazeppa  United States
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ACA amp with premium parts
You have my attention with your Pass/Hafler hybrid.

Recently swapped my ACA's from bridged to parallel.
Improvement over bridged was not subtle, the amps and my speakers (Omepa Super 3 XRS's) seem to get along better.

Are your boards a drop in to the DIYstore ACA chassis?
Don't have an enclosure to use, but separate power source is of course an option.

Do you credit the reduction in background noise to getting away from the SMPS, and losing the on/off notifications to the new boards?
Probably hard to tell, I guess.

Last edited by Mazeppa; 21st January 2019 at 04:03 PM. Reason: add "with your Pass/Hafler hybrid."
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Old 21st January 2019, 06:15 PM   #65
TungstenAudio is offline TungstenAudio  United States
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Default ACA Super Wolf Blood Moon Edition

Last night we were treated to an astronomically rare event Clear skies over Seattle as the Moon passed through the Earth's shadow. What a treat! While my neighbors seemed to be enjoying an outdoor viewing party, the low temperature (38 F) had me scurrying back inside to enjoy some appropriate mood music. Om, Massive Attack, Fields of the Nephilim, and of course Black Sabbath.

Sabbath's original album happens to be an excellent choice to highlight the capabilities of this little amp. Blessed with an unusually clean production on the CD edition which I have, the tracks offer mostly simple compositions of guitar, vocals and bass and drums, along with some wonderful atmospheric accents. The ACA in its present form is running with 24.5V rails and my preferred 12.4V bias. And it sounds spectacular. While it would be unreasonable to expect an 8 Watt amplifier to reach club volumes in my listening room, this one plays as loud as I typically like to go, and punches seriously above its weight. The inky blackness of the background lets the dynamics of a good recording come through. On the title track of Sabbath's first album, this results in an emotionally haunting dirge.

While Rudi's boards support several new features, I'm running them in a simple configuration. One feature that I am using, however, is a simple RC filter for the JFET buffer, as employed in the Zen V4 and suggested by the Mighty ZM for an (un)necessary turbo version of the ACA. The drain of the 2SK170 gets its power through 200 Ohms into 100 uF Organic Polymer. The PCB also holds a pair of 10,000 uF filter caps, but the position for the LM1084 is currently vacant. Grunt is supplied by a 22,000 uF reservoir cap and a 22V, 6.8 Amp secondary winding for each channel. I'm using a 1.82 Ohm, 25 Watt Dale power resistor to drop the reservoir voltage before it reaches the PCB. Measured ripple on the local 24.5V rail is about 5 mV.


To answer Mazeppa's question, I credit the reduction of background noise and the absence of turn-on and turn-off noises to the heavily filtered linear supply. The 300VA transformer borrows a design philosophy from the Naim gear that I also love, which is massive regulation through simple oversized power supplies.
Speaking of, all listening tests so far were conducted with my Naim CDX2 / XPS DR and Naim NAC 82 / dual HiCap DR on the front end and the venerable Vandersteen 2Cs filling up my living space.
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File Type: jpg ACA - Blood Moon.jpg (255.9 KB, 321 views)
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Old 21st January 2019, 07:34 PM   #66
TungstenAudio is offline TungstenAudio  United States
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Some construction notes

The DH-220 donor chassis has proven to be a good match for the enhanced ACA, and has plenty of room for a good sized transformer and reservoir capacitors. Probably the biggest effort in prepping the metal was enlarging the original power cord hole for a proper fused IEC inlet. Otherwise, the locations for the RCA inputs and speaker terminals are well suited for a stereo or bridged version of the ACA. Several of the other holes in the bottom plate may also be re-used for the new hardware. The best thing about the old chassis is the pair of well-sized heatsinks. With total power dissipation now running at 44 Watts on each side, the heatsinks are just nice and warm to the touch. No where near hot yet. The new PCBs fit conveniently between the pair of large internal flanges, and I've bolted the 25W power resistors to the rear surfaces.

I've recently adopted the practice of bolting the toroidal transformer onto a slab of Acrylic, which is in turn bolted onto the bottom plate of the chassis. I use rubber stoppers inside the center hole of the toroid to further dampen any mechanical vibration. Though I'm using a steel bolt through the center of the transformer, its flat head is recessed into the acrylic, so there is no electrical connection with the metal chassis. I've wired the dual primaries in the same manner as recommended for the various FirstWatt clone amps, in this case using a pair of 15 Ohm, 3 Amp thermistors. The bridge rectifiers on the secondaries are the GBPC type 35A, 400V. (These nasty little buggars drop over 3 Volts as bridge rectifiers, but are otherwise bullet-proof.) The DC negative side of the bridge rectifiers are connected to the chassis through separate CL-60 thermistors. No other connection is made between the chassis and audio ground.

I've used CAT-5e twisted pairs between the RCA inputs and the PCBs, routed carefully under the rear lip of the bottom plate. No other shielding of the input wiring was necessary. I did take the precaution of wrapping the AC power input wires with copper tape, and grounding that tape to the chassis.


Of course I am saving the unobtainium lateral power MOSFETs for another project.
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Old 23rd January 2019, 07:27 PM   #67
TungstenAudio is offline TungstenAudio  United States
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Default ACA-220

After a couple more days, the sound of the new build has improved noticeably. It is now less laid-back (in a good way) and has a greater sense of ease and flow. Something some folks call PRAT; it now has that and sounds more compatible with my Naim front end. There is also more tonal and micro-dynamic detail coming through.


Compared to the parallel-monoblock ACAmps that I have been enjoying, it is very, very close and even better in some ways. This build sounds like it has no problem driving my speakers, up to its physical limits of voltage and current. If anything, the inner details surpass the parallel-monoblocks. The lower bass may be slightly less full, but that sense changes from one recording to the next. I double-checked by hooking up the parallel-monos, and the background noise with no music playing is virtually the same. Both amps are incredibly quiet. However when music is playing, the newer build appears to present it from a blacker background. The subjective effect is greater clarity and dynamics.


The ACA-220 as it stands is going to remain unmodified for a while and used as a new benchmark, as I try out other ideas with other PCBs and chassis. Another DH-220 arrived yesterday (yes, It's a sickness), so I am free to tear down one of my other 220s to use for donor chassis and hardware.

Another build note: Rudi's boards measure 75 mm by 100 mm. There would be just enough space to install these in the diyAudio store ACA chassis, but the brackets would have to be modified to allow room for the standoffs at the four corners of the board.
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Old 24th January 2019, 01:26 AM   #68
TungstenAudio is offline TungstenAudio  United States
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This the current configuration of the ACA-220. The PCB also holds a pair of 10,000 uF caps plus two 0.22 Ohm resistors in a Pi filter network. As mentioned earlier, the position for the LM1084 is unused. Off board components include a 22,000 uF reservoir cap and a 1.82 Ohm power resistor.
The amp was built to accommodate several power supply variations, including different voltage transformers and an enclosed SMPS instead of a transformer. The next build will explore some higher voltage options.
Attached Files
File Type: pdf aca16 mods.pdf (902.2 KB, 91 views)
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Old 24th January 2019, 06:09 PM   #69
Mazeppa is offline Mazeppa  United States
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ACA amp with premium parts
Interesting work; good of you to take the time to share it with us.

My stock ACA's sound so good, I kinda hate to touch them.
I'll at least have to wait until I get the Aleph J that's currently under construction online before I mess with them.

Knowing how I am, though, the temptation to hot rod the ACA's will get to be too great eventually.
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Old 24th January 2019, 06:16 PM   #70
TungstenAudio is offline TungstenAudio  United States
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The stock ACAs really are great little amps, and were the ones that got me started building NP Class A designs. I also have a pair of Aleph J boards waiting for a suitable chassis to house them. I'm eager to hear how they sound, but my current obsession with the potential of the ACA circuit hasn't run its course yet.
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