I do quite a bit of rewiring of my guitars (pickups, caps, pots, etc) and have built some effects from the circuit diagrams, and it seems that every few weeks I run across a thread discussing the op amps that can be used in TS808 and TS9s. I have a few variations of tube screamers (doesn't everyone?), so I thought it would be fun to desolder that OP amp, replace it with a 8 pin DIP socket so I could plug various chips in, and go to town. First a couple of points: I randomly chose 6 (well 5) chips that were pin and voltage compatabile with the RC4558. Except for the direct replacement TI RC4558P, I had not read of anyone trying these chips. My goal was not to reproduce a "vintage Tube Screamer" sound - my goal was to determine which chip gave the "best" sound, realizing that as a blues/rock player Metalheads have different needs. Hopefully this post will help them and other types of players as well. To me, "best" means first and foremost that my guitar's tone stays true. Too much fuzz in the gain is what I wanted to avoid. I wanted to hear what I hear from my Mesa Boogie Lonestar Special when I plug into the dirt channel with some gain (but no drive circuit) pushing the valves - pure tube goodness from a pedal. I could have chosen many guitars (I have 8 electrics of various types), but it seemed reasonable to test with my favorite, the one I use the most. It is a completely customized 2011 Les Paul Traditional. When I say "customized", I mean all hardware and electronics were removed from the body. I installed Bare Knuckle Mississippi Queens (HB size P90s), new pots that I checked with a meter to get as close as possible to 500K, settled on .33uf caps with these pickups, as anything less was too bright and anything more was a bit too dark. I tested on the clean channel at moderate volume, so the only distortion was provided by the TS9. On to the op amps: 1 - TI RC4558P. I expected this to sound pretty much like the original chip and for the most part it did. With the gain cranked all the way up it seemed to me that the guitar's tone came through a bit more. The slew rate on this chip is 1.7 instead of the JRC4558D's slew rate of 1, so maybe that was the difference I heard. Mostly, it sounded just a bit better than a stock TS9, still pretty dirty. 2 - Analog Devices AD708JNZ. Beautiful tone. Still sounds like my guitar even with the gain at 10. High e string bent at the high frets still has some body. Not as "dirty" as the RC4558P but full with as much sustain. Slew rate on this chip is just .3. 3 - TI OPA2227PA. This chip has a slew rate of 2.3. Very nice and articulate, but seems to have quite a bit less gain at 10. Still, nice tone. 4 - TI OPA2604AP - A bit less definition and the tone is not as "true". Still pretty nice though. Gain is about like a 4558. Slew rate is 25. At first I wasn't sure I read that right, but I did. 5 - TI TL072CP. Seems noticeably louder with just slightly less breakup than a 4558. Lots of sustain too. Slew rate of 13. 6 - TI TL082CP. Sounds a lot like the TL072CP, which is not really surprising since they are part of the same family. Seems to have slightly more definition than th TL072CP, but just as much sustain. So who do you think the winner was, for my purposes, at least. I ended up going back and forth between the TL082CP and the AD708JNZ. In the end, I had to pick the AD708JNZ and I left that in the pedal and put it back together. I bought my first Tube Screamer in the early 80s, and I remember, before they achieved cult status, what I wanted; a pedal that would sound like a beautiful, driven, tube amp. With the AD708JNZ I played through the clean channel on a little practice amp and with the gain at around 7 it sounded eerily like I was plugged into the dirt channel of my Lonestar Special with the gain at around 2 o'clock. Put the TS9 with the AD708JNZ in front of the Lonestar's dirt channel and kick it in and what I hear is my TONE! Tons of creamy sustain and clarity. And the pure sound of my guitar, not the pedal. A little unorthodox? Maybe. But we all have to find our tone, don't we?