New FET distortion schematic...sensation?

[theman]

hi medved,

is it possible to use a different jfet for the upper jfet in the buffers ... something like a J202 or Russian KP303I?  these have higher i0. also, i'd like to ask what effect does the gain pot and feedback network have?  is there a way to get rid of the gain pot? the current design already has 6 knobs! 

thanks for all the feedback on this cool design ...

theman

[Medved]

Hi. By the way, where are you from? What Russian JFETs can you buy?
If you use high I0 upper jfet in buffer then output potential will be much highier than input one. It can be 2-3v higher- such buffer will clip upper half-wave.
Gain pot changes gain
When gain is max, lows and highs are reduced, when gain is min amplitude-frequency response is flat.
It adds one more degree of freedom in sound. I think that it's better to use 2 pots tonestack than removing gain pot. Nevertheless, it can be easily removed.

[theman]

i'm in california, usa. i don't have access to any russian jfets -- just have read that the KP303I is close to a J202. thanks for your suggestions. your gain pot sounds like a presence or contour knob -- not really affecting gain, but changing the frequency response of the circuit. for example, you said when gain is max, highs and lows are reduced -- does this produce a mid-boost or hump? for many, this is desireable. 

i agree with you on keeping this knob -- it is a pre-gain EQ in a sense, which is needed in many super-high-gain circuits for proper tonal shaping. any thoughts on comments on what's the best post-gain two-knob tonestack to use?  the vox/hotbox tonestack seems to be a lot different tonally than the marshall you are using ...

[Medved]

i'm in california, usa. i don't have access to any russian jfets -- just have read that the KP303I is close to a J202. thanks for your suggestions. your gain pot sounds like a presence or contour knob -- not really affecting gain, but changing the frequency response of the circuit. for example, you said when gain is max, highs and lows are reduced -- does this produce a mid-boost or hump? for many, this is desireable. 

No, it's not a contour or presence knob. It affects gain. Max gain of first stage is about 30-32dB, minimum- nearly 12-14db. What's the difference between mid-boost and hump? It's only your choice how much lows and highes are reduced...

i agree with you on keeping this knob -- it is a pre-gain EQ in a sense, which is needed in many super-high-gain circuits for proper tonal shaping. any thoughts on comments on what's the best post-gain two-knob tonestack to use?  the vox/hotbox tonestack seems to be a lot different tonally than the marshall you are using ...

The tonestack of xbananov... You can find it on his bsiab mod. I've posted a link...

[theman]

can you explain what the gain knob does to the tone? at min gain, do you still get a high-gain tone (if drive is cranked), and how does it compare to the tone when the gain is maxed? is there any filtering going on at all?  i thought that the feedback path from the output to the gate of the upper jfet is a high-pass filter.

[Medved]

can you explain what the gain knob does to the tone? at min gain, do you still get a high-gain tone (if drive is cranked), and how does it compare to the tone when the gain is maxed?

At min gain it is just an overdrive. And sounds like overdrive. At max gain... You have heard how it sounds at max gain.
At min gain and low drive it has clean sound with light crunch...
Ok, i'll write you some sounds soon.

is there any filtering going on at all?  i thought that the feedback path from the output to the gate of the upper jfet is a high-pass filter.

At min gain no filtering is perfomed, at high gain lows and highes are reduced. In mucascade feedback is positive, that's why low and high pass filters in feedback path boost mids by reducing lows and highs.

[Medved]

I've bought american strat $)
Here are some samples of clean channel from all strat's pickups:
http://webdrive.reborn.ru/egorov/Clean/StratB_r.rar
http://webdrive.reborn.ru/egorov/Clean/StratB+M_r.rar
http://webdrive.reborn.ru/egorov/Clean/StratM_r.rar
http://webdrive.reborn.ru/egorov/Clean/StratM+N_r.rar
http://webdrive.reborn.ru/egorov/Clean/StratN_r.rar

[Medved]

A sample of very new distortion design:
http://webdrive.reborn.ru/egorov/Bogner/bog_fet3.rar
(611kb)

[brett]

Hi
would somebody post a current schematic of this effect?
thanks

[markm]

I'd like to take a peak at this too!

[Medved]

There are some problems with tunning- it's only a sample device. I can show the scheme, but without oscilloscope it's hard to make it working in proper way.

[el84]

This 3d is VERY interesting !! 
Thank you boys 

Antonello

[Medved]

Sample of another device (practically full copy of DualRecto) with same clipping stages:
http://webdrive.reborn.ru/egorov/Slo/SloIII_1r.rar
(663 kb)

Scheme:
http://webdrive.reborn.ru/egorov/Slo/slo.PNG
Please, don't try to build it, if you have no oscilloscope. I can't help you to tune the scheme in this case.

[theman]

Could you make the drain resistors as trim pots and get it to work more easily? The architecture looks similar to many 4 stage tube preamps -- just different inter-stage filtering and attenuation -- which I believe contribute a lot to the character of the distortion. The Dr. Boogey and Cauldron of Gain (similar to Bogner XTC) are some examples. They also use a Marshall-style tonestack at the end, driven by a follower.

[Medved]

Could you make the drain resistors as trim pots and get it to work more easily?

Without osci or precise math - no. I can't explain it to you, because of my poor English.

The architecture looks similar to many 4 stage tube preamps -- just different inter-stage filtering and attenuation -- which I believe contribute a lot to the character of the distortion. The Dr. Boogey and Cauldron of Gain (similar to Bogner XTC) are some examples. They also use a Marshall-style tonestack at the end, driven by a follower.

The interstage filtering is exact copy of Slo100/DualRecto. The main difference from Dr.Boogey and others is clipping stages. J2 and J4 clipping is practically identical copy of tube stage clipping.

[Medved]

http://webdrive.reborn.ru/egorov/Bogner/bog_fet4r.rar
(1.02M)
One more sample.

[Nashtir]

oh my god..this pedal is simply awesome!!!will someone post a layout or a pcb for this wonderful effect?

[Medved]

Be patient, we are working on maths...

[stm]

Hi Medved, I gained a lot of understanding with respect to the operation of FET equations during the development of the Fetzer Valve article (http://www.runoffgroove.com/fetzervalve.html).  I offer you my help with the math and the preparation of the text in english.

Also, regarding the schem, are the D9b diodes typical signal diodes or are they germanium or power diodes?

[Medved]

D9b are germanium. Schottky are also usable (1n5917, etc).
About math. Let's do this way: you'll try to calculate the shown stage yourself (stage+load). We (xbananov (the author of a stage) and I ) have different points of view on calculations. 3 methods are better than 2, aren't they?
I'll try to explain you the basic concept. The main idea is not to allow the stage to clip hard the lower half-wave of stage output.
And, please, do calculations of source resistors with given drain resistor, not inverse...