Amp Camp Amp - ACA

Sorry...here are the pics (need to downsize).

Alex
 

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Alex,

Is the v1.6 your intro to SS Class A?

They (v1.6 mono blocks) were for me, and that's what I said, woohhhh!

Glad your project turned out well.

Hi Mazeppa - No, I have build kits before in several topologies! But I always wanted to try a NP design...and I’m glad I did. He is in my opinion at the very top of Amplifier design and a mentor also...a rare combination! Lucky for us...
 
Hi! I am waiting for the delivery of my two amp kits. In the meantime, I am thinking of what preamp to pair with the amp kit to control the volume control. I will appreciate very much your recommendations.
Hi RCV. I built the ACA for fun and by curiosity, but I had no preamp and I was not sure about which one to get in order to match well with my audio system. So many factors to consider : passive or active, solid state or tube, DIY or not, how much gain, cost, combination with the other elements. I have always used integrated class A/B amplifiers, switching to a class A power amp was really new to me. After finishing the ACA, I used the preamp section of a good integrated amplifier (fortunately it has preamp outputs) to test my built. Everything worked fine and I was rather pleased with the sound. However after a period of listening, I felt that it was not quite up to the level of my expectation and below my reference amplifier. Considering that it is a Nelson Pass design, I was hoping for more and I thought that maybe it was the preamp which was a limiting factor. After spending some time investigating different options, I finally ended up getting a Job Pre2 preamplifier. The difference was huge and it brought my system to a much higher level! I like the combination with my Klipsch FORTE III loudspeakers, of course it is subjective and my personal taste.
What I am trying to say is that the choice of the preamp should be done carefully and it will make the ACA really shine.
 

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I am trying to take it slow and absorb as much as possible learning and understanding all of the nuances that come with the territory but I think there needs to be a separate thread for preamplifier recommendation and matching to the ACA. I get that picking one out should be based on many things such as input source, speakers, diy or not, and budgethe and that can vary widely.

But man, I am still really struggling to narrow it down and a 658 page thread is a lot to weed through.
 
Look at it this way 73 and a 3rd, the journey may be as fun (or more fun) than the final product. 658 pages, great! A lot of the pages do not have much of interest in them, you do not need to read everything in detail, scan through it, read the good stuff. Remember there is a lot of information on these pages, including a lot of information on preamps.

I will give you a suggestion, try to establish a basic idea of how you want the signal to get to the amp, and follow a straight line (a theme if you wish) of thinking in your planning. For example, the ACA is a simple Class A design. I would combine it with other pieces (preamp, DAC) that follow the same rules, class A for analog, simple design. I think it would be a good idea to combine an NP designed Class A preamp with the ACA. Keep the signal pure and the path clean and do not forget volume control as part of the path. Be careful with power supplies. Understand that digital music coming through a USB can contain a lot of high-frequency junk in it which is passed right through your system. Make sure your DAC can clean the digital noise. Finally, you really do not need to spend a fortune doing this, what you need is careful planning. Keep notes as you go. You will never remember a lot of this stuff unless you keep a record.
 
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How big of a variac do I need to build the ACA?

Do not use a Variac if you are using the kit power supply or any other switch mode power supply (SMPS) to power your ACA.

Just to clarify the reasoning here - if you have access to one, a variac is an excellent piece of kit to have (although you can get by with a light bulb tester) if you’re building something with a linear power supply (eg. large transformer, followed by rectification, capacitor bank, etc) which is what you’d have seen used for the other (more power hungry) Nelson Pass designs built around here - the standard design for the ACA does not have a linear power supply.

Instead it was designed to use a switch mode power supply - SMPS for short, that’s the black ‘power brick’ that takes line power in one end and throws 24v out the other in this case. An SMPS operates in an entirely different fashion and will not react the way you might expect it to if you throw a variac on the line before it.

EDIT: As noted by Alan4411, this is assuming you stick with a SMPS, as per the original design and all the documentation.
 
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Look at it this way 73 and a 3rd, the journey may be as fun (or more fun) than the final product. 658 pages, great! A lot of the pages do not have much of interest in them, you do not need to read everything in detail, scan through it, read the good stuff. Remember there is a lot of information on these pages, including a lot of information on preamps.

I will give you a suggestion, try to establish a basic idea of how you want the signal to get to the amp, and follow a straight line (a theme if you wish) of thinking in your planning. For example, the ACA is a simple Class A design. I would combine it with other pieces (preamp, DAC) that follow the same rules, class A for analog, simple design. I think it would be a good idea to combine an NP designed Class A preamp with the ACA. Keep the signal pure and the path clean and do not forget volume control as part of the path. Be careful with power supplies. Understand that digital music coming through a USB can contain a lot of high-frequency junk in it which is passed right through your system. Make sure your DAC can clean the digital noise. Finally, you really do not need to spend a fortune doing this, what you need is careful planning. Keep notes as you go. You will never remember a lot of this stuff unless you keep a record.

I am totally tracking with what you are saying. Pretty much planning what you have laid out starting simple with a cd player or fm tuner as source input. Looking for stand alone preamp class A analog, possibly tube. I don't mind used or older gear if it fits right and helps with price. DIY is great too.
Seems like matching/managing/balancing voltage and gain is the trick. Also seems to be quite a wide variety of preamps to choose from.

Looking at a used Dared SL200a currently for $350. Anyone have feedback?
 
if you have access to one, a variac is an excellent piece of kit to have (although you can get by with a light bulb tester) if you’re building something with a linear power supply (eg. large transformer, followed by rectification, capacitor bank, etc) which is what you’d have seen used for the other (more power hungry) Nelson Pass designs built around here

For us newbies...can you explain how you'd use the variac with a linear power supply? Is it for testing purposes?
 
I am totally tracking with what you are saying. Pretty much planning what you have laid out starting simple with a cd player or fm tuner as source input. Looking for stand alone preamp class A analog, possibly tube. I don't mind used or older gear if it fits right and helps with price. DIY is great too.
Seems like matching/managing/balancing voltage and gain is the trick. Also seems to be quite a wide variety of preamps to choose from.

Looking at a used Dared SL200a currently for $350. Anyone have feedback?


I can't find useful tech specs on an SL200A, I can find comment out there on an SL2000A, such as:
Dared Mini SL-2000A Preamplifier
but no info on the gain of the unit. I'm using the ACAs as balanced mono blocks fed by a Schiit Freya using its tube stage with a gain of 2.5


Does a great job in a 3m x 8m room with a hard wood floor.


So, it's really a question of gain that you need and if you're like me, how you intend to line up the amp(s). If you're going a single ACA in stereo, you're choice is made - you'll be lining up the ACA's RCA inputs. Whereas if you're doing monoblocks, the balance XLR inputs are an option and you'll need a preamp that can play in that world.


There's no single answer because there are so many variables. Start off cheap (e.g. a headphone amp) or borrow a unit to try out is my suggestion.
 
I have had good luck buying inexpensive but fine equipment pre-used. However, I have stayed away from tubes because I am weary of tube replacements. I know tubes produce a great sound, but I stick to solid state. The variety available and buildable (DIY) available is what makes this a really interesting challenge.

And yes, I fully agree with Astromo.
 

Should be more than big enough!
As long as you do not mind drilling and tapping the heatsink holes for the Mosfets and board mounting.

I have mine built into a slightly smaller version as I wanted to have my power supply integrated and have a narrower front panel than the 'standard' ACA chassis. (Example only Express Delivery Full Aluminum Power amplifier Enclosure chassis case heatsink | eBay) and it runs nice and cool at 43 - 45 degrees Celsius.
 

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