Common GATE FET Overdrive

[DDD]

Hi men,
Please look at the common GATE FET overdrive schematic.
It seems that common GATE input stage is something new for the guitar overdrive stompboxes.

The circuit picture is published here: (post #5)
http://forum.gtlab.net/cgi-bin/yabb2/YaBB.pl?num=1355935946/0

Sound sample was recorded with: Jolana Tornado (old Czecho-slovakian electric) - Roland Cube-60 Clean Channel - Samsung mobile phone 
http://zalil.ru/34220253 (file name is IN_FET_1_001.3ga)
*** File type is .3ga, and it can be played with almost all of the Windows players.

[J0K3RX]

Dmitriy, very interested! Looking at this now...

[goamp]

Looks unusual, but interesting indeed. I don't know enough theory to explain how and why it differs from a minibooster, but I set up the first part of the schematic in LTspice. I used BF245A FETs ,because I have no model for the BF512 and got almost exactly the same voltages as stated on the schematic. The gain is about 20 and the tops of the waves are nicely rounded when overdriven. What puzzles me are the two diodes connected to the source of the lower FET which don't seem to have any function to me. Overall, a very promising and unusual building block for an overdrive.

[J0K3RX]

I will try to throw together a quick layout and build this unless there is already a layout which would save me some time... any suggestions for FET's?? I don't have any BF245's right now.

Is this by any chance similar to the AMT Heater HR-1? Probably not since I have not seen anybody dissect one of those yet...

Sound sample has good tone considering it was recorded on a samsung mobile phone

[pakrat]

Thanks for posting this Dimitry, it looks very interesting and sounds pretty thick! I guess I need to learn russian now....... 

[DDD]

Well, there's no need to learn Russian since there's time to learn Chinese  
I'll try to answer any questons in English:
1. Two diodes connected to the source of the lower FET are just to protect the FET from possible voltage spikes, etc. The diodes are biased reversely hence they did not affect the sound. Maybe, some hot pickups can "over-reverse" them, but I didn't notice any changes in the sound

[Thecomedian]

LOL. I've been trying to work on one of these myself.

[kingswayguitar]

i finally got it to play with real player
very interesting sound

[J0K3RX]

Common gate input stage might be something new for overdrive stompboxes but what exactly is the benefit of going this way vs. let's say a good old tube screamer type circuit? Obviously there is a difference in the circuit design but what exactly is different about the sound??

[kingswayguitar]

Common gate input stage might be something new for overdrive stompboxes but what exactly is the benefit of going this way vs. let's say a good old tube screamer type circuit? Obviously there is a difference in the circuit design but what exactly is different about the sound??

and furthermore, how does it play? as a guitarist, i like the high input impedence of a typical jfet. correct me if i'm wrong, but i seem to remember that common gate inpedence is lower.  too low? not sure, i'd have to try. hmmmmm...breadboard where are you??

[Thecomedian]

yeah common gate/base is low impedance input and high impedance output. That means it'll be more bassy.

[DDD]

The common gate input stage has low input impedance, about 4 kOhm as far as I remember. So, combined with input (coupling) capacitor, this low impedance forms a grounded capacitor, connected to the hot terminal of a pickup.
Thus we have serial LC-tank with the resonant frequency depending on the pickup inductance and input capacitor.
Playing with the inductance by guitar pickup selector and with the capacitance by the toggle switch (installed on the stompbox) we can vary the resonant frequency.
*** Hard rock and metal rock stompboxes usually have the same frequency responce of the input stages, formed with special RC-filters. Here we have the same without any additional components.
At the same time we can expect very low noise performance since we have the best impedance matching at the resonant frequency (R_inductive = R capacitive as per LC-tank theory).
The frequency response curves confirm the above assumption.
*** С1(4.7n) / R2(56kOm) circuit gives enough brightness to the sound. The sound is too much "dark" without C1 / R2. 
The lower picture shows frequency responce of the amplification section with two different input capacitors.