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Author Topic: The JFET Bender Preamp  (Read 4907 times)

tarahall

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Re: The JFET Bender Preamp
« Reply #15 on: July 26, 2017, 06:21:19 AM »



Attached below is a layout I did and an updated schematic. I took you suggestion and added resistors in series with the pots to limit their span and make them easier to adjust.

Would there be any difference/advantage in inserting the FETs between the resistor(s) R4 and/or R9 and the caps(s) C1 and C3 instead of between the caps and ground ?

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Ironsinthefire

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Re: The JFET Bender Preamp
« Reply #16 on: July 26, 2017, 04:56:43 PM »


It shouldn't be that hard to find P-channel JFETs.

(Refer to the schematic JFET_bender_7.gif) Without P-channels, use an N-ch on the first stage. Reverse diodes D1 to D5 except connect D5 to the +15 rail. Keep R3 and R4 the same and use negative bias on the Gate of N-ch Q1. Insert an inverting stage with a gain of one between the first stage and the tone stack. Use the stage after the tone stack as is.

Thanks!
Working on  another project, I was recently burned on a shipment of 2n5484 fets from eBay/China (they shipped mystery NPN bipolars with the Fairchild logo and 2n5484 silkscreened on them!) so I'm hesitant to work with parts I can't get from Digikey/Mouser, etc.  Anyway, I decided to dig deeper and found some 2SJ and 2SK fets in my parts bin which seem to work. I have a few more design theory questions about this circuit (basically wondering about the stage gains, the use of a single "grid clamping simulator diode" on the 2nd stage etc.) but will wait until I've organized my thoughts a bit more before asking to avoid wasting your time.
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Loudthud

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Re: The JFET Bender Preamp
« Reply #17 on: July 26, 2017, 05:06:20 PM »



Would there be any difference/advantage in inserting the FETs between the resistor(s) R4 and/or R9 and the caps(s) C1 and C3 instead of between the caps and ground ?

When you overdrive one of these stages, there could be some DC accumulation on C1 or C3 because of the asymmetrical clipping. This DC would interact with the DC bias on the Gate of the JFET. It's' hard to say if it would be audible or if it would sound good or bad. Try it.

FYI: The polarity of C3 might be an issue. The offset of the opamp could go either way. Short out C3. If pin 1 of the opamp goes positive, C3 is OK. If the polarity is negative, reverse C3. If you can find a 4.7uF film cap or a Non-Polarized electrolytic, you can use those without a worry.
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tarahall

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Re: The JFET Bender Preamp
« Reply #18 on: August 12, 2017, 09:16:43 PM »

The JFET is used as a variable resistor in a region where it's resistance changes at different current levels. MPF102 JFETs were used because they had a bendable resistance in the right range.

I post this as it may be of some interest to those following this thread.

On researching that presumably "CRATE" overdrive circuit idea in US patent 5647004 filed by St. Louis Music, I came across Scholz's US patent 4627094 which uses an N-Ch JFET (possibly a 2n4340) to vary the gain in the compressor IC (probably a TL072) using the gate's bias voltage being varied by a DBX 2252 rms level detector chip - see fig 2a in the patent. Not exactly the same structure as yours Loudthud I know, but it would obviously lend weight to that JFET variable resistor concept. 

I guess this patent this was the original circuit idea for one of the Rockman models. I know they used TL072's in the little headphone rockmans as I have two of them (a model 1 & a model 2), and the JFET used in those was a 2N4340 although those models don't use the DBX chip.

I have circuit tracings of the 2 models that I did using Express PCB's schematic program if anyone is interested in copies but I have not verified their complete accuracy - they were only used as guide for my curiousity :)
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