New JFET guitar preamp project

[opor]

Originally Posted by JP Hugsley

Hi Vance,

There are many JFETs available that are recommended by the manufacturer for audio applications; some of the more popular devices are: 2SK170, 2N5457, 2N5458, 2N5459, 2N4117, 2N4119, KSK30 (Fairchild), LS846 (Linear Systems), 2SK369, and 2SK373 (Bart seems to like the 2SK389 and 2SK170)

You will find that these devices are usually optimized for use as either high-gain preamplifiers or line-level amplifier signal chains. You shouild have no problem in finding the right JFET for just about any intended, low-level audio application you have in mind.

Many other JFETs are recommended for RF and Switching applications and, although they may be suitable for audio, they ususally emphasize, as well as supply data for, specifications that are of less concern to a purely linear audio application (e.g. input capacitance (impedance parameters), switching speed, RDS-on and maximum ID current).

I have gone into detail on JFET specifications for an "on-board" application where the power supply voltage (9 Volt battery) and current draw were paramount; however, in the case of an off-line application (e.g. a power amplifier front-end), relativley high-voltages and larger bias currents will often help with low-noise performance and lineratity.

Have Fun,
JP Hugsley

It is hard to get these JFET in my country . Is it possible for BS170 or 2SK117?

[Brett]

Get what you want posted in from overseas.

[unclejed613]

Originally Posted by JP Hugsley

Hello SharynMac58,

The funny thing about guitar pickups is that they don't exist in a vacuum. The series resistance, parasitic capacitance and series inductance of a pick-up play a major role in determining the overall "tone" of your guitar. However, the volume/tone control pot resistance (although minimally with 500k Ohm pots), the guitar-amp input impedance (again, minimally at 1.0 MegOhm input resistance) and the guitar cable capacitance (usually around 33pF/ft) effect the tone as well.

You're correct about the resistance loading cutting down on your "high" frequencies but, with the inherent series resistance of a hum-bucking pickup being around 9kOhm to 18kOhm, the pot and guitar amp loading effects will be minimal. However, it's the resonant effect of the guitar pickup inductance and guitar chord capacitance that can't (and shouldn't) be ignored. This resonance can actually enhance the tone of your pickup if crafted correctly.

You can experiment with a quality, low-noise, hi-Z input, low power-drain opamp to buffer you pickups as suggested by others, but you should try out different values of loading capacitance (say, between 100pF and 470pF) in parrallel with a 1.0MegOhm resistor shunted across the inputs of your opamps. I believe you'll find that, in this way, you will have very good control of your humbucker tonal characteristics.

Please remember that these are only suggestions, your "ultimate" tone is what sounds best to you and finding that tone may take many iterations of experimentation. That's the fun part!

JP Hugsley

next time i get a chance to build an onboard preamp, i have a couple of ideas i'm going to try, one of them being a variable cap to load the pickup with...

[Jarno]

And a resistor, that works too. I have both a resistor and a cap in my precision bass, with a DPDT center off switch.
Center is standard P-bass, down is the resistor a woollier sound with less treble, up is the cap which adds high end snap to the standard P-bass sound, I use it if I have to slap or for a more modern sound.

[JP Hugsley]

Hello Opor,

The BS170 is an Enhancement mode mosfet so the JFET circuit designs and methods described in this post will not apply without major bias changes. The 2SK117, however, is a JFET and may work very well as a project/experimental device.

It seems that the main issue you may have with the 2SK117 would be in a battery-powered, portable application. If you qualify and bias the 2SK117 as described in this post, you will end up with a relatively high DC bias current (depending on the device, possibly as high as 5.0mA) and suffer from low battery-life as compared to a J201 (possibly 1/10 the battery-life of a J201). If that doesn’t matter for your application, then the 2SK117 looks like a very good device for audio.

Please let me know how it works out for you.

JP Hugsley