Don Tillman's discrete FET guitar preamp

[fretman_2]

Here's my piezo preamp (buffer actually) design. The 2.2 meg resistors bias the JFET. I'm not looking for any amplification, just trying to handle the difference between the piezo output impedance and guitar amp input impedance.

 

[Higgs F. Boson]

Here's my piezo preamp (buffer actually) design. The 2.2 meg resistors bias the JFET. I'm not looking for any amplification, just trying to handle the difference between the piezo output impedance and guitar amp input impedance.

I'm working on the same type of project, using an MPF102 and the Hawes design, for use with piezos. I built without testing the transistor or breadboarding (stupid, I know) and I'm getting attenuation, so I plan to re-bias when I have the time.

 

[jaylow]

On Tillman's website, he says the following can be done to his circuit.
- Bypass R2 with a small value capacitor for a treble boost.
- Bypass R2 with a large value electrolytic capacitor for more gain.

What would be a recommended value for both of these capacitors? Also, can both of them be used simultaneously to achieve both effects? Sorry, I'm new to tinkering with circuits and I'm trying to learn.

 

[Big John Studd]

On Tillman's website, he says the following can be done to his circuit.
- Bypass R2 with a small value capacitor for a treble boost.
- Bypass R2 with a large value electrolytic capacitor for more gain.

What would be a recommended value for both of these capacitors? Also, can both of them be used simultaneously to achieve both effects? Sorry, I'm new to tinkering with circuits and I'm trying to learn.

You can't do both I'm afraid. Well, you can connect two bypass capacitors, but they will effectively form one capacitor...read up on "series and parallel capacitors" to see what I mean. Try values between 1uF and 10 uF. That will approximately double the gain.

 

[jaylow]

Since I can't do both, using a 1uF-10uF cap will net me double the gain. What value would I use if I opted for a treble boost instead?

 

[Big John Studd]

The value of the capacitor will affect where (with regard to frequency) the treble boost will kick in. Higher value capacitors will result in a lower frequency at which this occurs, and vice versa. You can perform calculations to determine exactly where, or you can just try different capacitor values and listen to how things change. To only boost treble frequencies I think you'll want to play around with values in the 0.01 uF range.

 

[jaylow]

Thank you sir. That's a big help.

 

[Big John Studd]

Absolutely. The gain of the preamp is basically -(R3||R4)/(R2 + 1/gm). The gm term is the transconductance of the transistor. So what happens when you add that capacitor across R2 is that it essentially replaces R2 in that gain expression with an impedance that varies with frequency...its magnitude decreases as frequency increases. And since R2 is in the denominator, as it goes down (with increasing frequency) the gain goes up...ie. treble boost! Not sure if that makes much sense. It is difficult to clearly explain without drawing some pictures and maybe elaborating on the math.

 

[jaylow]

I have attempted to build another one of these in the hopes of using it as a buffer for my pedal board (everything I have is true bypass) so I thought it might be helpful. It's a very nice sounding preamp with just a hint of colour and the 3db boost is a welcomed addition. The problem is that it's adding a lot of hiss in the background. It's currently powered by a 1spot in a daisy chain. If I power the preamp by itself with the 1spot, the hiss gets louder. Is there anything I can do to remove the hiss. It tends to get worse once I turn on my overdrive, compressor or distortion, so I'd like to try and nip it in the bud and remove as much hiss as I can.

 

[LightninMike]

To answer the question posed about noise due to using a 1 Spot power supply, it seems a lot of guys have the same issue.... Some work well, others are noisy....

Voodoo Labs makes a great low noise power supply.... lots of guys on the road use it daily, and touring stages want nothing but the best and most reliable