Help with Tillman JFET Piezo Preamp

[studiodog]

Hi folks. I'm hoping someone can point me in the right direction with this preamp project I'm working on. I've been trying to build a JD Tillman-designed piezo preamp using a single J201 JFET. I've assembled it about 3 times now and cannot get it to work - no signal at all at outputs!

The circuit is here.

I'm wondering if I've perhaps got the pins on the JFET wrong? From the schematic, it would seem that the Drain and Source pins should be interchangeable, but I don't know for sure.

On a related note, I've tried Tillman's JFET Preamp Cable design as well. My build seems to work intermittently, with periods of horrid static/noise invading the audio. Anyone experienced the same sisue and perhaps know what the problem is?

Thanks.

[Nigel Goodwin]

It's a dead simple circuit, nothing to wrong - apart from connecting the FET the wrong way (which is easy to do, as FET's often seem to have different pin connections, even for the same type!).

[studiodog]

Hi Nigel. Thanks for pointing out my obvious shortcomings. I think my confusion is determining from the schematic which FET pins go where. Perhaps you can assist here?

[Jarno]

This Google search, will get you datasheets for that FET, the pinout is in there.
It's important to note that these (and indeed other FETs) vary by quite a bit, so not every J201 will work well. You might need to select one from a batch.
Although, I don't think the pre will not work at all with a less suitable FET so something else must be wrong.

How did you connect it? You did notice that the electrolytic goes from +9 to ground, and is NOT connected to the output? Can you post the voltages on all three of the FET pins?

[studiodog]

Hi Jarno. I'm using a patch of Fairchild J201's bought off ebay. The seller provided the datasheet, so I know the pinouts on the JFETs are (L-R) D-S-G. I'm hoping someone can confirm the pin positions in the circuit schematic.

Currently I've wired the F201 as follows:

Pin 1 - to R3 and 9v+
Pin 2 - to R2
Pin 3 - + signal input

Is this correct? I'm guessing not, as I can't get a signal......

Thanks

[mjf]

hello.
drain voltage should be around 5v.....6v (from drain to ground).
......if it is not change r2 (source res) so that you get this voltages.... (try out other values for r2).
greetings

[studiodog]

Thanks, mjf! At least I know I'm not wrong on the pinouts. I will dig out the old Voltmeter and see if I can get a decent reading. Then I guess I'm back to searching the perfboard layout and my possibly dubious soldering techniques.

I'll get to work on thre weekend and report back with any news!

Cheers all,

[Jarno]

Pin 1 is also directly connected to the plus of the 4.7uF electrolytic, do you have that as well?

[Minion]

Ive built this circuit before , Ive noticed that some fets just plain don"t work well , I had to try a few j201 before I could get one that would bias properly with the posted resistor Values ..... also realize that this preamp doesn"t do much preamping , it only has 3db of gain ......

The first one of these I built I did point to point and then just wraped the thing in a Ball of tape , Looked bad but worked , I found that it didn"t really do much for me , I have since designed a Much higher Gain preamp that I now fit into all my Guitars.....

Cheers

[Jarno]

That's also in the explanation of the circuit, you need to select FETs, and those will work. But I agree, I built a number of them, with 2SK170s, J201s, 2N5459s, MPF102s but somehow the single ended approach (at least that's my theory) doesn't cut it. I've also built a preamp/overdrive pedal for bass (the flipster) and that also uses the single ended approach, it does have more gain (since there's 4 stages in there) but it also lacks "something". Somehow it feels like it's running out of steam, not enough dynamics. I am going to try some SRPP topologies now, and of course there's the opamp everybody loves to hate (but works fine, and sounds good as well, especially opa2134).