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Messages - Ironsinthefire

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Preamps and Effects / Re: The JFET Bender Preamp
« on: August 24, 2017, 09:46:19 AM »
I got the 2sj103 from my bag of fets, I think it's obsolete now in through-hole format.  I think it was a leftover from back when I used to repair Yamaha audio products. It's not too twitchy to adjust, I used single turn pots for my build.


The pinout requires criss-crossing leads for your PCB layout.

Preamps and Effects / Re: The JFET Bender Preamp
« on: August 23, 2017, 12:37:29 PM »
Up and running. This is a scope shot showing input and output of the first stage. Jfet is rounding off the bottom of the trace, when you push it harder the diode "grid clamping" starts acting on the top of the trace.

This is using a J103 fet adjusted to the 80% gain mark. Looks OK?

Preamps and Effects / Re: The JFET Bender Preamp
« on: August 22, 2017, 05:19:57 PM »
Design goals understood, thanks for taking the time to explain. Looks a good "best for least" solution.

I've got the board partially stuffed and the first stage working. I'm using a 2SJ something  for the FET and it seems to work, I've got some asymmetry on the scope,  but I think the FET specs might be a bit off. Found that what appears to be the SMD equivalent of the 2N5462 from Digikey- the MMBF5462 at 48cents each and a nice source of mini SMD to through-hole kludge boards for 25 cents each.

I should probably stop trying to gild the lilly, just build this thing stock and see how it sounds.

It would be cool to take a multi-pole switch and wire it to be able to switch the fets and grid clamping simulation diodes out so you could A/B the difference. (You could have the ground leg of the op amp feedback network switch to a trimpot to ground so you could match the FET in circuit /bypassed levels exactly )

Preamps and Effects / Re: The JFET Bender Preamp
« on: August 20, 2017, 10:30:53 PM »
Etched the PCB tonight, so I will probably be up and running soon. (Had a test circuit on a protoboard.)

A thought or two- based on your scope traces, it looks like the gain of the tube circuit you emulated with the bender is about 60X. That must be unloaded? In the tube Fender, the first stage is loaded by the tone stack and the gain is more like 43X. (going by the AC voltages off the Fender DRRI schematic, 37mV in =1.6V out) So, maybe better to adjust things to more closely emulate the original?

I'm guessing that your design process was to note that the tube circuit swings about 200V pk-pk at the limits, the op-amp/jfet is limited to 30V or less pk-pk by the 15 volt rails, so the gain of the op amp stage has to be scaled accordingly to get the right headroom. 200/30= 6.6,  so using your gain of 60X, the op amp's gain should be 60/6.6= 9X . I may try revising the gain of my build down, as the op amp won't get loaded down at all from the tone stack due the OpAmp's low output impedance.

Preamps and Effects / Re: The JFET Bender Preamp
« 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.

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.

Preamps and Effects / Re: The JFET Bender Preamp
« on: July 21, 2017, 05:01:55 PM »

Given the difficulty of finding appropriate P-channel FETs nowadays, would preceding the first stage with an inverting, unity gain buffer and switching the FET to an N-channel work? I guess you could follow the first stage with an inverting buffer to flip the phase back afterwards? I would guess that you'd have to move the input diode clamping circuit to after the first inverting buffer and maybe just have single diodes to the positive and negative rails of the first buffer for protection. Does that seem reasonable? Could do all that with a dual op amp and four resistors....

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