Jfet Boosters Help

[AM]

Hi everybody,
I am building a 3 in 1 effects box for my pedalboard. The last effect in the box is going to be one of the following boosters:
http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album14/album143/DOD_201_JFET_PREAMP_VERO_001.gif.html

http://www.aronnelson.com/gallery/main.php/v/rnfr/VERO/ANUBICSFETBOOSTER.PNG.html

I want to have the maximum input impedance and the minimum output impedance to drive the cable to my amp. I tried to do the calculations on my own but the results I get are a bit weird. I'm sure I'm missing something. Can somebody help me with this please?
Thanks in advance.

P.S.: I also just realized I don't have a 2SK117 transistor if I decide to build the Anubicsfetbooster. I looked at my Transistor Substitution Chart but I can't find any substitute. Does anybody know what else could work there?

[Gus]

http://www.vishay.com/fets-small-signal/ssfanp/

First you need to know what you want
define high input impedance  100k, 470K, 1meg, 10meg, 100meg, 1gig?
define the output drive needed, current and/or output resistance what is the worse case load that the preamp will drive? 10K, 100K, 1meg etc?
define the gain wanted, x what?

That will help define what you need for selecting a FET or using what you have.

[AM]

First you need to know what you want
define high input impedance  100k, 470K, 1meg, 10meg, 100meg, 1gig?

1 meg would be fine. As I said it would be the last effect driving a 30 ft cable  to an amp with the usual 1M input impedance.
Unity gain or a bit of boost to drive the amp a bit harder.
Do you have any idea what are the input - output impedances of the circuits I linked to above?

[Ben N]

For that application, why a jfet? Why not an opamp on 18v?

[Gus]

You did not post 30ft cable and its capacitance per foot.

A JFETs gate resistor is it's input resistor in cases like the above linked circuits.  Schematics are better than layouts for help.  The 201 is about 3.9meg input. the other1meg

Of the two linked I would build the anubics fet booster, you can change the input resistance changing the 1 meg value. I would leave it at 1meg you can go up or down.

Are you sure you want an JFET stage?  You could do this with an opamp at 9VDC to higher voltage DC as posted above at a low gain setting not to clip or a BJT circuit did you check the beginner project?


I see BF245B in the schematic(not the layout) I found using google

[AM]

For that application, why a jfet? Why not an opamp on 18v?

No particular reason other than that I have all the parts needed and I heard that the DOD one does a good job.

A JFETs gate resistor is it's input resistor in cases like the above linked circuits.  Schematics are better than layouts for help.

Sorry Gus. I saw the schematic being attached to the layout of the first one and it totally skipped my mind about the second one. Sorry.
I understand what you said about the input impedance. What would be the output impedance of the circuits then? I still haven't figured that out. Unfortunately I can't provide any info on the capacitance of the cable I use to go from pedals to amplifier. It's of good quality though.
Schemo links:
http://analogguru.an.ohost.de/193/schematics/Analogguru_FET-Booster.gif
http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album14/album143/DOD_201_JFET_PREAMP_VERO_001.gif.html

Are you sure you want an JFET stage?  You could do this with an opamp at 9VDC to higher voltage DC as posted above at a low gain setting not to clip or a BJT circuit did you check the beginner project?

I checked it. Could I replace the 5k pot with a 10k one? It would save me a trip to the store. What would be the result of that substitution. I also have a few MPF102. Could I use those without changing anything else at your circuit?
I read this article about boosters and the drawbacks of having the volume controls at the output. This raised some of the questions I have and thought a design without a volume pot at the output like the anubics might be more effective as far as lkeeping the output impedance low.
http://www.muzique.com/news/boosters-are-not-buffers/

[slideman82]

For that application, why a jfet? Why not an opamp on 18v?

Because JFETs sounds way much better than OA...

[Ben N]

JFETs have fairly high output impedances; driving a long cable is something best done by a device with a low output impedance. That's why a BJT would be better, and an opamp, preferably something like a 5532, would be best. Check AMZ for his super buffer, which parallels a couple of opamps for super drive capabiity.

If you especially have your heart set on JFETs, though, there is a hybrid FET-BJT buffer that RG Keen posted not long ago, with the obvious advantage of the JFET's high input impedance and the BJT's low output impedance. Or look at AMZ's notes on paralleling jfets.

[Ben N]

For that application, why a jfet? Why not an opamp on 18v?

Because JFETs sounds way much better than OA...

In a low-gain boost/buffer? Really? In what way? Do you have any evidence for that claim? I'm not sure, but it doesn't sound to me like AM is looking for coloration here.

[slideman82]

In that way, you're right, but I love the sound of JFET singles stages, reacts pretty good with overdrives and distortions.

[Ben N]

Have you seen:
http://www.diystompboxes.com/smfforum/index.php?topic=75862.0?

[AM]

Have you seen:
http://www.diystompboxes.com/smfforum/index.php?topic=75862.0?

I'm checking it right now. A guy who built it used a TL071. You recommended a 5532. Do all of them work without having to change other resistor-caps values? Is it just a matter of keeping the circuit the same and just socket different ICs until you are happy with the results? Thanks for pointing this one out.

[alanlan]

High input impedance - determined by input resistor
Low output impedance - if you've got a long cable and you don't want to lose high end, then either an op-amp buffer, emitter follower or source (JFET) follower will do the trick.

My personal choice would be an emitter follower.  If you want a volume control then put it between the JFET and the output buffer, not after the output buffer.

[AM]

High input impedance - determined by input resistor
Low output impedance - if you've got a long cable and you don't want to lose high end, then either an op-amp buffer, emitter follower or source (JFET) follower will do the trick.

My personal choice would be an emitter follower.  If you want a volume control then put it between the JFET and the output buffer, not after the output buffer.

Man, your answer was short, simple to understand and straight to the point. With a bit of additional googling (is that a word  )
I figured out what I need to do. Thank you!

[Ben N]

Have you seen:
http://www.diystompboxes.com/smfforum/index.php?topic=75862.0?

I'm checking it right now. A guy who built it used a TL071. You recommended a 5532. Do all of them work without having to change other resistor-caps values? Is it just a matter of keeping the circuit the same and just socket different ICs until you are happy with the results? Thanks for pointing this one out.

I'm guessing this is moot now, but just for completeness: I mentioned a 5532 specifically as an output buffer with very low output impedance (and great noise specs, too). It is a dual, so not exactly interchangeable with a TL071, but running it in parallel gives you an even lower output Z. (The single opamp version is the 5534.) The caveat is that its input impedance is quite low for an opamp, so it needs to follow something with a relatively low output-Z for it to work optimally. I figured that the other stuff in your box might provide that. If not, the TL071 (or the dual version, TL072) or similar would be better.

I strongly recommend you read the various articles on buffers and boosters at AMZ, to give you a handle on the basics.

[AM]

I'm guessing this is moot now,.....

Not at all. Thank you for your suggestions and for pointing out those links. Your input has been greatly appreciated.

[Gus]

http://analogguru.an.ohost.de/193/schematics/Analogguru_FET-Booster.gif

If you need more output current.

Direct couple the Base of a NPN High Hfe transistor to the drain of the FET place say a 3.3K in the emitter leg to ground, collector to 9VDC, move the output cap to the emitter of the NPN you might want to move the bias point up a little it helps to have a scope to check the output waveform under load and adjust the bias.

Like I asked what is worst case load for the booster