Come with me as I build a Pass ACA Amplifier kit

The ACA amplifier is a Nelson Pass design used to create basic concepts about amplifier design. It is low-powered (6-8Ws per channel), run in pure Class A, capacitor coupled on the output, and is even single-ended.

There is a long, and very detailed article from Mr. Pass which, to put it bluntly, far surpasses my more vacuum-tube orientated knowledge. Anyone who wants to know the nitty-gritty of this single-ended Mosfet amplifier should take a gander at the link for more information. For those who are a glutton for punishment here is the schematic:



My own requirements are modest wattage and an attempt to capture some of the magic that I hear with single-ended tube amps. The ACA amplifier can also be bridged, via an XLR input, to double the power, something I will need with my 86dB efficient Wharfedale speakers that are (in this case) luckily in a small listening room. ie, I don't need gobs of power for reasonable listening levels. Since my Classe Five preamp has XLR outputs, I will be going this route, building two stereo amps to use as bridged monoblocks.

Enough babbling - let's get building the first amplifier:

Here you can see the passive parts and the circuit boards. It's best to populate the small parts first - the resistors first, followed by the caps and solid-state devices.



Once the circuit board was stuffed - this will be another post - it was time to start assembling the amplifier chassis for PCB fitment, along with RCAs, XLR, switches, and binding posts.



Sadly my day was drawing to a close so I had to put off further construction until tomorrow. Stay tuned!

 

Dang, wish I could do that. It all looks like spaghetti to me.

 

From what I read on this famous SS amp is a way to get rid of tubes, but have that tube sound. Always wanted to build it, but need more power due to my great only 86.5dB efficient speakers.

 

Yesterday I only got an hour or two of amp work in.

Here you can see the stuffed circuit boards attached to the heatsink. The Mosfet outputs are mounted too.


and I did some more work on the back panel, which has all of the connections. Tonight I hope to have everything wired up, set the bias, and give the first amp a listen.

 

Great job so far! I've been hanging out on DIYaudio forums and almost pulled the trigger on the amp camp kit as well. I guess I'll live vicariously as you build these.
I'm curious to know if you've had any previous experience building electronics?
Jon

 

Nice job, so far. That looks like a fun build. Did you order the chassis(s) too, or are you doing a custom build?

 

The infamous Aleph current source. I've considered building a Pass Aleph 3 clone or something from Nelson Pass' firstwatt.com site. Just don't have the time.

 

I've built a few kits and quite a number of homebrew / DIY designs, mostly tube. The ACA is fairly easy to build though the chassis is small enough that my big hands get in the way. Also my eyesight isn't what it used to be, so dealing with small SS components is a little hit 'n' miss. I should get a magnifying glass setup.

The kit comes with a chassis - predrilled holes - which makes life a lot easier. The most difficult part of DIY-ing is the chassis, so I'll the easiest route that I can!

 

Powering up the amp for the first time is always exciting. But I ran into a snafu - the LEDs would begin to flicker. Measuring the voltage from the power supply brick, I could see it was fluctuating from 24Vs (good) down to 5Vs (bad). A check on the DIY Audio forum indicated this is the lap top power supply going in protection mode if there is short.

To troubleshoot I removed a power supply lead from one of the boards. And the remaining PCB still had a flickering LED. Once I found the board with the problem, I disconnected it from the heatsink. And then the problem went away. This was a clue that one of power transistors was being shorted against the chassis. Removing one of the Keratherm insulators I found a small pinhole - manufacturing problem? My own mistake? Luckily I had some more insulators in my upcoming Aleph J project box so I replaced them.

Whew - sorry for the long side story but it just goes to show you that nothing ever seems to go right the first time.

Here I am biasing the output mosfet:



And then once that was done, I was able to - FINALLY! - give it a listen:



Initial impressions: lower gain than the old workhorse Adcom 545. Instead of 9-10 o'clock on the dial, I have to turn up the volume to 1 o'clock to get the same sound levels. The ACA is very smooth, a little lacking in bottom end thump but still very articulated. Jazz - like Chet Baker's album Broken Wing - sounds fantastic, while harder rock albums seem to lack some of the macro dynamics. Of course we are only talking 6-8Ws here with 86dB Wharfedale speakers (6-ohm impedance) so not an optimal match. I wouldn't say the ACA sounds like a tube amp - some similarities: with a non-fatiguing top-end - but it also doesn't sound like your stereotypical SS amp either. Obviously some break-in may also be needed.

More later, when I build the second amplifier and can listen to these as bridged monoblocks.

 

How much heat is coming off that heatsink? I've wanted to build an amp similar to this but scale it up a little bit.

How you described the sound of this amp is almost exactly how Nelson Pass described most of the amps from his Zen series of DIY amps... Not the ultimate statement in bass but smooth and non-fatiguing from the mid range on up. I built a "Szekeres" headphone amp for my Grado 325's a few years ago which is a current source loaded mosfet amp. I usually like good instrumentation but this amp really put a spotlight on good vocalists.

 

The amp gets HOT - like tube amp (but not tube glass) hot - I can keep my hand on the heatsink but the skin doesn't feel comfortable doing so.

 

From the website:

• "Insane amount of detail, deep bass and crystal highs" - Starbender
• "Great power, bass and massive detail" - Waam68
• "Highly detailed, yet smooth and sweet" - Mitchba
• "I am amazed at the sound quality" - Neoinc

Agree?

 

Hard to say until I give 'em some more time. Also they aren't the greatest pairing with 86dB / 6-ohm speakers. I'll feel better when I'm running a pair bridged, which will be happening in a few days.

 

Nice work! Asking that amp to drive 86 db speakers is a way tall order though- not surprising it's losing bass articulation at anything other than conversational levels. I'll have to get on the stick and build mine and see what it can do with a slightly more efficient design. Should be good for over 100db on my khorns, lol.

 

Nice write up and impressions. Thanks.

 

Does bridging the amps cause issues with speaker impedance? i.e. A formerly 8 Ohm speaker is seen as 4 Ohms, to the bridged amps?

 

Very interesting work here, this might be a good idea for my Altec 604's.

 

In this case, power is doubled, but the damping is halved.

These may not be the best solution for my current speakers but thought it would be fun to build until I get the next project - an Aleph J amp - sorted out. That will be another post for another time.

 

Good read, looking forward to the bridge update. Love to hear one of these with my Cornwalls.

 

Over the weekend I managed to finish the second amplifier - no problems this time. I still have to install LEDs and the top and bottom part of the chassis, but I did manage to listen to them bridged, using the XLR output from my Classe Five amplifier. Very impressive.

 

First impressions of the bridged amplifiers: Neil Young - Live the Cellar Door sounds good on just about any stereo I've owned, but this may the best I've heard it, even compared to the days when I had an all tube chain and big UREI 813A speakers. Lots of micro dynamics, very realistic, and engaging. The amps sound way more powerful than the 12-16Ws it is making.

Some further listening - The Bee Gees - Trafalgar - revealed a slight metallic sheen that seemed to reduce with further break in. Very clean but not lean. Detail, studio reverb, etc was all there. Frequency response was very even, no warts here, and less "lossy" than some of the lesser tube amps I've owned. Single ended Mosfet is interesting, to say the least, not quite tube-y, nor is it "classic" solid-state (whatever that means). Just different, even more so than Class T (Tritpath) designs I've built. The old (and much need of a recap) Adcom 545, when put back in the system, sounded more indistinct, gray, and uninvolving, though it does have superior deep bass reproduction (and better damping and more power).

Gain is still a bit lower than I would like. Deep bass isn't as prominent as other amps, but again, 6-ohm and fairly inefficient Wharfedale speakers with 5" woofers here.

More later when I've had some more hours on them, and an audiophile friend is interested in hearing them too, so I want to get his response.

 

Blindfold your audiophile friend and do some A-B. Ask which has “more detail”, which is “more involving”. See if he/she hears what you hear.

 

a picture (in the evening) of the blue LED power supply indicators. The glare isn't as bad as the photos indicate, just my phone trying to compensate.

 

After a few weeks in the system, I'm quite impressed. Very neutral with a touch of sweetness. Some of the best depth I've heard out of a solid-state amplifier, along with very good detail retrieval. Some of the better tube amps I've built or owned are slightly better at separating voices and instruments, but the ACA amps do a very passable job, just not quite as much "breath" to the performers. Whether the tube amps are revealing realism or a coloration is an interesting question. Considering the capacitor coupling, low output - even bridged, and the low damping factor the ACAs still managed to control the Wharfedale Denton speakers quite well, with just some minor loss of bass control. These amplifiers just make me want to build an Aleph J amplifier that much more.

 

I just ordered 3 of the basic kits myself, and look forward to building 3 bridged mono ones for my 114db horn setup I amcurrently working on (These amps should rock!). I have never soldered more than a cap or 2 on a crossover with a friend directing me so this will be an experience. I hope I can do this! I see you still have the Classe preamp... I used to have the 4, and then upgraded to the 6. The best preamp I have ever owned, and I wish I still had it for 2 channel.