314159265358979323846264.... good rasons why?

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Hi JJ sorri to hear that.
As you have a A75 could you give me any pointers on where to find links on Diagrams and modifications or even your personal experience construction notes.

I have the information from the Pass site A75 part 1 and 2.
And I am collating all I can get from the forum.

Ps (at work at present so I may be wrong) I believe that on the A75 there are same 30 Ho. resistors to ground few have rised this point any way to get rid of them?

I will definitely provide any help I can if you do build one. There are also several threads on the forum about the A-75, and there are many other members around who have built the amp.

I bought my boards from AudioXpress, but I they have been out of stock the last couple of times I checked.

Not sure about the resistors to ground. The schematics shown in the A75 articles are correct, and any resistors to ground on those schematics are necessary for various reasons.

Tanks Jupiterjune

All the help I can get will be most welcome.

The reason for this tread is to put forward suggestions for reasons why Papa Nelson should have a stab at in proving the A75

Reading around I have found quite few comments on removing the 30Ho resistors from the output to ground believe they are necessary to reduce the DC offset to ground.

Maybe this would be a god improvement maybe especially if you have built one You have same more maybe much more valid points.

Another that came to mind is that there may be better new semiconductors to do the job.

In regard to my build of the A75 I will first Trying something more simple and I am at present collating information on the F5 Aleph X Aleph XJ.

I have found same good stuff burried under tons of info, EG I have in front of me 3 different drawings for the Aleph x supply voltages vary from 15+- to 24+-

Thats why I have started my other tread If you know where the good reliable information and drawings are please paste the links on that post.
Look like this tread is now dead.

I was expecting same suggestions on how to improve the A75 maybe like this:
"New bias circuit – A newly developed bias generator has eliminated three sources of variation in bias current in the output stage. It also allows approximately 20% greater power output for a given supply voltage."

Papa You know from where this came from Is not my work if inapropiate to use in this way You have my most sincere apologies.

Or maybe suggestion regarding the replacement of the power mossfets with T03 dudes
314159265358979323846264 + 1 reasons


The A75 is a great amp, one of Mr.Pass's jewels along with the A40 (but that is another story).......so are you planning on building it?

If so, are you willing to post pictures of your progress and share your experiences with the rest of the members?

I will donate a set of boards to get the ball rolling.

As for modifications to try, I suggest

A) Jfet front end

B) Different supply for the front end

C) A bridged version

..................but before you get into all that you should build the amp as it stands....you will find that it kills most of competition.


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Hi Jam
I will eventually build the A75
Tanks for your kind offer of support.
It will take me along time to get there as I am planing on
F5 aleph x aleph x100 first
At present I am collecting data and modifications.

I have made inquiry about the transformers those will be custom Jobs
1000VA with twin outputs at 24 volts taped at 20 volts this to give me flexibility with the regulated voltages.

I will try to approach the construction in a modular way same as for the Burning amplifier.

I will also try a different approach to the heat sink problem.

Papa Nelson keep saiing that the Moss fet runs much better at hieger bias.

But this is outside the scope of this tread

As you notes in one of my posts Papa wrote that one day he may be having a go at fixing the A75

The idea of this post is to collect suggestion in what we would like to improve on the A75

Which improvement would you like to see on the A75?

Please send me a PM with your address. The boards are yours ...........hopefully we will see the results of your experiments one day. I went thru the process of building most of Mr.Pass's designs and all I can say is that you are in for a treat.

I own Mr.Pass a lot for his advice, help and encouragement over the years ,he is truely a great man who is willing to share his designs and ideas with us. I hope you will enjoy the experience as much as I did.


Hi jam
Tanks for the offer.
You sure?
A75 is about 2 years away (I hope).
One part of me say get them the other say may be you should not as maybe some one deserves them more.

I will take you up on it but could you hold on to them till I get started.

Agree with you about Papa and the treat.
Wow Jam -- that is very generous.

Are those audioXpress boards or did you make your own? I am in the slow process of building a pair of A75 monoblocks. Chassis are done, and I have audioXpress boards, one populated, one not.

I was wondering if you might have more boards that you might wish to give away, or sell, or perhaps trade?

Also, I guess you have looked at using jfets in the front end of the A75. I think I can just stick them in for the input fets and perhaps adjust the bias resistors to get them to work. But perhaps you already know the answer as to what is required?

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