Any examples of Sag or Mis-bias applied to Op Amp circuits?

[John Lyons]

I'm looking for a way to misbias or Sag an op-Amp's
sound other than lowering the voltage.
Looking for extra texture and sloppiness
and possible gating.

[teemuk]

Here's one for "sagging" (Polytone): Notice 3K3 series resistors on power supply pins of the 741 opamp employed in the drive circuit.
http://www.murchmusic.com/Polytone%20Info/Schem1.JPG

For "misbias" look at e.g.
- SansAmp (Character?) circuit where you have user adjusted DC offset
- Carvin SX or Crate FlexWave circuits where offset depends on signal envelope

[bartimaeus]

with opamps, setting different bias points will generally just add asymmetrical clipping. one side will hard clip first. at least for most guitar pedal circuits. you won't get anything similar to "sag" on a tube amp.

to get sag, you'd probably want a bias supply that's poorly regulated. or perhaps even a power supply that's poorly regulated. you could try using only a very small capacitor on your power supply and bias. i wouldn't remove it entirely though as you want to remove any high-frequency noise from the power.

a much more elaborate idea would be an envelope follower circuit that adds with your bias supply. more circuit but it could sound cool...

[John Lyons]

Thanks for the links and clues folks.

[ronh]

I accidentally designed something like what you want to do.  Op Amp was biased by a bi-polar supply.  The negative side was from a 555 timer chip running as a voltage inverter. As the current draw went up the voltage would collapse.  At max current draw the voltage was -3V from -8V. Less current draw, less voltage collapse. Unfortunately the 555 chip would go up in smoke.  It sounded great while the chip was working. Different sounding overdrive.  Current goes up, voltage goes down.

[PRR]

Here's one for "sagging" (Polytone): Notice 3K3 series resistors on power supply pins of the 741 opamp employed in the drive circuit.
http://www.murchmusic.com/Polytone%20Info/Schem1.JPG ....

Seems to be noted as '071'? TL071 maybe? Now rare but available.

Different opamps will take to this trick differently. Some, a few hundred unbypassed ohms in the supply pins will send them into AM-band oscillation.

Bunch of '4558, LM358 lamp drivers, but note the '301 in the reverb drive. And with fancy compensation! (For reverb??)

[Frank_NH]

The Roland BC-60 Blues Cube Amp has a control option called Rectifex which simulates voltage sag of a tube amp rectifier.  Schematics are available online.  It's basically a discrete power amp circuit which takes an input signal from the preamp and uses it to vary the source voltage on one of the later jfet stages of the preamp.  I own two BC-60s and frankly the effect is rather subtle but maybe could be modified to exaggerate the voltage sag.  Also, maybe you could substitute an LM386 for the discrete power amp circuit.  By the way, the BC-60 is a fully discrete jfet/bjt transistor design (no op amps!).

[amz-fx]

Look for the now discontinued Tech 21 XXL pedal. It used a variable bias to change the harmonics.

Best regards, Jack

[Eb7+9]

from back in the day ... 

https://www.diystompboxes.com/analogalchemy/sch/diodeopamp.html

[Rob Strand]

The Roland BC-60 Blues Cube Amp has a control option called Rectifex which simulates voltage sag of a tube amp rectifier.  Schematics are available online.  It's basically a discrete power amp circuit which takes an input signal from the preamp and uses it to vary the source voltage on one of the later jfet stages of the preamp.  I own two BC-60s and frankly the effect is rather subtle but maybe could be modified to exaggerate the voltage sag.  Also, maybe you could substitute an LM386 for the discrete power amp circuit.  By the way, the BC-60 is a fully discrete jfet/bjt transistor design (no op amps!).

It's a unique idea because the audio doesn't pass through the sag amplifier.  The sag amplifier just loads the supply to create sag.   There's one quirky thing about the power amplifier, the output stage is AC coupled with quite a small cap.   Maybe they did that to prevent low frequency ripple on the power rails.  It might be worth trying a larger coupling cap to exaggerate the effect, you could also use a smaller load resistor for more loading.  (For an LM386 version I suppose you could roll-off the lows with the output cap but it might sound different.)

It seems the STAGE model does this in DSP?

[Frank_NH]

It's a unique idea because the audio doesn't pass through the sag amplifier.  The sag amplifier just loads the supply to create sag.   There's one quirky thing about the power amplifier, the output stage is AC coupled with quite a small cap.   Maybe they did that to prevent low frequency ripple on the power rails.  It might be worth trying a larger coupling cap to exaggerate the effect, you could also use a smaller load resistor for more loading.  (For an LM386 version I suppose you could roll-off the lows with the output cap but it might sound different.)

It seems the STAGE model does this in DSP?

The BC-30 and BC-60 from the 1990s were all analog solid state amplifiers and are highly regarded for their tone.  I like the BC-60 - it's versatile and gets many great tones with the Crunch control.  The Lead channel can get a bit "fizzy" for my tastes but maybe there's a mod that can tame that.  The newer Blues Cubes are DSP-Analog (onboard reverb and tremolo are DSP effects).  I have the Blues Cube Artist and it's my main gigging amp as it sounds great and is about 10 lbs lighter than the BC-60!

As for Rectifex, I have an LTSpice model of it and may fool around with the cap values as you suggested.  Would be interesting to have a go with it on the breadboard as well.

[Rob Strand]

I have the Blues Cube Artist and it's my main gigging amp as it sounds great and is about 10 lbs lighter than the BC-60!

Ha, yes, those class D amps help a lot.

As for Rectifex, I have an LTSpice model of it and may fool around with the cap values as you suggested.  Would be interesting to have a go with it on the breadboard as well.

I'm not quite switched on today, the LM386 version could use the input cap to roll-off the lows, maybe a better idea than the output cap.

[CheapPedalCollector]

Armstrong blue clipper has an unbalanced VREF, you could do the same and make it more extreme. Recently I found that opamps don't bias up very well under 1/2 supply voltage and won't at all usually under 1.5v

[teemuk]

Seems to be noted as '071'?

Sorry, yes 071 not 741.

Different opamps will take to this trick differently. Some, a few hundred unbypassed ohms in the supply pins will send them into AM-band oscillation.

Yes, even when the circuit board traces are long enough the chips might need close vicinity decoupling.

What helps the Polytone circuit work stabile is that it is the only stage drawing current from the node; the rail voltage fluctuation is not "coupling" to any other stages.

[teemuk]

The newer Blues Cubes are DSP-Analog (onboard reverb and tremolo are DSP effects).

Roland is commonly a bit mysterious of what they employ; e.g. they advertise their COSM but the millennia Cube series amps still also had an analog distortion circuit to soft clip the power amp input signal. If I'm interpreting some Blues Cube series diagrams correctly they also partially perform "tube emulations" with analog circuitry. "TubeLogic" can also mean just about anything.

As for Rectifex, I have an LTSpice model of it and may fool around with the cap values as you suggested.  Would be interesting to have a go with it on the breadboard as well.

Yes, it's a clever circuit concept. I personally prefer how Ampeg does it in SVT8-Pro though: There's just an envelope control for threshold of a plain symmetric diode clipper, and another diode circuit to emulate dynamic crossover distortion. Overall Ampeg features a much more realistic "tube power amp" simulation than the Blues Cube, IMHO, yet it is far more simple in terms of circuitry than the whole Rectifex power amp loading down FET preamp power supply thingie.

Anyway, Roland indeed has many interesting circuits under the TubeLogic category.