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David’s New Toy(s) – ACA Amp Camp 1.8 Class A Amplifiers

Published on 3/23/2022

ACA Class A AmpYou may have read the title of this New Toys blurb and thought to yourself, Amp Camp.  Sounds like DIY.  I don’t want to buy anything I have to put together, and then you wanted to stop reading right there.  But the fact is, you don’t have to be the one to put one of these amps together, and really, everything is “put together” by someone or something (robot?), so please continue with this article.  I can tell you now that I have seen these amplifiers selling on eBay, assembled, for just about the cost, or less, than the kit from DIY audio would run you.  But if you are at all able to use your hands, and your eyes, and a multimeter and a soldering iron, you too can have fun, and/or exasperation, putting these together.  I didn’t experience any of the exasperation myself, well, perhaps momentarily, but I managed to, for the most part, pleasantly while away a couple of days assembling three of these.  Ah, the joys of the Pandemic.  Some artists wrote new books, or recorded new albums, and me?  I added to the collection of audio gear that I have in my home.

But you may be asking yourself, why did I have three of these amps?  On the other hand, you may not care at all, but I will tell you anyway.  Way back in May of 2018 I heard about these Nelson Pass design Amp Camp class A stereo amplifiers. I ordered one.  The amps are designed to put out 8 wpc in stereo mode and 15 wpc in bridged mode. The amps also come with a single balanced input, for mono operation, and a pair of single-ended inputs for running in stereo- though you can use a single-ended input for mono operation as well.  You also get to choose between a silver or black faceplate.  What’s not to like?  Anyway, time passed. I moved in 2019 and never assembled my amp.  But I did bring the kit with me.  Then in August of 2020, I read that there was a 1.8 upgraded version of the ACA 1.6 amp.  I decided I should get two to use as monoblocs (it’s a sickness, yes) so I put in my pre-order, and sometime later the kits arrived and took their place on the shelf next to my ACA 1.6 kit.  Did I mention there was a pandemic?  Yeah, anyway, fast forward to one of our SFAS Friday Night Happy Hour Zoom events, and Stephen Scharf showed off his newly assembled ACA amps.  Oh yeah, went a bell in my head, I have a couple of those too.   Stephen mentioned that he had upgraded a couple of the resistors and emailed me a list of those items.  I was psyched.  But some more time went by.  I looked for Stephen’s email with the upgrade “premium” parts, found the email, but in the meantime, I had also stumbled around on at least a couple of different forums that discuss these amps, or all things Nelson Pass, and decided to get some Nichicon Capacitors in addition to the metal foil resistors, etc., that Stephen had recommended.  In for a penny, in for more shipping charges.  But, the total cost for the “premium parts” was probably less than thirty bucks to upgrade two of the amps.  I left the original 1.6 amp stock except for upgrading the capacitors.  I ordered most things from Mouser but did find a good deal on some of the bigger Nichicon capacitors from a seller on eBay.

aca R4 parallel board   

All that was left was sorting the parts, testing the resistors with a multimeter, and beginning the process of stuffing the boards.  And then soldering.  Each amplifier has two circuit boards, one each for the right and left channels.  The chassis, which had shipped from Italy, assembled with little difficulty- except for the fact that one of the chassis had threaded inserts for the top and bottom plate mounting screws but came with sheet metal screws that were the same as those used on the other two chassis.  It took me a little while to figure out why the sheet metal screws wouldn’t fit on that kit, followed by a trip to the hardware store for some miniature threaded screws to fit for the final assembly.


All of the instructions for assembling these amps are available online with lots of pictures and videos that you can watch to assist you in your build.  And, if you’re a forum-type person there are multitudes of forum pages dedicated to these, and all other Nelson Pass amps.  I dislike spending time on forums as I find they are rabbit holes and I can spend twenty minutes, or an hour, looking for something and sometimes never find what I’m looking for, or I find conflicting info, and then I’m no better off than when I started.  On the other hand, digging can occasionally produce nuggets of golden info, and the info I did find that was related to these ACA amps was platinum.  YMMV.

But you probably really just want to hear about how these amps sound, so I’ll tell you.  I first hooked these up in my breakfast nook, having built them on the dining table there while my wife was off the mainland for two months.  I grabbed a couple of cheap-o KLH model 970-A speakers that have been living in my garage and that I probably paid around $20 for when they were new nearly 20 years ago.  Just before I assembled the ACA amps, I’d assembled one of the KORG Nutube preamps, so I ran an RCA cable from an old second-generation iPod to the Nutube input and connected the Nutube via RCA cables to the ACA 1.8 amps running them as monoblocks after trying each amp individually in stereo mode.  The sound was… not bad.  Not impressive, but not bad.  Then I remembered reading (forums again!) that someone said the amps needed around 200 hours of break-in to really sing.  So, I let them run.  Fast forward a couple of weeks and Leslie borrowed my Parasound JC1+ monoblocks to try with her MBL 101 E MKii speakers.  (side note- she ended up buying a pair of the JC1+ amps!) Anyway, I decided it might be fun to try the ACA 1.8’s using their balanced input in mono mode with my Nola Viper Reference 2+ speakers. 88 dB sensitivity.  The Nola’s are known for liking a lot of power, but not like the 80 dB MBL’s.  How did the amps do with their 15 wpc while replacing the 450 wpc JC1+ amps?  Spectacularly well, that’s how.  The JC1+ amps run in class A up to 25 watts, so at moderate listening levels the sound quality was not night and day different between the amplifiers when I listened.  Even the bass was surprisingly enjoyable.  I found myself laughing on more than one occasion while listening to music and being impressed and then looking at the amp stands that disappeared under the JC1+ amps and their 83 lb. each weight.  The ACA amps look miniature by comparison. See the pic below.

ACA amp 1  in the system setup   ACA amp 2 in the system setup

Over the week or so that I lived with these amps in my main system I found myself with the thought running through my head that I could happily live with these amplifiers in my system forever, all the while reminding myself how much less electricity they were using while I was listening to articulately detailed highs and a midrange that had just the right amount air and ambiance. And as I said, the bass was more than adequate too.  With that said, most of the audiophiles I know are not satisfied with just adequate bass.  When I eventually put the JC1+ amps back into my system, there was no mistaking the presence of much more than adequate bass.  There was majesty in the lower octaves as the behemoth JC1+ monoblocs, with their 180 amps of peak current capacity and full power rated down to 2Hz (1300 watts @ 2 Ω), announced themselves as the ruling kings of amplification in my system.

But now that I had experienced the spectacular sound of the ACA 1.8’s I knew I had to find a place for them somewhere else in my house… to be continued.

As I said above, you can find these amplifiers already assembled, sometimes for less than the price of a kit.  Other times you can find someone who purchased the kits and never assembled them before selling them (Larry, you know who you are).  And other times you can find the kits new for incredibly cheap online. They are still available from DIY Audio (link here) for $336 each (currently out of stock but you can give them your email to be notified when they are back in stock) and while writing this piece I found someone on Etsy who will put one of these together for you for $200. But then you’d miss out on the fun of building it yourself. I mean come on, the pandemic won’t last forever (will it?). Get busy and create a memory you can cherish from these times!  Etsy 1.6 ACA

What I didn’t know when I purchased my ACA 1.8 amps (I’d not yet gone into the forums :-)) was that they are the same amplifier as the 1.6.  Identical parts except for the 3-position rear switch that comes with the 1.8 and a little bit of a wiring variant to connect all that.  There’s also a lettered backplate on the 1.8 that identifies all of the inputs and outputs and the functions of the three-position switch.  I ended up purchasing a DPDT switch for my 1.6 amplifier from Parts Express and wiring it to function the same way the 1.8 amplifiers are set up.  A label maker filled in for the backplate with lettering.  The 1.8 backplate was available to order separately for around nineteen bucks, if I remember correctly, but as it comes from Italy the cost for shipping was equal in price, and with the pandemic, I didn’t want to wait who knows how long for the backplate to arrive before I could start my assembly. I mean, after all, I’d already waited years to begin!

image of inputs   photo of ACA back plate

And so, in conclusion, I give these little class A amps from Nelson Pass a solid A rating for their sound and value. Of course, I did modify the amps with some premium parts that were not included in the kit, but the cost of doing that was minimal, it’s just that I can’t tell you how much those parts contributed to an overall improvement in sound quality compared to the stock parts that are included in the basic kit because I have not directly compared them.  However, I assure you that you can find firm opinions on the subject on any of the forums that discuss these amps.

Additionally, I’ve heard of a few people that run these as monoblocks and actually use two amps per speaker in a bi-amp configuration. These are small enough that anyone should be able to fit a pair, or four into their system. I would also recommend that you watch the video review by Steve Guttenberg on his Audiophilliac channel. There are some significant differences in the quality of the sound depending upon whether you run the amplifiers in stereo mode or in the balanced mono mode. Which mode would satisfy your audiophile sweet tooth might be good for you to know before you purchase one, or two, of the amplifiers.

Highly recommended!