diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Solid State (http://www.diyaudio.com/forums/solid-state/)
-   -   best small signal FET? (http://www.diyaudio.com/forums/solid-state/62936-best-small-signal-fet.html)

hesener 22nd August 2005 01:57 PM

best small signal FET?
 
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

jackinnj 22nd August 2005 02:55 PM

check out:
www.linearsystems.com

their LSK170 looks pretty compelling. there are SPICE models available on their website.

they have distributors all over Europe (and 2 in the U.S as well)

jcarr 22nd August 2005 03:58 PM

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

Panelhead 22nd August 2005 04:23 PM

Thanks for sharing this info
 
Quote:

Originally posted by jcarr
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

MRupp 22nd August 2005 09:50 PM

Quote:

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/medi...tp0805Lyra.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.

amplifierguru 23rd August 2005 05:40 AM

The 2SK389 is the most suitable dual matched high transconductance Nch JFET and, if you want symmetry a J109 is the corresponding Pch dual JFET. You can do a simple folded bipolar cascode off that with a bipolar EF output.

Too easy. Sounds great - wins awards!

Cheers,
Greg

EUVL 23rd August 2005 08:28 AM

> 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 23rd August 2005 11:44 AM

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? ;)))

Elso Kwak 23rd August 2005 12:23 PM

2SK389 and 2SJ109 have high capacitance and should be cascoded. Unless u like ur sound dark brown.

Ultima Thule 23rd August 2005 06:44 PM

Konichiwa Jonathan-san!

May I ask what an IRF510 makes in such a luxurious product as 4-2SE? :scratch2:

May it be something like
this headphone amp M0nty started in this recent thread? ;)

Well done and greetings to the high scores received in Stereoplay.de!

Gre Michael


All times are GMT. The time now is 04:15 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2