Greetings and Salutations

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I came across the link to this site while perusing the Pass Labs DIY section. I have been a long time fan of Nelson Pass and his designs, my most recent project is one he Co-Authored, the A75.
It's quite interesting that I also built another Class "A" Amp Co-Authored by Nelson Pass and Norman Thagard in 1995 published in Audio Equipment Authority.

My first DIY Audio project was an "All American 5 Tube Radio" an ancient reference design used in High School technical programs as teaching aids. My life long ambition is to build an Amplifier with a Power Supply big enough to arc weld with. :)

I am an Electronics Technologist by training, though for the last dozen years I have made it my hobby and became a Professional Purchaser currently in Pharmaceuticals. I dabel in circuit design for a few extra bucks and recently started doing a fair bit of work with Balanced AC Power systems. Which by the way in my humble opinion is the next best thing to sliced bread for the Audio and / or Video enthusiast.

Well that's a snippet from my Biography for what it's worth. There is one last thing, my user name says' it all, I like to keep "current" on Audio Ideas. :D
Re; Hpotter

Peter isn't it? I can put some stuff together on balanced AC Systems for you if you want. You can try this link as several good white papers are published there.

The Pass/Thagard project was a 100 Watt class A Amplifier published in Jan, Feb and March of 95. I will scan the schematic and post the PDF Monday night in Pass Forum under Pass/Thagrad thread.

As for building A75 over Aelph, you built one did you not? Actually the A75 is a lot cheaper to build. I will be building a pair of Aelph 2 monoblocks in the Fall.


I've built 3 pairs of A75s. The original articles appeared in Audio Amateur in 1992 and 93, 1994 was Zen article (I've got them all).

As to power A75 monoblocks could be compared with Aleph 2.
Output devices are doubled on A75. Complexity of front end board is like 10 to 4 comparing A75 and Aleph. You require FEB PS for A75 which is not neeeded on Aleph. Everything else is the same (transformers, caps chassis). Alephs are cheaper and way easier to build. I did mine p2p which I wouldn't do with A75.

I was thinking about changing some of my A75 into Aleph 2, but it would be a waste of some boards and components. The best way will be changing A75 into X Amps when their schematic become public (I've heard rumors about September).;) All you have to do is change the FEB, everything else stays the same. And Alephs sound better, more texture, definition and space, less solid state.

Here is my A75 Gallery link
and here is Aleph 5 link with some more comments on sound comparison. Aleph2 would have twice amount of output devices, all other parts the same.

And welcome to the Forum (I should've said it in the beginning). It's pretty nice place, and quite addictive to me (I guess you know what I'm talking about).;)
Re: A75 vs Aelph

Thank you Peter, it is a great board and very addictive. I feel the A75 to be cheaper as the PCB were only $20 and the Transistors only a buck something, compared to $2.50 or more for IRFP244. The over thing was the heat dissapation per transistor is lower on the A75 which would work with this chassis I built.

Are you suggesting I abandon the A75's and go directly to the Aelph II times 2?

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From looking at your heat sinks, they really would work better with more output devices. How many spaces you have available for them? How big is a fan, so I would have better idea about the size. For A75 I was using IRF230 and IRF9130 which cost me about the same as my current IRFP140 (I prefer those to IRFP244). I didn't realize you can get output mosfets for that project so cheap.

Now, the decision is up to you. If you want base control and slightly forward sound with lots of detail go for A75. If you want more subtle caracter with better textures, more pleasing sound, slightly laid back, kind of 3-dimentional space with notes hanging in the air, go for Aleph. When I came back from camping trip I had my A75 connected because Alephs were on a loan. After couple days I got them back and put in the system and when I started to listen, I knew they were totally different amps and I wouldn't switch back to A75s.:) Remember also, it was a commercial product from Pass Labs and they knew what they were doing when they decided to put Alephs on a market and not something along A75 amps.
All my listening to Alephs was done as mids/tweeter amps with cutoff freq. 100Hz, so I cannot comment on the bass, but the person who loaned them from me (he runs Audio Oasis) said they were better with bass than A75s. He also ask me to build Alephs for him.
More of my comments about choice here
Each of those rails holds 6 TO-3 transistors, there are 8 rails, two per bank of heatsinks. There are 4 3.5" 12VDC fans running at 6 volts, one at the bottom of each pair of rails. There is one 12 VDC 5.25 exhaust fan at the top of the Tower running at 8 volts. All fans below audible threshold at more than a few feet away.

There will be three platforms inside the chassis when finished, transformer, caps, P/S FEB, and Output Board. I will finish the corners with L channel dropped in from the top. The columns are for an archecturial detail to match my A/V racks.
So you can fit 48 output mosfets. Perfect number for stereo A75.;) You can built it and later when you built Aleph you could biamp or move A75 to a different system. The price of the boards is quite important too. I did mine myself and still have 2 complete sets available. OTOH for Alephs you could wire the circuit p2p and save not only on boards but on components as well (I did it and can say no problem). Probably 3 times less parts, not mentioning lack of any adjustments and output short protection. A75 doesn't have protection, and be prepared to spend one evening and half of next day for adjustments.;)
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