Modulating the Offset Null pins on an Opamp

[Vivek]

Is it possible to modulate the Opamp Offset null pins

1 and 5 on a 741

to try and implement some time based distortion ?

Like rectify the output, integrate on a cap, feed DC to offset null, make bias point change based on cap voltage, get different harmonics / amplitude based on integration cap voltage


Or feed some of the main output via a filter back to one of these offset null pins ?

[GGBB]

The Crowther Hotcake uses pin 5 of the TL071 to implement its "cream" feature.

[Vivek]

Thanks Gord,

This should be fun to dissect and learn from!!!


But it surely looks like an under-investigated idea and there might be lots of goodies lying in store for avid electronic spelunkers

[Vivek]

Earlier thread by Aion titled : TL071 offset null... what's this circuit doing with it?

https://www.diystompboxes.com/smfforum/index.php?topic=125157.msg1190214#msg1190214

[EBK]

You can use the offset null pins as the signal input pins if you like.  I need to dig up a link to some great reading material.  Give me several minutes.

Here: 
http://www.analog.com/media/en/training-seminars/design-handbooks/Amplifier-Applicatons-Guide/Section10.pdf

[Vivek]

Thanks Eric,

Very pertinent !!!


Would it be possible to implement time varying bias shift using the scheme I suggested :

rectify the output, integrate on a cap, feed DC to offset null pin, make bias point change based on cap voltage, get different harmonics / amplitude based on integration cap voltage


Or somehow make multi-stage distortion by using the offset null to also clip internally besides the external clipping diodes ?

[EBK]

You can modulate the bias on an op amp without getting fancy with any offset null pins.... 

(I will have more time to write later)

[Vivek]

Like Ampeg/ Crate bias shift circuits ?

[amptramp]

There are some op amps like the LM318 and the 5534 where there are compensation pins connected to the collectors of the input stage.  This has led to designers taking the normal inputs and tying them to the most negative voltage to shut them off and connecting the collectors or drains of external devices to them.  The LH0061 was a FET input version of an LM318 made by exactly this process - I know because I was the first customer in Canada for them.

These are not the same as offset and null pins but it does show that you can tweak the characteristics of an op amp with the right connections.

[Steben]

Bias shift effects occur even without fiddling with the opamp. Just an asymmetry somewhere and a cap that gets charged.
Or are you referring to fixed mis-bias?

[anotherjim]

Slow modulation of opamp bias. DC couple into a comparator. PWM fuzz? Some phasing action?

[Vivek]

I want the Opamp to change harmonic content with time, like some kind of sag.

[Steben]

I want the Opamp to change harmonic content with time, like some kind of sag.

Yes, I just think in practice it will be unsatisfying, unless it is a very very special opamp. Opamps clip harsh (if no latchup, than its gross) even with misbias. Asymmetric harsh will get you even order harmonics, but not as sweet sounding as a gradual change of harmonics. It will sound fuzzy in best case. Usable yes, if that is what you want.
One of the simplest things with sag is having an opamp or better a good old 386 power amp that draws variable current (its class AB) which let a resistor drop the supply voltage. Bias shift is something of a rather "pre-ampy" thing which is obtainable in easier ways as said before.

[Rob Strand]

I want the Opamp to change harmonic content with time, like some kind of sag.

Yes, I just think in practice it will be unsatisfying, unless it is a very very special opamp. Opamps clip harsh (if no latchup, than its gross) even with misbias. Asymmetric harsh will get you even order harmonics, but not as sweet sounding as a gradual change of harmonics. It will sound fuzzy in best case. Usable yes, if that is what you want.
One of the simplest things with sag is having an opamp or better a good old 386 power amp that draws variable current (its class AB) which let a resistor drop the supply voltage. Bias shift is something of a rather "pre-ampy" thing which is obtainable in easier ways as said before.

That's where the Hotcake technique comes in.  The opamp output stage is prevented from clipping.  In fact the signal is clipped but the opamp isn't clipping because the feedback loop still has control.   When a feedback loop loses control under clipping the feedback drives everything harder and harder into saturation. 
https://www.diystompboxes.com/smfforum/index.php?topic=125157

Feedback diodes and zeners do a similar thing.  However you can still clip non-inverting stages if the voltage on the + input is high enough to cause clipping.

[Rob Strand]

You can use the offset null pins as the signal input pins if you like.  I need to dig up a link to some great reading material.  Give me several minutes.

Here: 
http://www.analog.com/media/en/training-seminars/design-handbooks/Amplifier-Applicatons-Guide/Section10.pdf

You have to admire the rectifier on page X-17 - despite the quirks in the text and ill-defined drift and offsets.

[EBK]

You can use the offset null pins as the signal input pins if you like.  I need to dig up a link to some great reading material.  Give me several minutes.

Here: 
http://www.analog.com/media/en/training-seminars/design-handbooks/Amplifier-Applicatons-Guide/Section10.pdf

You have to admire the rectifier on page X-17 - despite the quirks in the text and ill-defined drift and offsets.

It is clever.  Makes me want to use power supply pins as audio outputs in a circuit.

[Rob Strand]

It is clever.  Makes me want to use power supply pins as audio outputs in a circuit.

Do a search for the name Alexander which appears in the references.  You will get a few hits (also a patent),

https://www.analog.com/media/en/technical-documentation/application-notes/58052492001115525484056221917334AN211.pdf