Let's discuss RAT OpAmp options

[stm]

Let me start saying that I haven't built a RAT, yet.  Searching gave me two suitable OpAmps, LM308 and TL070.  The former is bipolar while the latter is FET-based and has high slew rate. Aparently the key to the RAT sound lies in using an OpAmp with external compensation, where the compensating capacitor limits bandwidth and slew rate significantly to mellow out the supply rail clipping characteristics. In general, it can be said that there is some type of nonlinear filtering process going on as the filtering occurs by limiting the slew rate, rather than having soft clipping near the rails.

Even more, found that favorite compensating caps were in the order of 33pF for the LM308 and 180 pF for the TL070, which makes sense, since the TL070 is a much faster OpAmp and thus needs a larger compensation cap to slow it down to LM308 levels. Also, from people who said they tried both the LM308 and TL070, there was a preference for the TL070.

First line of investigation is find other suitable OpAmps which might be easier to find.

a) The LM741 can be excluded as it doesn't have the compensation pin (pin 8 )

b) NE5534 is said to be "too hi-fi" sounding. However it does have a compensation input, so it is a matter of using a larger compensation cap to see if it is possible to slow it down to attain comparable performance. An added benefit of this type of OpAmp is very low noise. In addition, it is possible to increase the output impedance by adding a 220R resistor (or another suitable value) in series wih its ouput (but not included in the feedback loop), if it sounds too hi-fi.

c) LM101 / LM201 / LM301: I'm puzzled I didn't find a report on this opamp being tried as a RAT alternative.  It is bipolar and it does have the compensating pin. This is something to further investigate IMO. Also, the LM201 is still available locally where I live.  THIS chip is what made me wonder if there are other "undiscovered" IC's that may sound as the original RAT.

d) LF355 / LF356 / LF357: no compensation pin, so probably not good.

e) TLC2262 / TLC2272: These are rail-to-rail opamps that come in duals per IC, not singles.  They don't have compensation input, nevertheless the TLC2262 has been extensively used by NY Tech 21 for its mild clipping characteristics in several designs (for instance Sans Amp GT-2 and XXL). The TLC2272 is low noise and faster than the 2262. It is clear that this type of IC behaves well when clipping into the supply rails, and it is also a decent and modern alternative for TS workalikes.

f) Discrete OpAmps, like Joe Davisson's Diode-Compression OpAmp. No compensating pin here, but due to much lower open-loop gain I would expect this beauty to behave well when it comes down to clipping, apart from the built-in soft compression, of course.  Also, I guess it is perfectly possible to add the compensating cap, maybe between the collectors of the first two transistors? Perhaps this is a sleeper with respect to RAT usage.

Another line of possible investigation relates to the compensation scheme used in OpAmps with the compensation pin (pin 8 ).  Currently the RAT employs what is called 1st Order Compensation, however it is possible to add a second cap (you should refer to the LM308 or TL070 datasheets) and make a 2nd order pole. Maybe this could be even more mellow than 1st order compensation scheme? And of course there is feedforward compensation, which I don't know if it might be better or not, as its main effect is to actually increase or extend the unity gain bandwidth of the opamp. Nevertheless the key here is to set the compensation frequencies lower than they're usually used.

P.D. I did search the forum and read the posts in the Diode-Compression OpAmp subforum as well.  It's just I'd like to summarize and round-up the different ideas floating all over and explore the new ones.  Based on the available information, the last word hasn't been said yet.

[Steben]

single-stage OP07 has low slew rate. It IS useful.

[Meanderthal]

 Weeellll.... if ya can mod the Rat to accept compensated opamps the floodgate of options opens wide...

[puretube]

http://www.diystompboxes.com/smfforum/index.php?topic=55214.0

[WGTP]

Your going to investigate the MOJOE?  Seems I remember someone saying the newer rats were using a different op amp, but I don't remember what it was.  Have you gone thru the TI parametric search?   

[Mark Hammer]

Weeellll.... if ya can mod the Rat to accept compensated opamps the floodgate of options opens wide...

You can ALWAYS stick a compensated op-amp in the socket of a Rat and leave out the compensating cap.  Would it sound like a Rat?  Debatable.  Could it sound okay?  Absolutely.  Part of the unique quality of the Rat lies in the ridiculous maximum gain applied to the top end in a highly slew-limited device.  But there are also other things at play, like the "break-point" at which the two ground legs divvy up the spectrum for differential gain, the use of feedback-loop gain-adjustment, the clipping diodes, the variable low-pass filter, and the FET buffer, all of which play a role in producing the characteristic sound.  All of those things remain in play if you slip the LM308 and 33pf cap out and slip any other pin-for-pin single op-amp replacement in there.

Some aspects of the Rat essentially provide an "upgraded" Distortion+, which will be sure to please a wide array of users.  The distinctive qualities of a Rat, however, over and above the simple straightforward distortion function, result from pushing a limited device too hard in a particular way.  If you need to have that distinctive quality, then you need to use  externally compensated op-amps, and pick them carefully.  The op-amp universe is big enough that I'm sure there are more candidates than the LM308.

[Processaurus]

Great ideas S.T.M., I think I'll socket my compensating cap in mine and try some of those suggestions.  The TLC2262 sounds especially compelling.

[brett]

I don't know what a Rat sounds like, but a TL071 sounds fine in that circuit.  Gain, grit, harmonics...everything I want.

[MartyMart]

I have a DIY rat with a TL070 in it and it does sound "right", my original rat has a 308
but I'm sure that I tried an LM301 also with decent results.
The 5534 didn't work for me, sort of got too "nice" sounding !
I've just recieved a "metal can" 308 from smallbear, so will be investigating this further

MM.

[WGTP]

The 5534 is sweeter than the TL070, but I have variable clipping options, so I can dirty things back up.  It seemed to have a nicer high end and tighter bottomend.  Haven't tried the 308, but it may not be pretty enough for me.   

[Sindran]

single-stage OP07 has low slew rate. It IS useful.

Is this what I have in my stock RAT2???
it says on the chip:
37C7 TLM
0P07DP

[stm]

Curious to see there are opposing opinions on the hi-fi 5534.  As usual this demonstrates that apart from personal tastes, the difference in guitar, amp and other gear can have great impact on how each person evaluates a pedal.

I agree that the modern TLC2262 is a nice alternative, but why leave out some proved options like the JRC4558 just because they're duals?

[dosmun]

single-stage OP07 has low slew rate. It IS useful.

Is this what I have in my stock RAT2???
it says on the chip:
37C7 TLM
0P07DP

That is what is the stock chip now.

[WGTP]

There was a thread awhile back IIRC, that discussed wheather the slew limiting/comp cap deal was an intrinsic part of the Rat or not.  In other words, is it still a Rat if you change certain parts of it.  Since the comp cap/slew limiting deal is (sort of) unique to the Rat, is it still a Rat if you use a non-compensated op amp, or even a non-308?  Another part of that arguement would be the dual shunt to ground legs off the feedback loop, or an op amp buffer to replace the Jfet.  I guess it depends on how your define a RAT.   

[col]

A quote from the Book 'Stompbox' by Art Thompson, published by Millar freeman Books. The passage starts 'Scott Burnham's story';

"I had the basic circuit roughed together and I'd found an op-amp I liked, the LM308N, which was an instrumentation amp used for seismic and medical sensors. I was experimenting with an EQ boost for this op-amp in order to pre-boost the treble so I could  use just a passive tone control to cut back the highs. i was bypassing the voltage divider that sets the gain when I picked up a resistor, looked at it and thought to myself, "yellow, violet, brown - thats 470ohm." I plugged it in expecting to get about 50dB of gain, but when I picked up my strat and hit a string , it went wooooo. I thought, " Holy shit, this is cool. What did I do?"
I looked real closely and realised that I'd plugged a 47ohm resistor instead of a 470ohm resistor (sound familiar ). That meant it had somewhere around 70db of gain , which, according to it's spec sheet was impossible from that op-amp. Trying to set the gain on this thing I had stumbled across a combination of resistors that produced this really weird high-frequency shelving boost that the op-amp couldn't possibly sustain. It didn't have enough slew rate to produce that much gain at those frequencies so it drove the op-amp into incredible slewing distortion. This usually is very bad , but in this case it gave the Rat it's yeowl - I've never heard any other stompbox make that sound."

So there you have it!

[stm]

Very interesting!

So the RAT was born as a happy accident.  I still wonder where it got its name 

[Steben]

Yes, the slew rate is important. But if it is slew rate, the modern OP07 is as good, I think the slew rate is sometimes even smaller than the LM308.
The OP07, however, has less noise AFAIK.

[tonefreak]

The RAT I modded didn't have the LM308.  I chose the TL071 and never looked back... sounds good to me.  I compared it to another RAT I was working on w/ the LM308 and the TL071 did sound a little different.  It's like vanilla icecream made by two different companies... they're both good and the flavor is basically the same, but there are differences.

Regarding the TLC2272, it really enjoy that chip in my TS808 projects.  I  A/B'd a bunch of ICs... RC4588P, OPA2134, TLC2272, TL072, LM833N and the LM1458 against the JRC4588D. IMO, the TLC2272 was the only one that improved the TS808 tone without changing the basic characterstic of that classic circuit. Some of the others sounded excellent, but changed the overall vibe too much. The TLC2272 created a richer tone and increased touch sensitivity... just more fidelity overall.  To me, the TLC2272 sounded like someone took the pillow off the JRC4558D.  Really hard to describe.

[dosmun]

The RAT I modded didn't have the LM308.  I chose the TL071 and never looked back... sounds good to me.  I compared it to another RAT I was working on w/ the LM308 and the TL071 did sound a little different.  It's like vanilla ice cream made by two different companies... they're both good and the flavor is basically the same, but there are differences.

That is the things that kinda bugs me about all of the mod guys and others saying the new Rats are way different than the old ones and sound terrible.  Pretty much the only difference is the chip and it doesn't change the sound that much. They still sound like a Rat to me.  It can be fun to analyze stuff but many get to wrapped up in the "mojo" thing and I doubt anyone would be able to tell the difference in a real world or live situation.

[tonefreak]

The RAT I modded didn't have the LM308.  I chose the TL071 and never looked back... sounds good to me.  I compared it to another RAT I was working on w/ the LM308 and the TL071 did sound a little different.  It's like vanilla ice cream made by two different companies... they're both good and the flavor is basically the same, but there are differences.

That is the things that kinda bugs me about all of the mod guys and others saying the new Rats are way different than the old ones and sound terrible.  Pretty much the only difference is the chip and it doesn't change the sound that much. They still sound like a Rat to me.  It can be fun to analyze stuff but many get to wrapped up in the "mojo" thing and I doubt anyone would be able to tell the difference in a real world or live situation.

I agree with you!  But many guitarists are a obsessive bunch and can tell the different where as the audience wouldn't care.  When working on a RAT new or old, I do a bunch of stuff and not just change out the chip.  I've noticed a few things in working with ICs.  One, they all pretty much sound the same OR they totally change the character of the pedal.  Two, there are things that you can do with the pedal that will induce a more drastic change than changing out the IC.

Not all modders go along with the hype or mojo... sometimes they're just doing what the clients ask or insist on having done.