jfet substitutions

[eschertron]

While I'm still thinking about it, I received a handful of 2N5485s for an eek!bay seller. I used the cascode [high voltage solid state preamp] schem (second page) from here.

As I suspected, the input headroom is about twice the jfet's Vgs_off. That's about 2vpp input with the 2N5485 that I had, as compared to the about 14vpp input for a j111 transistor. Once I got the cathode resistor (er, I mean the source resistor, Rs) to where it was about center bias, I measured an unbypassed gain of about 96 - as compared to the unbypassed gain of 20 with the j111. As I added capacitive bypass the gain went up, but not as fast as the increase with the j111. Not sure why. Comments?

I settled on a Rs value of 680R, but for really small-signal inputs I don't think it would be all that critical. I can see using this transistor (2N5485) to amplify really small signals (uV, mV) like microphones. Whereas the j111 can easily handle the full rail-to-rail output of a 9v powered pedal without clipping, so I am tempted to try it as the first stage in a guitar/line level amp. Not sure when, but maybe soon.

[loudthud]

In general, JFETs with lower Vgs(off) will have lower transconductance. They will also have higher Rds(on). So the J111 has way way more gain than the 2N5485. JM selects his JFETs for a low Vgs(off) and those parts also have lower gain. If he just measured Rds(on), he could find the same parts for his preamps and possibly save some time. In most low Voltage circuits, the tendancy would be to use a lower Drain resistor for the J111, and that would bring the gain down substantially. In this circuit, all JFETs get the same load resistor because of the cascode and gain for the J111 is so high, it's not a good choice. When you use unbypassed Source resistors to reduce the gain of a JFET, you quickly get to the point where distortion is quite small. If the bypassed gain is 50, don't try to reduce it to 10. 40 is about as low as is practicle. An article by Dimitri Danyuk (AES 2004) can be found where a method is described to match the distortion of a JFET to that of a tube.

[eschertron]

When you use unbypassed Source resistors to reduce the gain of a JFET, you quickly get to the point where distortion is quite small

You write that like it's a bad thing

Unless I'm slamming a triode input stage the distortion is quite low too, because I'm operating in a fairly linear region of the tube, right? My assertion is that the input stage of an amp (and I mean a buzzy, farty, fizzy rock guitar amp) will have very little to do with the end result of the amp's signal processing. And even when the stage is slammed, the pedals are probably forcing their brand of distortion down the amp's throat.

I haven't built any hybrid amps yet, but I am seriously thinking what my options are for when I need "just one more" 1/2 triode stage. I've played with R.G.'s mosfet cathode follower and understand (from this forum, and Merlin's excellent preamp book in particular) that there are times when the CF is adding a great deal of distortion so a mosfet is not a good choice.

There are probably quite a few circuit designers who, like good chefs, make sure that every ingredient can be tasted and adds to the gestalt. I'm not there yet. But I don't want to turn a deaf ear, and I do want to learn. So thanks!