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Pass ACA Class A Power Amplifier Review

anmpr1

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1)...many (not all!) of the 1970s Yamahas used a switch rather than jumpers to tie the preamp and power amp sections together. The switches will cause some fun of their own at this late date (i.e., they get dirty, of course, but the audible symptoms range all over the map).

2)I do hope Peter Aczel wrote "yakback" and not "yackback". ;)

3)...A friend used his CA-600 integrated amp as a preamp only... for his Polk Audio Monitor Series Model 10 loudspeakers (e.g.).

1) The Yamaha 1010 spec sheet claims jumper bars can "often pick up interference" and that's why they used shielded switches. Maybe. I guess if you live next to a Clear Channel AM station, or your neighbor is in to the 10-4 Good Buddy CB broadcasting scene... LOL

Yamaha states the pre out are active whether the amp section is uncoupled. This feature is present on the two analog outs on my Benchmark DAC3 HGC, and is useful for connecting a sub. Subs weren't popular during 1010 days, but Yamaha suggested the feature could be used for oscilloscopes and/or a remote power amp. Nothing wrong with flexibility.

Using the pre section of an integrated amp with an external power amp is certainly an option. It would be better if one could 'turn off' the power amp section when doing this. But I don't think that was possible for any of them.

2) Indeed he did. Of course Peter was more familiar with animals, possibly because of his hobby as a dog breeder. Not only that, but being a long time Madison Ave Adman (some would say Madman) he was much more literate and could spell better. :cool:

3) For those who might want to look down on Polk, or don't recall their early products, your friend's Monitor 10s were nothing to laugh at. Very respectable for the day.
 

mhardy6647

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3) For those who might want to look down on Polk, or don't recall their early products, your friend's Monitor 10s were nothing to laugh at. Very respectable for the day.
I've owned and still do own a pair of Monitor 7As (bought from Polk's first retailer, Soundscape in Baltimore) since 1978. I ain't sneerin'. Great products for the money in those days. Matt Polk & I share an alma mater -- although he was a few years ahead of me.

DSC_5417 by Mark Hardy, on Flickr

I also have a little... ahh... thing... for vintage Yamaha products of that era -- in full disclosure.

DSC_0241 by Mark Hardy, on Flickr
 

anmpr1

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I also have a little... ahh... thing... for vintage Yamaha products of that era -- in full disclosure.
Sure. It's an obsession. But you probably won't go to jail over it. And don't worry about what your neighbors say. All neighbors talk about the 'guy nextdoor'! o_O

Otari and Sherwood? Or Eico? Nice mix...
 

SIY

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Soundscape, that brings back memories. Cold Spring Lane, if I recall correctly.

The success of Polk had more to do with Sandy Gross than anything else.
 

scott wurcer

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It's a blatantly deceptive plot.

Not only do we know the residual is likely 60-80dB down and amplified by the output of the distortion meter for the "scope output jacks" (probably 1000 times at least for that X/over image), we also don't know the second trace's sensitivity (V/Div).

Without signal processing to get rid of the noise, I don't reckon I could get a clean X/over distortion residual even if I deliberately drop the bias right down for a pretty pic, but I'll have a go now I've repaired my distortion meter's self cancelling range switch array. (The most fiddly thing I have done this year)

The Benchmark plot is really no better than the hundreds of "night and day" "with and without" pictures in HiFi brochures over the years.

Pioneer (July 1985)
View attachment 67937
Yes for sure, comparing to a well designed class A/B amp with no crossover distortion would hardly make a point. BTW how does the non-switching come into this (I might have just missed it)?
 

mhardy6647

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Soundscape, that brings back memories. Cold Spring Lane, if I recall correctly.

The success of Polk had more to do with Sandy Gross than anything else.
Yes, sir. Walking distance (if one were desperate enough) from my alma mater -- but always better to drive. Would be a long walk, especially carrying a hifi component.

Still there AFAIK. 406 West Cold Spring Lane in Roland Park. Across the street,in those days, a good record store (Record and Tape Collector... I think) and one of my favorite Baltimore bars/restaurants: Alonso's. Oh, and Jay's Smoke Shop shared a building with Soundscape.
 

restorer-john

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Here is another random amplifier. Rotel RA-970BX left channel. Right was the same.

1W@8R. Same scale (2V/Div) for each trace. Distortion residual from scope output (analog THD meter) after 1KHz notch -60dB (x1000).

DSO:
1591865679568.png


Same signal on the trusty old CRO:
cro trace.PNG


If I drop the scale to -50dB (x316x) and increase the sensitivity on the DSO (500mV/Div), I can get a slightly better image:
RIGOL Print Screen11-06-2020 3_21_23 PM.786.png
 

MakeMineVinyl

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We usually test for crossover distortion components closer to full power, and at different frequencies.
 

maty

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MEASUREMENTS: Pass Amp Camp Amp (ACA) 1.1. The crossroads between objectivism and subjectivism, and reconsidering von Recklinghausen.
http://archimago.blogspot.com/2020/07/measurements-pass-amp-camp-amp-aca-11.html

Pass%2BAmp%2BCamp%2BAmp%2B-%2BTHD%252BN%252C%2B8R%2Bvarious%2Bpower%2Blevels.png


Pass%2BACA%2BSMPTE%252C%2BCCIF%252C%2BLinkwitz%2BIMD%2B-%2B2V%2B4R.png


[ Permit me to be blunt. This is the worst measuring amplifier I've done to date and these measurements will likely not be "outdone" by another device soon in this blog! ]


OWNER'S COMMENTS: Pass Amp Camp Amp 1.1 (by Mitch Barnett)
http://archimago.blogspot.com/2020/07/owners-comments-pass-amp-camp-amp-11-by.html

[ I purchased these kits seven years ago to drive high efficiency compression drivers, which are typically rated at over 113 dB/W/m. ]

[ What is interesting is that I have had several lower distortion amps like the Purifi 1ET400A and active speakers like the Kii THREE and Dutch &Dutch 8c here. ]

[ As Arch has alluded to, with that amount of 2nd harmonic distortion, this reminds me of tube sound. It has that “glassy” sound to the top end, which I am a sucker for. I know the objective measurements are not good, but listening to music, I don’t find anything really objectionable about the sound quality. So what if I like some sugar on top of my accurate sound? ]
 
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NTK

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...
OWNER'S COMMENTS: Pass Amp Camp Amp 1.1 (by Mitch Barnett)
http://archimago.blogspot.com/2020/07/owners-comments-pass-amp-camp-amp-11-by.html
...
[ As Arch has alluded to, with that amount of 2nd harmonic distortion, this reminds me of tube sound. It has that “glassy” sound to the top end, which I am a sucker for. I know the objective measurements are not good, but listening to music, I don’t find anything really objectionable about the sound quality. So what if I like some sugar on top of my accurate sound? ]
Just be aware that "some" has been predetermined and baked-in for you by Mr Pass.
 
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Vuki

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MEASUREMENTS: Pass Amp Camp Amp (ACA) 1.1. The crossroads between objectivism and subjectivism, and reconsidering von Recklinghausen.
http://archimago.blogspot.com/2020/07/measurements-pass-amp-camp-amp-aca-11.html

Pass%2BAmp%2BCamp%2BAmp%2B-%2BTHD%252BN%252C%2B8R%2Bvarious%2Bpower%2Blevels.png


Pass%2BACA%2BSMPTE%252C%2BCCIF%252C%2BLinkwitz%2BIMD%2B-%2B2V%2B4R.png


[ Permit me to be blunt. This is the worst measuring amplifier I've done to date and these measurements will likely not be "outdone" by another device soon in this blog! ]


OWNER'S COMMENTS: Pass Amp Camp Amp 1.1 (by Mitch Barnett)
http://archimago.blogspot.com/2020/07/owners-comments-pass-amp-camp-amp-11-by.html

[ I purchased these kits seven years ago to drive high efficiency compression drivers, which are typically rated at over 113 dB/W/m. ]

[ What is interesting is that I have had several lower distortion amps like the Purifi 1ET400A and active speakers like the Kii THREE and Dutch &Dutch 8c here. ]

[ As Arch has alluded to, with that amount of 2nd harmonic distortion, this reminds me of tube sound. It has that “glassy” sound to the top end, which I am a sucker for. I know the objective measurements are not good, but listening to music, I don’t find anything really objectionable about the sound quality. So what if I like some sugar on top of my accurate sound? ]

The horror & heresy :D:D:D
 

anmpr1

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I get the idea that someone might want to build something they can then use. Accomplishment, pride of ownership, burnt fingers, the smell of solder, and all that. I've done it and enjoyed it. But if one is looking for that 'glassy sound on the top end' with a lot of distortion, why not just get the real thing? One can (or at least could--they are probably now out of sight like all vintage tube gear) grab an old Dyna ST-70 and purchase one of the numerous rebuild kits. Then you will have five times the power and at least twice the S/N. Not only that, but it will really glow in the dark.

And I think we have to admit, the name David Hafler is certainly as audio 'street cred' as Nelson Pass. Certainly Dave and his gang of designers (Erno Borberly, Ed Laurent, Jim Bongiorno, Herb Keroes et al) were arguably a whole hell of a lot more influential in audio than anything Nelson has ever concocted.
 

mhardy6647

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I hates me some "glassy sound" -- not something I associate with single-ended amplifiers (i.e., with higher levels of low order, even order harmonic distortion) nor with the ACA.

To me, glassy treble is emblematic of bad/early solid state. There's an autoformer-coupled McIntosh MC-2100 here that is hard and shrill from top to bottom. More like broken glassy. Horrible sounding amplifier -- I've never understood their popularity.

I likes me some smooth treble. Like buttah.

Just wanted to mention that. That is all.
 

anmpr1

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To me, glassy treble is emblematic of bad/early solid state.
The entire 'subjectivist' vocubulary is questionable. Harry Pearson is credited with trying to clarify it, but IMO he just muddied the waters. Glassy, dark, liquid, hard, soft, grainy and all the rest. I think much is simply imagined. However, there is no doubt that using a limited use amplifier with a problematic loudspeaker can produce weird sounds.

At a dealer showroom I once listened to a low impedance power hungry Acoustat loudspeaker driven by a tweako tube amp. Bad combination. You could definitely hear the differences between that and Jim Strickland's beefy MOSFET amp designed with the Acoustat in mind. The tube amp was obviously distorting badly.

One problem I have with this little Pass 'toy' is that it's impractical as hell. 7 watts at 7% distortion? But some people take this sort of thing seriously.
 

mhardy6647

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However, there is no doubt that using a limited use amplifier with a problematic loudspeaker can produce weird sounds.
Yup. Synergy is a vector quantity ;)

PS The ACA has plenty of power (even at reasonable distortion levels) for loudspeakers with 100+ dB sensitivity.
 

anmpr1

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PS The ACA has plenty of power (even at reasonable distortion levels) for loudspeakers with 100+ dB sensitivity.

Yes. But that's sort of like saying an old VW microbus has plenty of power and handles great--as long as you're driving downhill in a straight line.
 

pozz

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RichB

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Yup. Synergy is a vector quantity ;)

PS The ACA has plenty of power (even at reasonable distortion levels) for loudspeakers with 100+ dB sensitivity.

Pass ACA THD+N_vs._Output_4+8R.png


If I read this right, a 100 dB per watt speaker would deliver 94 dB at .25 watts.
THD+N (dB) is 94 - 54 = 40 signal and 54 dB of noise. One would think that is audible.

- Rich
 
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