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Need help with ROG: fetzer valve (gain stage) + buffer

Started by Alexius II, September 22, 2010, 11:35:34 AM

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I must qoute myself, somebody doesn't read all the posts >:(
Quote from: mensur on September 25, 2010, 06:08:07 PM
Use MPF102's for both, voltage gain and as buffer.
Use 680R for RS(voltage gain) without bypass cap, BTW. Cs reduce internal resistance of the channel, hence more gain.As for Rd use 2.2K.
These are resistance values foe MPF102 FET (with -4.5V, 13.5mA Idss characteristics).
From my cacs. it will bring you to -2.12V of Vg(humbacker sweetspot) and 3.46mA Id.
Gain will be around 3 to 4x.As for buffer use same character. FET with 2.7K Rs, everything else remains the same.I don't like bipolars as buffers, you don't need much current there.
You are right :tu:
Quote from: Alexius II on September 26, 2010, 04:11:09 PM
I was a kid and bit too young to remember anything from the old Yugoslavia, but they say that were the good times  ::)

Anyway, thank you both. I went through some online theory pages and found a few other equations that helped me understand the few missing relations. I think I get it now... I hope :duh

I will now go again through the calculations for my transistor:

These are the equations I used for the gain stage:

Vgs = -Id*Rs
Vds = Vdd-Id*(Rd+Rs)
Id = Idss*(1-(Vgs/Vp))^2
(the characteristic curve is drawn from this equation)
gm0 = 2*IDSS/VP
gm = gm0*(1-(VGS/VP))
Av = (gm*Rd)/(1+gm*Rs)
(if there is no Cs)

My transistor:
Vp = -4.5V
Idss = 13.5mA

I choose Vgs to be 1/2 of Vp, so
Vgs: 0.5*(-4.5V) = -2.25V

Id = 13.5mA*(1-(2.25V/4.5V))^2 = 13.5mA * 1/4 = 3.375mA
Rs = Vgs/Id = 2.25V/3.375mA = 666ohm (closest is 680R)

gm0 = 2*13.5mA/4.5V = 0.006mS
gm = 0.006mS*(1-(2.25V/4.5V)) = 0.003mS

Then I choose Rd. I understand that with larger Rd come larger voltage gain and output impedance + lower current.

Rd = 2.2k  (18V/2200R = 0.0081A = 8mA)
Av = (0.003mS*2200R)/(1+0.003mS*680R) = 2.17 (= 6.7dB)

Ok, I hope everything is OK this far.

Then comes the buffer (aka "source follower" or "common drain amplifier")  ::)
Only the Rs is necessary to bias it. I've read some theory about it, but I'm not sure. The way I understood it, you have to choose Rs in a way, that at the source you have half the voltage (Vcc). If this is correct, I then used the identical MPF102 transistor with calculated Id = 3.375mA (for Vgs = 1/2 Vp) inserted in the equation:

1/2 Vcc = 9V = 3.357mA * Rs
Rs = 9V / 0.003357A = 2680R = 2.7k

Is this ok, or am I making things up?  :loco  ;D

Alexius II

No no, I did read it well...  ;)
I just wanted to confirm the same results by calculating it again myself.  :tu:
(active learning as opposed to passive)

Well, thank you very much (puno ti hvala!! 8|), you've helped me gain a lot of new knowledge. :tu:

Now that calculations are behind me, I can finally draw the new schematic:

The final thing I now must do, is to draw and etch a PCB :tu:  (I'm tired of perfboard)

Well, while I'm at it...

I have an enclosure measuring L/H/D = 12 x 4 x 18 cm.
I would like to mount the preamp in this enclosure, together with the power supply.
Power supply would be a simple one, small 15V trafo, around 1000uF smoothing, LM7818 regulator.

I think this would be ok:

Would the close proximity of transformer create any serious noise in the preamp?
Distance between the transformer and preamp PCB would be around 10cm  :-\

I could always use a separate power supply, but I would like to have a single practical unit, that works by itself  :)

J M Fahey


No problem bro(nema problema burazeru :tu:),
Here's layout, and the final schematic:

Alexius II

He he, I just finished drawing my layout and there you go, done it before me!  :)
I have no experience with drawing a layout myself, so I'll trust your comments about this. So far I only etched pcb's that were drawn by others. I thought this could be my first  :)

What I did is kind of a more compact version.
I first drew everything together with jacks and pot wiring:

I tried to keep the path (traces) of guitar signal as short as I could. Also, a while back I did a few mini amps and preamps from runoffgroove.coma with this kind of "star" grounding and it worked realy good (and dead silent), so I thought of using it here as well. All grounds (pcb, in_jack, out_jack, pot) will be joined at the power supply (0V) with bolt/nut. And just in case, input, output and pcb-to-pot wires will be shielded. (helped me a lot with high gain pedals)

Then I removed all the components and flooded the empty layout space with the ground plane.

I am aware this can create a few benefits (and possibly problems), but I'm not experienced enough to predict what can go wrong. Do you maybe see a problem with this?

And just now I noticed a new resistor at the input in your diagram: 4.7k... what does it do?  ???
I can easily add it in my layout, there is enough space in the input area.

I also used a power filtering cap, but 100uF instead of your 1000uF.
There is still enough space for the bigger one. I guess the bigger the better (filtering).

Alexius II

Hello again after some time!  :)
I built the preamp and it works great  :tu:
I used mensur's input resistor (4k7), though I still don't know why is it necessary...  ???

The next thing I will try to do is to include a simple fender-like "tone stack", a.k.a. bass/mids/treble eq.

The generic schematic looks like this:

So if I understand correctly, I should place the tone stack input directly at the source of the buffer.
I suppose the 1uF "output capacitor" can be easily omitted, because the three capacitors inside the tonestack (C1, C2, C3) already block DC voltage?

And then again, if I need low impedance output, I can just place another gain stage + buffer after the tone stack, right? This would also serve as "recovery" for the signal loss of the tone stack. As for volume control - I think 1M voltage divider between the tonestack and the second gain stage is pretty common. If I were to use an J201 transistor (very low Vp) for the second stage, I think I could even get some "dirt" out of it ;D

I will try to play with these ideas some more and post my results... hopefully with some photos  :D


hi everyone,hey alexius... unconsciously i make the same proyect  ;D,i dont see this post... but its the same stuff, fetzer valve+buffer (i did the buffer with a opamp)...
i want to add some tonestack stage too,but i have a lot of problems with the awful signal loss... but adding some new gain/buffer stage after the tonestack add noise too,so... what's the solution for this problem?? if i find this answer i will post it  ;)

greetings from Chile(yeah... miners and earthquake...)  8|

Alexius II

Hi luchin!
Yeah, a lot of us tried to do some project like this after reading about and building a Fetzer Valve... ROG are great! :tu:
Well, while I like fets more than opamps for dirty tones, I never tried an opamp for a buffer (and I don't need to, as I have a LOT of fets available). The type of buffer shouldn't make much difference noise-wise. You say that post tone stack gain adds a lot of noise? What kind of power supply are you using? A cheap power supply can bring all sorts of nasty things into your sound... xP ...but if you are using a battery, the problem is probably elsewere. (do you have a schematic?)

I've also been looking at THIS "fender-ish" circuit. Something to try and fiddle with, when I have time :)

At the moment I'm spending most of the time tinkering with my dirty metal preamp (dr.boogey & mods). When I'm satisfied with it, I will return to this clean preamp with fender tone stack. After that I will also make a marshall(ish) preamp and integrate all three into a "head" powered by a LM3886 or something. That will have to be more than enough sounds for my home/practice amp  ;D

J M Fahey

Hola Condorito  :tu:
Don't worry too much about that:
the tonestack loss , in practice, can be considered about 10dB or 1/3 of the original signal; so following it with a 3x gain stage just about compensates it.
In practice too, the "recovery" stage usually has 10x to 20x gain, to recover losses and add a little extra gain.
That's why volume controls often give you maximum power already around "6" and not on "10", as to have some reserve gain for weak pickups/player/old strings/etc.
With that moderate gain, you will not have noise problems.
Good luck, vecino.


alexius i worked the vibin' champ on my protoboard... its perfect!!,i just change the first j201 for a mpf102,for a "clean" sound and i place a 25k pot instead the R1(15k resistor)... to have a mid control(tonestack calculator helps me)... but this circuit its all what i need
... i use this circuit at 15V and sounds awesome with a tda7294,now i'll do the pcb and switch the channels of my ss amp,and finish the job!!!

but really really thank you alexius for this info... i search a lot of this circuits,but never find something like this...

many thanks once again for you help

holaaa JM fahey,como le va vecino?... se nos fue Bielsa y en Chile aun estamos reclamando por el nuevo presidente de nuestro futbol...
como decimos aca.. "mensa cagaita..." jajajaj,saludos compadre...y claro eran como 10dB de atenuacion segun el tonestack q habia hecho,gracias por su ayuda tambien...

Alexius II

Hey luchin, I'm very happy that "vibin champ" proved so useful to you! 8|
I also searched for a "fender" sounding fet circuit a lot and this seemed very simple/useful  :tu:

I have yet to build it (still haven't found the time), but my plans are similar:
I will use a "mid" pot (around 20 to 30k) and some lower gain transistors (MPF102/2N5457).

Good luck, and use it well!  ;)

Alexius II

Hello again,
I have another question for those who know their stuff   8|

I'm in need of a "signal splitter" with separate volume controls - and what I found is THIS little circuit, which seems to do more or less what I need. :)

There goes my question: is there an "easy way" to use the schematic of the previous circuit (DC coupled amplifier+buffer) and add a couple more buffers (buffered outputs)? I know that a gain stage + jfet splitter should work, but would DC coupling also work here? I was thinking of using the exact same circuit as before, but using three (instead of one) identical buffers (schematic below). Is this doable? ???

I won't modify the previous "gain+buffer" build, this will be an all new project in a separate enclosure, so I can start from scratch :)

Thanx in advance  :tu:


I came upon this a bit late, and I'm not entirely sure what you're after, but I think I see a possible problem. I'm somewhat of a noob, so you'll probably want a second opinion.

The circuit in your picture (second one) seems to be missing a bias for the splitter inputs. You can see R2 and R3 providing an evenly divided bias in the first circuit, where in the second it's missing.

DC coupling of gain stages can be trouble. I'm not much of an authority, but I'd recommend against it for audio. It's usually used where precise measurement or high frequency is important.

Hope that helps. :)

J M Fahey

The three buffers *are* biased, to the drain voltage of the first one.
Bias is DC so in this case DC coupling is ok.
Besides, those output buffers have unity gain, so they won't amplify DC offsets; in fact, their source voltage will be a couple volts over their gate voltage, quite acceptable.
As-is, it looks like it will work with no problem.


Just a quick question re resistors to set Vd & Vs for voltage biasing in the schematic. If I want to run at 12V or 19V (I have these power supplies), should I just use the calculators on the ROG Fetzer Valve revisited page after working out the Vp and Idss for my MPF102? The values in the schematic are twice that of the average values on the ROG page.