Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: Loudthud on September 25, 2012, 10:10:37 PM

Title: Look Ma, NO POTS!
Post by: Loudthud 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.
Title: Re: Look Ma, NO POTS!
Post by: J M Fahey on September 26, 2012, 01:22:16 AM
Very good   :dbtu: :dbtu:
Title: Re: Look Ma, NO POTS!
Post by: KMG 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.
(http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/00S.jpg) (http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/00.jpg)
(http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/01S.jpg) (http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/01.jpg)
(http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/21S.jpg) (http://milas.spb.ru/~kmg/files/projects/dualrectifier/test/compl/dyn/21.jpg)
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.
Title: Re: Look Ma, NO POTS!
Post by: J M Fahey on September 26, 2012, 06:29:18 AM
Cool :dbtu:, but please explain the graphs.  ???
Title: Re: Look Ma, NO POTS!
Post by: KMG 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.
Title: Re: Look Ma, NO POTS!
Post by: Loudthud on September 26, 2012, 11:18:36 AM
Quote from: KMG 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.

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.  :)  :)  :)
Title: Re: Look Ma, NO POTS!
Post by: KMG 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.
Title: Re: Look Ma, NO POTS!
Post by: Loudthud on September 26, 2012, 11:59:15 AM
Quote from: KMG 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.

Please list those benefits and note which ones you think will be lost. Then build the circuit to confirm.
Title: Re: Look Ma, NO POTS!
Post by: Roly 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 }
Title: Re: Look Ma, NO POTS!
Post by: KMG 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.
Title: Re: Look Ma, NO POTS!
Post by: Roly 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 (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.   ???
Title: Re: Look Ma, NO POTS!
Post by: Loudthud 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.
Title: Re: Look Ma, NO POTS!
Post by: KMG on September 27, 2012, 04:10:36 PM
QuoteI 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
Title: Re: Look Ma, NO POTS!
Post by: Roly 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.   :-\
Title: Re: Look Ma, NO POTS!
Post by: J M Fahey 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.
Title: Re: Look Ma, NO POTS!
Post by: KMG on September 28, 2012, 10:51:40 AM
QuoteThe servo integrator time constant in fact is too fast !!!
It's right!
Some results of simulating two JFET stages with & without servo matched by gain & operating point. Both works without "grid current"
V(out2), V(bias2), V(servo2) - vith servo loop.
V(out1), V(bias1) - vithout servo loop.
First - oscillating of servo loop due to improper loop parameters.
Second - as I said, servo loop increases operating point shift even in steady loop state (noservo_servo_end.jpg).
Title: Re: Look Ma, NO POTS!
Post by: Loudthud on September 28, 2012, 12:39:02 PM
I did some experiments with a matched pair of J201s. (Guessing Idss within 2%, Vgoff within 5%). I used a 1.5K Source resistor on one side and the servo circuit on the other. 33uF capacitors bypassed the Sources. To be fair, I used a dual 50K pot to adjust the Drain resistor of both JFETs. That way when the pot was adjusted to give equal Drain voltage at DC, both JFETs would have very close to the same gain. The generator was connected directly to the Gates, no stopper and no coupling cap (important). Signals with Hi/Lo times of 40/60, 60/40, 20/80 and 80/20 at amplitudes of 200mV and 600mV were applied at 1KHz. At 100Hz you could start to see a little tilting on the servo side Drain, maybe the time constant needs to be longer,

Source voltage was measured for each duty cycle at supply voltages of 9V, 20V and 30V. In the vast majority of cases the shift in Source voltage taken as a percentage of change from the DC value was something less than twice as much in the servo circuit. I plan to retest with a coupling cap between the generator and the Gates. This should make the integrator see a consistant DC Source voltage and only shift bias when the signal clips.

In the interest of full disclosure, changes had to be made in the circuit at 30V. 33K was added in series with both sides of the Drain pot and 680 ohms added from Source to ground on the servo side because the opamp couldn't swing low enough with the 1K between it's output and the Source.

Opamp selection: A TL072 could be used but they don't like the output to swing very close to the rail. That could be fixed with a resistor from the Source to ground. A JFET input opamp would allow the integrator resistor to be increased. I'm sure better parts are available.
Title: Re: Look Ma, NO POTS!
Post by: KMG on September 29, 2012, 05:50:05 AM
Schematic of simulation project & waveforms.
Title: Re: Look Ma, NO POTS!
Post by: Roly on September 29, 2012, 07:43:23 AM
Quote from: J M Fahey
talking about different subjects !!
...
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 !!!

Yep.

Just assembled in LTSpice; will poke with stick to see what it takes to get it to bite.  Given that there is only a small trace of 440Hz at the op-amp output, if this is going to oscillate I expect it to be at a very low frequency, slow "motorboating".

Well we all seem to be in agreement on this point.  I would rather see a few seconds time constant.


My model used a 2N3819 (since I don't have the MPF102 model on this 'puter, but I think this is close) and the LM324 because a) its output goes to ground and b) is slow.
Title: Re: Look Ma, NO POTS!
Post by: J M Fahey on October 02, 2012, 03:48:50 AM
@ Roly: maybe it's not enough for steady state oscillation, but hit it with a single narrow pulse (say, 1V peak) and watch it ring.
Not at an audio freq. but way below.
I do not simulate (too modern for my abilities) but test designs (actual components on a wood, nails and bare wire breadboard) by wreaking havoc  touching some input with my finger and listening to and scoping what happens.
Old school? You bet !!  ;)

@ loudthud.
Yes, maybe you can not pull that source resistor down enough even by straight grounding it, and it would not be an Op Amp defect, of course.
At least for the experiment, hook the Op Amp to +/-15V rails, so it can sink below ground if needed, and see what happens.
Title: Re: Look Ma, NO POTS!
Post by: Loudthud on October 02, 2012, 05:58:29 PM
Quote from: J M Fahey on October 02, 2012, 03:48:50 AM
@ loudthud.
Yes, maybe you can not pull that source resistor down enough even by straight grounding it, and it would not be an Op Amp defect, of course.
At least for the experiment, hook the Op Amp to +/-15V rails, so it can sink below ground if needed, and see what happens.

The circuit I copied from used +/- 15V supplies, but I'm committed to a single rail power at the moment. It's easy to tell if the opamp is out of gas, the opamp inputs will be at different DC voltages because the loop isn't closed. The resistor to ground across the source cap works fine. The JFETs I'm working with need at least 3V bias, not a problem for the opamp. At times I see low frequency ringing on the opamp output, but if the R1/R2 voltage divider is set properly, the shift in bias point is minimal. Maybe a small resisitor in series with the source cap would improve phase margin. I'm allowing the preamp rail to sag and that also effects the bias point.
Title: Re: Look Ma, NO POTS!
Post by: Roly on October 02, 2012, 11:40:59 PM
@JM - Yes, well I have been known to say the ultimate simulation is when you build it.   ;)

I've just had a rather instructive episode.  I was asked to knock up a FET buffer with isolated transformer output.

The actuality came very close to the sim with only a couple of minor variations, one of which was a small but unpredicted rise in response up into the supersonic, but hardly worth talking about.

What really surprised me was that I tried a small cheap 3k:3kCT tranny purely on spec, with no strong hope that it would be sufficient.  I think these were originally used as driver transformers in small push-pull transistor amps, and I've often wondered why these driver and output transformers are still available cheaply when amps like that haven't been made for years, at least since the LM380 appeared.

Well I've got 2.3Megs presented to the guitar, no surprise there, but out of the tranny I'm getting 40Hz to 88kHz (my upper measurement limit) +/-1dB on a volt in.  NAD is -80dB at 100, 1k and 10k, and magnetic sensitivity to external fields is very low - all way better than I expected.

I've long been a "birdsnest" guy, throw it up against the universe and see what sticks, but sims have became part of my process 'coz I can't physically mock up every circuit that gets thrown at me on a forum.

{Following the floods here we had an object example of what happens if you feed a sim garbage data - you get garbage results!  These guys never heard of "GIGO".}


@Loudthud - sometimes it pays to relax one of your constraints for a while until you get a better handle on the problem, i.e. introduce a -ve op-amp supply for the time being so you can see the full sweep of your results and not have the bottom half hidden by being clipped off.  When you can see the whole picture an answer as to how to do it with a single supply may present itself (or it may emerge that the complication of providing a -ve supply may be worth it).
Title: Re: Look Ma, NO POTS!
Post by: Loudthud on October 03, 2012, 11:52:53 AM
I'm not seeing any problems with a single rail design. The opamp is happy. Time constants could use a little tweeking, but a negative rail won't fix that. JFETs really start to open up when you get above 15V. All I can say is that a single rail design is more tube like, warts and all :) The subsonic artifacts of a tube amp are often overlooked.

Now I need a chip power amp with 100K input impedance (at least) and a Presence control.

Title: Re: Look Ma, NO POTS!
Post by: Roly on October 04, 2012, 06:16:32 AM
Okay, a couple of sims in passing.  These are plots of what happens when the FET-servo is presented with a step to try and get some insight into what the low frequency stability is like.

The first is a 1 volt step up followed by the same down again after half a second.  Note that I have extended the time constant of the op-amp integrator from 0.1uF to 1uF.

The second is perhaps more a realistic 100mV step.

In the first case we can see that the FET saturates/limits for a significant time (due to having a gain of x30, similar to a 12AX7, but with a limited voltage swing available in the Drain circuit).

In the second case the Drain circuit doesn't actually saturate/limit but there is still that well damped oscillation.

I don't know that this proves anything, other than confirming what we would expect anyway, but it does suggest to me that input DC coupling might not be a good idea, and that a suitable coupling cap should be used with a much shorter time constant than the integrator servo loop.
Title: Re: Look Ma, NO POTS!
Post by: J M Fahey on October 04, 2012, 11:44:30 AM
@ Roly. That's the point.
Time constants should be chosen to work for you, not against.

@ loudthud
QuoteJFETs really start to open up when you get above 15V. ..... The subsonic artifacts of a tube amp are often overlooked. ....  Now I need a chip power amp with 100K input impedance (at least) and a Presence control.
Agree, I use FETS with as much as they can stand.
At least 25V and a few types as high as 40V.
Nothing new, it's what those killer old Yamaha and Roland "Twin type" amps did. And old Randalls too.
Subsonic artifacts: although themselves can't be heard, mixed with audio they produce a very audible "tremolo type" modulation.
How?
Suppose a FET (or transistor, or Op Amp) has, say, 25Vpp swing capacity, and is clipping, so it's clamped at that value ... and you add a, say, 8Hz subsonic.
Yea, not a steady oscillation but a ringing waveform.
Say it goes from 20Vpp down.
The audible signal will have only 5Vpp available, and will be modulated, like it or not, by the 20Vpp subsonic, same as having an 8 Hz tremolo, simple as that.
That said "tremolo" lasts a few milliseconds is true, but we can't deny it adds "something" audible to note attack.
JM2C