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Question power supply aca/boz
Question power supply aca/boz
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Old 11th February 2019, 10:15 AM   #1
Giovanni malluzzo is offline Giovanni malluzzo
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Join Date: Nov 2017
Default Question power supply aca/boz

Hello everyone.
I made an aca and a boz. I am very satisfied with the performances, but I would have some questions:

1) I read somewhere that aca works better with regulated power supply. My power supply is: 19V transformer for 24V continuous (modified aca), fast Mur820 diodes, 22000uf samwha screw capacitors, and a 2K2 ohmite resistor in short circuit. I ask if you have a scheme of a possible regulated power supply, and what are the advantages compared to my power supply.

2) I put fast diodes on the aca and schotty diodes on the boz. After some listening tests I noticed higher performances (three-dimensionality, soundstage, scanning instruments in space, natural tone without "S" hissing). But I read somewhere that someone prefers slow diodes, I read in an article that even Mr. Pass prefers slow diodes. I kindly ask your opinion on this, especially to understand if I'm on the right track or I'm wrong.

Thank you very much for the answers, and sorry for some uncertainty in the translation with Google translate.
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Old 11th February 2019, 10:38 PM   #2
mjf is offline mjf  Austria
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perhaps look into this thread:

ACA amp with premium parts

post 21 and so on.........
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Old 12th February 2019, 12:48 AM   #3
6L6 is offline 6L6  United States
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Question power supply aca/boz
1) You could try a regulated supply, but your existing supply seems like it is very good!

2) I think you are on the right track.
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Old 13th February 2019, 04:38 PM   #4
TungstenAudio is offline TungstenAudio  United States
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Join Date: Jan 2014
Location: Kenmore, WA
A Class A amplifier places different constraints and demands on its power supply (PSU) from the more typical Class A/B amp. The biggest difference is the constant current demand from the Class A amp. Even with a small to medium size amp such as the ACA, the constant current draw will change the relationship between the bridge rectifier, reservoir capacitor, filter resistors and final smoothing capacitors. Each of these parts affects the final voltage, including ripple, that powers the amplifier circuit.

The choice to use a voltage regulator as part of a Class A PSU is also not as simple as it would be for a Class A/B supply. While I am currently working with a set of PCBs that support the use of an integrated regulator (LM1084), I have not installed the regulators yet. Before I do that, I want to better understand how the individual parts of the PSU affect the performance and sound of the amplifier.

One of the issues that I have run into with my recent builds is the voltage loss from the GBPC style chassis mount bridge rectifiers which are commonly used in the FirstWatt and clone amplifiers discussed on these forums. The first build (ACA-220 v1) used an Antek AS-3222 toroidal transformer feeding a 24000 uF reservoir capacitor through On Semiconductor GBPC3504 bridge rectifiers for each channel. This resulted in about 27 Volts at the terminals of the reservoir caps, rather than the 28+ Volts that I was expecting. While 1.0 or 1.2 Volts may not seem like much, it eats into the headroom needed to produce a solid, low ripple 24.6 Volts to the amp.

After some experimentation, I have currently settled on 1.5Ω Ohms for the power resistor after the reservoir cap. This was about the minimum value that still provided good ripple reduction. A value of 2.26Ω yielded lower ripple, but at the expense of lower voltage to the amp. Space on the modified ACA boards permits a pair of 10000 uF, 50V filter caps, with a pair of parallel 0.22Ω resistors before the second of the two filter caps. The 1.6 Amp quiescent current of the modified ACA accounts for the voltage loss through the resistors, and also through the rectifiers. This overall PSU setup seems to be a "sweet spot" for good sound, resulting in an amplifier that is a pleasure to listen to.


It would be possible to alter this setup by eliminating the 1.5Ω power resistor, connecting the reservoir cap directly to the first of the two on-board filter caps, and installing the LM1084 adjusted to output 24.6 Volts. Due to the necessary voltage headroom to ensure that the regulator is not operating at its minimum dropout voltage, the final PSU output voltage is about the same using the regulator with CCRC vs unregulated with CRCRC. I may try the regulator sometime later, but for now I am busy trying other things.
Yes, it is a lot of work to replace a good SMPS.
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