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Nelson Pass amps

audiopile

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I think the GFA-555 was designed by Nelson Pass (not the later versions of the 555). So were the GFA-565 mono monsters. And according to a test report in Stereophile ?? so was the GFP-565. Both the preamp and power amp 565's have issues with leaky (literally electrolytic leaking out of the capacitors) . The bad capacitors didn't show up until many years down the road (Looong out of warranty) -but with the power amps the fluid from the caps can short stuff out and result in pretty impressive failure. AFAIK - all Adcom equipment from that era needs to be recapped. I've got a GFP-565 here that also hisses - but pulled the covers and it has many obviously leaky capacitors. One of these dayz -I'll get around to recapping it and try it again - it's obviosly not working right as it sits .
 

invaderzim

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I would think that any that age would need recapped to get them back to the original sound.

Nelson has confirmed that he did do the GFA-555 but then also indicated that he did others but not the specific models.

"I designed the 555. I believe the Mk II added a couple
of what were considered improvements, and it seemed
to me that it included triple darlington outputs instead
of 2 follower. I might have the schematic around, but
all told it was a simple bipolar design with a current
sourced input diff pair followed by a common-emitter
gain device that was also current sourced. This drove
the followers.

It was typical of all the 5XXX amps that I did the first one
and then they improved it until it had too many parts,
and then I would design the next in the series "

The 545 and 535 may have just been downsized versions of his original design. I even read one writeup where someone added additional transistors to the 535 to up the power since there were open spots on the boards for them.

I've read where the GFA-555II was actually designed by someone else, not just an evolution of the 555 and Nelson himself said he didn't recognize the schematic for it on the same thread the above quote came from.
 

Sal1950

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Is GFA-535 a NP design? I have a 2535 which is 2x 535s in one chassis, I know it is "well regarded" but I find it too warm sounding and it runs out of steam pretty quickly even when driving efficient Klipsch speakers. It did help to mask some of the JBL-L100s ills though at low to mid volumes.
My understanding is he designed the original GFA 5X5 line with the version II being tweaked by Adcom. He later also returned to design the GFA 5X00 line of mosfet amps. I don't know how closely Adcom held to the original 535 with their multich versions. I'm looking for a link to a pdf I have here.
I found it.
https://linearaudio.nl/sites/linearaudio.net/files/pdf/DiddenInterview_MMM110(1).pdf
 

Sal1950

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AFAIK - all Adcom equipment from that era needs to be recapped. I've got a GFP-565 here that also hisses - but pulled the covers and it has many obviously leaky capacitors.
None of the 535-555 or 5X00 amps have experienced the cap failures the 565 did. ???
I've current got 5 here running powering my multich rig, I had them all open a few months ago to set bias and all looked A-OK.
 

audiopile

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I was selling a set of original B&W 801 (air suspension woofers) -suggested to the potential buyers that they bring along whatever amp they intended to use with the 801 ( those speakers were NOT a easy load to drive). Guys showed up with a GFA-555 -they didn't buy my speakers -but I was very impressed with the 555. I think I've owned five or six GFA-555's since then -none of these had a cap problem -sounded great-so I wasn't going to second guess Papa's parts choices. To be clear: This is not design flaw in some Adcom products -it is a parts failure caused by the OEM capacitor manufacturer - I think one version of the GFA-545 can also have this issue because it used the same capacitors. This is not a broad range issue for Adcom products -but limited to a few models over a time period when their Taiwanese manufacturer was using one cap companies products.
 

Sal1950

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This is not a broad range issue for Adcom products -but limited to a few models over a time period when their Taiwanese manufacturer was using one cap companies products.
IIRC All but the latest production runs of the GFA 5X00 amps were made in Japan. Later production was switched to Taiwan.
They all came from the factory with stickers on the back stating county of origin but most fell off very soon. On the 5400's you can spot the difference by the top panel. Japan has cooling slots, China a large number of round holes.
 
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audiopile

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I'm between GFA=555 -so can't check those -but the GFP-565 sitting here with it's covers off is made in Taiwan. I'm pretty sure that Adcom never had a factory as such- but farmed out production to whatever assembler seemed to be able to do the job for them. It's interesting how Taiwan has developed into a prestige assembler from a reputation a few decades back of being several steps below "made in Japan" .
 

bigx5murf

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I've only owned one Nelson Pass amp, it was an old Nakamichi stasis receiver. It's 50wpc did seem to be more bass heavy than most of my other amps, even ones with much more power.
 

despoiler

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It is a much more involved build and made harder since there isn't a complete kit but going with something like an M2 or M2x is in a completely different league from the ACA. I miss my M2 a lot.

The M2X definitely has my eye. I think the build would be super fun. Endless hours of comparing the sound of the different boards.
 

cjfrbw

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Yeah, those M2 variants on DIY audio look very interesting. Some of the guys didn't find the various inputs any better than the stock Pass concept of toshiba jfet buffer, however, but some did.
 

despoiler

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Steve Guttenberg has been running a multi-part interview series with Nelson Pass. This is the second one. He goes into his thoughts on subjective vs objective measurements, design philosophy, and blind testing later in the interview a little before 24 minutes. It's worth your time IMO.

 

Jmudrick

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Nelson on the creative use of second harmonic distortion and negative phase .

 

Forman

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Is GFA-535 a NP design? I have a 2535 which is 2x 535s in one chassis, I know it is "well regarded" but I find it too warm sounding and it runs out of steam pretty quickly even when driving efficient Klipsch speakers. It did help to mask some of the JBL-L100s ills though at low to mid volumes.


The 535 is a bipolar design, so its not a Pass design.
Both input and output is BJT. 2SD1047/2SB817 outputs. Inputs are 2sc2362 .

Looking at the schematics the 2535 does not seem to made up of 535s. The 535 is a 3-stage amp while the 2535 is a 4-stage amp. Input and output devices are the same, but the rest is different.

The old bipolar Adcoms are preferred by some because of the warm sound. Just like old NADs. Muddy.
Warm sounding amplifiers often has a lot of distortion in the highs. And/or poor high frequency response. In old gear an obvious reason is parts degradation or bad connections.

Your amp may need service.
 
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SIY

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The 535 is a bipolar design, so its not a Pass design.
Both input and output is BJT. 2SD1047/2SB817 outputs. Inputs are 2sc2362 .

Looking at the schematics the 2535 does not seem to made up of 535s. The 535 is a 3-stage amp while the 2535 is a 4-stage amp. Input and output devices are the same, but the rest is different.

The old bipolar Adcoms are preferred by some because of the warm sound. Just like old NADs. Muddy.
Warm sounding amplifiers often has a lot of distortion in the highs. And/or poor high frequency response. In old gear an obvious reason is parts degradation or bad connections.

Your amp may need service.

Pass did bipolar designs for Adcom, e.g., the 555. The sound is in no way "warm," it is (like any other well designed amp), neutral. That assumes it's not broken, of course. Distortion in the 1985 vintage one I have on hand isn't spectacularly low, but well below anything humans will hear.
 

Forman

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Pass did bipolar designs for Adcom, e.g., the 555. The sound is in no way "warm," it is (like any other well designed amp), neutral. That assumes it's not broken, of course. Distortion in the 1985 vintage one I have on hand isn't spectacularly low, but well below anything humans will hear.


You are absolutely right.

I dint´t realize how old they are. I thought mid nineties, and that turned out to be pretty far from correct.

I have never listened to the 555, but I always thought it was a MOSFET. Just goes to show how stupid it is to assume.

After looking at the GFA-555 schematic, its pretty clear that they are based on the same design. The fact that Pass managed to get that kind of performance with BJTs in a circuit that simple is impressive. Then again, so is the rest of his designs.
 

SIY

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You are absolutely right.

I dint´t realize how old they are. I thought mid nineties, and that turned out to be pretty far from correct.

I have never listened to the 555, but I always thought it was a MOSFET. Just goes to show how stupid it is to assume.

After looking at the GFA-555 schematic, its pretty clear that they are based on the same design. The fact that Pass managed to get that kind of performance with BJTs in a circuit that simple is impressive. Then again, so is the rest of his designs.

Yeah, it's a very simple straightforward adaption of the classic Lin topology. Boring and plain vanilla, but it works, and does exactly what an amp is made to do- make a small signal larger without screwing anything up enough to be audible. And reliable as well.

I *think* (but do not know) that their MOSFET designs were done by Erno Borbely.
 

tmtomh

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My understanding, including from a personal email exchange with Pass himself, is that he designed the original MOSFET-based 5x00 series just as he'd done the original bipolar-based 5x5 series. And as others have said, he had no part in the modifications made for the mk ii versions of some of those models.

I have a GFA-5400, Japan-made version, that I picked up used about 5-6 years ago for $150. My local tech had completely gone through my 535 and my 565 preamp and replaced tons of caps. I paid him $40 to go through the 5400 and check it out, and he said he cleaned it but it was totally fine otherwise - caps all good, performing to spec as-is.

It is on my list to get it serviced one of these days, though, as those caps will need replacing eventually.

However, I am mighty tempted by the emergence of such highly performing Hypes-Based amps now coming down below the $1500 (or I guess $750 per channel) price point, and so I might just save up for one of those. I do love that 5400, though - I don't know if it's the MOSFETs or what, but I slightly prefer it to my 535 (yes I know they have different power capabilities as well).
 

tmtomh

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My understanding is that Pass Labs' AB products generally are biased higher and run hotter than most other Class AB amps, meaning they run in Class A longer before switching to Class B operation. I believe Class A vs Class B operation is something that can be audible (if there is evidence to counter that, though, I'm happy to see it) - and if so, IMHO that would go a long way towards explaining why Pass amps sound different.

However, I believe Pass also likes simple circuits and minimal use of negative feedback; so his amps might also sound different simply because they have higher harmonic distortion, owing to the reduced use of negative feedback. His older Adcom designs have very low distortion (or at least distortion on par with well-performing other amps of the time). But his new first watt and similar designs sometimes are not designed with low distortion measurements in mind.
 

fitero

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Oh and a question for you guys. Do any of you know which First Watt amplifier might work well with my JBL 4367?[/QUOTE said:
I ran my 4367s with a 200WPC, AKSA amp and briefly with a 35WPC Primaluna Integrated HP model. While the tube amp drove it to more than adequate volumes in my small room, it never was able to generate the marvelous. visceral, tuneful bass that I obtained with the AKSA amp. I guess that it depends upon your taste in music. With the Primaluna it was just nice, while with the AKSA it was FUN!
 

Sal1950

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My understanding, including from a personal email exchange with Pass himself, is that he designed the original MOSFET-based 5x00 series just as he'd done the original bipolar-based 5x5 series. And as others have said, he had no part in the modifications made for the mk ii versions of some of those models.
That is spot on correct info. ;)

I'm running five of those in my system, (2) 545 II's for FL FR RL RR, a 5400 for the Center, and (2) 535 II's for the 4 overhead Atmos speakers.
They all sound the same to me. :)
 
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