I think you missed the point. The ACA isn’t a "Holy grail" audio product. It was designed to give the first time builder the greatest chance of building something they would enjoy building, owning, and using. And be safe in the process.

Awesome job, Stephe. Beautifully clean work as usual. Excellent detective work, and smart idea floating the boards and the vent holes on the bottom. Definitely agree that Pass amps have a certain sound. The midrange seems to be more prevalent in them, and they’re very well balanced using a somewhat low parts count. I like certain SS amps but being driven by tube preamps. I believe I read somewhere that Nelson intended First Watt amps to be driven by tube pres. You’ll have 10K subs in no time. Your channel is fantastic, and your ability to teach how tube amps work is totally top notch. Love that you just tell it how it is too. I work at a shop fixing guitars and tube amps for a living, and I’ve still learned things from you. You’ve built a great little community. 🔊😊🎶

I ordered and built one when my push pull EL34 valve amp was in the repair shop. I needed a very low watt amp to drive some 105db horn speakers. I'm not an electrical engineer - I'm a draughtsman that can follow a schematic and an ex welder that can handle a soldering iron and make a good job of it. It sounds very good when put together. It does what it's supposed to. The full kit is very well sorted with all the bits required delivered and nothing missing. The instructions on assembly are are how all kit amps should be - very clear and plenty of photos. The DIY Audio supplied case goes together well and everything is pre-drilled and tapped. The finished product looks very professional. The low voltage makes it very safe for novices. The vast majority of kit failures reported on the forum are usually down to poor soldering or some through hole missed entirely. The amp seems pretty robust and most of the problems reported get fixed pretty quickly. The popularity is down to the above, a relative amateur can put it together safely in a day and have something that sounds good. As a single ended transistor amp it sounded better than my push pull tube amp. A pair run as monoblocks sound even better. But I still want to upgrade to single ended tube monoblocks so looking to see the progress you make on that.

I just finished my ACA. I ordered the components from the diyaudiostore, the psu from Germany and the case from Italy (I live in Belgium). My expectations weren’t very high, but the sound turned out to be quite good. It must be the original parts that are the amp’s saving grace.

Your review of that ACA amp is the most honest, correct thing I've ever heard about the Pass DIY stuff. I bought the same amp and put in a much heftier power supply and it sounds like typical solid state gear - like the speakers have a heavy wet blanket over them. I also built a First Watt F5. It sounds better than the ACA but is blown away in every category by a couple of single ended tube amps I built with 6V6 and EL84 tubes. They don't even have expensive output transformers. Just Edcor's for less than $40 each. Tube amp designs have been around and basically unchanged for what, 100 years and solid state gear just doesn't come close. Each of those SE tube amps puts out about 4 watts per channel. They sound louder, effortlessly dynamic, better bass and soaring high frequency response and transparency than any solid state amp I have had. I can understand why most people buy solid state amplification. But what I'd call audiophile quality with high resolution speakers? I don't get it. The only reason for solid state amps is the introduction years ago of low efficiency speakers. The argument that well you didn't buy the right parts is total baloney. I order parts from reputable companies like Digikey and they are ALL made in Asia. Nelson Pass has said it's not the parts that make a difference - its the design. Maybe I don't have the right speakers for solid state. I have a pair of old Klipsch Forte's which on tube amps are as close as I can get to a live acoustic instrument. I have been an acoustic guitar player for 60 years currently playing classical and flamenco. I have a very good hand made guitar. I know what a guitar sounds like and you don't get that with a solid state amplifier. An acoustic guitar, even a cheap one, produces harmonics when you pluck a string. For some reason, I don't know why, that is not passed along in a solid state power amp. It does with tubes. Electric guitar players who know what they are doing use tube amps. Solid state guitar amps sound dull where a tube amp has what I call a dynamic organic sound. OK so I never owned a $4,000 solid state amp. That's OK. Each of my tube amps cost about $400 in parts and I have the pride of something I built myself that sounds like real music. Too many people have never heard live, unamplified music, like a skilled acoustic musician or a symphony orchestra. They've been raised on rock n roll and terrible quality amplified sound.

I agree with you Stephe on the Miles Davis Bitches Brew album. I tried to figure out what Miles was trying to do but could never figure it out.

I've watched several of your videos and just enjoy your delivery. I am not skilled at all in electronics but always learn something from you.. thanks so much for your content!

Well done! Just did an extensive listening comparison between the ACA (two of them in mono - both bridged and parallel), the Pass VFet and the Hiraga Super 30. Ran them with a Cary tube preamp and a Schiit SS preamp for the front end. Results were as one would expect. I was comparing them using some DIY 4 ohm coax speakers (80db efficiency - not easy to drive), a pair of Fostex full range speakers (about 89db efficient) and a pair of Cornwalls (102db efficient). The ACA’s certainly did not embarrass themselves, and are well worth the money invested. They need a low output impedance preamp to perform at their best. My friend (who owns the ACA/Schiit gear - he asked me to listen and give my thoughts seeing as I built the ACA’s) had ordered 4 of the AliExpress clone boards (prebuilt, and they were modded after arriving as per DiYAudio recommendations. I put two SMPS units in a chassis and used them to power the 4 boards that were connected in parallel so he had an amp that was optimized for 4 ohm speakers. They also required a low output impedance preamp as they had an input impedance of 5kohm - low… These low powered amps, really sound at their best when driving efficient full range single driver speakers. That’s when the “magic” happens. These are not high end amps, nor do they claim to be. They are a fun and safe diy kit and you end up with a unique product.

Nice video, Stephe! Good to see you were able to hear what this amp can do a little better. I'm not sure if I have seen this amp ever referred to as a holy grail type of amp and if people have referred to it that way then they would be sadly be mistaken. I think it is popular due in part to it's sound because it does sound better than a lot of class AB amps out there. The First Watt and of course Pass Labs class A amps sound much better. I have a DIY Nelson Pass Burning Amp 3, pure class A solid state that puts out 35-40 watts, and use a tube preamp ahead of it (6SN7's and tube rectified) and the combo is quite nice.

Love that you are branching out in SS even though TUBES RULE. Love how honest and uncolored by others that you are. Keep up that spirit. I hope to see you start trying more DIY speakers and maybe even doing some chip amp stuff too. Recently found your channel and have subscribed, BTW. I have an old Zenith console that I have work to do to and take all apart. Planning to reuse the AlNiCo speakers into open baffles as well. Greetings from Newnan. DOraville? REally? LOL

So nice to see someone who can admit mistakes and the petal plus is letting viewers learn from your mistakes. Kudos. I have now built a pair of these and when matched up with a true balanced preamp (I am lucky enough to to have a fully balanced all tube pre for this) the sound is fantastic and the power is decent. I do use the 24v Meanwell power bricks. Still, I am a SET guy so I am mostly using 2A3 SET monoblocks or an EL 84 based triode strapped SET. I like the sound of tubes. Someday, someday, perhaps I will build a 300B differential amp to pair with that balanced pre, if for instance I win the lottery.

Well done Stephe. Finding a solution a problem is fun. I agree that the lower end DIY Audio gear is rather basic and lacking from that level upward. Its place seems to be for beginners and dabblers. Their place seems to be to support the idea of creating enthusiasm that leads to better things

Nice Stephe 👍 Enjoyed the video. Guess since you tried this clone , you shoud try a FM300A clone as well from China as the official FM300A was costing 30,000 USD at those vinatge times and supposed to be a Holy Grail sounding AMP. Thanks for the video...😊

I did a lash up amp of the original rev 1 diy audio ACA boards years ago and it’s a very nice sounding amp. Definitely a fun worthwhile project. The main aim of the project was to get people with no background in electronics into building an amp that was reasonably good sounding and safe to build as the power supply was a couple of laptop bricks. I was a bit surprised with your initial findings. Try it with the bricks &see what you think?

Great video. Your amps have two inverting amplifiers connected together which means a non inverted output. The ACA amp has a non-inverting buffer followed by an inverting output stage. So as you said the output needs to be inverted to maintain absolute phase with the recorded music. Otherwise the speaker cone will be moving inwards when it should be moving outwards and visa versa. Apparently this makes an audible difference on things like drum beats but I have not tested it myself.

I built the official ACA and thought it was a little flat sounding with weak bass compared to a little 5 watt class ab amp I built. Made me think about selling a kit lol.

Another great and honest video.

Bob Carver's theory is that a lot of the differences in the sound of solid state and tube amps is down to the speaker output impedance (ss is low, tube is high). So Bob's Sunfire amps have two sets of speaker outputs on the main L/R channels, one labelled 'Voltage' and the other 'Current', the only difference being a 1ohm 50w resistor added inline to the 'Current' positive speaker terminal. This means it should be relatively cheap and easy to just add a pair of 1ohm resistors between the speaker outputs of any solid state amp and the speaker inputs to get the same effect. FYI, I own a Sunfire TGA-7401 amp (7 x 400w) and the 'Current' output does sound different. Ideally it is used bi-wired with the 'Voltage' going to the bass drivers and 'Current' to the mids and tweeters. I found this set up useful for harsh recordings as the 'Current' output appears to roll off the high frequencies.

I find that the amp works way better with a really good preamp that has lots of game sounds significantly better than using the volume out of like a blue sound node.

Gotta love anyone with Tetenol C-41 color film processing chemicals in the background. Analog respect. Great video. Thank you. I built the ACA Mini and am intrigued by the reversed output polarity. ACA mini is nice sounding. I have not taken the tube plunge yet but really appreciate your channel and your clear explanations. If you don’t mind, what is the tube amp in the video?

When you run it bridged it actually cancels out the distortion so it is a much different sounding amp. There are parts upgrades that take this thing a million miles past where it is including changing out some parts values. You can use different output transistors you can run it up to 36 volts if you protect the JFET with a resistor. I think in its stock form it's just okay... But if you're not going to do anything else with it as far as modifications or bridging then honestly you're missing out on the fun!

If I had to guess, the idea is to reverse the phase of the harmonic distortion

A word about "grounds." (Note: Anyone associated with OSHA must not read this. If you do, the ghost of Nikola Tesla will come to you at night, and strap you to the output of the biggest Tesla coil you can imagine.) You know those "ground adapters," that enable you to connect 3-prong plugs to ungrounded outlets? When I worked for KUAT, we called those "ground busters." They allowed us to use the Tek 465 with the chassis HOT.

Also, if you want to take one last shot at it, There are several mods to upgrade this amp. One of which, installing IRFP150 mosfets. It was certainly making interesting video. Comparing tube rolling to mosfet rolling. LOL Another thing, this amp was not designed to be the best sounding. The idea was it was supposed to be an easy circuit for a new Builder to complete. The idea was to not improve on it in such a way that it would make it more difficult to build. I suggested it before and I will give it another shot. Try the ACA mini instead. It is pretty darn cheap and still relatively easy to build. I think that it is still less than $150. Also with the mini, you get authentic Toshiba JFETs As well as harris parts for the output stage. The last interesting part of it which would be a cool addition to a video, There is a jumper to switch between 3rd harmonic dominant distortion or 2nd harmonic dominant distortion.

Do you do repair work on vintage audio gear? I have a mint condition H. H. Scott 130 preamplifier that needs a selenium rectifier replacement and an under chassis electrolytic capacitor replacement to maintain the cosmetic appearance of the unit. Please advise as to cost ... -Edward

Interesting - thx 🙏

Which Input-Fet did you use on the China Boards? The Pinout between 2SK246 and 2SK170 is different. If you used a 2SK170 as in the schmeatic you need to turn it 180°. Tha ACA is a neat little amp and totally beginners friendly because of the low voltages involved and it sounds very good. It also leaves room for improvement when you want to dig deeper without the risk of burning $$$. The downside for some indeed is the low wattage so it runs better on high sensitive speakers or you build the double version.

I thought it was supposed to be a 8watt amp?

Can you describe the workings of the circuit? I think I see upgrade paths via passive parts subs in the signal path. I have a xa25 and aleph 3 and hope to get a sit3. Nelson helped me do some mods on the xa25 it’s simply the most transparent amp I ever heard.

I like the look of this amp! Really cool. The thick aluminium front panel looks good. But it's 1 stage short! I'm used to the output being in phase with the output, not inverted. I dunno; 3 watts at 3% distortion? Pretty lousy for a transistor amp. Being a transistor amp it would have mostly the objectionable odd harmonics, which are much worse than the even harmonics of valve amps. The frequency domain readout of the harmonic distortion looks pretty awful. Are they even & odd harmonics? - I can't read the small writing on the screen. What's this talk of a switch to change between even & odd distortion? I've never heard of that before? I don't get the extensive heatsinking. A laptop psu of 19V @ 3A is only 60 watts maximum. This is a push-pull amp so why would it get so hot? It's not a class A amp! The 'soft clipping' is weird & suspicious. What might be the damping factor of this amp? I have not built or used one of these amps, but I would definitely not buy TWO of them in order to get the 'best' sound out of them. Waay too much hype & bs around this amp. I don't think it should be elevated to a status any higher than being just a cheap, easy-to-build power amp & that's it. I like the cabinetry, but I would build something much better into it.

Hi Stephe, did you build the bnc to rca board for the suite. I'm asking because I just got the analog discovery and the instructions from the suite says to make your own. Thanks!

LOL Well we've all pushed the wrong button, clipped the wrong lead SOME time ;) Nelsons made some nice things, But I'm not so sure this is one of them - - ok, it might sound clean and pure driving a single driver fullrange, but its not really a low impedance source do drive a speaker with an Xover in IMHO Though I like tube things, and low feedback amps its not quite my cup of tea.. I guess the idaea was a quick easy project you can run from a laptop power unit?

3-4wrms amp, what speakers would a person hook up to these amps? In my experience, using ferrite magnet speakers with an SPL efficiency of 85-92dB left me needing more volume. How does one overcome the inefficiency of todays speakers. Is using AlNiCo magnet speakers of yesteryear the solution?

The new boards are supposed to run off 24 volts it was a previous model that ran on 19

use a sig gen to see if the input and output are to me the output looks like it should be positive ill ask nelson

I may be being really stupid here...but what is the point of this amp? Only it has no volume control, so not really a pre-amp, yet really low power, especially being solid state, I just don't get what this thing is meant to be used for.🤷 Does it not have headphone output, seems like that's more a plausible use case for this thing. I get extremely low wattage tube amps, but this just seems really oddball.

hi,can you help us do a review on youtube