I made an onboard Tillman bass preamp with a 2SK117 for a 2 humbucker bass. It sounds great but the signal clips when I play a string hard. I increased voltage to 18v and biased different voltages at R2 and it didn't help 
I changed R1 to 500K is beacuse I was told to lower the resistance if the pickups are wired directly to the preamp. the values below are in the preamp with 18v power. The drain to ground reads 9v.
Q1 2SK117
R1 500K
R2 130 Ohm (5K trimpot)
R3 6.8K
R4 50K pot
C1 47uf
C2 47uf
maybe add components or change some values to stop the clipping? do I need to change R3?
I changed R1 to 500K is beacuse I was told to lower the resistance if the pickups are wired directly to the preamp. the values below are in the preamp with 18v power. The drain to ground reads 9v.
Q1 2SK117
R1 500K
R2 130 Ohm (5K trimpot)
R3 6.8K
R4 50K pot
C1 47uf
C2 47uf
maybe add components or change some values to stop the clipping? do I need to change R3?
QUOTE I increased voltage to 18v and biased different voltages at R2 and it didn't help
hello.
the gain of the 2sk117 is much bigger than of the original jfet...........
I changed R1 to 500K is because I was told to lower the resistance if the pickups are wired directly to the preamp.
this can help only a little bit............you can use 1 meg.
the values below are in the preamp with 18v power. The drain to ground reads 9v.
this is an acceptable drain potential..............
Q1 2SK117
R1 500K
R2 130 Ohm (5K trimpot)
R3 6.8K
R4 50K pot
C1 47uf
C2 47uf
maybe add components or change some values to stop the clipping? do I need to change R3?
you can lower r4 down to 20k or so..............if it doesn't help try it with 10k.this load res/pot attenuates the output volts .
.......and you can use a 1k res in series with the gate (for radio freq. attenuation).
greetings
hello.
the gain of the 2sk117 is much bigger than of the original jfet...........
I changed R1 to 500K is because I was told to lower the resistance if the pickups are wired directly to the preamp.
this can help only a little bit............you can use 1 meg.
the values below are in the preamp with 18v power. The drain to ground reads 9v.
this is an acceptable drain potential..............
Q1 2SK117
R1 500K
R2 130 Ohm (5K trimpot)
R3 6.8K
R4 50K pot
C1 47uf
C2 47uf
maybe add components or change some values to stop the clipping? do I need to change R3?
you can lower r4 down to 20k or so..............if it doesn't help try it with 10k.this load res/pot attenuates the output volts .
.......and you can use a 1k res in series with the gate (for radio freq. attenuation).
greetings
That Tillman JFet one even clips with a guitar signal and doesn"t add much gain , The circuit just doesn"t seem suitable if you are looking for clean tones .....
I would suggest useing an Fet opamp based preamp , confugured as a Buffer or with a small amount of gain if you need it ..... I use a simple Opamp preamp that I threw together in my Guitar and it sounds great ..... I can post it for you if you want ?
Cheers
I would suggest useing an Fet opamp based preamp , confugured as a Buffer or with a small amount of gain if you need it ..... I use a simple Opamp preamp that I threw together in my Guitar and it sounds great ..... I can post it for you if you want ?
Cheers
the gain with this 2SK117 is tremendous, even without adding another cap in parallel with the source resistor. I have to use the active input on the amp. I already have an opamp DIY preamp. this 2SK117 jfet has a unique cleaner sound than the J201. the J201 was easy to bias without distortion but the 2SK117 biases differently I see
I tried that and it didn't work. I think the pickup signal is too strong. when I put the pickups in parallel mode the clipping is barely audible. when I switch to series mode the clipping is really bad. Before I give up on the 2SK117 I am going to put a 220nf film cap on the input and replace R1 500K with a 5M variable resistor to see if the 1M - 5M range will help
Well if clipping is the only problem, just lower the input signal level by putting another resistor in series with R1 and taking the FET gate input from between the two resistors.
It makes a simple voltage divider, or attenuator. If a new R0 is put above R1, in series with it, in your schematic, and the FET's input is taken from between R0 and R1, then it sees the input voltage times R1 / (R0 + R1). So, for example, if R1 is 3 Meg and R0 is 1 Meg, you'd get 3 / (1 + 3) or 3/4ths of the original voltage. Increase R0 (and/or decrease R3) until there's no more clipping. It does, of course, lower the overall gain.
Cheers,
Tom
It makes a simple voltage divider, or attenuator. If a new R0 is put above R1, in series with it, in your schematic, and the FET's input is taken from between R0 and R1, then it sees the input voltage times R1 / (R0 + R1). So, for example, if R1 is 3 Meg and R0 is 1 Meg, you'd get 3 / (1 + 3) or 3/4ths of the original voltage. Increase R0 (and/or decrease R3) until there's no more clipping. It does, of course, lower the overall gain.
Cheers,
Tom
Leave the source resistor there.
You can alter the bias point of the FET by altering it's value, and altering the input resistor (gate to vSS) will set the input impedance, without changing anything else.
I tried it and its still clipping. The clipping is not as bad though. Also adding the other resistor from power supply to the gate is causing the pickup switches to make popping noise. This 2SK117 is trickier than I thought
QUOTE=PaulBass
when I put the pickups in parallel mode the clipping is barely audible. when I switch to series mode the clipping is really bad.
.......yes,it looks like the signal is too strong for use with this kind of jfet.
you can use an input volt attenuator with two res..........not very smart but should work.
for example: gate to source r1=500k and in the signal path in front of r1 try 330k and increase it to 500k or so if it is too small.
greetings
when I put the pickups in parallel mode the clipping is barely audible. when I switch to series mode the clipping is really bad.
.......yes,it looks like the signal is too strong for use with this kind of jfet.
you can use an input volt attenuator with two res..........not very smart but should work.
for example: gate to source r1=500k and in the signal path in front of r1 try 330k and increase it to 500k or so if it is too small.
greetings
With the bass at its highest output (both pickups together in series) the DMM reads at 12.3K DC resistance and the voltage peaks at 15mV when I played the bass hard. I put 1M in front of R1 and left R1 at 500K. since both resistors are in parallel the sum is 350K going into the gate. I got rid of one the batteries, I think the extra voltage was contributing to the high gain clipping. I made the drain to ground 4.5V by adjusting the source resistor. It made a difference. Now I only hear slightly audible soft clipping when both pickups are together in series. When I play hard I don't get that horrible distortion anymore. Looks like I'm almost there
QUOTE ......... and the voltage peaks at 15mV when I played the bass hard.
good morning.
15mv is not very much.........perhaps an o-scope could read it better.
.....I put 1M in front of R1 and left R1 at 500K.
i would use lower res values because the input attenuator and the gate capacitance make a low pass filter (roll off only a few kHz).
example: r1=220k........330k and r5=500k or so. and the input impedance still remains high enough.
the bigger (18) psu voltage should not be a problem..........
greetings
good morning.
15mv is not very much.........perhaps an o-scope could read it better.
.....I put 1M in front of R1 and left R1 at 500K.
i would use lower res values because the input attenuator and the gate capacitance make a low pass filter (roll off only a few kHz).
example: r1=220k........330k and r5=500k or so. and the input impedance still remains high enough.
the bigger (18) psu voltage should not be a problem..........
greetings
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I agree, a cheap DMM is no substitute for an o-scope. wish I had one.
I tried 100K, 220K, 330k and 500K in front of R1 and it went back to that over-the-top clipping again. switched back to 1M and the clipping stopped. so I guess somewhere between 500K and 1M is the clipping threshold. I'll try a 1M trimpot there. I got the clipping to totally stop by raising the source resistance (R2) to 1.5K, drain resistance (R3) now is 22.1K which makes the Vd 4.7v from 9v volt power supply
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