AX100 100W Aleph-X Monoblocks

[macka]

Hi Graeme,

I should have both channels ready to do some evaluations with different gain settings tonight.

In comparison to the X250.5 the X Jfet Aleph has noticably less gain. I guess the X250.5 must be set up for 30 db of gain.

It may only seem marginally more but on the volume control settings its a lot more gain and there is more source noise.

My preference is certainly for lower gain with high sensitivity loudspeakers.

Ian

[tryonziess]

One note. Do you think the actual thickness of the individual heatsinks you used made that much of a difference overall in the efficiency of there cooling ability. I would guess that square foot area to the atmosphere would be the most important thing. I was going to replicate this chassis design with 1/16 inch aluminum plate salvaged from old street signs. After I finish my A-75/300 Pass/Thagard amp your amp will be next.
I was waiting for quite awhile for Nelson to publish any corrections to Grey's original schematic and or the current model schematic. Since Nelson came out with the AX.5 series does not the AX original become old stuff. Just wishing here I know the folks at Pass Labs have to eat and pay bills. They already do far more than all the other designers combined. Thanks all Tad

[gl]

Tad,

The material I used is .187 (3/16") thick. As they stand, the sinks I built do the job. They don't rise above 45-50C even in summer. But they're not efficient because they're too thin. The heat doesn't spread out enough so a great deal of the sink plates aren't being utilized maximally. Each plate is hottest right next to the transistor(s) as you would expect. The heat then radiates outward and because of convection the heat pattern tends upward toward the top of the plate. So imagine a parabolic heat pattern around each transistor. The ends of the plate are not hot at all. The reason it works is that the total surface area of the sink is probably double or triple (probably lots more) that of the factory version.

I placed the transistors laterally at the 33% and 66% points on the plate. If I did it again I would place them at the 25% and 75% points.

I would also use .25" plates instead of .187. That would spread the heat better and the plates could be made smaller. As it is the plates I have could probably be reduced an inch in height and two inches in width without causing much of a thermal change overall.

1/16" material is not going to work. I don't like raining on anyone's parade but I wouldn't recommend it.

Nelson talked about publishing an upgrade to the original Aleph-X design but he has been silent on this for quite a while. I don't believe that he will ever publish the factory XA schematics. That said he has said on several occasions that original Aleph-X is very, very close to the factory version. And that was before the refinement process set in. So I think it's safe to say that the Aleph-X still holds a place of serious value-for-effort-expended in the DIY world.

Cheers,
Graeme

P.S. Ian, yes I believe the the 250.5 is in the 30dB group.

[kropf]

Originally posted by gl
The 2SJ74 and 2SK170 are still available and probably will be for a few years.

I have been unable to find these JFETs in other than industrial quantities. Could someone post possible sources?

Thanks,

Jeremy

[gl]

Erno Borbely Audio is always quoted as a source. You can Google for that. Then there's mcmelectronics.com. They haven't had the 74's for a while. But here you go for 170's:

http://www.mcmelectronics.com/produc...oductid=2SK170

And there are other places. Search this site or Google the net. Don't give up. People on this site offer these parts for sale or trade once in a while too.

GL

[tryonziess]

I just checked and ampslab sells the jfets in limited quantities. Just google ampslab.com and go to components and jfets.

Graeme, So it seems that as usual thick heavy aluminum is the material of choice for the sinks. Mo money!~!! I like this hobby it gives me direction for my working efforts. I guess I could take up golf or building motorcycles. There is always something to buy. And I do love spending money. Just can not seem to find an unlimited source of it. Maybe melting a few hundred soda cans and molding some diy heatsinks would be a worthwhile effort. Again thanks Tad

P.S. Where is the next great amp project going to come from?

[Nelson Pass]

Get them while you can.

[macka]

I managed to get both channels running tonight with the Jfet front end as per Graemes modifications.

There are two differences: feedback bypass caps are 4.7 pf and I run 4 x IRFP240 and 0.75R sources resisters.

Subjectively its always are hard call but I would describe the result was silky smooth, perhaps a tad plump in the bass.

I certainly prefer the 26 db gain but would like to lower the gain to 20 db.

Perhaps Nelson can suggets the best approach?

Ian

[macka]

I have tried halving the value of the feedback resisters from 220k to 110K as this was quite simple to implement .

Input voltage sine wave 100 hertz

I measured the gain at 9.6 times

Input voltage differentail 140mv

Output voltage differential 1345mv

Using the above equation gives 19.64 db gain.

So what happened subjectively?

It made a very noticable difference.

I would describe the result as less powdery (as in soft), with far more prominant extension of bandwidth and clarity and the bass was truer, more firm.

I dont think there is a right or a wrong answer to any of this as it also depends on one's own system and environment.

Perhap spmewhere in between the two gain settings would be interesting.

Ian

[gl]

Tad,

Yes, thick and heavy is the rule for heatsinks - particularly for class A applications. The trick is to get the heat away from the transistors as fast as possible. That means thick hear the transistors at least, lots of radiating area, dark color, as few thermal interfaces as possible in the heat path, etc, etc, ... . The bulk of the cost for Al heatsinks is the cost of the raw material. There's just no getting around it. If you find surplus heatsinks or raw Al you'll be looking at similar numbers. It's like a law of physics. And yes, fun hobbies also seem to be the expensive ones. Another law of physics maybe.

As for the next great amp project - we'll all just have to see what comes along. My preference is for something that has 100W to 200W per channel. I need that as a minimum for my speakers. The project would also have to have demonstrably better sound than the AX100/AX100J project. I watch the Firstwatt developments with interest. It is tempting to try scaling one of them up to higher power output. For now it's fun just listening to what I have. But I know that sooner or later I will be smitten by "Power-amp AcquiSition Syndrome" or PASS.

Graeme