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Old 14th March 2019, 08:28 PM   #121
TungstenAudio is offline TungstenAudio  United States
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Join Date: Jan 2014
Location: Kenmore, WA
Default ACA-220 v2

One of the things I was hoping to achieve with my newer ACA builds was an amplifier in one chassis with just a pair of ACA PCBs that sounded as good or perhaps better than my ACA parallel bridged monoblocks. I took two different approaches toward this goal. The ACA-220 v1 was one approach, and worked quite well. The second approach was to increase both the rail voltage and the transconductance of the power MOSFETs Q1 & Q2.
For v2, the PCB components were updated as follows:
Q1, Q2 - IRFP140
R16 - 499Ω
Deceptively simple, but the remainder of the improvement was to come from the power supply.

The results were worth the effort in the end. V2 has a more lifelike 3-dimensional soundstage, bass that extends deeper yet feels completely integrated with the rest of the music, and an ease of tonal clarity and dynamics that exceed what I have been hearing with the monoblocks. Most importantly, music from this amp is engaging to the point of making me forget whatever else may have been on my mind as I sat down to listen. With this amp I am able to dial up my volume control a little further, and the sound simply extends both forwards and backward in my listening space without the noticeable compression that the original ACAs tended to exhibit when pushed near their power limit.

The journey to achieving the V2 sound proved to be a bit longer than originally anticipated. Sometimes that is part of the fun. I originally wanted to get around 30V power rails for the amplifier circuit, but that goal proved elusive. I started with a pair of 200VA transformers with 25V secondaries, a set of 24000 uF, 63V reservoir caps, and 0.25Ω power resistors after the reservoirs. Didn't work. I barely got 30V at the reservoir caps, sometimes less, and there was a noticeable hum from the power supply. Aside from the faint hum, the imaging didn't seem to lock in place, as it has with V1. The transformers themselves were losing almost 2 Volts in their secondary windings, and the bridge rectifiers were eating another 3+ Volts. Substituting the newer Vishay rectifiers that I listed in my BOM for V1 gave a small but noticeable and repeatable improvement, resulting in 30.2 Volts at the reservoir caps. Increasing the power resistor to 1.0Ω also gave a noticeable and repeatable reduction in hum from the power supply. The 1.0Ω power resistors also provide a convenient way to measure the supply current entering the amp – 1.6V translates to 1.6 Amps. The 1.6A measurement also appears with the appropriate voltage across the 1.5Ω power resistors in the V1 build.
Finally I installed a 300VA transformer (Antek AS-3224) with 24 Volt secondaries and got the same 30.2V at the reservoir caps, but an improvement in sound. Yes! On the right track. Now that I had settled on a lower voltage at the reservoirs, I changed them to the same 36000 uF, 40V caps used in V1. The final piece of the puzzle was to look at the CRCRC configuration as a big C followed by two cascaded RC filters. The V1 build has 1.5Ω / 10000 uF as its first RC. When I changed the V2 build to have 1.0Ω / 16800 uF as its first RC, the sound I had been chasing fell in place. The voltage at the amp power rails is 28.4V, and I have set the Vds of Q1 to 14.4V.

There seems to be a common agreement among folks who have built both the ACA and one or more of the larger FirstWatt clones, that while bridged ACA monoblocks sound greater than the sum of two amps would suggest, the larger FW clones transcend this sound in different ways. I suspect that V2 may bridge some of this gap between the different types of amps. I intend to find out, and have PCBs and parts for both the Aleph J and M2x.
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Old 15th March 2019, 01:04 PM   #122
Mazeppa is offline Mazeppa  United States
diyAudio Member
Join Date: Mar 2009
Location: Harrison, Arkansas
ACA amp with premium parts
My builds are as close to the 6L6 guides as I can get, but our paths are similar.

I started with 2 ACA's, working on an Aleph J now, and have parts on hand for an M2x. Decided not to mess with my ACA's until I get something else going.
I will be watching for your progress on those builds as well.

BTW, I too noticed compression setting in from my ACA's when the volume knob of my unity gain preamp got a little too close to 11.
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