Random musings on intentional op amp misuse

[EBK]

Probably not terribly original, but....

I would assume that some op amp specs are designed to protect the op amp from damage, but some are merely designed to constrain behavior to "normal". 

What if the abnormal behavior of a tortured but still functioning mediocre-spec'd op amp made some new unique (perhaps awful) sounds possible?

For example, a 741 datasheet says the following:

For proper operation, the power supplies must be properly decoupled. For decoupling the supply lines, a 0.1-µF
capacitor is recommended and should be placed as close as possible to the LM741 power supply pins.

What if we not only ignored this, but actually powered the op amp with a really large AC signal riding on the DC? 

[EBK]

Reading through a paper book chapter called, "Unusual applications for op amps."
Interesting stuff. https://goo.gl/hBkaE3 See Paul's links below.
Or maybe Merlin's link.

[anotherjim]

That a very interesting document, new to me - Thanks  -  I always wondered if you could "play" compensation pins to some purpose -  must have a deep look into that.

[PRR]

> "Unusual applications for op amps."

The link you gave throws an error in FireFox. I think it just blows its small mind.

With Chrome I found the true url:

ftp://ftp.analog.com/pub/cftl/Amplifier%20Applications%20Guide%201992/Section%2010%20Unusual%20Applications%20for%20Op%20Amps.pdf

You may prefer to see the whole book by chapter:

http://www.analog.com/en/education/education-library/amplifier-applications-guide.html

[EBK]

Thanks, Paul.  I had trouble deGoogling the long URL I had (Google likes to hide where it is going to take you, it seems), so my solution was to reGoogle it.  I'm glad you were able to double deGoogle it for the masses.   

[merlinb]

With Chrome I found the true url:

ftp://ftp.analog.com/pub/cftl/Amplifier%20Applications%20Guide%201992/Section%2010%20Unusual%20Applications%20for%20Op%20Amps.pdf

That link don't work for me either! This one does:
http://www.analog.com/media/en/training-seminars/design-handbooks/Amplifier-Applicatons-Guide/Section10.pdf

[EBK]

This is starting to get spooky!

[EBK]

Some more fun:
http://www.edn.com/design/other/4373768/Use-an-op-amp-as-a-set-reset-flip-flop

[tubesimmer]

Not outright misuse, but I've been getting into a bit of torturing.   
I've been testing the sound of various op amps driven deep into overdrive vs using diodes for clipping.

Bought a dozen op amps from Sayal, ranging from a $0.29 TS27L2IN all the way up to a $6.30 OP270.
Plugged them into a Marshall Guv'nor clone that relies heavily on its LM833 being overdriven.
Wow, an astonishing array of sounds.
I've never been a huge fan of distortion circuits w/clipper diodes post op amp to ground (i.e, not in the feedback loop TS style), but after hearing what's possible with a couple of the amps driven to the point of breaking is just shy of incredible in terms of tone & dynamics.

It's very well worth trying out, I'd highly recommend it.

[EBK]

I seem to have more ideas without time to fully development them these days.  I want to throw this half-baked idea out here for a bit of feedback.

Basically, the idea is to modify the positive power rail in an op amp circuit, keeping the voltage within the min/max operating levels at all times, but otherwise making it time-varying in some way, possibly tied to a LFO or as some function of the input signal.

My thinking is that, for normally-expected output levels that don't reach the saturation threshold (now a moving target), the varying supply would have relatively little effect on the output waveform, but as the normally-expected output level crosses the lowest saturation threshold, the output waveform would pick up more and more "modulation", for lack of a better term.

I have no particular aim for what I would want this to sound like.  I'm just looking for something new and relatively controllable, even if it ends up sounding rather harsh.

Mainly, I'm wondering if I risk destroying the op amp while trying this out.   

My other question is whether the op amp could have some trick up its sleeve to thwart my evil plan (like internal low pass filtering of the supply rail, perhaps).

All thoughts are welcome.

[tubesimmer]

Basically, the idea is to modify the positive power rail in an op amp circuit, keeping the voltage within the min/max operating levels at all times, but otherwise making it time-varying in some way, possibly tied to a LFO or as some function of the input signal.

How about tying the signal output to a corresponding drop in voltage supply; emulating tube amp 'sag'.

Last year I was playing around with a simple resistor in series with the +ve supply to the op-amp, making the supply voltage circuit-current dependent.
Mixed results, needs more investigation, maybe a cap in parallel to soften the response time...

[samhay]

>Basically, the idea is to modify the positive power rail in an op amp circuit, keeping the voltage within the min/max operating levels at all times, but otherwise making it time-varying in some way, possibly tied to a LFO or as some function of the input signal.

The gain is not set by the supply voltage, so I don't see how you would achieve anything other than mashing the signal against the positive rail in a modulated way.
If that's desirable, then can I suggest an alternative? Leave the power supply alone and instead monkey around with the bias voltage (Vb, Vref, whatever you call it) instead. You may find this is (much) easier to implement, and I don't think you will hear the difference.
Alternatively/in any case, look up bootstrapping.
e.g. http://www.edn.com/design/analog/4361336/Bootstrapping-your-op-amp-yields-wide-voltage-swings

[EBK]

The gain is not set by the supply voltage, so I don't see how you would achieve anything other than mashing the signal against the positive rail in a modulated way.
If that's desirable, then can I suggest an alternative? Leave the power supply alone and instead monkey around with the bias voltage (Vb, Vref, whatever you call it) instead. You may find this is (much) easier to implement, and I don't think you will hear the difference.

Almost.  I think the result would be slightly different in that I would have to remove the AC component of vbias from the unsaturated portion of the output. (Im allowing for the possibility that I haven't thought your comment the whole way through though...).  What I'm hoping to do is take advantage of the op amp's power supply rejection ratio to somewhat mask the variation of the supply except where the output saturates.

Alternatively/in any case, look up bootstrapping.
e.g. http://www.edn.com/design/analog/4361336/Bootstrapping-your-op-amp-yields-wide-voltage-swings

I will take a look later tonight.  Thanks!

[PRR]

> wondering if I risk destroying the op amp

That would be a 19 cent tragedy.

[EBK]

> wondering if I risk destroying the op amp

That would be a 19 cent tragedy.

And $1 worth of entertainment if it somehow starts smoking before it burns out.

[PRR]

Sadly very unlikely to smoke.

[amptramp]

Sadly very unlikely to smoke.

Challenge accepted.

[samhay]

^I can smell a new type of DIYSB competition brewing.

[EBK]

I was reading the "Op Amp Subtleties" section of the Amplifier Applications Guide linked above (somewhere), and I noticed this little gem regarding output voltage phase reversal:

This effect in BiFET operational ampli-
fiers can cause a very dangerous lock-up
condition in mechanical system applica-
tions where the amplifiers act as elements
controlling a system's servo loops. In one
particular application, a customer who
used BiFET amplifiers in the servo loops
of a shaker table controller noted that he
could consistently launch 100 lb. objects
across his lab every time he induced a
failure mode in his system. In one way,
he was quite pleased that he had
stumbled upon a new catapult system and
wondered aloud if there would be a mili-
tary application for it . On the other hand
and not surprisingly, he was quite
alarmed by the effect.

[ashcat_lt]

Yeah, the effect of modulating Vref is exactly the same as mixing the modulator signal into the input.  You will "hear" the modulator on the other side.  This should probably be obvious, but I did have to try it before I went "Oh.  Duh."  With low enough modulator speeds, you can strip it out by carefully sizing your coupling caps, but at audio rates...  One might try mixing an inverted copy of the modulator back into the output of the opamp stage, but that only works up til the opamp starts clipping, then its output loses coherence with the modulator, and the peaks of the negative wave will still end up poking through in the final output. 

The effect of wiggling the V+ seems like it would work better, but I tend to think you'd still hear the modulator in the same way and for the same reason that you hear buzz from a poorly filtered power supply - basically because the opamp output is actually the power supply itself modulated by the input signal!  That's how all active amplifiers work.  Now, of course, usually when a PS hums, it's wiggling both V+ and Vref since Vref is just a divided copy of V+.  If you decoupled those so that Vref stayed stable while the opamp supply wiggles, things might be different, but I still think you end up hearing it.  Definitely worth a shot, and if it works and does anything interesting without too much mod bleed, I'd be happy to hear about it.

A while back, I got to thinking about how we normally use an opamp with a maximum output of (usually less than 9V) with enough gain that it wants to output hundreds of volts and then clip that back down with diodes to something right around a volt.  So I thought, why not just use a lower supply and forgo the diodes?  Quickly found out this is mostly because there aren't a lot of common opamps that will even light up with much less than 9V supply.  I did find that the LM324 works with power supplys down to 3V.  It will swing to something like a diode drop from the top rail, and close enough to the bottom to say it's all the way, so the output at 3V is right around that of an LED to ground clipper, but if you bias to half the supply, you get a little asymmetry for free.  It actually sounds pretty darn good, too!  I had a kind of silly design based off of this, but I never got around to building it.