best small signal FET?

hesener

Member
Paid Member
2005-03-11 11:09 am
Munich, Germany
Hi all,

after seeing the new Lyra Connoisseur line stage and test reports, I'm thinking about building a similar thing (just can't stop dreaming.....)

and now, after extensively searching the solid state category here on diyaudio, my question still is: What would be the best input device for a line stage? Candidates are 2SK170, 2SK30, .... other than noise figure and linearity, what would you look for?

Bipolars - there were lots of bipolar suggestions, but I would rather go with FETs.....

The topology I am thinking of is a input differential pair, followed by source followers to get a low output impedance (using monoblocks close to the speakers, the cables can get long-ish). Signal swing should be ~3V out, the gain of the whole thing does not need to be very high... sorry it's hard to be more precise but - as several people pointed out - the circuit and the devices correlate a lot.

thanks a lot for your help!
alfred
 
hesener:

If you want to use a similar topology to the 4-2SE line amp, for the input I'd recommend a dual-JFET with a high Vds (40~50 volts), in addition to good noise and linearity. The 4-2SE can produce about 11V output, and the input JFETs should preferably work well with a healthy amount of Vds across it. The input JFETs' load is the low impedance of a common-base stage, so small capacitance is perhaps of less importance.

The input JFETs should also have low gate leakage currents with the operating Vds applied. Regarding gate leakage currents of JFETs in general, P-channel JFETs tend to behave better than N-channels, but FWIW, I've chosen N-channels for the inputs of the 4-2SE.

>The topology I am thinking of is a input differential pair, followed by source followers.

Hmmm. Although the 4-2SE does operate differentially, it is essentially a much-augmented folded-cascode topology, loosely similar to what the AD829 would be if it were made from discrete semiconductors and had a JFET input. And a bit more complex than what you have in mind.

How informative was the article about the circuit topology, componentry or physical structure of the 4-2SE?

regards, jonathan carr
 
Thanks for sharing this info

jcarr said:
hesener:

If you want to use a similar topology to the 4-2SE line amp, for the input I'd recommend a dual-JFET with a high Vds (40~50 volts), in addition to good noise and linearity. The 4-2SE can produce about 11V output, and the input JFETs should preferably work well with a healthy amount of Vds across it. The input JFETs' load is the low impedance of a common-base stage, so small capacitance is perhaps of less importance.

The input JFETs should also have low gate leakage currents with the operating Vds applied. Regarding gate leakage currents of JFETs in general, P-channel JFETs tend to behave better than N-channels, but FWIW, I've chosen N-channels for the inputs of the 4-2SE.

>The topology I am thinking of is a input differential pair, followed by source followers.

Hmmm. Although the 4-2SE does operate differentially, it is essentially a much-augmented folded-cascode topology, loosely similar to what the AD829 would be if it were made from discrete semiconductors and had a JFET input. And a bit more complex than what you have in mind.

How informative was the article about the circuit topology, componentry or physical structure of the 4-2SE?

regards, jonathan carr


Thank you for passing this on to the masses. I would love to cobble something together that can approach your designs.
BTW, is the article on the 4-2SE available online?

George
 
Originally posted by jcarr

How informative was the article about the circuit topology, componentry or physical structure of the 4-2SE?

The article can be downloaded here: http://www.stereoplay.de/sixcms/media.php/188/stp0805Lyra.pdf

I could translate the relevant passages in a few days but maybe somebody else is quicker? In the meantime you may try to get a reasonable translation from babel fish.
 
> How informative was the article about the circuit topology, componentry or physical structure of the 4-2SE?

JFET folded cascode input stage, BJT second stage, IRF510 driver stage, partly hard wired & partly PCB, 24 discrete regulators directly under circuit PCB, teflon caps and wires, Shalloc selectors, 190VA tranformers, 20000uF caps on damping materials, Schottky diodes, RIAA filter partly passive and partly active, .....

Well, pretty detailed, but not sufficiently detailed for easy re-engineering. And they said you polished all electrical contacts, including AC sockets, in the demo.

They did say also it sounded better than anything on the planet -- even better than Pass Xono. (I am only quoting the article.) Their reference was Naim NAC552.

But Jonathan, why would you prefer transformers & regulators to batteries ?


Patrick
 

hesener

Member
Paid Member
2005-03-11 11:09 am
Munich, Germany
thank you!

thanks a lot for all your ideas and inputs, this helped a lot! will start working on the circuit now... and it will probably be based around the 2SK389. for the rest - don't know yet, as my power amplifiers have a lot of gain so the line stage doesn't have to.

and yes, the 4-2SE is a masterpiece, as pointed out in the following thread... anybody know the japanese word for carpenter? ;)))
 
hesener said:
The topology I am thinking of is a input differential pair, followed by source followers to get a low output impedance (using monoblocks close to the speakers, the cables can get long-ish). Signal swing should be ~3V out, the gain of the whole thing does not need to be very high... sorry it's hard to be more precise but - as several people pointed out - the circuit and the devices correlate a lot.

If you like the simple idea of a differential input stage driving an output follower, have you considered using a step-up transformer for the differential input stage instead of an active stage?

It'll give you 100+dB common-mode noise rejection even from unbalanced sources, it can take either balanced or unbalanced input, it gives you ground isolation, extremely low noise and distortion, and Power Supply Rejection Ratio? What power supply? :D

se
 
Martin Rupp:

For whatever reason, I couldn't get babelfish to translate the pdf. Babelfish returned a message about a "bad template" of something like it.

??

Greg:

>The 2SK389 is the most suitable dual matched high transconductance Nch JFET<

Absolutely not, as the design of the 4-2SE is just as much about the physical structure of the circuitry as it is about the schematics. During the design process I was revising the physical structure against the schematics and the schematics against the physical structure, over and over until I achieved an optimization level for both that I was nominally satisfied with.

Rather than the 2SK389, I'd look for the most electrically suitable dual-JFET that you can find in a compact, round metal-can package. Now this doesn't mean that the 2SK389 is a bad part, because it isn't. But it isn't suited for what I had in mind for the 4-2SE.

hth, jonathan carr
 
Hey Kwak -

a folded cascode is a cascode all the same . The load is LOW and the capacitance is not a big issue!
Crap about dark sound - dark sound doesn't win awards! What have you ever done....

Oh.. Johnathon has something superior ... but it's a secret? Oh revelation ... is that in your mind of minds... never mind.

Oh.. the B***s*** that flows in Audio.
 
>The 2SK389 is the most suitable dual matched high transconductance Nch JFET<

Absolutely not, as the design of the 4-2SE is just as much about the physical structure of the circuitry as it is about the schematics. During the design process I was revising the physical structure against the schematics and the schematics against the physical structure, over and over until I achieved an optimization level for both that I was nominally satisfied with.

Rather than the 2SK389, I'd look for the most electrically suitable dual-JFET that you can find in a compact, round metal-can package. Now this doesn't mean that the 2SK389 is a bad part, because it isn't. But it isn't suited for what I had in mind for the 4-2SE.

hth, jonathan carr [/B][/QUOTE]


My SE (Steve Eddy) buffer uses the 2N5566 as an input JFET. But this is a cascoded single ended, current sourced, dc coupled, discrete circuit. Not differential.
I like the idea of a folded cascode, but the AD829 is not an option. Too low current.
Thanks for giving us a target to shoot for. Hope a reasonalbe diy option comes along.

George

George
 
jcarr said:
For whatever reason, I couldn't get babelfish to translate the pdf. Babelfish returned a message about a "bad template" of something like it.

Hi Jonathan,

Babelfish won't translate the entire PDF. Instead, open the PDF in one window. Then use the "Text" tool in Acrobat to select text. Copy and paste into the window with babel.altavista.com running in your web browser. (I think the word limit is 150 words at a time.) Then copy those results (one chunk at a time) into a word processor or text editor.

Here's a sample to show the method works:

Stereoplay is admits for it, after inexpensive High end to search. Like that the test of a 25000-Euro-Vorstufe was violently disputed in the conference of topic. Read here exclusively, why the Lyra Connoisseur 4-2 L SE found the way in the long run nevertheless in the booklet.

I guess it only sort of works, because then you need to translate the translation! :xeye:

Cheers,
Charlie Hansen