AX100 100W Aleph-X Monoblocks

[gl]

Hi Ian,

Very interesting results you have there. Especially the subjective difference between the two gain settings. I don't think that I'm tempted far enough to try the 20db setting myself though.

Graeme

[macka]

Hey Graeme,

These Jfets been talkin to me and they now sound different.

I am not sure what the ids of your Jfets are but I suspect I have a bit too much open loop gain in the circuit to to be carving off that much feedback for just 20 db of gain.

After burn in it now sounds a bit sterile with 20 db gain

From reading the X'd Zen articles I gather it works best with simple 2-3 gain stages where the stages are set up for maximum linearity before applying super symmetry.

High open loop gain and high linearity are opposing maxims if I understand what Nelson has written.

I am wondering if some local feedback in the form of source resisters might be beneficial for a 20 db gain JFet design.

I will look at this more closely over the weekend.

Perhaps Nelson can throw light on the subject.

Ian

[gl]

Hi Ian,

You bring up a number of design goals that do indeed interact. Many on this site have discussed (and continue to discuss) these all to death. Nelson has talked about using 10dB of SuSy feedback a number of times. I don't want to misinterpret his words but he seems to favor this as at least a nominal value or initial value. So take the rest of your design process from there.

Your idea of using Source degeneration resistors to reduce the sterile sound is certainly interesting. I have no opinion. So you'll need to try it and let us know how it sounds. My only thought is that the AX (XA) circuit is very interactive. It seems like varying any parts values at all in this circuit produces some degree of sonic change. There are so few parts in the core circuit that this seems inevitable because of interactions. So let your ears be your ultimate arbiter - not your scope.

Cheers,
Graeme

[Nelson Pass]

You can play with the open loop gain and feedback without
difficulty. If you simply want to reduce gain, reducing the high
value feedback loop resistor and increasing the input signal
resistor values is a start, and will improve the bandwidth
slightly. Usually acompanying this is adjustment of the resistors
to ground at the inputs, which can trim the amount of feedback
a bit.

Degeneration off the Sources works as advertised, but can
make the sound more clinical. There's no way of predicting this
effect, but it's easy enough to adjust against listening - all it
takes is good taste and lots of patience.

[macka]

Hi Nelson,

Thanks for the tips. I will investigate this with a goof bottle of red over the weekend.

I noticed that since installing high speed rectifiers on each secondary I lost a few volts and the rails are now 22.9 volts.

The output source resisters are 0.75R using a total of 16 IRFP240.

Do you see any advantage in reducing them to 0.47R?

I have not measured it yet but the ouput stage obviously plays a part in the overall gain.

Ian

[kropf]

Originally posted by 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. GL

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

gl and tryonziess,

Thanks for the tips. My SJ74's and SK170's are on the way from ampslab.

Jeremy

PS. http://www.grandata.co.uk/ has SK170-BL in stock. No SJ74.

[macka]

Well I'll be damned, 'I'll be damned.

This thing is judging my entire system..

Firstly over the past few days I have sensed an opening up of the sound, an air of transparency. The amps have been running continuously for 4 or 5 days now. Great room heaters!

However, until this morning I was not aware of its unnerving capacity to distinguish between my various signal sources, notably digital. This had made using auditioning various musical selections a bit pointless on say SACD or DVD audio with different machines.

At the moment the amps are telling more about what's right or wrong with my transports, dacs and universal players than what may or may not be the best set-up option.

This is not just subtle but chalk and cheeze, its brutal in some instances. Thing such as differences in dynamic contrast, bass definition that I felt were subtle really jump out in your face now.

I suppose that good news and as they say if its not broke don't fix it.

I guess I need to talk to Russ White about his Opus dac.

iMac

[macka]

Graeme,

The amps continue to impress.

Why it sounds so much more open and transparent from day one is beyond me.

iMac

[EUVL]

There is still a long way to go in the journey of experimetation.




Patrick

[macka]

Patrick,

I think your posting in the wrong thread again you unless you feel you know more about the subject than Nelson and what I have been doing specifically.