A JFET buffer embedded in a guitar cable

[enrico.menotti]

Some time ago I had the idea of assembling a guitar cable with an embedded JFET buffer. Today I found out the following link:

http://guitarnuts2.proboards.com/thread/3150/jfet-buffer-cable

There, the author exploits exactly my idea. Now, the problem is the specific JFET to be used. The MPF102 used in the above quoted link is not rated as far as its noise is concerned - see the data sheet at http://www.onsemi.com/pub_link/Collateral/MPF102-D.PDF.

I found out some indications about very low noise JFETs; the best candidate is the IF9030 by InterFET. But, its I_DSS is so high that it seems to me impossible to polarise it correctly by using just a resistor from gate to ground and a resistor from source to ground (and connecting the drain to the supply voltage). In this way the V_GS turns out to be too close to the pinch-off voltage, at least if one tries to obtain low values of I_D (say, 5 mA).

Now I am starting this thread to ask for advice about a low-noise JFET suitable for a source follower configuration.

[PRR]

> not rated as far as its noise

JFETs at similar current will have similar hiss. For the current you want to drive cable, this hiss will usually be significantly less than a guitar's self-hiss.

If you can derive a Gm (Forward Transfer Admittance) *at the current you will run*, invert that to a resistance and calculate the hiss voltage across the audio band. Typical values for JFETs at a part-mA is about 500uMho so 2K Ohms and about 0.7uV-1uV audio hiss. Guitar tends to be 50K+ and 3uV hiss.

We might also note that guitars were developed to be reasonably low-hiss when feeding 12AX7. Which never had a for-sure hiss spec. But was typically 1uV-2uV. This did work well. It is pretty likely the next stage after your buffer, even if solid-state, has hiss hardly lower than 12AX7. The buffer JFET is not the system noise floor.

[jubal81]

I did this a while back on a PCB using SMD parts and a dual opamp setup in parallel. Worked pretty well. Mic wire with second lead carrying the voltage.

http://www.diystompboxes.com/smfforum/index.php?topic=106832.0

[amptramp]

You can set up a Tillman circuit with the source resistor, transistor and gate resistor are located in the cable (or in the guitar) and the battery, drain resistor and coupling capacitor are in a box at the end of the cable opposite the guitar.  This eliminates the need for a power lead in the cable.