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Author Topic: Look Ma, NO POTS!  (Read 8436 times)

Loudthud

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Look Ma, NO POTS!
« on: September 25, 2012, 10:10:37 PM »
I ran across this circuit on an old Sunn schematic. It's a servo to set the operating point of a JFET so you don't need a pot. Resistors R1 and R2 form a voltage divider, the voltage at the center will be the Drain voltage. The voltage divider will track changes in supply voltage and the opamp will adjust the Source voltage to bring the Drain voltage back.

R3 needs to be low enough that the opamp can still supply the Drain current and not have to swing it's output too close to ground. If the JFET needs a source voltage too close to ground, add a resistor across the 47uF cap, lower than needed to bias the JFET and let the opamp pull it positive to reduce Source current. If you want to add a treble peaking R/C or Fetzer resistor in the Source, add it between the 47uF cap and the Source.

I built this circuit on a proto board with a J201. When I plugged in a MPF102 the bias adjusted itself and the gain was very close to where it was with the J201. The bias adjusts perfectly changing the supply from 9 to 32V. The bias can drift when the output is clipping. You'll get a little jump in bias when the signal cuts off. A slight adjustment to the R1/R2 ratio will fix this.

J M Fahey

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Re: Look Ma, NO POTS!
« Reply #1 on: September 26, 2012, 01:22:16 AM »
Very good   :dbtu: :dbtu:

KMG

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Re: Look Ma, NO POTS!
« Reply #2 on: September 26, 2012, 02:56:32 AM »
Accurate emulation of grid current (shift of operatin point) often means more than a shape of clipping.
Look how tube stage respond on "burst" input signal.



K1 - Anode voltage
K2 - Input voltage
K3 - Grid voltage
K4 - Cathode voltage
Output signal  "floats" in time at a constant input level.
Average current varies almost 2 times.
Amount of changes depends on interstage circuitry (impedance, time constant etc).
That's why guitar players often say that tube sound "live" and the sound of semiconductors "dead" (uniform).
This refers not to the color of harmonics but to reaction on the sound picking.

J M Fahey

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Re: Look Ma, NO POTS!
« Reply #3 on: September 26, 2012, 06:29:18 AM »
Cool :dbtu:, but please explain the graphs.  ???

KMG

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Re: Look Ma, NO POTS!
« Reply #4 on: September 26, 2012, 07:34:35 AM »
I don't understand what do you mean by "explain the graphs".
But the processes is the following:
When grid current begins, constant amount of voltage at inter-stage coupling capacitor rises.
This lead to creating virtual negative biasing of the control grid. Stage operating point shifts closer to cutoff.

Loudthud

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Re: Look Ma, NO POTS!
« Reply #5 on: September 26, 2012, 11:18:36 AM »
Accurate emulation of grid current (shift of operatin point) often means more than a shape of clipping.
Look how tube stage respond on "burst" input signal.

This thread isn't about tube emulation. In fact, I left those components off the schematic. It's about JFET preamps for novice builders who don't even own a DVM or know how to measure anything. If you want to over analyse things, tell me why the opamp doesn't oscillate or if it will with changes to time constants in the loop.  :)  :)  :)

KMG

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Re: Look Ma, NO POTS!
« Reply #6 on: September 26, 2012, 11:30:58 AM »
Adding servo you'll loose most benefits of jfet stage. In this case it'll sound like simple opamp with clipping diodes.
Simpler tunung but poorer sound.

Loudthud

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Re: Look Ma, NO POTS!
« Reply #7 on: September 26, 2012, 11:59:15 AM »
Adding servo you'll loose most benefits of jfet stage. In this case it'll sound like simple opamp with clipping diodes.
Simpler tunung but poorer sound.

Please list those benefits and note which ones you think will be lost. Then build the circuit to confirm.

Roly

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Re: Look Ma, NO POTS!
« Reply #8 on: September 26, 2012, 02:48:22 PM »
@KMG - it's very late here and I'm quite tired, but I don't see what this has to do with "emulating grid current", and I don't understand "In this case it'll sound like simple opamp with clipping diodes."

Without calculating it, it looks to be like quite a long integration time with 0.1uF and 220k, and followed by the 47uF on the Source, so there should be little or no AC signal through the op-amp.

"Adding servo you'll loose most benefits of jfet stage." but ... for the AC signal it will still be distorted by the FET characteristic ... won't it?  It's only a slow DC bias servo ... isn't it?

Puzzled?  ???

{I'll come back to this after some sleep.  xP }
If you say theory and practice don't agree you haven't applied enough theory.

KMG

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Re: Look Ma, NO POTS!
« Reply #9 on: September 27, 2012, 07:12:27 AM »
By benefits I mean closeness to vaccum tube operation, i.e. absence of deep feedback, existence of "input clipping" (by gate-source diode).
I apologize for  hasty conclusions about comparing this idea with opamp limiter, but after the simulation would like to note the following.
Servo in this case acts as a positive feedback to the clipping duty cycle for stages with "shifted" operating point.
In stages with "shifted" operating point without servo clipping duty cycle does not change significantly without "input clipping".
Also it works as a positive feedback for "input clipping".
In stage with servo clipping duty cycle while "input clipping" changes faster than in stage without servo.
V(out1) & V(servo1) for stage with operating point shifted to saturation, V(out2) & V(servo2) for stage with operating point shifted to cutoff.
« Last Edit: September 27, 2012, 07:23:45 AM by KMG »

Roly

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Re: Look Ma, NO POTS!
« Reply #10 on: September 27, 2012, 12:32:59 PM »
I dunno, maybe I'm missing the point here, but I took the original post to be a follow-on from;

http://www.ssguitar.com/index.php?topic=2713.0

...where we were presented with a fairly abominable and unstable JFET-isation of a Boogie Mk4 valve pre which had a pot for every FET Drain load (apparently because the "designer" didn't seem to know what he was doing, a key point being the use of a 9 volt supply instead of something more reasonable like 18 volts).  I therefore assumed, perhaps wrongly, that this servo was another way (apart from yer actual proper design) to rid a FET preamp of trimpots while still coping with wide FET spread.

Quote from: Loudthud
It's a servo to set the operating point of a JFET so you don't need a pot.
...
The bias adjusts perfectly changing the supply from 9 to 32V.

While I take your point about clipping producing a DC shift in bias I am still at a bit of a loss to see how it applies to the first stage or two of a properly biassed amp, FET, transistor, or valve/tube.  Pete Townsend aside, I don't expect any guitar to clip the first preamp stage, and if it's being driven by a hairy-legged fuzzbox the point seems a bit moot.  Obviously if you want the stage to clip then a servo defeats the idea, but I thought this idea concerned an amplifier, not a fuzz box.

Still confused.   ???
If you say theory and practice don't agree you haven't applied enough theory.

Loudthud

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Re: Look Ma, NO POTS!
« Reply #11 on: September 27, 2012, 03:14:31 PM »
Tip: If you want to set all the JFETs in your preamp stages to the same drain voltage, you can use the same voltage divider (R1/R2) for all the opamps.

Cathode bias will shift in a tube amp if it is clipping non-symmetrically or the operating point is off center. I will experiment later today driving two JFET stages at the same time. One with servo bias and one with ordinary source bias and compare bias shifts (source voltage).

To make an input stage "pedal friendly", I try to duplicate what the output clipping of the first stage looks like in reference to the input. With a 30V preamp supply rail, it looks like the gain needs to be less than 10. I use diode clipping on the input because gate current causes the output to clip too sharply and gate current doesn't always start when the output clips.

KMG

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Re: Look Ma, NO POTS!
« Reply #12 on: September 27, 2012, 04:10:36 PM »
Quote
I use diode clipping on the input because gate current causes the output to clip too sharply and gate current doesn't always start when the output clips.
2010
http://www.amtelectronics.com/support/articles/why_tube-like_cannot_measure_up_to_tube_inside/
Russian original text dated 2006.
http://www.sugardas.lt/~igoramps/article68/article.htm

Roly

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Re: Look Ma, NO POTS!
« Reply #13 on: September 28, 2012, 09:28:54 AM »
Quote from: Loudthud
tell me why the opamp doesn't oscillate

That may have been rhetorical question, but for the benefit of other readers, the reason that the op-amp won't oscillate is because it is wired as an integrator with a capacitance directly between output and inverting input, and therefore is in a very highly negative fed-back state, and acts as a (very) low pass filter.


KMG - I don't dispute what you are saying; but given that there is next to zero chance of clipping either the first or second stage of a Boogie Mk4 (unless driven by a pedal) I just cannot see how it relates to the topic of this thread.   :-\
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Look Ma, NO POTS!
« Reply #14 on: September 28, 2012, 10:33:42 AM »
Guys !! guys !! Don'tfight !!
You are all right, only talking about different subjects !!
1) @Loudthud: you are right, this is an *much* better way for proper biasing without selecting or adding (Ugh!!) trimmers.
Which to worsen the kludgyness are added as drain loads.
If anything, their place is in the *source*, if we want to use the word *biasing*, otherwise we are adjusting the *load*, a very different thing.
2) @KMG, you are right, specially when referring to clipping type, even more so when considering grid/gate rectification, shifting bias/symmetry/duty cycle/gain "everything at the same time".
Simple here we are not talking about that (yet  ;)  )
3) @ Roly: in fact that Op Amp + FET stage *can* oscillate (and probably will if given proper loop gain and phase shift).
The servo integrator time constant in fact is too fast !!!
220K+.1uF mean around 8Hz , which *looks* too low compared to guitar frequencies, but means an  incredibly short period compared to the duration of any note played.
So *if* the stage is clipped (OK, not the first one, but definitely the 3rd or 4th), the Fet will *start* to react "like a tube", doing everything I mentioned above .... and immediately ( a few milliseconds later) be brought back in line to proper biasing, because the servo will do *anything* necessary to achieve that. That's what KMG is saying, using different words.
As of phase and oscillation : to check it consider the drain going up (positive pulse). The servo is an inverter, so its output goes down, the source resistor goes down also, the Fet gess less negative bias, the higher current makes the drain go low, so we have opposition to original positive pulse= feedback is negative.
*But* both the integrator cap and the 1K+47uF both produce phase shifts .
We'd still need a 3rd phase shift stage to turn this into a classic phase shift oscillator, I mentioned this only to see that apparently simple circuits *can* have "hidden" quirks .... "hidden in plain sight"
Maybe a pedal or preamp with 3 or 4 of these stages becomes way *too* sensitive to grounding or supply decoupling , motorboating with no apparent reason.

And yes, a servo will correct a clipping Fet so much, that probably it won't sound too different to a clipping Op Amp stage.
But maybe that's stuff for a different post.
Personally, I find buying a few extra FETs and measuring, at least, Vp (pinchoff/cutoff voltage) well worth the time and effort (15 seconds, only need -15V , a 15K resistor and a socket ... which in my case means 3 rows in a protoboard).
After matching them, can use a fixed source resistor and let them clip anyway they want to.
jm2c.
 

 

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