Pass ACA Class A Power Amplifier Review

[pma]

Excellent rejection of power supply mains frequency and its multiples should be mentioned. When I did several tests with mathematically added non-linear distortion with similar profile as of this Pass amplifier, almost nobody or just nobody was able to tell the difference in the ABX listening test. Correlation of SINAD with subjective sound evaluation is highly questionable.

 

[maty]

[PDF] http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

It need loudspeakers at least with 98 dB/W/m. Really, more than 100 dB/W/m.

https://www.doctorproaudio.com/content.php?2273-calculators-proaudio-sound-dmx&langid=1#calc_spl

I want: 80 dBSPL continuous + DR15 = 95 dBSPL

* 98 dB/W/m, 2 watts, random phase, 3 m -> 94.5 dB


With 0.1 % -> near to 110 dB/W/m and... true 8 Ohms.


Therefore, some measurements made by amirm for this amplifier are inappropriate, which will only works properly with very few speakers. And the vast majority of those who have built these amplifiers do not use appropriate speakers, unless they listen in the near field and in low volumes, with commercial music with very/little dynamic range (DR < or << 10 dB).

 

[VintageFlanker]

Is the distortion audible?

With so much of it: definitely.

 

[JohnYang1997]

Excellent rejection of power supply mains frequency and its multiples should be mentioned. When I did several tests with mathematically added non-linear distortion with similar profile as of this Pass amplifier, almost nobody or just nobody was able to tell the difference in the ABX listening test. Correlation of SINAD with subjective sound evaluation is highly questionable.

Distortion over 0.x% is quite audible in change in sound. One may still not pick out as distortion but the tonality and texture change. 0.01% is the point no coloration is perceived. Let alone that the amp is not even loud enough. Class A is really doing no help in any way.

 

[restorer-john]

I think people need to consider that the design was clearly just a proof of concept, junk-box amplifier and likely never intended to take on this highly profitable life it has subsequently been saddled with. Kitset vendors have climbed aboard the DIY gravy-train, exploiting the ignorant 'maker-movement', peddling the parts in an over-priced Italian case and now it costs an almighty US$329, which is a sad joke and does nothing to further perceived value in build-it-yourself projects.

The original BOM is here (for NP's 2012 PDF)

(per channel)

Basically, the entire concept of building a little junk-box amplifier as an afternoon project for fun, has been bastardized by profiteering hangers-on and that's hardly NP's fault. His gifts to the DIY community are numerous, respected and sometimes polarizing, but universally appreciated.

In terms of performance, it's a joke THD wise- we all know that, but if it cost just $30 for the parts you didn't have in stock, and the rest you pulled out of your junk bin or scavenged, it was no doubt a pile of fun.

 

[restorer-john]

Therefore, some measurements made by amirm for this amplifier are inappropriate

Maty, the measurements are implementation agnostic.

The amplifier is tested and its performance parameters are reported without fear or favor. It is up to the reader to then determine appropriate usage scenarios in order to exploit the characteristics of the device.

 

[RayDunzl]

At least it maybe doesn't have any crossover distortion.

https://guides.diyaudio.com/Guide/Amp+Camp+Amp+V1.6+Build+Guide/5

 

[pma]

Distortion over 0.x% is quite audible in change in sound. One may still not pick out as distortion but the tonality and texture change. 0.01% is the point no coloration is perceived. Let alone that the amp is not even loud enough. Class A is really doing no help in any way.

Do you have any proof based on ABX tests with music samples (not single sine waves) with known and defined distortion, defined non-linear transfer function? If yes, please post it or give a link to a public source. Nothing else should be applied than a transfer function non-linearity, it means no changes of frequency response and residual noise would apply.

 

[maty]

This amplifier only yields 0.1% with 0.1 watts. And 1% with 2 watts. << amirm measurements.

 

[martijn86]

So looking at measured performance, this is mostly a DIY learning experience with a 'technically working' result. Not much to actually want to own as your main amplifier (you'd think). I do however think that this is not incompetent design but rather on purpose. They control PS noise and noise in general very well but seem to be targeting an audience that like their amp to sound 'wooly' with lots of harmonics. This would then be a successful failure, like drifting, you loose traction all the time, set no fast laptimes, but you do it in style. The only thing missing in that analogue is the fact that drift cars tend to make ample power.

 

[audimus]

I think people need to consider that the design was clearly just a proof of concept, junk-box amplifier and likely never intended to take on this highly profitable life it has subsequently been saddled with. Kitset vendors have climbed aboard the DIY gravy-train, exploiting the ignorant 'maker-movement', peddling the parts in an over-priced Italian case and now it costs an almighty US$329, which is a sad joke and does nothing to further perceived value in build-it-yourself projects.

The original BOM is here (for NP's 2012 PDF)

View attachment 38106
(per channel)

Basically, the entire concept of building a little junk-box amplifier as an afternoon project for fun, has been bastardized by profiteering hangers-on and that's hardly NP's fault. His gifts to the DIY community are numerous, respected and sometimes polarizing, but universally appreciated.

In terms of performance, it's a joke THD wise- we all know that, but if it cost just $30 for the parts you didn't have in stock, and the rest you pulled out of your junk bin or scavenged, it was no doubt a pile of fun.

View attachment 38107

Sigh, you just cut short at least 10 pages of pinãta bashing including snarky comments on NP’s choice of clothes and hairstyle and his education and talking style, etc., etc by introducing a dose of reality. Taking all the fun out of the forum...

 

[PierreV]

It's a bit like a liquor store - you get to pick your poison. But there isn't a trace of dishonesty about it.

 

[MZKM]

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

Stereophile measured it; excellent performance except super high THD in the bass, but we are less sensitive to that.

 

[restorer-john]

Sigh, you just cut short at least 10 pages of pinãta bashing including snarky comments

And to be fair, Amir was perfectly up front about the ACA's intended purpose:

It is a DIY design by the famous designer, Nelson Pass, meant to be starter amplifier for people new to audio electronics to build.

We aren't showcasing bad design or poor performance against spurious claims here. But that said, this little exposé certainly does put into perspective what it can, and can't do.

 

[MZKM]

Is the distortion audible?

Axiom Audio did a THD audibility test on some people, at 1kHz they found -30dB to be roughly the threshold for music (albeit many factors). Amir just posted the warmed up chart showing ~28dB; so yes, it can be audible.

Take note that many people here like to strive for absolute inaudibility, rather than inaudibility with most music. A DAC with 90dB SINAD is usually more than enough, but since music varies, it’s good for to aim for absolute inaudibility.

 

[miero]

@amirm Would you add a link to Nelson's article about this amplifier into your review:
- http://www.firstwatt.com/pdf/art_amp_camp_1.pdf

 

[dropbear]

I am trying to conduct something meaningful and wise into my post, but I simply cannot.

Take this as an opinion of someone outside DIY, but I am outraged, a whole point of DIY kits should be that for very little money (if you are willing to learn as you go) you can build something exceptional (maybe not measurements-wise) for the price, but this seems like a waste of money, time and effort. A betrayal of the whole DIY concept.

The shit on the product description page was merely a bonus, pardon me, I need to go puke now. If I got my feet wet into the audio this way as portrayed by Mr. Pass, I would probably end up as one of those buying audio cables for 1000$ because "The first nanometer of the copper grain of each strand is the most important grain", I believe Audioquest would wholeheartedly agree. I simply cannot give it a pass...

 

[Ralf Stocker]

This device is an insult to the AP! Well, the upper measuring range must also be approached.

 

[solderdude]

Since this is a DIY device no one would be stopping an owner to rip out the guts and use the otherwise nice enclosure to build yourself a better performing amp if one so desires.

 

[maxxevv]

Since this is a DIY device no one would be stopping an owner to rip out the guts and use the otherwise nice enclosure to build yourself a better performing amp if one so desires.

Think a budget set of 50~100W IcePower modules would definitely measure better when put into that case.