MOSFET outputs for an A-40?

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roddyama

roddyama

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Joined 2002
I have most of the parts for an A-40 and plan on actually building the amp, but as always, I have been sidetracked by new technology. Having discovered this fabulous forum and read what I could on the Aleph's (This will be my next project), I began to wonder...

Can the IRFP 240's be used as the output devices for an A-40 amp? If so, how would I alter the A-40 circuit to drive them correctly, and make them work well?

I also wonder how different the 240's would sound vs. the Darlingtons?

Thanks for your comments.

Rodd Yamashita
 
F

FBJ

Member
Joined 2002
There a mod for the Pass A40 to change the output BJTs to mosfets in one of the TAA back issuses. It's late and I have to get up early but I will get the issuse number for you tomorrow. And if I can work my CAD program and figure out how to post a drawing I will do that.
Also off the top of my head, I remember Mr. Pass responding to that article about the Mosfet used may not be higher wattage enough for the heat disspation. And the ones he recommended are the higher wattage types. Can't remember the part numbers but I noticed some for sell in the for trade section at this site.
:eek:
 
roddyama

roddyama

Ex-Moderator
Joined 2002
AA 3/87

FBJ,

Thanks for your reply.

I know that article. It was a letter from Randolph Vikan called "EZFET" from AA 3/87. The thing is that I'm sure that FET technology has come a long way since 1987. I know that the Aleph outputs are single ended. What I'm wondering is what would be a more up-to-date version of converting an A-40 to an FET output, with possibly IRFP 240's and it's complement what ever that may be.

Your right, Mr. Pass responded with a warning on the wattage of the 2SK134 and 2SJ49.

Rodd Yamashita
 
F

FBJ

Member
Joined 2002
It's great to meet a fellow TAA-er or aX-er now. But I agree with you. The IRF mosfets may have higher gate capacitance but should work though I haven't tried this.
But Mr. Pass usually criuses in this forums around lunch time where I lived I'm should he will have some ideas.
:)
 
gpsmithii

gpsmithii

Member
Joined 2003
Re: AA 3/87

roddyama said:
FBJ,

Thanks for your reply.

I know that article. It was a letter from Randolph Vikan called "EZFET" from AA 3/87. The thing is that I'm sure that FET technology has come a long way since 1987. I know that the Aleph outputs are single ended. What I'm wondering is what would be a more up-to-date version of converting an A-40 to an FET output, with possibly IRFP 240's and it's complement what ever that may be.

Your right, Mr. Pass responded with a warning on the wattage of the 2SK134 and 2SJ49.

Rodd Yamashita
Click to expand...


Nelson Pass said:
Norm Thagard did an excellent job updating the
A40 with MOSFETs, in TAA (now Audio Express) and
I recommend checking it out.

One of these days, I'll get around to the same sort
of project, trying to make it simpler than Norm's
Click to expand...


Is this the article Mr. Pass is referring to? When I look at the back issue info on the AX website there is no discernable reference to it under the 1987 TAA index.

Thanks!
 
N-Channel

N-Channel

Member
Joined 2003
Nelson Pass said:
Norm Thagard did an excellent job updating the
A40 with MOSFETs, in TAA (now Audio Express) and
I recommend checking it out.

One of these days, I'll get around to the same sort
of project, trying to make it simpler than Norm's
Click to expand...


gpsmithii,

Originally posted by gpsmithii
Is this the article Mr. Pass is referring to? When I look at the back issue info on the AX website there is no discernable reference to it under the 1987 TAA index.
Click to expand...

I believe the article Nelson referrs to is the A75 from TAA 1994. It was a 2-part article: part 1 on the amp circuit itself; part 2 on the power supply.

There are a number of DIYers that have built this amp, including myself, but I still have to finish mine. Haven't gotten past construction of the case. Nice heatsinks, but no front, back or top/bottom.

A couple of years ago I asked Nelson if the A75 could be run with a "heavy AB" bias. He said not a problem. With that in mind, I cooked up my (unfinished) version of the A75, I call the AB60 (Class AB, 60W into 8W. I made a couple of substitutions: for the IRFP240/9240 200V MOSFETS, I substituted IRFP140/9140 100V MOSFETS. Couple of reasons: I wanted to use what I had on hand, and didn't have the $$ at the time to get the '240s &'9240s. I used the original A75 boards from Old Colony Sound, and mounted them on two nice heatsink assemblies from Thermalloy (now Aavid, I believe).

Still need to do the remainder of the chassis, but the original plan was to make the amps into two 2-channel modules, using two modules per amp chassis, to be run with either balanced or unbalanced intputs. This would open up alot of configuration posibilities: 4 Channels @ 60W each; 2 Bridged channels at 240W each into 8W; 2 channels at 60W, and one channel at 240W.

Power is still going to be two synchronized half-bridge SMPSs, fed by an Active PFC front end. Anyway, here are a few pics of the almost-finished modules. For more info on how SMPSs can successfully be used with audio circuits, take a peak at the Power Supplies forum. I believe I can make this Audio-SMPS combo work well.

I have the Amp modules mostly done, and here are the pics.

Steve
 

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gpsmithii

gpsmithii

Member
Joined 2003
Nelson Pass said:
Norm Thagard did an excellent job updating the
A40 with MOSFETs, in TAA (now Audio Express) and
I recommend checking it out.

One of these days, I'll get around to the same sort
of project, trying to make it simpler than Norm's
Click to expand...


I believe the article Mr. Pass is referencing is from the 1999-2000 issues of TAA. The article spans 6 parts. The folks at AX were kind enough to research them up.
 
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