Tonal Effect of Different Amp ICs in Overdrives

[bluewaterpig]

Hey guys...back again with another noob question so that you guys can bestow your wisdom upon me.

From what I've gathered, the amp IC has a pretty important role in an overdrive pedal's circuit. The JRC4558 is your TubeScreamer, the LM308N is your RAT, etc.

I don't know if I have one specific question, but...how big of a role does the amp IC actually play in the overall sound of the effect? If you took a TS-808 and replaced its IC with an LM308N, would it sound like a different pedal? I'd also like to see if we can make a list of the most common amp ICs, which pedals they come stock in, and what their tonal characteristics are.

Thanks in advance everyone. 

[smallbearelec]

If you took a TS-808 and replaced its IC with an LM308N

That's one change that won't work, because the pinouts are different. The 308N is a single op-amp, uses an external capacitor for frequency compensation; more modern chips use an internal cap. The JRC4558 is a dual op-amp, internally compensated.

There Are many dual op-amps that will drop in place of a JRC4558. From my Product FAQ:

"TLC2262, TLC2272, LM833, TL072, RC4558P, NE5532, OP2134PA, OP2604AP.

Note that if you are willing to breadboard the circuit so that you can easily swap other components, you'll find that changing values and types of coupling caps, types of clipping diodes and the configuration of the diode loop, and the gains of the stages, create Much more pronounced changes in tone than changing chips. The "Bible" concerning these mods is R. G. Keen's article, "The Technology Of The Tube Screamer," at GEOFEX.com. Welcome to DIY pedal madness..."

[teemuk]

Are you overdriving the chip?

Because if you are, then acknowledge that different chips have different characteristics in how close to supply rails their output can "swing". Rail-to-rail opamps, as name implies, pretty much clip to rail limits, other opamps may clip a few volts earlier. How early... depends on the chip. Some chips also handle ovedrive more decently than others (IOW, they just "clip" instead of things like latching to rails, inverting polarity, oscillation, etc.)

If you are not ovedriving the chip and the overdrive is created within feedback loop (e.g. clipping diodes in the loop) then the differences between chips aren't really noteworthy because you are running them within closed negative feedback loop, which tends to even out all differences in performance.

Then we have an issue whether the chip has BJT or FET input... This can make a difference between inherently lowish input impedance or inherently very high input impedance. Depending on this the opamp might or might not load the source stage, which could then have tonal effects (e.g. excessive attenuation).

Some opamps are also noisier than others. In an overdrive circuit (which usually bases its operation on HIGH gain) the noise will be amplified (by same HIGH gain ratio) so this characteristics may become somewhat important.

But it really depends on many factors. Opamps have differences, but their operating priciple is pretty much based on negative feedback loop "normalizing" their performance so that any opamp practically has the same predictable and consistent performance. (For this reason we don't really consider opamps to have "tonal characteristics", they should all perform identically.... within their limits, that is). If you for some reason break the loop then the differences might start to portray more clearly.

If you took a TS-808 and replaced its IC with an LM308N, would it sound like a different pedal?

No, It would not.

You might hear small differences in sound, or you might not. Tweaking the overall circuitry around the opamp stage has vastly greater effect on tone than swapping one opamp to another of different type.

Wikipedia essentially summarizes the issue very well:

The popularity of the op-amp as a building block in analog circuits is due to its versatility. Due to negative feedback, the characteristics of an op-amp circuit, its gain, input and output impedance, bandwidth etc. are determined by external components and have little dependence on temperature coefficients or manufacturing variations in the op-amp itself.

[Guitar Poppa]

Hello

Teemuk said almost everything that has to be said about that question. I just would add something, that is more a general markers than an accurate information :

In overdrives like the Tube screamer, or disto boxes like Rats, the best ICs are not the best as in a Hifi point of view... It is not new, and common in the guitar world.

The 4558s, especially the asian JRCs are very slow, even when controlled by a negative feed back, they give a signal that sounds warmer, less shrill, more progressive in its transition to overdrive than fast chips like, for example, NE3552 or OPA2604. Furthermore, this effect depends on the gain : the more you set the gain control, and the more the bandwith will be reduced. That is  the "good" reason to use them in stompboxes like the Tube screamer.
Some people would add : there is also a "real" reason : these chips are cheap !
You can try other common Ics, with the same pinout. The sound will be slightly different, but not essentially (the other components, that define gain maximum and tone control, and the diodes themselves will be more influencial, as teemuk said).

In the case of distorsion boxes, with the diodes loading the IC, the influence of the IC is more audible. The rat is a good example. This circuit is high gain and top boosted. This requires much of the op amp. The 308 is a good compromise between gain, bandwith and price. Very high clipping, next to the rail, may give a shrill distorsion, too much bandwith may cause oscillations at maximum gain, A lack of gain and speed may damp the biting  that characterizes the Rat...

The best is to built a standard circuit, with sockets for the IC and the diodes, and try different models : the best sound will be the one you prefer, until you change your guitar, your amp, yout way of palying, your perception and your culture !

I dont think that hi-class OpAmps like the OPAxxx are so adapted to guitar gears, and they are a bit pricey... Just try something with this common models :

>>>> Single OpAmp :
Low class : LM308, LM741 ; Middle Class TL071, ; Upper Middle Class : NE5534

>>>> Double OpAmps :
Low Class : nn4558 ; Middle Class TLO72 ; Upper Middle Class : NE5532

"N" suffix should be low noise...

>>>> Diodes (silicium, other technologies are other stories)
1N4148, 1N4448, 1N400n...

Good trials !
Guitar Poppa

[Electron Tornado]

Take a look at this video:

[Hatredman]

Well the guy also BUSTED the True Bypass myth.

https://www.youtube.com/watch?time_continue=5&v=MOBn75-WN30