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ACA amp with premium parts
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Old 24th February 2020, 06:05 PM   #231
TungstenAudio is offline TungstenAudio  United States
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I can see a way to carve up the power plane and use the LED resistor as a substitute for R16. It's more work than I would care to take on, especially since the cuts need to be duplicated on the top side of the board. There is a chance of making the board unusable in the process. Then you need to repeat the process for the second channel.
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Old 24th February 2020, 06:25 PM   #232
Kokanee is offline Kokanee  Canada
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ACA amp with premium parts
Thanks but no thanks, I know my limits as a beginner. I try to employ the KISS method whenever possible. I am going to build a PS giving 20 - 24 V, set the bias to around 1/2 of voltage and see what happens.

MM
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Old 22nd March 2020, 05:04 PM   #233
elwood625 is offline elwood625  United States
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I just finished building a stereo ACA with premium parts, I used the schematic and parts list described in this thread. It is in a Hafler DH-200 enclosure with a Meanwell SMPS power supply mounted inside the case.
It is currently playing music, breaking in, about 5 hours old right now.
What do you set bias to for this amp? I set both channels to 12 vdc at pin 2 of Q1. I thought I read you were setting it to 12.4 vdc.
Is this your recommended setting?
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ACA v1.6 monoblocks, Balanced Zen Line Stage, M2x, Aleph J, H2, B1 Korg triode, 12AX7a Phono amplifier, Balanced B1 Korg triode, 6FG6 "Magic Eye" tube VU meter
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Old 22nd March 2020, 05:20 PM   #234
TungstenAudio is offline TungstenAudio  United States
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Nice, the DH-200 is a great old amp, and a very suitable chassis.
The bias current of the ACA is set by the resistor values R1 through R4, and also R15. With the values I'm using, I have seen 1.6 Amps quiescent current in all of my boards. The adjustment is for the voltage at the Drain of Q1, which I like to set at 1/2 Vdd, plus 0.4 V.
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Old 22nd March 2020, 06:15 PM   #235
phomchick is offline phomchick  United States
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Quote:
Originally Posted by TungstenAudio View Post
Nice, the DH-200 is a great old amp, and a very suitable chassis.
The bias current of the ACA is set by the resistor values R1 through R4, and also R15. With the values I'm using, I have seen 1.6 Amps quiescent current in all of my boards. The adjustment is for the voltage at the Drain of Q1, which I like to set at 1/2 Vdd, plus 0.4 V.
Why +0.4 V? Won't you have minimal distortion at full power with the bias at 1/2 Vdd?
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Old 22nd March 2020, 07:50 PM   #236
TungstenAudio is offline TungstenAudio  United States
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I'm mainly following Nelson's lead on the +0.4V offset from 1/2 Vdd. But more recent measurements with my distortion analyzer show a favorable harmonic content (dominant H2) to the residual distortion signal using this guidance. I have been looking at 5W output.
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Old 23rd March 2020, 05:39 AM   #237
Loudthud is offline Loudthud  United States
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Quote:
Originally Posted by phomchick View Post
Why +0.4 V? Won't you have minimal distortion at full power with the bias at 1/2 Vdd?
The ACA clips softly on positive excursions of the output. The 0.4V offset gives slightly more output before you hit the rails (hard clipping).
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Old 26th March 2020, 01:27 PM   #238
Mituisho is offline Mituisho  Hong Kong
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Is it safe to use a 4700uF 25V (instead of 35V) cap for C1 if running the amp on a 24VDC rail?

I am using Rudi's pcb.
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Old 26th March 2020, 02:23 PM   #239
TungstenAudio is offline TungstenAudio  United States
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^ Yes, that will be Ok.
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Old Yesterday, 08:16 PM   #240
dfoulk is offline dfoulk
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Howdy to all,

I just remounted the amp camp in a B&K ST-202 case which has larger heat sinks than the ST-140. I never used the case that came with the kit, just the heat sinks. I just bolted the kit heat sinks to the case ones. I am using an Antek AS-3225
transformer with the 25 volt secondaries set up as dual mono. I ran the output of each bridge rectifier through a Hammond 159ZJ choke, which is a 10 mH 5A choke. That goes to 20,400 uf (3X6800 uf caps) of filter caps with .2 uf poly by-passes for each channel. Wanted to add another 6800uf to each side, but just ran out of space. I am only getting about 22.5 volts on the rail under load, but it is nice and quiet. I have run this amp on the 24 volt brick, but I like this linear supply better, sounds more solid. I have a 1000uf Silmic II for C4. For Q1 I have a IRFP 140, and for Q2, a IRFP 044, with 12.7 on the bias. I also ran the grounds to an inch and a quarter square of copper, 1/4 inch thick, which is isolated from the chassis with a nylon bolt. The purple shield from the transformer is bolted to the actual chassis, together with the power ground via a CL-60.

I have found that isolating the ground that way seems to add some clarity and dimension to the sound in my setup. Also have 4700uf 63 volt Nichicon for C1, a bit big, but fits. It has been playing for 45 minutes and the rear sinks are warm, but not hot.

It sounds good enough to put up the solder and iron for a spell, and work on fixing our broke dishwasher :^)!
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