Scott Hampton JFP Jfet gain module... on a Bipolar +-15 volt Power supply?

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MikeClev

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Joined
Mar 18, 2009
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184
Location
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Hello People!

I am in the process of knocking together a 8 into 2 line mixer as seen in this article, and the +24 volt power supply has been causing a few headaches. If I want to put in things like headphone outputs and other gimmicks later, every other circuit I have runs on bipolar +/-15 volts or thereabouts.

The schematic for the JFP gain module can also be seen at the same LINK, upper centre of the page.

What would happen if I hooked up a JFP module to +12v instead of +24v and -12v instead of ground? I know some smart alec is going to say 'Just wire it up and try it!' but I would like to know if there is are theoretical grounds for possible problems. The main thing I can think of is that (and my understanding of JFets is very basic) perhaps the Gate would not like being at 12 volts higher that in the unipolar power supply scenario. Perhaps a capacitor in front of the gate would block DC voltage and prevent this problem.

Advice would be greatly appreciated. I think a Bipolar PSU powered JFet gain module would very useful to have!

Thankyou in advance!
 
Greetings again to anyone who is in fact interested...

I am putting together a test rig featuring a JFP as a line amp in a small mixer, to test whether the Hampton JFP will work on 'standard' Bipolar power supplies. he JFP tested fine on a +24 single rail, output trims to 12v (half supply voltage) as it should do.

Hooking up the Bipolar supply with +12v going to where +24 usually goes and -12v to where 0v should go allowed me to trim to 0v. Good! This was with no modifications to the circuit. The only drawback was that input was now sitting at about -5v, as there was a 10Mohm resistor from input to -12v, so I did not test audio. Routing the resistor to ground resulted in the circuit not trimming correctly, with the output sitting at about +16v, input sitting at 0v.

I am currently re-doing my circuit to incorporate a DC blocking capacitor (0.01uf Wima MKP10) to prepare it for an audio test. Wish me luck!

I still think a JFP on +/-15v will be a great thing to have!
 
Success. Sounds great!

The Scott Hampton JFP is running fine and sounding great on +12v and -12v. Next thing is to crank up the voltage to +15v\-15v. I need to remove a 25v cap first...

Edit: before and after schematic showing the small modification that got this working.
 

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thank you exactly what i needed, I am about to do the same thing!

what about losing the coupling caps on the output if its trimmed to 0v?
 
Just stumbled across your post - exactly what I was hoping to do, as well. I'd like to crank out quite a few JFP single stage modules, and use them for various nefarious purposes.

Glad to hear you got it working on bipolar 12V, but I'm curious how it went with the higher rails? Ideally, I'd like to run them on +-16V for API 500 series use, heck maybe even +-24V for the 51X series (probably require some component substitutions, I'm guessing without studying it).

Congratulations on your bipolar condition.
 
Are people getting clean-ish output on these? As proposed in the schematic mine produced a lot of dirt, changed the FET to another one, lowering the gain significantly, cleaned it up, think I used J310.
 
Hello!

Yes, I'm getting a pretty clean output on a +16/-16v supply. I haven't tried going any higher than that. I wouldn't personally remove the output capacitor unless you are certain the following stage can cope with a bit of DC offset, as the output will drift over time as the components warm up.

I built a pair of JFP / Hampton Preamps with Lundahl 1530 input transformers and they are just about my favourite preamps right now. They have tons of gain... I must measure it some day.

Strangely, I built another pair copying my first pair and they don't work... they draw far too much current and the rails drop to about 12 volts. They both have the same problem so it must be due to something I changed, but what? I need to investigate further!

-Mike
 
Very glad to hear you have these working at +-16V ok...
I think I'm going to try it and then give +-24V a shot. I know these draw some real current, so I suspect heats sinks are in order. I have plenty of different NPN options to try, and probably a hundred or so 2N5457 FETs, as well as some other N-channels. I'll let you know how it goes.

Hope you figure out your problem... Good luck. I still have a FETboy I haven't fixed from a couple years ago, so I know these can be fiddly, as simple as they are. Should be easier a stage at a time.
I hear they work well as a DI, and would like to try that.

I have one of Scott's tube pre kits I built a few years ago, and it is really stellar, btw. Highly recommended (HVTP2, I think?).

Frank
 
Dug my build up last night, and I used a plain jane MPF102 for the fet. I have two gainblocks in series, with a pot in the middle. Clipping starts at 3.5volts (one side, so about 7volts p-p) output voltage.
Used a BD type output transistor, not the to-92 one which is in the original article.
 
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