A new X preamp.Call all eXperts!!!

[nar]

So here's the thing.I believe possible to build a new preamp for myself,using the X benefits.So here's my design options.Should be able to convert Unbal.inputs to balanced outs.I desire to use a 2 stage design:jfet 2sk389 inputs,in differential pair.Then cascoded by Jfets too.Output will definitely be mos,with high bias ( relative to preamp's function.) Gain must be variable,from 0dB to 20dB approx.Feedback will definitively be X type.

I believe this research is to get best sound possible,with low distortion,high output swing capabilities in low loads ( such 600 ohms,for example),very low noise indeed.

The power supply will be classical one,with high current bridge+capacitors,CRCRCRC pi filters and mosfet regulation.I will provide my ideas,but hell what do you think you guys of this?Would be so nice to fit a pair of XA-100...already studying.

Any feedback ? Go ahead.I kindda' like it (Thanks Nelson!)

Anael

[Nelson Pass]

It looks like you've answered all the questions you might
have, so probably all you need is encouragement.

[Won]

Weren't there noises a while ago regarding a tube-based X input stage? Grey?

-Won

[nobody special]

Why JFETS? You can (as i have) use a cascoded single-stage current-sourced differential pair of MOSFETs with X feedback.

[GRollins]

Yeah, I've got an X preamp coming. Gotta do something to go with the Aleph-X, right?
There are several reasons I haven't put it out yet. Number one being that this has been one of the worst summers of my life, though hopefully some of the dust is settling. Two is that I intend to do a set of four variations: 6SN7, 6DJ8, JFET, and MOSFET. Builders can take their pick as to which suits their needs and/or prejudices. I've got the two tube versions pretty much ready. The solid state versions are lagging behind. I'll get there.
Reason number three is that I went and started this little thread about an amp and I wanted to kinda let that settle out before I stirred the mud any further.
Reason number four is that I got sidetracked into building a bunch of speakers. One pair of Ariels, two pairs of ME-2s, a modified pair of CS-1s (old KEF design), and three subs (maybe more before all is done...you can never have too many subs). Parts aren't all in yet, so I'm amusing myself by doing the woodworking. And believe me, Ariels take a bit of fiddle-factor to put together. ME-2s aren't that bad. CS-1s are just boxes, although I gave into the urge to piddle with the basic design. Two of the subs are transmission lines; the third is a bass reflex. Anyway, point being that I've been kinda busy.
Did I mention that I've also been writing? Oi! I need more hours in the day...
As far as topology goes, I see no need for cascodes in linestages. Yes, linearization, Miller, distortion, yackety-yack, and blah, blah, blah. Whatcha going to do with all that gain, eh? Burn it off as feedback? Not me. I've also got a neat little non-NFB distortion-buster trick that I'm using. And it's X, of course. And balanced. Etc.
Like I said, I'll get there. Just lemme get the sawdust swept up first...

Grey

[eLarson]

Originally posted by nobody special
Why JFETS? You can (as i have) use a cascoded single-stage current-sourced differential pair of MOSFETs with X feedback.

I suspect the 2sk389s were picked for their low noise. If I recall correctly ("watakashi-no kyoku ga tashika naraba") the 2sk389 is a dual monolithic N-channel JFET, so you'll get the best match possible between your two amplifier halves.

Erik

[Petter]

You can adapt the X100 design work (separate thread) to work as a preamp. That would give you a "one stage" amp with relatively low output impedance.

Your jfet input will be inherently low current and thus you likely need an output buffer. Bipolar emitter follower would likely work better for you than FET follower especially if you are planning to have zero offset at balanced output ....

Petter

[nobody special]

As far as topology goes, I see no need for cascodes in linestages. Yes, linearization, Miller, distortion, yackety-yack, and blah, blah, blah. Whatcha going to do with all that gain, eh? Burn it off as feedback? Not me

Yeah, I agree... It was more for the point of doing it. My plan is to get as many variations of the balanced zen line stage together as possible, and make PCB's that will plug into the same chassis, and test each subjectively. The planned topologies are:

1. Completely standard BLS-
2. Current sourced BLS- otherwise standard
3. Current sourced BLS with 2X+ bias
4. Current sourced BLS with X feedback
5. Current sourced / cascoded BLS
6. Current sourced / cascoded BLS with X feedback

This is to allow me to try all the "flavors". There has been so much talk here in the past as to the sound of cascodes, minimalist vs. active current sources, etc. that i wanted to try it all out for myself. I first must finish my Aleph X's. I hope to publish what I find to a website eventually. Unfortunately, I don't have access to a distortion analyzer, so my testing will have to be on a subjective basis.
NS

[nar]

Which power,V gain and bias will you run your Aleph-X ?Just to have an idea...

I mostly think about a 2 stage version now,with 2sj109V input differential,cascode I think is not necessary,but would work well with an other Jfet,so convenient to use in fact (I just link its gate to its source pin,those connected to the 2 common sources pin of the differential.I think about something like 2sk246,or equivalent if I decide to go for a N version differential.)

I intend to use a V version for the 2sk389 or 2sj109,with higher bias possible(but not at idss anyway.Damn!!!...)

Output will be a 610 or 9610,with Voltage gain.Feedback will be adjustable,to allow for a 0 to 20 dB gain setting.

...

So the output will be mostly like an aleph design,but with no Aleph link and resistors as the input of power amp.is quite an easy load compared to a speaker.

I remember a note from Nelson saying that eliminating resistors to ground at the input would be nicer for a better unbal.to balanced conversion.So will have to work perfectly well with unbalanced sources,mostly all mine!! And yours too I guess...

I just thik about a mini XA for line levels in fact.But with no Aleph rear topology.I guess if i want no DC offset I should load outputs to ground.But I think 100 Ohms value will not do it.Nelson,should it be OK if the load resistors are 1000 times the value of output impedance of the preamp?

We should think about this value against input impedance of the power amp following,if we don't want to see any loss bandwith nor gain...

Sorry for long post!!!Any feedback GREATLY APPRECIATED guys

[Won]

Another thing I was curious about were the push-pull style topologies pictured in the patent. Anyone venture to try these designs?

-Won