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Pass ACA Class A Power Amplifier Review

amirm

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#1
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:

Pass ACA Class A Amplifier Audio Review.jpg

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:
Pass ACA Class A Amplifier Back Panel Connectors Audio Review.jpg
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.

Amplifier Audio Measurements
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:

Pass ACA Class A Amplifier Audio Measurements.png


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:
Best stereo amplifier tested.png


Let's dig into distortion spectrum at lower power level of just 1 watt:
Pass ACA Class A Amplifier 1 watt Harmonic Distortion Audio Measurements.png


Second harmonic is more dominant than at 5 watt but it doesn't stay there that long. Here it is at 2 watts:

Pass ACA Class A Amplifier 2 watt Harmonic Distortion Audio Measurements.png


Our 32-tone signal resembling "music" paints an ugly picture as a result:

Pass ACA Class A Amplifier Multitone Audio Measurements.png


I was pleasantly surprised how quiet the unit is though:
Pass ACA Class A Amplifier snr Audio Measurements.png


Frequency response is also very well extended:
Pass ACA Class A Amplifier Frequency Response Audio Measurements.png


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:
Pass ACA Class A Amplifier Power into 8 ohm Audio Measurements.png


Oh gosh again.... Only 7 watts of power with SINAD of 38 dB?

Things don't get much better with 4 ohm load:

Pass ACA Class A Amplifier Power into 4 ohm Audio Measurements.png


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:
Pass ACA Class A Amplifier THD vs Level vs Frequency Audio Measurements.png


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:
Pass ACA Class A Amplifier THD+N versus Frequency Audio Measurements.png


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.

EDIT: 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.

EDIT 2: See this link for bridged mode measurements: https://www.audiosciencereview.com/...ass-a-power-amplifier-review.9741/post-262036

Listening Tests
Is this thing on? 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.

Conclusions
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 please donate generously using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 
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amirm

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#4

Cahudson42

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#8
More than once I almost bit on getting one of these kits. But fortunately, I always realized there were a lot of better things to do with $300. Still, when you see and hear Mr. Pass, you can see why the kit is popular, and his 1st Watt and Pass Labs stuff sells..


For $22 more you can get a Yamaha as301:
Yamaha A-S301BL Natural Sound Integrated Stereo Amplifier (Black) https://www.amazon.com/dp/B00MXUCN0A/

Hope Amirm tests one of the as301/501/701/801 sometime soon:)

And this for $130:
Yamaha R-S202BL Stereo Receiver https://www.amazon.com/dp/B01EMQI2CU/


Amirm, great to see a 'break' from DACs for some amps!
 
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tomchr

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#9
Wow... Yeah. You won't make best buddies with the DIY crowd with a review like that. Reality bites sometimes. :) Thanks for the review.

I know it's possible to bridge the ACA. I would be interested in seeing the FFT and THD+N vs output power for bridged mode. Many believe bridged or balanced provides better performance, and I can make a theoretical argument for why that'd be the case for a high-distortion amp. Thus, this test would provide some insight, which would be of interest for regular readers here.

Tom
 

amirm

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#10
Is the distortion audible?
It would not get loud enough for me to tell. See the listening test section. I would be shocked if detail was not lost/sound did not get bright though...
 

tomchr

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#11
Is the distortion audible?
I had the opportunity to compare the ACA (bridged) to my Modulus-686 last summer at the Vancouver Island Audio Fest. Compared with a precise amp (the Modulus-686 provides 90 dB better THD than the ACA) ACA definitely compresses the sound stage and muddies up details. I know at least one participant in that Audio Fest who ended up getting one of my Modulus amps instead of the ACA.

Tom
 

miero

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#12
@amirm Does it have high distortion also to a higher impedances? Maybe it could be used as a power amplifier for headphones :)
 

amirm

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#13
I know it's possible to bridge the ACA. I would be interested in seeing the FFT and THD+N vs output power for bridged mode.
I thought I had a cable for this but can't find it. Will need to make another one to test it.
 

amirm

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#14
@amirm Does it have high distortion also to a higher impedances? Maybe it could be used as a power amplifier for headphones :)
It would take some screwing around to mate my headphone jack with speaker terminals. If I have time (and motivation :) ) I will test.
 
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#17
Thank you so much Amir. When I read what you write, it is clear, I can understand your reasoning, and I can follow the discussion. Whenever I read something where I can’t readily follow what’s being said, I become suspicious. You have filled huge gaps in my understanding.
I believe Dr. Pass holds a PhD in physics, so he’s obviously an erudite fellow, but by his self description his technical writings are primarily for entertainment purposes. Your writings are practical and useful.
 

Sal1950

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#19
The amps designed to have a particular sound and be used with high efficiency speakers.
To my mind a waste of money, kit or not. If you really want to build something, the same amount of
money will get you a 50wpc Akitika GT 102,
https://akitika.com/measurements.html

  • Output Power: greater than 50 Watts per channel into 8 Ohms
  • Small Signal Bandwidth: typically wider than 5 Hz to 100 kHz at -3 dB points
  • Slew Rate (typical) 14 Volts/microsecond
  • Damping Factor @ 1 kHz wrt 8 Ohms: >80
  • Signal to Noise Ratio: typically 136 dB below 50 Watts into 8 Ohms, A-weighted, referred to a shorted input.
  • Harmonic Distortion: typically 0.003% at 50 Watts into 8 Ohms at 1 kHz. Clipping occurs at a bit more than 60 Watts into 8 Ohms at 1 kHz.
  • Intermodulation Distortion: 0.004% typical, SMPTE 4:1, 60 Hz, 7 kHz
  • Separation: >90 dB at 1 kHz, >70 dB at 10 kHz
  • Input Impedance: 51 K Ohms
  • Sensitivity: 1 Volt RMS input produces 20.59 Volts RMS output
  • Dimensions: 15” Wide x 10” Deep x 4.5” Height (includes height of the feet)
  • Input Power: 120 VAC 60 Hz, IEC connector (power cord supplied)
  • Input Power Fuse Rating and type: medium acting 3 Amps or 3.15 Amps, 5x20 mm
  • 220-240 Volt operation also possible with the V240 kit, which uses a 1.5 Amp fuse
  • Weight: 15.5 lbs
  • Shipping Weight: 19 lb

 

bunkbail

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#20
The amps designed to have a particular sound and be used with high efficiency speakers.
To my mind a waste of money, kit or not. If you really want to build something, the same amount of
money will get you a 50wpc Akitika GT 102,
https://akitika.com/measurements.html

  • Output Power: greater than 50 Watts per channel into 8 Ohms
  • Small Signal Bandwidth: typically wider than 5 Hz to 100 kHz at -3 dB points
  • Slew Rate (typical) 14 Volts/microsecond
  • Damping Factor @ 1 kHz wrt 8 Ohms: >80
  • Signal to Noise Ratio: typically 136 dB below 50 Watts into 8 Ohms, A-weighted, referred to a shorted input.
  • Harmonic Distortion: typically 0.003% at 50 Watts into 8 Ohms at 1 kHz. Clipping occurs at a bit more than 60 Watts into 8 Ohms at 1 kHz.
  • Intermodulation Distortion: 0.004% typical, SMPTE 4:1, 60 Hz, 7 kHz
  • Separation: >90 dB at 1 kHz, >70 dB at 10 kHz
  • Input Impedance: 51 K Ohms
  • Sensitivity: 1 Volt RMS input produces 20.59 Volts RMS output
  • Dimensions: 15” Wide x 10” Deep x 4.5” Height (includes height of the feet)
  • Input Power: 120 VAC 60 Hz, IEC connector (power cord supplied)
  • Input Power Fuse Rating and type: medium acting 3 Amps or 3.15 Amps, 5x20 mm
  • 220-240 Volt operation also possible with the V240 kit, which uses a 1.5 Amp fuse
  • Weight: 15.5 lbs
  • Shipping Weight: 19 lb

I hope amirm will measure this kit or @tomchr's LM3886DR sometime in the future.
 
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