Pass ACA Class A Power Amplifier Review

[Goodman]

There have been plenty of MOSFET amps that are transparent and quiet, not sure what the difference between a "good" amp and "great" amp are once they are transparent. Maybe @restorer-john knows of some "great" MOSFET amps and can chime in.

Sure, I would appreciate comments and opinions from from restorer-john.

 

[tmtomh]

My first sentence was meant as a quip or a joke. Not mean spirited. Great accomplishments can be appreciated but also can be over-rated. More to the point using mosfets on the output stage has rarely, if at all, resulted in great sounding amps.

My bad - I should have been clear. While I was responding to your comment, I actually meant to agree with you and was not directing my "not mean spirited" point at you. Sorry!

 

[restorer-john]

I've never had a problem sonically, with MOSFET amplifiers. In the early 1980s the Hitachi (2SJ49/50 and 2SK134/135) were used by everyone who could get their hands on them. From high-end home to commercial PA amps, they were truly a game changer for reliability. As long as they were kept stable. And they were expensive, even then.

Some very highly regarded (still to this day) amplifiers (think Halfer/Perreaux etc) used them for the best part of a decade. Toshiba in the late 80s released the J115/K405 pair specifically for top performance audio and statement amplifiers from many manufacturers incorporated those devices

But much advancement in bipolars meant the diffused emitter/ring emitter/LAPT devices from Fujitsu, Sanken etc took the stage IMO.

It also depends on what you want to define a MOSFET amplifier as being. Just the output stage? Pretty much every MOSFET output stage is driven with bipolar drivers. Or you've got FETs in the front end only and MOSFETs for the output stage with everything else being bipolar.

I will say this however. All my favourite amplifiers and preamplifiers have FET front ends. All my favourite phono stages heavily FET front ended. All my favourite integrated amplifiers have FET front ends. Not many preamps have FETs for the output stages, but I do prefer them. Mostly, that means we get true DC coupling from one end to the other, a more constant (high) input impedance across the band, mostly (not always) less noise and lower distortion.

My own collection is huge and spans mainly the 1970s-1990s with a bias towards big Japanese gear, so read into it whatever you like- because that was the era where the mere presence of a "FET" badge on the front panel would make audiophiles drool.

Like this Class A (switchable) Marantz. Genuine 35W in Class A (auto switchable to Class A/B) and it gets so hot, it has a warning sticker across the top panel (not shown in this internet pic)...:

They even picture the FETs (there's 12 in the PM-94 output stages)

I don't run my PM95 in Class A (it has a manual switch). They sound exactly the same to me as Class A/B. Honestly I cannot hear any difference except less power of course in Class A. The THD is the same at the same powers. There is no visible crossover distortion residual in either mode- it's buried in low level noise. The wasted power and serious heat of Class A is just not worth it with those two FET output amps. I should get out the PM-95 and measure the idle consumption in Class A and A/B just for fun.

 

[rdenney]

B&K amps also use MOSFET output stages. I don't know what they have upstream of that. They also use predrivers biased to Class A, so if they are merely switched on, they are making the heat of a 60-watt light bulb. There was talk of "MOSFET mist", but I wonder to what extent that attribute would survive controlled listening tests. But mist notwithstanding, they have always been highly respected and thought to be an affordable entry into high-end sound (whatever that is).

Looking at their performance, however, they do nothing in the range of frequencies I (or my cat, or the neighborhood dogs) can hear that is different from what, say, a Hypex amp does, within their linear range. But they do some things those big Class D amps don't do: 1.) They run out of power compared to the bigger Class D amps, 2.) they produce audible quiescent hiss, 3.) they cannot be bridged, and 4.) they prefer loads at 4 Ohms or above (and that's 4 actual Ohms, not just the nominal rating, and high current rating notwithstanding). The distortion is a bit higher, but not audibly so. My Reference 125.2 is the logical descendent of the famed ST-140, and switching to an amp based on the Hypex NC502MP as I have been considering, would result in twice the power, lower distortion, lower quiescent hiss, and stability down to 2-Ohm nominal loads (say, my stacked Advents, though I'm no longer using them that way). The Hypex has a bit less voltage gain, but my preamp will provide 12 volts RMS at peak.

But the comparison doesn't matter, because those who are committed to the notion that high-end traditional amps are necessarily better will state that my B&K isn't good enough to represent traditional A or A/B amplification compared to Class D approaches, so obviously the best of the Class D amps will be better than my "budget" B&K. That is belief talking, though, not data.

Rick "still using traditional stereo equipment" Denney

 

[Sal1950]

Like this Class A (switchable) Marantz. Genuine 35W in Class A (auto switchable to Class A/B) and it gets so hot, it has a warning sticker across the top panel (not shown in this internet pic)...:

John, could you share what that warning sticker reads, I'm just curious?
Nelson's first amp designs for Adcom (GFA 5X5's had bi-polar outputs. The next ones he did (GFA 5X00's) were MOSFET's He claims the MOSFET's sounded better? I have 5 of them in my 5.2.4 rig, 4 bipolar and 1 Mosfet, I can't hear any difference, they all sound fully transparent to me.
YMMV

 

[restorer-john]

John, could you share what that warning sticker reads, I'm just curious?

"When in operation the temperature of the set will rise. Never place any object on the set to maintain ventilation" Japanese English

A little picture of a few books with a big cross throught it. I took my stickers off as it would impede ventilation itself! But left them on the PM-64IIs.

The PM-94s were renowned for cooking themselves. That's how I got my PM-94 in the first place cheap. It had overheated, caused a bunch of dry joints and vaporized a whole pile of MOSFETs and PCB tracks. Fun repair, but worth it. The First run of PM-94s had normal height feet, the latter runs had taller feet to allow more air to get in underneath. The also touted a special 'flourine' coated/anodizing (can't recall exactly) on the heatsinks. It was a greenish blue colour instead of black. Still didn't stop them getting too hot if you ask me.

My MOSFET PM-95 was obtained in a set of three pieces with the Reference CD transport & DAC (CD12/DA12LE).

The PM-95, PM-94 mk1 and mk2, PM-84 (mk1 and 2), PM-64II all had similar stickers, as did some of their Esotec Class As. They all run extremely hot in Class A (Quarter A as they called it). I sold my PM-64IIs a while back and only kept the PM-94 and PM-95. Dad has got a PM-84 at his place. They did a short run of the PM-84 Limited where they put Toshiba MOSFETs in the power amp stage instead of the bipolars that the standard PM-84 had.

I'd really like to find a gold PM-94II sometime. My 94 is black- same as the one pictured above. The side panels were cheaper and dark rosewood colour- not rich and red like the one below. The problem with the side panels is they wanted to hide the screws, so the panels have a captive steel hook and slot arrangement near the top and a flat steel plate underneath which is screwed into the wood and also into the base. It's really not strong enough to support the 25kg weight if you hold the amps by the wood sides (which everyone does).

 

[Sal1950]

"When in operation the temperature of the set will rise. Never place any object on the set to maintain ventilation" Japanese English

Well thank you very much kind sir!
I love the look of your black one myself.

 

[anmpr1]

Great accomplishments can... be over-rated.
...using mosfets on the output stage has rarely, if at all, resulted in great sounding amps.

The 'sound' of amps can be highly overrated. That said, it's why men such as Erno Borbely, Jim Strickland, and even Julius Futterman, were interested in MOSFETs. Because everyone knows those guys wanted to build lousy sounding amps.

 

[Goodman]

The 'sound' of amps can be highly overrated. That said, it's why men such as Erno Borbely, Jim Strickland, and even Julius Futterman, were interested in MOSFETs. Because everyone knows those guys wanted to build lousy sounding amps.

OK then, would you use any of these amps now? You forgot to mention David Hafler All good mid-Fi. Which High end mosfet amp would you recommend?

 

[ShadowFiend]

Halcro.

 

[anmpr1]

1) OK then, would you use any of these amps now? 2) You forgot to mention David Hafler All good mid-Fi. 3) Which High end mosfet amp would you recommend?

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.

 

[Sal1950]

To tell you the truth, I don't even know what 'high end' is supposed to mean anymore.

Easy, Expensive.

 

[mhardy6647]

Easy, Expensive.

and/or esoteric and/or quirky.

 

[RichB]

High End is "Tremendous cosmic power, teeny weenie living space".

- Rich

 

[Goodman]

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 DH200 was an exceptional amp. In my salesman days, I did all my sales demos with Threshold power amps. One day I loaned out the Threshold 400, and used the DH 200. It actually sounded much better on the particular speakers I was selling that day, but not on other speakers. Go figure! That made me aware of speaker and amp pairing being important and quite unpredictable, but maybe Amir can fix that.

 

[jtgofish]

I read a lot of Nelson Pass's articles and never felt he needed anyone to defend him except/may be the way he talked about the negatives of negative feedback. Even on that, I believe he simply might have exaggerated it a little to make his points, not that he is against it. Surely he used it too, though perhaps more sparingly. On the topic of distortions, he's one of the few amp designer who actually elaborated on the warm, punch, cool kinds of sound signatures that people talked about and if anything he himself does not seem to buy a lot of those audiophile claims/arguments one way or another, other than less 0.1% is better than 5% sort of deal.

I bought and build the ACA amp and will do the F5 soon, but I don't do it for "sound quality". Mr. Pass never claimed the ACA amp would "sound better" than other amps that offer lower distortions, he posted the distortions vs output graph so we all know what to expect. He rated the amp 5 W into 8 Ohms and a little more if you use a larger PS and accept a little higher THD. So I have no idea what you are defending him for, specifically.

There are much better choices than the F5.A very boring sounding amplifier.It ticks all the audiophile boxes but somehow fails to involve you in the music being played.Which is not to say it sounds sterile but there just seems to be something lifeless sounding about it.

 

[anmpr1]

The DH200 was an exceptional amp. In my salesman days, I did all my sales demos with Threshold power amps. One day I loaned out the Threshold 400, and used the DH 200. It actually sounded much better on the particular speakers I was selling that day, but not on other speakers. Go figure! That made me aware of speaker and amp pairing being important and quite unpredictable, but maybe Amir can fix that.

Since we really don't listen to amplifiers (unless the transformers buzz), it only makes sense to speak about an amp/loudspeaker interface. I tried several low powered amps with Acoustats, but the less powerful ones didn't do the trick. With the more powerful Acoustat amplifier they gave a much better sonic presentation. With something like the 1 ohm Apogee speakers that were popular back then you needed a Krell, or something similar. A Futterman OTL mated well with some of the 16 ohm British speakers, but became unstable (and blow up) with anything much less than 8 ohms.

Once my dealer hooked a tube amp from an outfit called Esoteric Audio Research to a pair of 2+2s, for a customer demo. I was in the store that day, and it was clear to me that something funny was going on. I was used to the TNT 200 'sound', and those tubes were distorting as the loudspeakers asked for more current. At least that was my guess. But the distortion was 'pleasant' in the sense that it wasn't what you'd think of as hard clipping. It really was a 'liquid-ish' sound.

 

[HornJunkie]

Hey Guys,

does the ACA work on a 16 Ohms JBL Horn-Driver (2445J)?
I can’t find measurements with 16 Ohms load…

best regards,

florian

 

[NTK]

Hey Guys,

does the ACA work on a 16 Ohms JBL Horn-Driver (2445J)?
I can’t find measurements with 16 Ohms load…

best regards,

florian

Performance at 16 Ω shouldn't be worse than at 8 Ω. (16 Ω is supposed to be an "easier" load since it draws half the current.) The only issue is the max output will go down to half. But with 111 dB SPL/W (4 Vrms) sensitivity, I doubt insufficient power would be a problem. The ACA has low noise and lower distortion at low output, so if there is an ideal use case for the ACA, this would be it.
https://jblpro.com/en/products/2445j

For example, if you are listening at 91 dB SPL, the amplifier output will be 0.01 W, which, with a 4 Ω load, will be 0.04 W. At that output level, THD+N per Amir's measurement was ~65 dB, and is only about 17 dB worst than a March Audio P122 (Hypex NC122MP based). If you compare THD+N at 5 W, the ACA is ~64 dB worse than the P122!

 

[mhardy6647]

The ACA should be fairly happy with a nominal 16 ohm load. My Frankenaltecs are more-or-less 16 ohm loudspeakers (with ca. 104-ish dB per watt @ 1 meter sensitivity) and pair well with a slightly tarted up ACA that has been on a long-term stay here (although not really, technically mine).