This is a review and detailed measurements of the Pass ACA Class A stereo power amplifier. It is a DIY design by the famous designer, Nelson Pass, meant to be starter amplifier for people new to audio electronics to build. There are many kits available. The one loaned to me from a long time friend of the forum. The kit seems to cost around US $320 or so.The ACA sample I received comes in an attractive black case:The design is a Mosfet output stage with a single stage buffer. Very modest level of feedback (9 dB?) is provided to keep distortion somewhat under control. As you will see, the gain is very low requiring full 2 volt of DACs to drive it to max power.Here is the back side:I did not mess with it but I am assuming the single XLR allows bridged mono operation.Power is provided by a very beefy external switching power supply which outputs 24 volts with up to 5 amps or 120 watts. All of that is put to "use" in generating copious amount of heat. I was told by the owner to let it warm up for a few minutes. As you will see in the measurements, this actually made the performance worse, not better.Overall, this is a very nice package if it performs.As usual, we start with our dashboard with 5 watt output into 4 ohm. Normally this is well below the max power but not in this case as we see later:Gosh... What happened to aiming for high fidelity? This is a distortion-factory if there ever was one. The notion that it is mostly second harmonic does not hold true either. Welcome to the bottom of our amplifier ratings:Let's dig into distortion spectrum at lower power level of just 1 watt:Second harmonic is more dominant than at 5 watt but it doesn't stay there that long. Here it is at 2 watts:Our 32-tone signal resembling "music" paints an ugly picture as a result:I was pleasantly surprised how quiet the unit is though:Frequency response is also very well extended:You may be able to make an AM radio out of this thing.Back to ugliness, here is our power versus distortion+noise into 8 ohm load:Oh gosh again.... Only 7 watts of power with SINAD of 38 dB?Things don't get much better with 4 ohm load:We now have distortion climbing into near 10% category at max power of just 7.4 watts/channel.There was little variation of response with respect to frequency:Distortion level is so high that it is masking secondary sources of it.Making sure this unit matches the reference, I compared them to what Nelson Pass has published:We are getting very good agreement. So this is what the unit is supposed to do. Nelson stays with 1 watt output which shows the performance to be better.: I noted on the dashboard that the gain was much lower than the spec. I am told that version 1.6 which I tested now has a spec of 10 dB for gain so we are good.: See this link for bridged mode measurements: https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-262036 That is the expression that immediately came to mind after I hooked up the ACA amp to my Sonus Faber Concertino bookshelf speaker. Granted, the Concertino is spec'ed at 86 dB efficiency but come on, I could barely hear it. There was no bass naturally. But not much above that either. And this was with my Topping DX3 Pro at full volume.For grins, I plugged the US $74 tiny little SMSL SA100 Amplifier which I recently reviewed into the setup. Now the speakers came alive, with lots of bass, great dynamic range, and detail. Yes, the amp started to cry when I turned it up much but until that point, it left the ACA so far in the dust it was not even funny.As a starter hobby amplifier to get some sound coming out of a speaker, with relatively few components, the Pass ACA Class A Amp does the job. Build it, have fun with it and then put it aside and get a proper amplifier. Amplifiers need to have a lot more power. And much less distortion. We are not even close here folks.Do I need to tell you that I can't recommend the Pass ACA for hi-fi listening? I hope not.------------As always, questions, comments, recommendations, etc. are welcome.I suspect the above review is going to make me many enemies in DIY audio circles. So I need hire a bodyguard and get some serious insurance for my bodily parts from Loyd's of London. Both cost good amount of money so pleaseusing: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/