Rat Op Amps

[joelster]

I know that this has been discussed around here quite a bit but I'm just trying to distill it down a bit.

I know that there are several op amp options for the Rat. I also know that some require external compensation caps and some don't. So...

Are these all of the options available or did I miss some? Additionally, are these the correct comp cap values?

Rat Op Amps:
Model      Comp Cap
OP07           None
5534           22pF?
LM741        None
TL071         None
TL070         180pF
LM308        30pF  (edit: had 47pF by mistake)

And finally, in a more subjective vein, what's your favorite?

Thanks!

[GGBB]

First, the comp cap for the OP07 is the same as the LM308 which is 30pF (not 47).

I think any pin compatible single op-amp in general is a potential candidate as far as drop-in replacements in production RATs goes.  If you are rolling your own PCB, the world is your oyster.  The question is more about what sounds good, or what sounds like a RAT, which are different questions depending on your perspective.  If you want it to sound like an original RAT, use an LM308 - there are no alternatives.  If you want it to sound like late model RATs, use an OP07.  If you want some other kind of sound - experiment.  The datasheets for the op-amps will tell you whether or not the op-amp uses a comp cap - the value of which is hard to pin down for the RAT circuit - experiment.

[joelster]

Right, 30pF. Didn't check my own work there...

I've made a PCB with sockets for the comp cap so that I could roll a few different chips in there.

[ashcat_lt]

I used a TL080/180pf in mine.  Practically speaking, it's about the same as the 070, but...

I actually A/B'd against both a 70s original (not sure which opamap) as well as the PodFarm model.  There is a noticeable, but fairly subtle difference.  The original is a bit nastier at higher gain.  Like, more ragged or something.  It sort of "folds" in a way that mine doesn't.  The PodFarm model is (perhaps not surprisingly) a bit smoother, and more consistent across the range of the gain pot.  It was pretty close all around, though.  Not that it was a particularly scientific test or anything...

[joelster]

Thanks, guys!

Here's what I've got. Not gonna think about this any more...

[Mark Hammer]

In one of Art Thompson's books, he has sections where the developers of various classic pedals or early FX companies describe how they came up with stuff.  In the case of the RAT, a mistake was made in reading the value of a resistor.   Here's what someone posted on TGP:

Here's a quote from the Book 'Stompbox' by Art Thompson in which they interview Scott Burnham, one of the creators of the Rat:

"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 sh*t, 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. 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, the Rat relies, fundamentally, on exploiting the weaknesses of a particular chip.  I'm sure that other better chips can still produce interesting distortion in that circuit, given that a big chunk of the heavy lifting is done by the diodes.  But nailing the Rat tone probably also relies on use of that specific op-amp.

[joelster]

Great... so now I gotta get some LM308s too...

[Kipper4]

I've only ever used the LM308 and I loves em.
Classic pedal.

[mthibeau]

I have built 3 or 4 Rats with different clip mods, but always used LM308 (dip version, not the can) and they sound stellar..

[closetmonster.]

If you flip through the RAT build reports on tonepad, they list quite a few replacements that sound good. From what I have read, many chips will sound very good in place of the 308, but most of them change the tonal characteristics to the point where it doesn't sound like a rat.

Got a few LM308's on the way to finish off my rat build.

[Paul Marossy]

In one of Art Thompson's books, he has sections where the developers of various classic pedals or early FX companies describe how they came up with stuff.  In the case of the RAT, a mistake was made in reading the value of a resistor.   Here's what someone posted on TGP:

Here's a quote from the Book 'Stompbox' by Art Thompson in which they interview Scott Burnham, one of the creators of the Rat:

"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 sh*t, 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. 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, the Rat relies, fundamentally, on exploiting the weaknesses of a particular chip.  I'm sure that other better chips can still produce interesting distortion in that circuit, given that a big chunk of the heavy lifting is done by the diodes.  But nailing the Rat tone probably also relies on use of that specific op-amp.

Very interesting.