ACA amp with premium parts

I think you might be pleasantly surprised at what the ACA can do with the standard 24V SMPS brick. If you read through the main ACA thread in the sticky section of this forum, you will notice that a few builders have incorporated enclosed SMPS supplies inside the chassis. The 'standard' ACA Mini Dissipante 2U chassis is a bit of a tight squeeze for this, and usually requires that the SMPS be mounted on its side. This type of construction requires that one be comfortable and familiar with the means to safely bring AC line voltage inside the chassis. As a starter kit, the standard ACA keeps the AC line voltage safely enclosed inside the PSU brick.

If you build a second ACA, then you have some very attractive options available to drive your speakers with a pair of monoblocks, rather than a single stereo amp. Those options are definitely worth exploring with the stock brick. The illustrated ACA 1.6 build guide shows how to do this.

+1
 
I believe the improvement in sound quality using mono amps would be significant with challenging loads, but I'm using very efficient Omega speakers (98dB @ 4/6 ohms) and listen at relatively low volumes (80-85dB peaks) out of respect for my neighbors. Do you believe the mono option would offer significant improvements over a stereo unit in my situation?
 
Indeed, using monoblock amps is about more than just volume. Having a separate power supply for each channel yields significant improvements in soundstage imaging and inner details of the recording. Somewhat lower impedance speakers may benefit from the parallel bridged configuration, where inputs and outputs of both boards in a chassis are tied together. The primary modification I recommend for this is increasing the input impedance of the amp to 20 kΩ and changing the feedback resistor to 90.9 kΩ. These are the R11 and R12 mods, and will make it easier for your preamp to drive the monoblocks.
Go back a few pages in this thread to see how I changed from self-inverting bridged amps to parallel.
 
Domino,

I have Omega Super 3XRS's (94db@8 ohms nominal) hanging on the end of my ACA's. Which flavor of Omega's are you running?
Tried both a single amp in stereo and 2 in mono, bridged and parallel both.

Tungsten knows whereof he speaks. My system sounds better at all volumes with 2 ACA monoblocks; parallel being my preferred configuration.
Bass and midbass in particular are really good, best I've had of all my various system iterations over the years. Rhythmik L12 sub crossed at about 50Hz is in the mix as well.

Note: My ACA's are unmodified V 1.6's.
 
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Hello James,

I have been busy supporting a German DIYer building a "7.4.1 Home Cinema Soundstage" in the past weeks.
My support resulted in a very compact version of Michael Bittner's "SYMASYM 5.3" (see image #1).
My DIY friend has already built a couple of PCBs, and he is absolutely content with the SYMASYM's performance and its sound!!
As a "Thank you" he is offering me an ALU-case "for Nothing" (see image #2) that he designed himself, using the Sketchup Software,
and use for his Home Cinema SYMASYMs.

I have now time to proceed with the build of a new ACA#1, this time using your modifications (your resistor values, for example, …).
See Image #3 and #4, taken this morning, when I tapped the heatsink and adjusted the bias / quiescent current for the ACA.

As you can see from the picture, my new ACA is driven from a 2 x 19VAC / 160VA INDEL torroid transformer, the rectified AC voltage being
backed up by 2 x 10.000µF Nichicon electrolyt capacitors with a R/L filter between them, and then fed into a LM1084 PSU.

In effect: this is a dual-mono build, and I cannot imagine any better / simpler / lower-cost PSU for the ACA#1! This is my impression !
My HP / Agilent DMM and my oscilloscope show a ripple of about 20mV on the adjusted 24VDC. This is not bad at all!

As soon as I have built the second channel, I will tell you about my sound-impressions.

Best regards - Rudi
 

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Hi Rudi, that's looking good!
Dual-mono is definitely the way to go. There should be plenty of room in that chassis. What bridge rectifier are you using? If I read things correctly, you are only getting 1.2V drop across the bridge. Mine drop about 3 Volts, and I would really like to have some of that voltage back!
 
Hi James, the ACA is a very robust and versatile topology. Since you already know how to work with tubes, here is an idea you may find interesting.


Very interesting. In order to make a proper (non-) sissy version, the 12AU7 really needs at least 200V B+ supply, or better still, 300V regulated. I would go with a Mullard 6189 per channel, with matched triode sections connected in parallel for lower noise and better drive capability. Maybe a single ECC99 would work for both channels. The filament supplies need to be independently regulated for each channel.


This also gives me ideas for replacing the input JFET with an easier to source component like the LND150 depletion mode MOSFET, which would bias very similarly to the 12AU7. Bonus for not having to build regulated filament supplies, which can be a bit of a pain since most HV transformers just barely put out enough voltage on their filament windings to allow for proper regulation.
 
Premium power supply for ACA (and other projects) 2 x APS-1102AProgrammable AC/DC Power Source.
I'm going to get only one. Semi-premium :)
You could get one defsit3 for the price of the aps-1102A.
But premium not high end power supply with ability to measure power supplys too. & much more.
 
Gentlemen, find attached an image of the schematic of the PSU that I am using for my current ACA#1 - build.

Best regards - Rudi_Ratlos

P.S.- Btw: I have a spare set of ACA#1-PCBs (shown in a prior post) that I am using myself and that I will offer you.
My offer also includes: 2 suitable heatsinks, "hard to get components" like: 2 x 2SK170BL, "Futaba emitter-resistors" 0R47 and 0R56, 2xZTX450, 4xNichicon 10.000µF/35VDC,
2 x Nichicon Muse_Gold 4.700µF, …

If you are interested in my offer : give me a PM and I will send you an instruction manual how to build the "Standard ACA#1" and which modifications it will take to do the
TungstenAudio - ACA#1 with my PCB.

Best regards- Rudi_Ratlos
 

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Good idea with the LND, but there is a practical limit to usable HV without a negative bias supply. An ACA with 12AU7 input tube @ 90V is estimated to have ~0.9% negative phase H2 @1W into 8 ohm. Have fun. :cheers:
Good point about the Cathode follower being DC coupled to the input of the IRFP240. I'm used to AC coupled tube gain & follower stages. I don't have any transformers on hand for that low a B+ (90V).
 
My sim shows 140-150V DC as highest practical limit and each triode draws ~6.7-7ma. For testing, a couple of back to back 500mA 12V transformers (from wallwarts) will give you ~80-100V AC. Add > 200V silicon diode bridge, a tube rectifier, a couple of capacitors and a small dropping resistor to get 90-100V DC. The 12V AC also doubles as heater supply. Just my 2 cents. :)
 
Rudi's Regulator

Hello Rudi and Fellas.
Your showed Regulator is interesting me.
Not fully clear is the position of L1 in schematic?
Should it be placed between 3,3 ohm resistors or one option: C-R-C, C-L-C
filtering?
I have some Hmd. 10mH 5A for use in MoFo, so i could use/build your regulator
to use there or build a premium ACA. ;)


Tungsten Audio thank you for that thread and interesting mods on ACA.


Should dismantle a #1 version board and optimize to your options. :p


Actually i have worked to bring a F5HA as "Mighty Power Amp" to live-
it's singing current in background.