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Author Topic: The JFET Bender Preamp  (Read 9065 times)

Loudthud

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The JFET Bender Preamp
« on: April 17, 2015, 05:54:14 PM »

This thread is not about a distortion pedal called the Tone Bender.
It is about a preamp that emulates a Blackface Fender tube preamp.

In a previous thread I explored ways of getting opamps to act more like a 12AX7. Link: http://www.ssguitar.com/index.php?topic=2381 This is a continuation of the same idea except a JFET is used in the feedback network. A JFET is ideal for this circuit because it doesn't have a dead zone where it stops conducting current if the bias is set right. I hope some of you will try this idea in some exsisting designs to add some low level even order distortion. I call it the JFET Bender because it bends the straight line input-output relationship an opamp normally produces.

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 originally tried 2N4391-3 but the pot setting was way too touchy. I had to install multi turn pots. The MPF102s have an Idss usually between 8 and 12mA. The Curve Tracer shot shows how Voltage and Current behave around zero volts. When the Voltage swings negative, the curve is fairly linear but as it swings positive, the JFET starts to go into constant current mode and the current levels out. Adjusting the Gate Voltage lets you adjust the amount of bend and thus the amount of distortion.

The distortion generated is mostly pure 2nd order at low output levels. Some 3rd order creeps in as the output of the stage reaches 10V pk-pk. The distortion rises to about 10% (this is adjustable via the pot) when the output of the opamp starts to clip. Because the gain is not the same on plus and minus halves of the waveform, one side clips before the other. To adjust the pot, set the pot near zero Volts. That's where maximum gain is. Apply a signal and adjust the pot until the gain is reduced to about 80% (-2dB) the max value. Or just adjust it by ear for a little gain reduction. If the pot is set too far negative, the JFET cuts off and the gain falls to 1. You can find a place where there is severe clipping on one side of the waveform if desired.

For those paying attention, the gain reduction caused by an N channel JFET in the first experiment is backwards compared to what it's like in the first stage of a tube guitar amp. What is really needed is max gain on positive inputs (closer to vertical line on the curve tracer) and a slow reduction in gain for negative inputs (the curve of the JFET lays over horizontally). I played guitar through a couple of stages setup backwards like this with a tone stack in between and it didn't sound right. It just didn't have the Fendery clean tone I was looking for.

Next I tried a J201 connected differently to invert the distortion. The Drain is grounded. When the Gate and Source swing positive, you have the bulk resistance Rds(on) in parallel with the Gate to channel diode. This is fairly linear until the diode starts to conduct. When the Gate and Source are pulled negative, you get the Idss curve and it lays over (resistance increases) in the third quadrant. The disadvantage of this circuit is that there is no adjustment and the resistors in the feedback network have to be adjusted to get the current and voltage in the right range for the J201. This circuit sounded better as a first stage but there was not enough distortion at the gain I wanted for the first stage. Distortion was in the range of 1% to 4% at 1Vrms input depending on the impedance in the feedback network. Too big a resistance in series with the JFET in the shunt arm of the feedback network limits the amount of distortion the JFET can generate.

Lastly I looked for a P-channel JFET that had favorable characteristics. J174 and J175 were nowhere close. The 2N5462 looked good. It is a near compliment to the MPF102. This worked in the original circuit, all that was needed was to tweek the pot for positive bias on the Gate. A 2N4343 was also used, it worked about the same.

Since a tube preamp has mostly inverting stages, a non-inverting opamp preamp will have to flip the distortion on alternate stages. I think not doing this is why the first try didn't sound very good. So a P-channel JFET is in the first stage and an N-channel JFET is used in the second stage. The second stage needs a gain around 50.

When the gain is high, there is not much Voltage available for the JFET to distort. On the second stage the feedback resistors set the gain at 100 and the pot is adjusted to get 6dB of gain reduction. This gets the distortion up to around 10% before the opamp starts to clip.

The preamp is configured to mimic the Fender Blackface preamp. The first stage has low gain to provide about the same headroom as a tube, the diodes on the input clip the incoming signal so as to be pedal friendly and protect the opamp. The diode on the second stage is inverted because the input stage opamp is non-inverting.

To test the circuit out, I used a Peavey Mark III Standard from the 80's. It has two opamp based preamp channels, one with a FMV tone stack, and a handy power amp input on the front panel. The power amp is a 260D (130W at 4 Ohms) with Voltage only feedback. Without the capacitors in series with the JFETs, there was too much subsonic feeding through to the power amp so the caps were added to limit subsonic response.The amp gives a very convincing Blackface tone for both bass and guitar. I especially like it for bass, but the caps could be increased slightly to give a little bit more low end.
« Last Edit: July 25, 2016, 08:27:44 PM by Loudthud »
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Roly

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Re: The JFET Bender Preamp
« Reply #1 on: April 17, 2015, 10:17:19 PM »

Thank you for a full and illuminating report on your investigations.  This moves things forward.

 :dbtu:
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Matec

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Re: The JFET Bender Preamp
« Reply #2 on: April 18, 2015, 03:56:33 PM »

Hello Loudthud.

Glad to have found someone who has ideas and research close to what I have done for many years.

I'm not a master in the art (as mr. KMG), but I noticed a parallel between our work.

Now I saw this and its other posts will follow them, and occasionally make some suggestions

Cheers
Matec
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Loudthud

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Re: The JFET Bender Preamp
« Reply #3 on: April 22, 2015, 05:26:01 PM »

I thought I would provide a little background on how the JFET Bender emulates a triode. Some of this has been covered before in this thread: http://www.ssguitar.com/index.php?topic=2381

The first scope shot is an X-Y display of a Chinese 12AX7 with a 100K plate load to a 300V supply. Input is the horizontal axis, the plate is the vertical axis. As the input moves negative (to the left) there is a reduction in gain. This is where the JFET needs to become a higher impedance to reduce the gain. The next photo shows the JFET Bender. To make a clearer display on the scope, the 1uF capacitor in series with the feedback resistor is shorted out. To better match the 12AX7, the input clamp was reduced to three diodes as shown in the schematic. The last scope photo shows what happends when the bias is reduced on the JFET taking it out of it's nonlinear region. The input clamp is still working on the right side of the input.
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tarahall

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Re: The JFET Bender Preamp
« Reply #4 on: February 18, 2017, 12:54:49 AM »

how critical are the FETs? Is it the gate voltage that is the important factor?
If I dont have MPF102's will other FETs suffice - eg:2sk170, 2n3819, 2sk117 etc?

TIA.
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Loudthud

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Re: The JFET Bender Preamp
« Reply #5 on: February 18, 2017, 04:37:26 PM »

As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.
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tarahall

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Re: The JFET Bender Preamp
« Reply #6 on: February 19, 2017, 06:37:11 AM »

As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.

Thanks for the heads up.

I have a selection of N-FETs on hand (2sk170, 117, 30, 2n5457, 3819, J201, MPF106) so I'll check their Idss and find some within that spec. The P channel 2n5462s I have some on order.
I also have some LND150 depletion mode mosfets but I'm not sure they would be suitable.

Cheers
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tarahall

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Re: The JFET Bender Preamp
« Reply #7 on: March 05, 2017, 04:33:22 AM »

I breadboarded this up using a 3819 and a 5462. It certainly had the crsipness and brightness I have come to expect from the last blackface deluxe I played thru recently. The 200k pot adjust seems to be really touchy though. I tried using an 18k and 12k in series across the Vcc to ground (and Vee to ground in the MPF102 side) and then bridged the 12k with a 100k linear pot (which I had on hand) and fed the wiper to the gate of the FET. It definitely helped with the ease of adjustment
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tarahall

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Re: The JFET Bender Preamp
« Reply #8 on: March 11, 2017, 09:28:38 PM »

As a general guideline I would say you want a JFET with an Idss roughly between 2mA and 10mA. The 2N3819 should work fine. If Idss is higher, the pot becomes very touchy, you might need a 10 turn trim pot. This all depends on the resistances in the feedback network. Lower impedances work with higher Idss parts. Just keep the ratio of the resistors the same.

Are the gate voltages for the 2n5462 (+3.3) & mpf102 (-2.8) the optimum for your circuit or are they just indicative of the voltages you needed to set for the specific devices you used?
I'm curious as we all know how varied the tolerances and ratings of JFETs of the same type can be.

I have tried adding your JFET idea to Rod Elliots's Project 27 preamp (mentioned elsewhere on these forums) and the results are certainly encouraging although I had to adjust the gain of the first 2 OPAmp stages upward. It made the preamp brighter with less mid-range wool and a crispness and bite one expects from a typical Fender front-end.
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Loudthud

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Re: The JFET Bender Preamp
« Reply #9 on: March 12, 2017, 03:18:02 AM »

  Are the gate voltages for the 2n5462 (+3.3) & mpf102 (-2.8) the optimum for your circuit or are they just indicative of the voltages you needed to set for the specific devices you used?
I'm curious as we all know how varied the tolerances and ratings of JFETs of the same type can be.

Those Voltages are just to let you know what a typical might be. Each preamp needs to be adjusted for the JETs actually installed. The best way I found to adjust the pots is the "Gain Reduction" method. Set the pots to the zero Volt end, apply a signal and monitor the output of the stage with a scope or DVM. Adjust the pot for a 20% reduction of the signal level (first stage) or a 50% reduction (second stage).


I have tried adding your JFET idea to Rod Elliots's Project 27 preamp (mentioned elsewhere on these forums) and the results are certainly encouraging although I had to adjust the gain of the first 2 OPAmp stages upward. It made the preamp brighter with less mid-range wool and a crispness and bite one expects from a typical Fender front-end.

IIRC Rod's preamp used a Baxandall tone control network. The Fender type network will certainly sound different. I chose the gains of the two stages to mimic the headroom found in Fender amp and a distortion profile where distortion slowly rises and reaches about 10% before the opamps hit the rails.

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.
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tarahall

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Re: The JFET Bender Preamp
« Reply #10 on: March 12, 2017, 10:54:07 PM »


IIRC Rod's preamp used a Baxandall tone control network. The Fender type network will certainly sound different.

Actually the tone stack in Rod's Project 27 (see attached) is FMV. The Baxandall stack is used in his Project 94.

Project 27 is the one that I used as a test setup - I bought 2 of the boards for experimentation purposes. In the circuit schematic, I replaced resistors R6 & R11 with a 1.5k and 820r respectively and mounted C3 & C8 externally with the JFET network on a daughter prototype board. I set the gate volts at +3.3 and -2.8 as an initial test (aurally) although I realize they would need to be adjusted to suit the specific FETs.
 
I didn't worry about the diodes at this stage as I don't use OD pedals or active circuit guitars. The results using a standard Strat with Seymour Duncan vintage PUs were extremely encouraging.

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tarahall

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Re: The JFET Bender Preamp
« Reply #11 on: March 17, 2017, 05:04:18 AM »

I tried this with a James (passive Baxandall) tone stack circuit derived from a Goldentone/Fender Blonde hybrid combination. It sounded promising too.

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Ironsinthefire

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Re: The JFET Bender Preamp
« Reply #12 on: July 21, 2017, 05:01:55 PM »

Loudthud,

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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tarahall

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Re: The JFET Bender Preamp
« Reply #13 on: July 25, 2017, 10:04:35 PM »

Loudthud,

Given the difficulty of finding appropriate P-channel FETs nowadays

I got some 2n5462's from here:

www.utsource.net

HTH
Gavan
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Loudthud

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Re: The JFET Bender Preamp
« Reply #14 on: July 25, 2017, 11:29:54 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?

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.

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