What do VGS off and IDSS in JFETs mean for audio amplification?

[antonis]

Oldies but Goodies..



From here

[antonis]

I dunno if you've ever taken a gander at Douglas Self's book, Small Signal Audio Design, it contains some truly insane configurations that can get up to 61M ohm input impedance with just five MPSA42 transistors. I don't think that's ever something someone in the guitar pedal world would need, but even the three transistor bootstrapping configuration gets you to a 6.1M Zin. I feel my neurons buckling under the weight of the information in that book every time I open it up.

IMHO, almost all those really valuable informations should be assesed under the prism of author's ulterior intentions..

e.g. implementation of high voltage - low gain BJTs, like MPSA42, is only needed for high voltage / large current / low noise designs..
Almost all high impedance author's configurations can exhibit much higher impedance by the use of high gain BJT's..

[R.G.]

Re: Tillman preamp
In simulation, the following give what looks like very good results for "typical" devices.
2N4393 / MMBF4393
2N5484
2N5485
J202  (!)

[Rob Strand]

J202  (!)

But not the J201.

Tillman doesn't bias well with a J201,
https://www.diystompboxes.com/smfforum/index.php?topic=130258.msg1263159#msg1263159

Improved biasing which matches 2N5457.

Bias Scheme 1:
https://www.diystompboxes.com/smfforum/index.php?topic=130258.msg1263128#msg1263128
Bias Scheme 2:
https://www.diystompboxes.com/smfforum/index.php?topic=130258.msg1263261#msg1263261

[R.G.]

But not the J201.

Yep. That's what the exclamation point was for.

Tillman doesn't bias well with a J201,
https://www.diystompboxes.com/smfforum/index.php?topic=130258.msg1263159#msg1263159

Yep, as I mentioned earlier, you have to change the resistor values to get into its low Vgsoff and low Idss range. I didn't try all the JFETs in my sim model library, but those worked pretty well. The extremes of Vgsoff in any of them will be a problem, but then that's what JFETs do any time you rely on them to set their bias. I picked those JFETs from my article on matching JFETs at geofex.

[Rob Strand]

Yep, as I mentioned earlier, you have to change the resistor values to get into its low Vgsoff and low Idss range. I didn't try all the JFETs in my sim model library, but those worked pretty well. The extremes of Vgsoff in any of them will be a problem, but then that's what JFETs do any time you rely on them to set their bias. I picked those JFETs from my article on matching JFETs at geofex.

I'm yet to see extremes on real parts so it should be reliable.

Old parts like 2N3819's and MPF102 do vary quite a bit but for a given manufacturer even then the parameters aren't so variable. Unfortunately the manufacturer to manufacturer variation makes it difficult to come-up with a single spice model for these.  (I no longer have enough parts covering the different brands to do good statistics.)

FWIW: For the sim on the second circuit I posted  the VP spans of 1/1.5 to 1.5 times nominal then there's some correlations for IDSS and Beta.  Sometimes I use different correlations.   I find most parts sit in that window.

[mzy12]

Re: Tillman preamp
In simulation, the following give what looks like very good results for "typical" devices.
2N4393 / MMBF4393
2N5484
2N5485
J202  (!)

Thanks! I'll see what's available in my region of the world and order accordingly (obviously I'll check the datasheets myself too)

Improved biasing which matches 2N5457.

Good luck getting 2N5457s at a reasonable price these days 

[amptramp]

It may be possible to get matched JFET performance from a TL071.  The inputs go directly to a pair of p-channel JFET's but if you take the output from the offset null current mirrors, you might be able to get not only a decent JFET matched pair, but proper emitter follower buffers as well.  You may not need the output stage of the op amp at all.

[PRR]

TL071.  The inputs go directly to a pair of p-channel JFET's but if you take the output from the offset null current mirrors, you might be able to get not only a decent JFET matched pair, but proper emitter follower buffers as well. 

[Rob Strand]

Good luck getting 2N5457s at a reasonable price these days

The idea behind the biasing tricks is actually to make other JFETs act like the 2N5457's.  The example I gave was getting J201's to act like 2N5457's.   It's fairly extreme since they are quite different parts and as expected the part values had to change.  For JFETs more similar to the 2N5457 you don't need to go so far.

The old JFETs as we knew them are drying up for sure.   There are other options which might need some circuit tweaks for best results.    There's also SMD options.  IIRC you can still get SMD versions of the 2N5457 as MMBF5457.   A month or so ago there was a post on how to make adapters from SMD to TO-92 using small pieces of vero board.

The main motive for keeping good spice models for the old parts is so you can gauge what the old circuits did, then you can match the performance with other parts.

[mzy12]

IIRC you can still get SMD versions of the 2N5457 as MMBF5457

I couldn't find either in stock from the usual suppliers I order from. (I'm in Ireland so who has what available to me at a reasonable price is obviously very from the vast majority of users here, whom I presume are mostly in America). However, digikey still has some available, so I'll see if I can move some of my next order from Farnell/Mouser around to get free shipping from them

I already have some adapter boards. SMD soldering doesn't faze me haha.

[R.G.]

There are some BF-series JFETs in my sim. BF244 comes to mind, but I'll have to check. Are those easier for you to get?

[mzy12]

Unfortunately not! Both the BF244 and 245 are even more unobtainable for me haha. Thanks for trying anyways 

[Rob Strand]

Unfortunately not! Both the BF244 and 245 are even more unobtainable for me haha. Thanks for trying anyways 

What about 2SK stuff?   The 2SK30A-GR and 2SK30A-Y are still around.  The 2SK30A-Y seems like it wouldn't be too far off.  The 2SK30A-GR has a higher VP (Vgs_off) so it would need some tweaks.

A 2SK30A without any suffix would be hit and miss.  If the supplier doesn't specify it you might have to ask.