JFET preamp?


2004-03-23 10:43 am
JFET preamp?
I think that, within short time, Jfets will be the buzzword in the Chipamp forum:D In short time you will see preamps popping up, based on Jfets:D Not a bad thing at all:) In the meantime , take a look at this:
http://users.verat.net/~pedjarogic/audio/gainclone/gainclone.htm Just for inspiration:) I guess you will be hearing 2SK170, quite a few times now:D Yep, those darn Hitachi devices rock's! If you can get them;)


BTW here is a bit more to chew on;) : http://www.borbelyaudio.com/
The preamp originally cited was a common source amplifier with gain. A simple common source JFET amp will have a fair amount of distortion (~0.5-1% if done right, assuming low gain and and output voltage of ~1-2V). Most of the distortion is low order (2nd and 3rd harmonics). The preamp was originally being used with an electric guitar, though, so the distortion in that application was probably irrelevant for the most part. The common source buffer cited in the next post is a neat idea for overcoming the relatively low impedance of an inverting gain clone. Distortion can be pretty low (0.005% simulated).

If you are using a low slew rate power opamp and need some extra gain to avoid slew rate problems, it might be a better idea to just give in and use an opamp if you want a modest gain boost and are concerned about adding distortion. If you don't care, then a simple JFET commmon source amp will work. There will be a big turn-on thump, though, due to the single ended capacitor coupled output.
I did a simulation today using a PN4303 (halfway decent audio FET with reasonably low noise and fairly low transconductance). With the standard model in ORCAD, a 680 ohm source resistor and a 6.8k drain resistor gave me a gain of almost exactly five. For 200mV in (1V out), I got 0.3% THD, almost all 2nd and 3rd harmonic. This agrees pretty well with what I've gotten in real life. With these values, the drain voltage was fairly well centered using a +30v supply. For real - life FETs, you'd have to select devices or jigger the values around for proper gain and output voltage centering, but the results look reasonably encouraging. Last time I looked, Electronic Goldmine had the PN4303. Mouser also has them from Fairchild and Vishay. If you go this route, I'd spend the extra money and opt for the Vishay devices.

BTW, you never really mentioned wheter you wanted a gain block or a buffer in front of your clone, though since you originally cited the common source amp, I assumed the former. If you want gain, how much?