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Pass ACA Class A Power Amplifier Review

restorer-john

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1) Sure. If I owned it already and it was working. Why not? In one of my systems I use an amplifier that was designed by Dave (and Ed Laurent) from the late 50s. It works fine. Pretty much stock (no new board design, however with new tubes and other discrete components)--the original stuff probably isn't going to work well, electrically). FWIW I owned both the Hitachi amplifer and Strickland's TNT 200. The latter was excellent at driving the rather difficult Acoustat lousdpeakers of the day. The Hitachi was less powerful, and succumbed to a Central Florida electrical storm one evening.

2) Erno designed Hafler's early amplifiers, so I did mention Hafler, just didn't use the name. Also, Jim Strickland (Acoustat) was responsible for later Hafler designs. Julius Futterman is said (by Harvey Rosenberg) to have been working out an idea to use MOSFETs for tube based amplifiers, and I believe that design was 'finished' by George Kaye.

3) I would never recommend a used amp over a new one. Unless you are able to fix it. Where are you going to get Hitachi MOSFETs now? Or Sony VFETs? As to current amps? To tell you the truth, I don't even know what 'high end' is supposed to mean anymore. @ShadowFiend mentoned Halcro. The Bascom King hybrid amp uses MOSFETs. I suppose those names are 'high end'. I'm sure their are others, but I don't follw that scene closely.

However, to be square, one must specify what MOSFETs one is talking about? What is available today? I'm sure someone like @restorer-john would know. But I don't.

For those who can 'hear' differences (I can't) the following is from an interview with Erno:

EB: So, we moved back to the States in 1978 and I worked for Dave again at the David Hafler Company. I designed the DH101 preamp and also the DH200 (Fig. 1) power amp, which was the first one to use MOSFETs in the US.

JD: These were the Hitachi lateral MOSFETs?

EB: Yes, for me these sound the best of all MOSFET types. At low bias they sound kind of soft, and come very close to tubes. They have relatively
low Gm but the negative tempco is a plus. Vertical types such as the Toshibas have higher Gm but a positive tempco, which makes it more difficult
to stabilize the quiescent conditions. They can sound quite good as well, especially in the bass department, although for full-range I would always
prefer the Hitachis. And the Hitachis can sound good with only 100mA, while the Toshibas would need at least
double that.


I don’t like the IR-type power MOSFETs, although they have a very high Gm. When I used them I had an extra identical device on the heatsink as
the bias regulator to keep the positive tempco under control. They measure well but they are not my favorite for sound quality, unless you go to very high bias currents or other topologies such as Nelson Pass is using.

The Hitachis are only available on the secondary market and only then, through pulls from big commercial/PA amps that used them through the 1980s. You can pick up amps that use them for very little money and maybe get a few dozen or more depending on the models. I picked up a 200+200W Australian built PA amp with 8 Hitachis in it for $5, just for the MOSFETs to fix a big Perreaux.

The Toshibas used in the amplifiers above are NLA, but my Japanese supplier has plenty of NOS. The Exicons were good, but again, NLA. But I've got a ton of most of the MOSFETs I'll ever need I think.
 

AllenW

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Seems about right. :p

Doesn't make me want to get my hands on my friend's Aleph 30 any less tho. ;)
I got a clone mono pair of those on aliexpress for about $500 shipped and they sound excellent.
 

solderdude

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It shows that most ears aren't anywhere near as sensitive to distortion as most think they can actually hear.
It also shows that, as long as you are not nearing max. output power masking of human hearing does an excellent job.

It all depends on the loudspeaker and listening level as well as used music as well.
 

levimax

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The Hitachis are only available on the secondary market and only then, through pulls from big commercial/PA amps that used them through the 1980s.
I recently built an amp which was originally designed for Hitachi 2SK1058 & 2SJ162 and found drop in replacements from https://www.profusionplc.com/index.html in UK. The amp has been working well for years now so I think they are OK. Specs are as good or better than the originals.
 

despoiler

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It shows that most ears aren't anywhere near as sensitive to distortion as most think they can actually hear.
It also shows that, as long as you are not nearing max. output power masking of human hearing does an excellent job.

It all depends on the loudspeaker and listening level as well as used music as well.

This. The article below will twist a few minds at how hard distortion is to hear when it's buried in the mix. There is also a thread on ASR for the Klippel distortion test. I encourage everyone to give it a try. I slightly bested Amir (trained listener) and I could only pick out distortion as low as -30db.

 

gvl

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It has been sitting on a shelf half done for some time but I finally got annoyed enough looking at it so this weekend comes the push and voila, another ACA amp is born. Connected everything, turned it on and here comes the sound and folks, I’m blown away, blown away. I couldn’t stop listening to it for quite some time as it felt like I was in the same room with the band playing and old papa Nelson was sitting right there in the corner smiling. Not. Nowhere close. I didn’t expect much but this is below my modest expectations. It does produce tubey sound with blooming mids that is fun to listen to for a couple of minutes, but then you realize how dirty everything sounds. I ran it through somewhat sensitive Klipsch bookshelves and it could get pretty loud. I plan to get some measurements just to see if it performs as expected and I didn’t make a mistake but given the simplicity It’s probably what it is. It’s the original design, perhaps the revised version is a bit better but it should be pretty clear if you want to buy one to listen to music it’s probably not a very brilliant idea. I had fun building it with my son and he got to try new things and we’ll have memories doing it together, but I can’t see any application for it, even the garage is a bar that’s a bit too high.
 

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MakeMineVinyl

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It has been sitting on a shelf half done for some time but I finally got annoyed enough looking at it so this weekend comes the push and voila, another ACA amp is born. Connected everything, turned it on and here comes the sound and folks, I’m blown away, blown away. I couldn’t stop listening to it for quite some time as it felt like I was in the same room with the band playing and old papa Nelson was sitting righ there in the corner smiling. Not. Nowhere close. I didn’t expect much but this is below my modest expectations. It does produce tubey sound with blooming mids that is fun to listen to for a couple of minutes, but then you realize how dirty everything sounds. I ran it through somewhat sensitive Klipsch bookshelves and it could get pretty loud. I plan to get some measurements just to see if it performs as expected and I didn’t make a mistake but given the simplicity It’s probably what it is. It’s the original design, perhaps the revised version is a bit better but it should be pretty clear if you want to buy one to listen to music it’s probably not a very brilliant idea. I had fun building it with my son and he got to try new things and we’ll have memories doing it together, but I can’t see any application for it, even the garage is a bar that’s a bit too high.
I think those heatsinks are a bit under rated for all those transistors. :rolleyes::p
 

MakeMineVinyl

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Not if it’s sitting in a closet unused. It didn’t get alarmingly hot though over 1hr or so, so maybe ok.
I was just kidding! Those heat sinks should be good for something like a 100-200 watt class A/B amp.
 

AudioTodd

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It has been sitting on a shelf half done for some time but I finally got annoyed enough looking at it so this weekend comes the push and voila, another ACA amp is born. Connected everything, turned it on and here comes the sound and folks, I’m blown away, blown away. I couldn’t stop listening to it for quite some time as it felt like I was in the same room with the band playing and old papa Nelson was sitting right there in the corner smiling. Not. Nowhere close. I didn’t expect much but this is below my modest expectations. It does produce tubey sound with blooming mids that is fun to listen to for a couple of minutes, but then you realize how dirty everything sounds. I ran it through somewhat sensitive Klipsch bookshelves and it could get pretty loud. I plan to get some measurements just to see if it performs as expected and I didn’t make a mistake but given the simplicity It’s probably what it is. It’s the original design, perhaps the revised version is a bit better but it should be pretty clear if you want to buy one to listen to music it’s probably not a very brilliant idea. I had fun building it with my son and he got to try new things and we’ll have memories doing it together, but I can’t see any application for it, even the garage is a bar that’s a bit too high.
Despite all of the ranting and raving on both sides of the ACA “debate” I get the feeling from having read what Pass initially published about this that you got exactly what he intended you too!

If people like to listen to them too, good for them!

This is entertainment, as Pass says!
 

gvl

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Today I hooked up the amp to a pair of Monitor Audio Silvers and actually it didn’t sound all that bad at lower volumes. Still not clean but kind of mellow and comfortable, I could listen to it and it didn’t bother me at all. I think I’m intrigued enough to try building some other Nelson’s amps with more power and less distortion.
 

LTig

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Today I hooked up the amp to a pair of Monitor Audio Silvers and actually it didn’t sound all that bad at lower volumes. Still not clean but kind of mellow and comfortable, I could listen to it and it didn’t bother me at all. I think I’m intrigued enough to try building some other Nelson’s amps with more power and less distortion.
I'd rather keep clean power amps and use something in front to add distortion at will.
 

AllanMarcus

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Folks are selling dirty water as tasting good. What else can I say?
Coffee is dirty water. Lot's like the taste. Lots do not.
 

AllanMarcus

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Well, it appears I won an ACA at Burning amp! Shipping for free too! I should get it soon. I look forward to making it as I haven't made anything electronic since my mainline a few years ago.
 
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