Op Amp Evaluation -IMHO

[WGTP]

You guys know way more about this stuff than I do.  I figure if nothing else, the list of models that work would be helpful.  The differences between op amps are pretty subtle, yet at times they seem to jump out at you, it's weird.

Most op amps have caps in the loop that roll off the highs and/or low input impedance that lowers the highs from the guitar.  So, does this self restrict the bandwidth.  (I thouhg bandwidth had to do with how much of the stage a band took up.)  

I wonder how the different parameters effect the distortion.  bandwidth, slew rate (I think that R.G. speculated that might have to do with recover from clipping), rail to rail capabilities (for both input and output)(which seems to be related to cmos op amps), etc.

The cmos amps has repeatedly feedback at the lower harmonics.  I was surprised that the 353 started it too.

The other Burr-Brown I tried the opa2340 seems a little quieter with less gain, but it may break up more than the opa2604.

Do you all think the "feedback" test relates to op amp gain?

Of course I haven't tried different brands of the these same chips.

I also haven't tried my tests at high volumes.

[WGTP]

Final installment of the op amp evaluation

I didn’t really find new magic in this bunch either.  I really like this circuit (Gus’ modded Muff Fuzz #1/Gary’s Mockman) and all the op amps sound good in it.  It seems to maintain sustain even when you turn it down and the distortion decreases.

New concept - “Distortion Reluctance” - the 4558 doesn’t have much, but others seem more reluctant to break up.  This seems to be the thing that may separate the 4558 from the others.  Since distortion is the point, low DR would seem to be a good thing, as long as it’s the kind you want.  I know some don’t like op amp distortion characteristics, but this sounds as good, or better than of the others I’ve built.  

Late Entries

TLV2262   Cmos crunches up nice, quiet lower gain mid-rangey
TLE2072   Excelsior upgrade to TL0xx series, highest slew rate evaluated, high noise/gain, bright, smooth highs, cool
OPA2350   Burr-Brown CMOS, high slew rate, average gain, OK   
OPA2228   Burr-Brown  similar to 2604, not as much gain

[RDV]

I put an Burr-Brown OPA2134 in my SD-1/808(analogman mod) and boy did it wake up and become more full-range sounding, almost, but not, metally. Sweet.

Regards

RDV

[WGTP]

I have finally settled on the OPA2134 for the Mutant Muff too.  It seems to breakup more than the OPA2604.  The OPA2350 is also a BB and CMOS, but not as much gain, it woulds work well for situations when MAX drive isn't needed.

[Fret Wire]

I'd like to try some different OA's in a new DS-1. Has anyone found any suitable subs for it's inline chip? I keep hearing there's a 4558 and 5532 version of it.

[petemoore]

Is there a schematic available for this ckt?

[Dragonfly]

I bumped the other thread, but i should have bumped this thread instead - so i did !

[WGTP]

FLASHBACK - I was reading the document I wrote in '04 and still have pretty much the same perspective.  Alot of the differences between op amps can be minimized, adjusted, corrected, addressed, etc. by chaning the resistors in the feedback loop or to ground for gain adjustment.  The cap in the feedback loop or elsewhere can be moded to address some high frequency issues.  The input cap can be adjusted for bass or the cap/resistor to ground off the feedback loop.  The CLIPPERS used have a MUCH more detectable effect than op amps.  The difference between 2 SI's and 2 LED's doesn't require back and forth A/Bing to hear.  Ge's, SI's, LED's, Jfets, Mosfets (body and mosfet) all are pretty easy to hear.  As Mark, et al have said it is probably the clipping threshold that mostly accounts for the noticable difference, but transfer function seems to play a role as well. 

Certainly try several different op amps to get a feel for it.  I think some probably work well in some distortons and others better in others.
A diode in the feedback loop clipper with an inverting input op amp may not reveal the differences as well as diodes to ground.  A non-diode clipper like a Black Cat might be the best way to hear differences, as the distortion is all coming from the op amp.

Here is the circuit I used with some clipping variations.  http://www.aronnelson.com/gallery/v/WGTP/AlteredMuff.jpg.html?g2_imageViewsIndex=1

IIRC Jack at AMZ indicated that putting a small resistance in series with the diodes minimizes the difference between op amps.  I "think" it reduces the "shock" to the op amp from the clipping.   

The more I mess with this stuff, the more I think it is about pre/post eq, as R.G. and others have been telling us for a while now. 

[R.G.]

If you plot the current per voltage for diodes, you find that they all have some amount of roundness where they start to really conduct. They don't suddenly change from fully off to fully on. The roundness is different for different diodes, as you'd expect by the differences in their device physics.

But what really matters is the relative size of the signal you put across the diode versus the size of that rounded area.

Silicon has a "knee" that's about from 0.45 to 0.65V for "average" junctions, of which there are none, but it makes a good illustration. That 0.2V difference in diode voltage takes it from almost off to almost fully on. If you use signals below about 25mV, you can use the silicon diode knee as a variable resistor and have distortion under a percent or so. That is, a diode doesn't distort tiny signals to any noticeable degree AT ALL if the signal is small compared to the size of the knee, no matter where in the diode characteristic it's biased.

Thomas Organ Vox amps used four silicon diodes as a variable pass resistor in their tremolo - it's one of the better sounding tremolos in a solid state amp.

As you make the signal bigger compared to the knee, the distortion becomes first noticeable, then big. Eventually the signal gets so large that the knee only encompases part of the signal. This is the typical back to back diode pair. The signal is biased at 0V on the diode characteristic, and there is no distortion because the diode knee isn't reached until the signal hits 0.45V or so, and then that part of the signal pushes the diode into its knee, and the part of the signal bigger than that gets clipped.

When the signal get big enough to try to push out the top of the knee, the diode can no longer change to smaller incremental resistance, so the signal is flat-topped there. Signals that just push into the knee, or barely through it are softly distorted.

Now what happens when you put a signal that is 10V p-p unclipped into a diode clipping pair? It zips through the diode knee so fast into hard clipping that there is no noticeable knee region at all - you get a nice square corner in the clipped signal. And how is that different from a signal that "wants" to be 100V p-p? It isn't. Both are indistinguishably clipped to razor sharp corners.

Somewhere between the signal being about the same as the diode knee voltage and maybe 3-4 times the knee voltage you go from soft clipping to hard clipping and get into diminishing returns. It's the relative size of what the signal is (or would otherwise be) without clipping versus the size of the diode knee that matters.

A series resistor changes things. A diode goes from a big resistance, maybe 100's of k to M's before it turns on to a few ohms when it's fully on. If you put a resistor in series with the diode, the diode still does the same thing, but the sum of the diode and the added resistor cannot get smaller than the added resistor. You've limited how hard the composite diode/resistor can turn on and limit the signal. So a resistor in series with a diode will force the resulting signal to never fully flat line on top, no matter how hard you drive it. At big signals, the diode becomes a switch between high resistance for low voltages to the added resistance for voltages more than the diode knee.  You can sit and play with adding different resistor values to each diode, using a variable one, adding pots, caps, etc ad nauseum. Each tweaks with the resistance one or both diodes get to when you trip them over their knees. Likewise, subbing in different materials changes the base voltage before the knee starts and the extent of the knee.

But it's the relative size of the signal (or what it would be if it didn't get limited first), which translates into the vertical speed in volts/uS of the signal traversing the voltage through the diode knee that determines how sharp the diode knee looks to the signal. That's why you can make diode/resistor shapers for bigger synth-style signals. The diode/resistor nets are concocting a big, loose, "knee" that changes resistance over a big range. That's why the amplitude of a triangle put into a sine-shaper network is critical - it relies on the soft clipping of the knee region to round it off.

And then there's pre/post EQ...