Pass ACA Class A Power Amplifier Review

[RichB]

I built this amp many moons ago and enjoyed the experience. I thought it sounded okay but of course the measurements are an eye opener. The paradox of High-End Audio.

Steve Guttenberg just posted a video about the Amp Camp Amp. It captures the total experience of building this amp very well. I have always liked Steve and his reviews even though some of his positive reviews (Border Patrol DAC - an audio scam IMHO) are questionable. He will at least review low cost gear and acknowledge that good audio doesn't need to be expensive.

Audio does not need to be expensive to measure poorly

- Rich

 

[peanuts]

Robert and Steve

what should you pair with the worst amplifier? the worst speakers ofcourse, ZU baby! this one even has a 2" exit 500hz capable driver as a 10khz 1st order supertweeter lol.

 

[richard12511]

I built this amp many moons ago and enjoyed the experience. I thought it sounded okay but of course the measurements are an eye opener. The paradox of High-End Audio.

Steve Guttenberg just posted a video about the Amp Camp Amp. It captures the total experience of building this amp very well. I have always liked Steve and his reviews even though some of his positive reviews (Border Patrol DAC - an audio scam IMHO) are questionable. He will at least review low cost gear and acknowledge that good audio doesn't need to be expensive.

I can't help but this review is completely dominated by Steve's huge bias for Nelson Pass designs. I don't think this amp would hold up well at all in a blind test, given that it's the worst amp ever measured here. I think even Nelson acknowledges that this is a bad amp, and that it's designed more for teaching people how to build amps, and not really designed to be a quality amp.

 

[anmpr1]

...even Nelson acknowledges that this is a bad amp, and that it's designed more for teaching people how to build amps, and not really designed to be a quality amp.

If that's true, if that's what NP thinks, then how sad is that? I mean, if you're going to go to all the trouble to get a group together and show them how to solder, why would you not have them build something useful and helpful?

Compare with the thinking of David Hafler. Or Stu Hegeman. Or Hermon Scott and Harry Ashley. Or later designers such as Jim Bongiorno and Marshall Leach. I have one of Dave's 60 year old kit amps that runs circles around this little Ampcamp toy. Good grief!

 

[Trousersnake]

I recently built the 1.8 ACA version ( bored during shut down ) and I have to say I think it sounds really good. I have many amps to use in different systems ( krell , Luxman , Nakamichi PA 7 ) and keep going back to the pass diy. I think it’s the top end that is so appealing. Is it a monster power amp ? No but it’s more than loud enough and detail is superb. This little amp images amazingly well and has pretty good base surprisingly ( Krell pre amp) . I will say that when the amp was first complete and I did the first test it didn’t sound as good until after proper biasing was completed, and that took some trial and error for the completed product I use today. So that’s my 2 cents for someone new to DIY and not new to this rewarding disease of audiophiles.

 

[ta240]

I recently built the 1.8 ACA version ( bored during shut down ) and I have to say I think it sounds really good. I have many amps to use in different systems ( krell , Luxman , Nakamichi PA 7 ) and keep going back to the pass diy. I think it’s the top end that is so appealing. Is it a monster power amp ? No but it’s more than loud enough and detail is superb. This little amp images amazingly well and has pretty good base surprisingly ( Krell pre amp) . I will say that when the amp was first complete and I did the first test it didn’t sound as good until after proper biasing was completed, and that took some trial and error for the completed product I use today. So that’s my 2 cents for someone new to DIY and not new to this rewarding disease of audiophiles.

I think your preamp may be helping a lot too. The gain is pretty low on this amp when used in stereo so without a strong signal going in it needs to be really cranked up which increases the distortion a lot. Putting something with gain before mine made a pretty big difference in the sound.

If that's true, if that's what NP thinks, then how sad is that? I mean, if you're going to go to all the trouble to get a group together and show them how to solder, why would you not have them build something useful and helpful?

Compare with the thinking of David Hafler. Or Stu Hegeman. Or Hermon Scott and Harry Ashley. Or later designers such as Jim Bongiorno and Marshall Leach. I have one of Dave's 60 year old kit amps that runs circles around this little Ampcamp toy. Good grief!

He didn't say it sounds bad he just said something along the lines of the design focused more on easy build than ultimate sound. He's a class A guy and those generally aren't easy builds with external simple power supplies that don't deal in mains voltage.

 

[bunkbail]

I recently built the 1.8 ACA version ( bored during shut down ) and I have to say I think it sounds really good. I have many amps to use in different systems ( krell , Luxman , Nakamichi PA 7 ) and keep going back to the pass diy. I think it’s the top end that is so appealing. Is it a monster power amp ? No but it’s more than loud enough and detail is superb. This little amp images amazingly well and has pretty good base surprisingly ( Krell pre amp) . I will say that when the amp was first complete and I did the first test it didn’t sound as good until after proper biasing was completed, and that took some trial and error for the completed product I use today. So that’s my 2 cents for someone new to DIY and not new to this rewarding disease of audiophiles.

Which speakers you pair it with?

 

[Trousersnake]

Which speakers you pair it with?

Tekton Design lore reference

 

[Trousersnake]

One other thing I should point out is this is my bedroom system 18’ x 18’ room and I sit 10’ from the speakers , I think it works really well in this type of situation, as for a big room ? I can’t say , but I just can’t seem to take this little amp out of my system , I have switched it out for different amps and keep going back to it.

 

[AudioTodd]

The designer invents, the engineering is tasked with creating an actual product.

A guru is just a designer and/or engineer that is adored by followers that like the products or the stories that are told (or made up) around the product.
These individuals are the ones that feel an engineer/designer needs to have an honorary name. Some stay humble others make use of their honorary 'title'.

Old, old, old...

The Girl in the Chair Story:

• A girl was placed on a chair across the room.
• A mathematician, physicist, and engineer were placed behind a line at the other end of the room. They were told they could walk to the girl but each step had to be half the size of the first.
• The mathematician immediately started wailing that it was theoretically impossible to ever reach the girl, but turned around to the blackboard babbling about limit theories and starting writing all sorts of intricate equations to see if a new proof could be found.
• The physicist cried "not in the macro world" and began mumbling about discontinuities in space-time, uncertainty principles and such, then also turned to begin scribbling on the blackboard.
• The engineer took a big step forward, then another...
• The mathematician and physicist, seeing that, turned and yelled "Don't you understand, you can never reach the girl!"
• Without pausing or looking around the engineer replied "I'll get close enough!"

And that, my friends, is why I became an engineer.

Also why an engineer should not get offended by being told they are no scientist!!!

Each discipline has its place!

 

[almico]

I just built two of these as well as a Pass B1 Korg preamp. I've been running them through a pair of Altec 604Cs and they sound simply amazing. One does fine by itself, but I get another 3dB running them bridged mono. I hooked them up to a pair of Klipsch R-51m and they sound pretty good with them as well.

FWIW, I'm used to single-ended tube amps. I have a 3W/ch KT88 cathode follower powering a pair of Altec A5 VOTT for my TV/music room. (Correct, no wife.) I get >100dB SPL with the volume knob on 5, maybe 1W. I wouldn't know what to do with 50W/ch let alone 300. The 288 CDs are 115dB efficient and the 515Bs are 98dB/W/M.

I grew up on classic rock, ELP, Yes, Floyd, Allmans, Weather Report, and much more eclectic. These days I listen mostly to acoustic music, I flinch at compression. I like 40s jazz, 60s folk even choral, opera and classical.

Yes, the Amp Camps start to break up around 6 on the volume knob, but I really don't need or want that much sound. I'm getting about 90dB on 4. Plenty. And the sound stage is huge with the 604Cs. It's just a 3D wall of music that wraps around, gives me a big hug and puts a smile on my face.

And FWIW, I spent 15 years chasing audio holy grails and slaying deficiency demons. When I built my A5s, and added Hiraga X-overs, time stood still. It was bittersweet, because in a moment, I finally found the sound I was looking for...and a major hobby died. I stopped looking for things that were wrong with my system, and started enjoying what was right about it...oh yeah, and the music. I've spent the past 10 years just listening. The only thing I've changed is tubes.

But I'm opening a 2nd coffee bar in a very small space and it needs tunes. The Altecs won't cut it. So like Al Pacino in the Godfather, "just when I thought I was out, they sucked my back in". I built these two amps for the bar and am still looking for the right speakers. Volume is not a requirement. Open, airy Class A sound is. These Amp Camps are perfect.

It's interesting how much the coffee and audio worlds share in common. Both industries have two factions, those that like to measure and quantify infinite parameters, and those that like to experience them. In coffee you have extractions yields, TDS, particle size distributions, etc. In audio its frequency curves, impedance, Signal to noise, THD, SPL, decibels etc. With both we are trying to quantify what we taste or hear. In coffee, when the rabbit hole conversation gets too deep, the inevitable question, "but how does it taste?" pops up. With audio, it's, "but how does it sound?" Numbers can direct us to weaknesses, they cannot tell us what tastes or sounds good.

My A5s measure horrifically. Big mid hump and a steady falloff after 10k. It's truly ugly. But they sound amazing and I can listen to them non-stop for days. It literally sounds like Ella is sitting in my living room and singing just to me. Ultimately, that's all I really care about.

Now, back to your regularly scheduled program!

 

[Sal1950]

In coffee, when the rabbit hole conversation gets too deep, the inevitable question, "but how does it taste?" pops up. With audio, it's, "but how does it sound?" Numbers can direct us to weaknesses, they cannot tell us what tastes or sounds good.

My A5s measure horrifically. Big mid hump and a steady falloff after 10k. It's truly ugly. But they sound amazing and I can listen to them non-stop for days. It literally sounds like Ella is sitting in my living room and singing just to me. Ultimately, that's all I really care about.

I know nothing about coffee, but I do know audio.
I never slam anyone's gear but if you feel your A5's sound good with the errors you report, they are far from reporting to you what is on the source.
You may enjoy them, that's your call, but that's not High Fidelity.
And yes, measurements can and do tell us how gear sounds, but I won't insult you with describing the sound of your system, you did that yourself.
"My A5s measure horrifically. Big mid hump and a steady falloff after 10k. It's truly ugly."

 

[amirm]

But I'm opening a 2nd coffee bar in a very small space and it needs tunes. The Altecs won't cut it. So like Al Pacino in the Godfather, "just when I thought I was out, they sucked my back in". I built these two amps for the bar and am still looking for the right speakers. Volume is not a requirement. Open, airy Class A sound is. These Amp Camps are perfect.

You need that in a coffee bar??? Our company does a lot of commercial sound and there, we opt for efficient, and cool running class D amps. If you think the average coffee drinker can appreciate any difference there, well, you and I need to talk.

 

[anmpr1]

You need that in a coffee bar???

I have no idea about @almico 's eatery, but in some places it's less about the 'sound' (really, how can anyone judge sound quality in a restaurant?) and more about ambiance with 'mystique'. A cozy atmosphere to relax and have a good time. Below is a link to a Japanese establishment (now closed as its owner left the planet). Lowther and Altec speakers, homemade tube amps, Garrard 301/Grace oil damped arm (with coins to add tracking force help)/Denon 102 mono cartridge and plenty of old jazz records. My impression was that the gear was more for exploring 'camaraderie' within a nostalgic setting.

Not sure about the electrical code in Japan-- exposed wires and high voltages might be considered a danger around all that high proof alcohol!

http://www10.big.or.jp/~dh/codo/tennai/tennai.html

I get the idea of 'pride of ownership' from something hand made. I've done that. As far as the little Amp Camp thing? If I'm going to spend time and effort with construction, I want the finished product to be something I can be proud of, I want it to be something that at least approaches practicality, and something that isn't on the borderline of ridiculous.

 

[617]

I just built two of these as well as a Pass B1 Korg preamp. I've been running them through a pair of Altec 604Cs and they sound simply amazing. One does fine by itself, but I get another 3dB running them bridged mono. I hooked them up to a pair of Klipsch R-51m and they sound pretty good with them as well.

FWIW, I'm used to single-ended tube amps. I have a 3W/ch KT88 cathode follower powering a pair of Altec A5 VOTT for my TV/music room. (Correct, no wife.) I get >100dB SPL with the volume knob on 5, maybe 1W. I wouldn't know what to do with 50W/ch let alone 300. The 288 CDs are 115dB efficient and the 515Bs are 98dB/W/M.

I grew up on classic rock, ELP, Yes, Floyd, Allmans, Weather Report, and much more eclectic. These days I listen mostly to acoustic music, I flinch at compression. I like 40s jazz, 60s folk even choral, opera and classical.

Yes, the Amp Camps start to break up around 6 on the volume knob, but I really don't need or want that much sound. I'm getting about 100dB on 4. Plenty. And the sound stage is huge with the 604Cs. It's just a 3D wall of music that wraps around, gives me a big hug and puts a smile on my face.

And FWIW, I spent 15 years chasing audio holy grails and slaying deficiency demons. When I built my A5s, and added Hiraga X-overs, time stood still. It was bittersweet, because in a moment, I finally found the sound I was looking for...and a major hobby died. I stopped looking for things that were wrong with my system, and started enjoying what was right about it...oh yeah, and the music. I've spent the past 10 years just listening. The only thing I've changed is tubes.

But I'm opening a 2nd coffee bar in a very small space and it needs tunes. The Altecs won't cut it. So like Al Pacino in the Godfather, "just when I thought I was out, they sucked my back in". I built these two amps for the bar and am still looking for the right speakers. Volume is not a requirement. Open, airy Class A sound is. These Amp Camps are perfect.

It interesting how much the coffee and audio worlds share in common. Both industries have two factions, those that like to measure and quantify infinite parameters, and those that like to experience them. In coffee you have extractions yields, TDS, particle size distributions, etc. In audio its frequency curves, impedance, Signal to noise, THD, SPL, decibels etc. With both we are trying to quantify what we taste or hear. In coffee, when the rabbit hole conversation gets too deep, the inevitable question, "but how does it taste?" pops up. With audio, it's, "but how does it sound?" Numbers can direct us to weaknesses, they cannot tell us what tastes or sounds good.

My A5s measure horrifically. Big mid hump and a steady falloff after 10k. It's truly ugly. But they sound amazing and I can listen to them non-stop for days. It literally sounds like Ella is sitting in my living room and singing just to me. Ultimately, that's all I really care about.

Now, back to your regularly scheduled program!

Where is your coffee bar?

 

[Sal1950]

The Japanese are heavily into classic horns and SET amps. Many believe it's the path to enlightenment (Zen)
Check out this antique lump.
http://www10.big.or.jp/~dh/codo/arm/arm.html

 

[Sal1950]

But I'm opening a 2nd coffee bar in a very small space and it needs tunes. The Altecs won't cut it. So like Al Pacino in the Godfather, "just when I thought I was out, they sucked my back in". I built these two amps for the bar and am still looking for the right speakers. Volume is not a requirement. Open, airy Class A sound is. These Amp Camps are perfect.

Get yourself a pair of classic Klipsch Heresy's for that task. Should offer just the sound your going for with the Amp Camps

 

[almico]

You need that in a coffee bar??? Our company does a lot of commercial sound and there, we opt for efficient, and cool running class D amps. If you think the average coffee drinker can appreciate any difference there, well, you and I need to talk.

Ha, good point. But this bar is only 300sqft in toto. There is barely enough room for counters, fridges and equipment, let alone customers. Definitely no seating!

This space is located on Main St in Doylestown, PA, right in between the two courthouses in the midst of the legal district. It will be take-out only and perfect for our Covid times. I'd like to take credit for that, but planning started before the earth stood still. I thought about pulling the plug, but I had already spent $1000s on variances and designers, when an Italian friend of mine told me an expression from his homeland: "Quando balli, continua a ballare." When you're dancing, keep on dancing.

So this little system is for me more than my customers. I will be the one spending 8-10 hours a day there for the foreseeable future. I need to have something to be able listen to decent music. And if my customers happen to start tapping their feet during their early morning pickup, so much the better. It will all be part of the experience. People can stop in for their morning mocha with Miles.

FWIW, I care very much about good coffee, I even roast my own. I just expanded that end of the business and bought a $140,000 coffee roaster, (used for $36K), to get into the wholesale/online business. Sounding more like high-end audio by the minute?

 

[almico]

The Japanese are heavily into classic horns and SET amps. Many believe it's the path to enlightenment (Zen)
Check out this antique lump.
http://www10.big.or.jp/~dh/codo/arm/arm.html

I think that arm would be a perfect match for my Presto Pirouette

Once I fell down the horn rabbit hole I never climbed out.

Get yourself a pair of classic Klipsch Heresy's for that task. Should offer just the sound your going for with the Amp Camps

Alas, Heresy's won't fit. Maybe some LS3/5a's if I can find a pair reasonably.

 

[BDWoody]

FWIW, I care very much about good coffee, I even roast my own. I just expanded that end of the business and bought a $140,000 coffee roaster, (used for $36K), to get into the wholesale/online business. Sounding more like high-end audio by the minute?

Outstanding! Where do you source your beans? I've been using a Hottop roaster for many years, and enjoy trying a few lbs of something new each time I order...

Treated myself to 5lbs of Kona that I'm working through, but my go-to's are Sumatra Mandheling, and the Tanzanian Peaberry. There is a lot of good coffee out there. Best of luck with the new shop!