AX100 100W Aleph-X Monoblocks

[carpenter]

Originally posted by macka




Once I have finalised everything the wiring will be tidy up.



Macka

Tidy, or not, that's a nice pic to look at. Can you get a close-up of the diff pair?

[gl]

Hi Ian,

William offers good advice as always. He's one of the most experienced people around regarding JFET front ends on Aleph-X's.

Regarding your questions:

1) William addresses this well. It's also the reason why I picked the BL parts. I don't know if Nelson or any other manufacturers Idss grade their JFETs within the Toshiba color grades like BL for instance or whether they they design their circuits so that they can just take parts out of the bag. I really doubt that they hand select front end JFET's for sweet spot performance on an amplifier by amplifier basis. I am just taking J109BL's out of the bag.

2) The 10p cap was originally put in because it was there on the original Aleph-X. I have tried no cap and I have tried twisted wires (in fact that's what's in the pictures at the beginning of the thread if your eyes are good enough to see it). I couldn't hear any difference so I've deleted these parts.

Thank you for the photo. A very professional looking job. I deliberately used scatter wiring to connect the driver board to the output section to avoid potential coupling problems.

The Aleph-X, either the homebrew or commercial version, is an exquisite balance of bare sufficiency and excess. On the excess side it is a high power class A circuit with excess current and heat compared to most power amplifiers. On the sufficiency side the signal passes through only two transistors between the input and output, and there are very few parts that directly touch the signal. The input stage design is carefully balanced against the output stage design. So much bias current here vs. so much input cap there vs. so much voltage swing etc, etc. The whole amplifier circuit is a collective sweet spot. Change almost anything and you risk leaving that place.

OK. That being said, I figure that if you want to change the input section you need to make it look just like the old one to the rest of the circuit. That means 1) driving the output stage with the same voltage and current parameters, and 2) changing the input drive requirements as little as possible. That's where the parallel J109BL idea came from. It provides most or all of the requirements of this argument AFAIK. I'm not sweating the transconductance thing for now because the Aleph-X has two gain stages, not one like a UGS or a ZV9 etc.

I don't know if the AX100J circuit will work. There's only one way to find out and that's to try it. Although Nelsons comment above is encouraging, he really only refers only to a parallel JFET implementation being better than the MOSFET implementation. He doesn't actually refer to this specific implementation. Something could still be amiss.

Cheers,
Graeme

[EUVL]

> The whole amplifier circuit is a collective sweet spot. Change almost anything and you risk leaving that place.

But then there are XA-30, XA-60, XA-100, XA-160, XA-200, all with different number of output devices, different open loop gains and bandwidths, different voltages & biases on the output devices, .....

So which one is the sweet spot, or is the sweet spot large enough that you can still change a few things ??

> I'm not sweating the transconductance thing for now because the Aleph-X has two gain stages, not one like a UGS or a ZV9 etc.

That I do not understand. Would you care to explain ?
What do you mean by sweating the transconductance thing ?

> I couldn't hear any difference so I've deleted these parts.

I wonder whether you see any difference in your 10kHz square wave response......


Patrick

[gl]

Hi EUVL,

Exactly the point. Each of those XA amps has it's own collective sweet spot.

I was saying that I believe that there is enough transconductance in the parallel JFET input stage shown to do the job here.

Yes I have seen the the very small difference. But I can't hear the difference so I'm deleting them. Others have said the same.

GL

[macka]

Thankyou William and Graeme,

I will start organising some Jfet.

Can anyone refer to a link for P channel Jfet testing.

I have the Borbely N channel, how is the diagram reversed?

regards

Ian

[Zen Mod]

Originally posted by macka
Thankyou William and Graeme,

I will start organising some Jfet.

Can anyone refer to a link for P channel Jfet testing.

I have the Borbely N channel, how is the diagram reversed?

regards

Ian

http://www.passdiy.com/howto.htm

[daly41k]

GL or anyone

Has anyone got jfets from Linear systems? They advertise in Audioxpress. I haven't tried them but they may have sources of the power J's.

dave

[macka]

Originally posted by daly41k
GL or anyone

Has anyone got jfets from Linear systems? They advertise in Audioxpress. I haven't tried them but they may have sources of the power J's.

dave

Yes. They only have N channel type at the moment. So you could built a Aleph with P channel out put fets as discussed elsehwere.

macka

[macka]

In terms of tweaking I have found the 220uf capacitors in the current share loop critical for that special result.

This was discussed ages ago but I tried tonight Panasonics FC 220 uf 63 volt with and without Panasonic PP film 0.0068 uf bypassed. Nice with the bypass if perhaps a bit prominant in the extreme top end. Without quite plain and uninteresting.

Tomorrow I will try bypass with 0.046 Panasonic PP film and see how that goes. I also have some Elna Silmic and some KZ Gold Nichicon Muse to try.

I know some people hav tried BG series but they never really captured my imagination.

macka

Macka

[gl]

Hi Ian,

The Nelson Pass paper on DIY opamps on the PassDIY site has instructions for matching MOSFETs, JFETs, and BJT's.

I'm very interested in your experiments on bypassing those particular electrolytics. Like you say there has been lots of discussion on this in the past. But I don't recall any conclusions or even consensus. I also don't recall ever seeing any Pass Labs or First Watt schematic that showed these particular caps being bypassed either. However, there are Pass Labs schematics that show electrolytics that are in series with the signal being bypassed with small film caps.

Are you allowing time for the electrolytics to break in before adding the bypasses and are you listening before and after?

Graeme