j109, j111, j112, j113, or BF256b for AMZ buffered splitter?

[focusbob]

Hi, I would like to build some AMZ buffered splitters, but would like to use a current production through hole jfet instead of the j201 the layout calls for (would also prefer to avoid the smd j201 + adapter board option, if possible). j109, j111, j112, j113, and BF256b are the available options on Mouser. Which would be best for this project? Thanks!

[11-90-an]

Welcome to the forum.. 

As far as I know, j112 and j113 should be fine... not sure about the others... probably would have to tweak some values, though...

[LightSoundGeometry]

Welcome to the forum.. 

As far as I know, j112 and j113 should be fine... not sure about the others... probably would have to tweak some values, though...

agreed. you will have to lay the circuit on a breadboard and get the drain resistor to bias the fet between 5-7 volts

[pacealot]

Mild necro-bump to mention that I breadboarded this successfully with a pair of J113s without altering any values. J111s, however, did not work (I didn't bother to try to tweak it with them, as I just went with the J113s).

In fact, I'm now trying to figure out what I could possibly do with over a dozen J111s which I can't seem to use for anything I'd originally bought them for!

[Rob Strand]

In fact, I'm now trying to figure out what I could possibly do with over a dozen J111s which I can't seem to use for anything I'd originally bought them for!

With higher Vgs_off devices you might be able to get it to work by removing the R2 resistor, or at least making it larger until the DC output voltage is about 4.5V,

http://www.muzique.com/lab/splitter.htm

[pacealot]

Thanks, Rob — that hadn't occurred to me. Removing R2 brought it down to around 7.4V — still not where it ought to be yet, but it does actually function as a buffer now!

[Rob Strand]

Thanks, Rob — that hadn't occurred to me. Removing R2 brought it down to around 7.4V — still not where it ought to be yet, but it does actually function as a buffer now!

You might be able to scrape a bit more by reducing the source resistor.  (more current means lower Vgs)

[pacealot]

You read my mind! I started dinking and got it down to ~4.6V with 150Ω. Thanks, Rob! My poor neglected J111s thank you as well....

[Rob Strand]

150 ohm is pulling quite a bit of current, ID = 4.6V / 150 = 30.7mA.

You can trade a little bit of bias voltage for a fair amount of current.

Rs    ID             Vgs
220   23mA      5.05V           
390   14.5mA   5.64V
470   12.3mA  5.80V        <----- maybe around here  40% of current for 1.2V bias shift
1k      6.4mA     6.4V

You might see the waveform compressed a bit due to the high Vgs_off of the JFET.
(I roughly estimated IDSS = 175mA and VP = -7.91V)

[pacealot]

That sounds like a very good trade-off. I hadn't quite gotten around to crunching the numbers—or reading the current draw—yet. Thanks yet again. This time my batteries thank you as well!

I'd thrown together a quick ROG Vp/Idss circuit together earlier to measure the J113s, and when I stuck a J111 in there, I think I got readings right around where your estimate was....

[pacealot]

And for the specific use this particular buffer/splitter will be used for (splitting a super-saturated mono fuzz output into both sides of a stereo bass rig), a bit of waveform compression will probably be hardly noticeable!