Powersuply sag emulation in a stompbox??

[d95err]

There was a discussion a while back about the Roland Blues Cube amp which has a dynamic "sag" emulation circuit that uses an envelope detector to manipulate the supply and bias voltages to one of the JFET gain stages. Perhaps this circuit could be extracted into pedal form?
http://www.blueguitar.org/new/schem/misc_amp/roland_blues_cube30_60.pdf

For those into tube amps, I can recommend one of the Sustain & Compression Enhancer kits from Kevin O'Connor (www.londonpower.com). It's a very simple circuit that adds real tube amp compression ("sag") at any volume level.

[Stellan]

If the OP is looking for "tube amp sag"- that implies a class a/b output circuit that dynamically increases current demand when there is an input signal. This draws from a "higher than it should be" impedance power supply which will dynamically drop the supply voltage on an attack transient, then restore it as the note decays. Zooming out a notch- the overall effect is compression. Simply using a compressor to emulate this sort of behavior seems to work pretty well.

"tube amp sag" is precisely what i am looking for, and a compressor does emulate it, but i always felt like there is something about the attack using a tubeamp with tube rectification that i couldn´t find in a compressor. I haven´t tried more than a couple of different comps though. Whick circuit(s) do you think would come closest??
Would a studio tupe comp be better at this than those usually used in stompboxform?? I think a big part of my disliking compressors is that that ones i have tried (orange squeezer, EMMA compressor and a marshall compressor) destroyed everything i liked about the pickattack, making it very "unnatural" sounding.
The Punisher sounds like an interesting circuit, but im not sure i understand what it does, so with the sparse time i have for playing around with pedals i don´t think i wan´t to start tweaking cirkuits that i have no undrstanding of whatsoever 
Anyway, I really appreciate all the feedback from you guys and will definately try some of these things out this sommer when i have no examns to study for

[reverbie]

Two things:

"But it will not have an initial pulse of current that declines rapidly to whatever can be sucked from the now-in-line used electrolyte. How to emulate the short-term ability?

Easy - put a capacitor on the output. With the cap there, at times of no or low load, the cap charges to or nearly to the open circuit voltage. When a pulse of current is needed, the cap supplies it, but this rapidly falls to the regulated voltage minus the I*R loss in the series resistance. The cap discharges until the power supply takes over and stays there until the load drops and then begins charging back up"

isn't this really similar to setting a compressor pedal's sensitivity down? Or couldnt you put the compression on some sort of micro-delay (ala recovery time) so it doesn't have the same impact on the initial attack...sort of like mimicking the discharge of that cap's reserved energy before the ensuing compression?

i also remember someone on a japanese site implementing both an envelope filter and compressor together to get some convincing sounds.

[The Tone God]

The Punisher sounds like an interesting circuit, but im not sure i understand what it does, so with the sparse time i have for playing around with pedals i don´t think i wan´t to start tweaking cirkuits that i have no undrstanding of whatsoever 

Feel free to ask questions. It helps everyone.

Punisher is a envelop controlled power supply that will drop the amount of current or voltage, depending on which version you use, based on your input signal. Basically you play hard and the current/voltage drops. What I like about it is that you can build it into an external enclosure and that you can adjust the response of the sag.

Andrew

[flo]

Perhaps a limiter is more appropriate to mimic "sag" instead of a compressor.
Compression is usually done by "sensing" the signal after the gain-controlling element (feed backward).
Limiting is usually done by "sensing" the signal before the gain-controlling element (feed forward).

[Caferacernoc]

Perhaps a limiter is more appropriate to mimic "sag" instead of a compressor.
Compression is usually done by "sensing" the signal after the gain-controlling element (feed backward).
Limiting is usually done by "sensing" the signal before the gain-controlling element (feed forward).

Yup. And if placed after the distortion and used sparingly it does have a similar effect to tube sag. If you use it before the distortion or overdrive, country style, then it just evens out your overdrive tone. Not as much varience to your pick attack. I hate that sound.

[reverbie]

What would dB boosting the signal and then setting a limiter around original unity gain do?

[flo]

So what actually happens in a tube when power supply sag happens on the attack peak of a guitar signal?
The plate voltage drops a bit and then ... what?
What does it impact on most?
Which tubes does it impact on most: Preamp or power amp?

The transconductance/gain-factor is still the same because that's a constant, independent of plate voltage, right?

[petemoore]

So what actually happens in a tube when power supply sag happens on the attack peak of a guitar signal?
  The output is increasingly non-linear with the input waveshape, amplitude, and frequency differences occur.
The plate voltage drops a bit and then ... what?
  It   gasps    for    voltage    then   comes  back up.
What does it impact on most?
Which tubes does it impact on most: Preamp or power amp?
  Preamp tubes only need drive a small force into an input of tube, output tubes drive the 'motors' [speakers], this puts a variable load on the PS, attack peaks make the V drop, capacitors 're-top off the tank'.
   The transconductance/gain-factor is still the same because that's a constant, independent of plate voltage, right?
   With 0 plate voltage the tube output is 0.

[DougH]

If the OP is looking for "tube amp sag"- that implies a class a/b output circuit that dynamically increases current demand when there is an input signal. This draws from a "higher than it should be" impedance power supply which will dynamically drop the supply voltage on an attack transient, then restore it as the note decays. Zooming out a notch- the overall effect is compression. Simply using a compressor to emulate this sort of behavior seems to work pretty well.

"tube amp sag" is precisely what i am looking for, and a compressor does emulate it, but i always felt like there is something about the attack using a tubeamp with tube rectification that i couldn´t find in a compressor. I haven´t tried more than a couple of different comps though. Whick circuit(s) do you think would come closest??
Would a studio tupe comp be better at this than those usually used in stompboxform?? I think a big part of my disliking compressors is that that ones i have tried (orange squeezer, EMMA compressor and a marshall compressor) destroyed everything i liked about the pickattack, making it very "unnatural" sounding.
The Punisher sounds like an interesting circuit, but im not sure i understand what it does, so with the sparse time i have for playing around with pedals i don´t think i wan´t to start tweaking cirkuits that i have no undrstanding of whatsoever 
Anyway, I really appreciate all the feedback from you guys and will definately try some of these things out this sommer when i have no examns to study for

In my experience, guitar pedal compressors tend to exaggerate the initial "squeeze" of the note attack. Studio comps may be worth looking into. I've got a presonus single channel comp I use for my acoustic rig that is pretty nice. It is much more subtle than a typical guitar pedal. I haven't used it with my electric much, but something along those lines may fill the bill. It has preset comp options but maybe something that was a step up which gives you individual control of attack/release/etc compression parameters might help. In my experience, "good" tube amp sag has a fairly subtle initial "squeeze", but a long release. But that's a taste issue I'm sure. A better comp that gives you more control may be something worth looking at.

[BTW, pretty typical reaction from this crowd- ask for the time and they tell you how to build a watch... The question is not "how to create power supply sag in a stompbox".  The question is "how to create the effect that power supply sag causes in a tube amp- in a stompbox". Two completely different issues.]

[Stellan]

hehe, yeah some of the information given is slightly over my head, but I do appreciate all the information, as I want to learn as much as I can just to satisfy my curiosity. I actually have a vocal compressor in the preamp i use for my studio mic, but it cant be used independent from the preamp. i am gonna try and see what it does though.

[Andi]

[BTW, pretty typical reaction from this crowd- ask for the time and they tell you how to build a watch... The question is not "how to create power supply sag in a stompbox".  The question is "how to create the effect that power supply sag causes in a tube amp- in a stompbox". Two completely different issues.]

That wasn't the way the original question was phrased, though, was it?

So while many of the initial answers - mine included - weren't what the OP was after, they were answers to the question as posed. If the initial question had been what you just claimed it was the answers would have been different, which puts a bit of a hole in your point-scoring attempt there...

[Stellan]

I was probably unclear in many of my posts, which is at least partly because english is not my first language. Anyway, it is nice to get a discussion going about what happens in different circuits instead of just a "build this" kind of answer, which is also nice, but not really educating. After all, the main reason I build is to learn about every aspect of tone. It is deffinately not to save money, cause I don´t 

[ericohman]

Would anyone recommend me to try to build ToneGod's "Punisher" (Dynamic Power Supply Sag Simulator) connected to a full range boost, all in one pedal?
(Or punisher connected into a compressor.....OR just a compressor alone maybe?)

..to get a "tone" that has more sag feel to it when used with a driven tube amp (JTM45)

[Steben]

simply to revive this a bit:
IMHO, power sag in tube amps as good as always suggests push-pull amps and surely always in class (A)B. Class A has nearly constant current draw, so there is no variation in in power supply load. A dead battery effect only in class A circuits (most preamps/fuzzes) lowers the complete power settings, giving lower output and perhaps bad (desired) bias.

In power sag on the other hand increased signal gives real increased current draw which gives voltage drop in power circuit. This lowers not only the maximum output but also the dynamic range and sometimes shifts a bit the bias. Power sag to me can definitely be heard on Disraeli gears by Cream with clapton half driven rhythm guitar sound on SWLABR, Outside Women Blues and Take it Back. And Led Zep's What is and What should never be on the chord attacks at the end.
Noodling gives clean sound but strong chord strikes gives mushy sound.
In a way, you could emulate this with an expander in front of a very soft compressing clipper. Expanding the drive, with a little compression however in output

If there are circuits that remind of these effects I stumble upon two things: the Mu-amp and the CMOS inverters.
The Mu-amp is pretty linear (current source effect) till its starts smooth clipping. This actually means nice cleans. If the voltage supply drops, the bias drops as well, since the drain is set at supply/2. This means the headroom lowers and clipping comes faster with bigger signals.
CMOS inverters are less linear in the clean zone, but they have another feature: with lowered supply the clipping gets even softer, meaning lower output but softer clipping. real mushy chord.
Problem however: current draw is still set, since it is class A. This means you still need a signal rectifer that converts the input to negative DC which should be summed with the power supply. Or you add a "large current biased" dummy class B push-pull stage, that dynamicaly load the supply.
This can be a buffer or a greedy on current opamp stage.
Greedy on current === low impedance load.

A TL071 has 1.5mAmps of idle current draw (With grounded output this goes to 60mA! ) and mu-amp let's say 0.5mA. That is 2mA in total.
A signal of 1V p-p and a 1k load gives 1mA p-p.
This means going from 0 to 1V signal peak there is 2mA going to 3mA. With a supply resistance of 1k on a 9V supply, this means supply voltage drop going from 2V to 3V. In other words supply goes from 7V to 6V.
In a nutshell ... blabla