Looking for true high-end Class A design

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DutchMark

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Joined 2004
#1
Hi,

I have recently purchased some Martin Logans Sequal series. I previously owned a 300B SE amp, which does not do the job as expected.

I now wish to build a nice stable really high-end Class-A amp using all FETs (not just the power stage). What is the best musical design class A you ever heard, and do you have the full schematic (or building instr. if possible)?

Thanks for helping,
Mark
 
#6
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acenovelty said:
Take a look at the Krell KSA 50 pcb thread. While not meeting your other ideas, it is one of the best musical design class A designs that will drive those window panes without effort.
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Hi,

I have had a look at the schematic, and all transistor is not what I am after. Coming from a 300B amp, I intend to build something tubelike, hence the wish for a JFET design.

Thanks for your help,
Mark
 
#7
BobEllis said:
Of course any of the Pass designs meet your criteria but the A75 may be better able to deal with the Martin Logans. See passdiy.com under legacy projects. I am very happy with my A75s.
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Hi,

I like the A75 design a lot. It is exactly the kind of design I was looking for. There are a LOT of FETS in this design for just 75Watts of power. With more modern FETs I would expect that the amount could be reduced. Have you ever seen a variation to this design like that? Also I would like to avoid non-silent cooling methods.

I understand you built one (must be quite a job). Do you use active cooling? Did you go with the mechanical design as documented?

How would you rate this amps sonic qualities?

Nice to hear from you,
Mark
 
#8
300B amp
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How about use 96pcs (close to 100 pcs) of Western Electric 300B?
This pair of speaker (2pcs, left and right) is powered by 4 amp blocks.
 

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#12
96 x300B

lumanauw said:
Each amp block has this : 24 of Western Electric 300B's. That means total of 96 of 300B's powering a pair of speaker 😀
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Hi,

I'll check with my energy company if i can do that 🙂

It's great, but seems somewhat unpracticle and for sure is beyond my budget.

However... i am truely impressed !!

/Mark
 
#13
DutchMark said:

Hi,

I like the A75 design a lot. It is exactly the kind of design I was looking for. There are a LOT of FETS in this design for just 75Watts of power. With more modern FETs I would expect that the amount could be reduced. Have you ever seen a variation to this design like that? Also I would like to avoid non-silent cooling methods.

I understand you built one (must be quite a job). Do you use active cooling? Did you go with the mechanical design as documented?

How would you rate this amps sonic qualities?

Nice to hear from you,
Mark
Click to expand...

There's no reason to build any amp with forced air cooling if you have the heat sinks available. Mine are on passive sinks with 38 40 mm fins in 25 cm high, 350 cm deep. It takes a lot to get rid of the heat passively. These sinks can handle about 200W/side so you could increase the bias, but in my case I ran the rails up higher. Back then I was convinced that 75W would not be enough and I went for some headroom in class AB.

Nelson Pass commented that the design really only needed half as many output devices. I run 6 IRFP240/9240 pairs and haven't had a problem My speakers have always presented fairly benign loads.

A caution though - most modern plastic devices are not as rugged as their TO-3 can equivalent. With a hard to drive load such as the Martin Logans, consider at least 9 pairs if not the original 12. (rule of thumb, 50% more plastic package devices than TO-3's) Especially if you increase the rail voltage.

On two of my channels I replaced the IRFD110/9110 with IRF610/9610. The 610/9610 have lower input capacitance, which theoretically means greater bandwidth potential. I cannot tell which is which, so there must not be much sonic impact. I also changed the zener current setting resistor values and power rating to accommodate my 65V front end rails and 58V output rails.

I run in the full folded cascode mode. Very sweet top end and plenty of bass authority. All the detail that is in the recording show through. Poor recordings are even more problematic than with lesser amps. I have a channel dedicated to each driver in an active 2 way Focal 6W4254/TC120TD5 system that crosses over to the subs at 60 Hz. Alain DuPont also built an A75, and has built other Pass amps - perhaps he can compare them. I think he may even have Martin Logans. I have been happy enough that I never seem to get around to building the A30 and AX that I have gathered parts for.

Hope that this is helpful.
 
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