JFET bass preamp clipping problem

[PaulBass]

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?

[mjf]

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

[Minion]

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

[PaulBass]

Originally Posted by Minion

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

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

[PaulBass]

I tried everything can't stop this 2SK117 from clipping. going back to the J201

[phase_accurate]

A drain voltage of 9V isn't ideal IMO. I would try to reduce R3 a little in order to lift the drain voltage and reduce the gain (and maybe increase R2 as well).

Regards

Charles

[PaulBass]

Originally Posted by phase_accurate

A drain voltage of 9V isn't ideal IMO. I would try to reduce R3 a little in order to lift the drain voltage and reduce the gain (and maybe increase R2 as well).

Regards

Charles

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

[Nigel Goodwin]

Just connect the drain directly to +9V, and take the output off the source - this gives a unity gain buffer - which if you're not wanting gain (and why would you?), is all you need.

[PaulBass]

Originally Posted by Nigel Goodwin

Just connect the drain directly to +9V, and take the output off the source - this gives a unity gain buffer - which if you're not wanting gain (and why would you?), is all you need.

do I leave the source resistor there or remove it?

[gootee]

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