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Pass ACA Class A Power Amplifier Review

mitchco

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I still have it. I will have to ask the owner to make sure he is OK with it as there is always some risk of damage.

@amirm did the owner get back to you to see if you can re-bias the amp? Without knowing if the amp is biased properly or optimally, we don't know if this is the lowest distortion profile of the amp or not...
 

Xulonn

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For what it's worth I'm going to be building Tom's Modulus-86 next.

How did you level match for your listening tests? Did you match within 0.1dB? Were the tests at least semi-blind? How did you manage the psychological influences? You apparently expected the Pass ACA to be "distorted" as in "harmonically rich" and it was.

Your comments reminded me of my reactions to retail salon high-end system auditions in the 1980s. I found many demos in that era to sound dry and analytical as audiophiles and hi-end audio salesman thought they were emulating "live" vocal and small ensembles. In reality, most of those recordings were made in dead silent studios with completely different acoustics than the venues where I actually experienced such music. To this day, I take issue with audiophiles claiming that such recordings can be made to sound "just like live" in the sense of a real world live performance with an audience.

Many such recordings can create an illusion of the performers being right there in your listening room, but that illusion is created by talented and skilled recording and mastering engineers who can be "artists" as much as the musicians themselves, because they are the ones who actually "create" the recordings. I have experienced many listening sessions - at my home, in stores, and at the homes of friends - that create an incredible feeling of "being there." But it was never the same as going to a small venue and listening to live performances reinforced with Electrovoice, Peavey or similar monitors.

The closest I have ever come to live and recorded sounding similar is with live events in small venues with acoustic instruments and no electronic amplification. And no audio system has ever reproduced perfectly the concerts I have experienced over the years by attending the Chicago Symphony Orchestra, San Francisco Symphony Orchestra or S.F. Philharmonia Baroque. Or the acapella men's group, Chanticleer, singing in a small historic church, a California Mission cuurch, or a cathedral (where I sat next to the group's recording person, who was in the aisle seat about 10 rows from the front, using a digital tape recorder and a pair of microphones mounted on a skinny pole about 8 feet above our heads).

I walk the line between objective and subjective by accepting the objective as "what is physically there with respect to an audio signal or sound waves," and the subjective, which is my personal experience and/or "perception." However, I draw the line at claiming that differences in my "perceptions" are representative of the character of the actual sound waves that vibrate my tympanic membrane, or claiming that I can hear physically non-existent "differences" in sighted listening comparisons, differences that scientific research has demonstrated cannot be detected.

It will be interesting to hear about your experience with the Modulus amp, both about the assembly process and your subjective comparison of the sound to that of the Pass ACA. Unlike DAC's however, which shouldn't produce audible differences if properly designed and manufactured, the "sound" of those two very different amplifiers will very likely be influenced by their synergy with your speakers and your room.
 
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amirm

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Just out of curiosity amim how far is your listening position from your speakers? Where are your speakers in the room? I don't remember seeing any numbers to give context to some of your subjective review.
For this test, I did NOT use my regular system. I hooked up a different bookshelf speaker on my bench and sat maybe 2 to 3 feet from it. There simply was no power, bass response, etc.
 
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amirm

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@amirm did the owner get back to you to see if you can re-bias the amp? Without knowing if the amp is biased properly or optimally, we don't know if this is the lowest distortion profile of the amp or not...
Yes, he has given me permission and is anxious for results. I just have not had the time to do it. :)
 

KSTR

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Please note that the single trimmer "P1" as per schematic on page 2 of this thread does *not* adjust output stage bias current. Bias current is established in a fixed fashion with a simple control loop that measures the voltage drop accross the output degeneration resistors. The bias current is not extremely tight controlled but "stable and predictible enough for the purpose".
The trimmer mainly affects zero signal quiescent output *voltage* (before the coupling cap) and that obviously has huge impact on max power before clipping. Ballpark start value is half-supply but one surely needs tweaking to get late and symmetrical onset of clipping, visualized with an oscilloscope / waveform display.
 

Thomas savage

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I don't understand this,

Amir measures a DIY build amp that comes up better than spec.. and generally a deal of respect for Mr Pass has also been expressed .

Folks go nuts bashing the review , forum and amir with anything they can find to hand.

Wtf lol
 

mitchco

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Personally, I am not going nuts :) To me it is as expected, so I am not surprised. As @KSTR mentions, just want to check that the P1 trimmer is set as best as possible. Probably won't lower distortion, (my mistake I was thinking of another Pass amp) but may get a bit more power before onset of clipping. Won't know for sure until checked.
 

Thomas savage

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Personally, I am not going nuts :) To me it is as expected, so I am not surprised. As @KSTR mentions, just want to check that the P1 trimmer is set as best as possible. Probably won't lower distortion, (my mistake I was thinking of another Pass amp) but may get a bit more power before onset of clipping. Won't know for sure until checked.
Ha ha you are very hight on the SINAD chart , put most ( me certainly) to shame Mitch.
 
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amirm

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Personally, I am not going nuts :) To me it is as expected, so I am not surprised. As @KSTR mentions, just want to check that the P1 trimmer is set as best as possible.
The owner has repeatedly assured me he has set the bias correctly and has even given me the page and line how this is done. Now, how stable the pot is after travelling to me, I don't know. I will measure.
 

tomchr

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I had a similar thought re Faraday cage.
I wasn't thinking Faraday cage, actually. There will always be some openings for RF to creep through. I was thinking in terms of reflections within the chassis and resulting return loss, hence my comment regarding VSWR.

Tom
 
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amirm

amirm

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OK, I measured the bias and looked at distortion.

The measurements read 12.1 volts but as soon as I touched the potentiometers, the voltage went down to 12 volt. So the bias was set correctly.

I then changed the bias from 10 volts to 16 and it made at most 1 dB difference in distortion. And the 12 volt setting seems to be the optimal one.

Measurements were made at 1 watt by the way and SINAD would only vary from 41.8 to 42 and change, or 40 and change.

Summary
The ACA Amp as built is operating optimally and the bias instructions are correct. Our measurements had bested prior published ones anyway so we were good then, and are good now. :)
 

DonH56

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I'm not going nuts. I've been there for a long time.
 

pma

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I have built something similar, 18 years ago, and published at Rod Elliott's pages

https://sound-au.com/project83.htm

With 40V supply voltage, it was able to give a bit more power, 17W/8ohm. It was a power follower with voltage gain close to 1. Distortion characteristics were probably similar as that of the ACA. They depended on power supply voltage, quiescent current and load impedance.

Reading this thread and being curious, I have wiped out the dust of that project (better say I had to re-build it on an universal PCB), connected to 24Vdc power supply, set idle current to 1.5A and measured distortion with 4ohm load. The result of THD vs. output voltage is attached, I think it is similar to Amir's measurement of the ACA.

1573739284981.png
P1030422-1.jpg
 

maty

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The key is that they work up to 1% THD like the tube amps -> very high sensitivity speakers if you want to listen to very good recordings (DR15) in mid / far field. With modern commercial music (with < DR10), near field & in phase, only high sensitivity.
 

pma

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The power follower is not that bad as expected in CCIF IMD 19+20kHz distortion. Attached is the plot for 5Wpeak/4ohm. It is fast and not prone to slew induced distortion or switching distortion. However see power supply related lines.

1573741281844.png
 

tomchr

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However see power supply related lines.
Also see all the IMD products caused by the mains hum. If you cleaned that up, I bet you'd find a dramatic increase in sound quality.

Tom
 

pma

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Also see all the IMD products caused by the mains hum. If you cleaned that up, I bet you'd find a dramatic increase in sound quality.

Tom
You are talking the same thing. IMD mains related lines. The reason is very low PSR rejection of such circuits, together with high PSU ripple due to high idle current. That's why ACA uses SMPS instead of linear PSU (transformer, bridge rectifier, filter). SMPS does not have mains line issues, it may have HF EMI issues. You may also go with high filter capacitances for linear supplies, which helps, but again it is a kind of overkill, similar to produced heat.
However, it was the project just for fun. Same as ACA, same as ZENs. These are not serious amps, these are toys, or better say excellent heaters where sound is just a byproduct :D. So it goes.
 
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