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Re: Geoff's Amateurish Pedal Tinkering Workshop
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SlewerRat. I made this uhh, around Thanksgiving of '18. The name is a spinoff on a controversial comment the mayor of Toronto said around that time. The idea was that I wanted to build a RAT but I didn't want to have to source an LM308 opamp. Luckily, Ibanez figured this thing out in the eighties when they made their Fat Cat, which is essentially an improved RAT isn't it? It uses a different opamp.
I'm going to go into a bunch of detail so I have something to copy paste into my blog, at least for Chris this is probably not any new knowledge...
So the reason the RAT and the obsolete LM308 opamp have such a legendary relationship is that the opamp has a pretty abysmal performance. Its slew rate, which I understand is the rate at which it can uh... slew... its signal... is very low by general standards. So that gives the RAT its signature sound. The opamp you could say is very 'low resolution'. For anything but making rock music its a garbage part.
In the Ibanez Fat Cat they instead used a NE5534 opamp which has a much better performance, higher slew rate, but you can actually tune the slew rate of it by putting a cap joining legs 5 and 8. And that's what they did, there's a 1nF cap there doing the job. So it's still performs like a RAT.
Cool! So I can make a RAT with existing layouts, without having to source an LM308. All I have to do is add a cap in there and I'm open to use this much more common opamp.
But wait... let's take it one step further... Let's put a switch on there, so I can switch different caps and vary the slew rate!
Okay, so now there's three slew rate settings. One is the 1nF, like in the Ibanez Fat Cat, which I'm calling RAT mode, under the assumption the engineers at Ibanez did their maths correctly and chose the one that slows it down to the LM308 slew rate.
One is no capacitor. So the NE5534 opamp is performing at its normal speed, which is much faster than the LM308.
And one is, I forget what cap I used, maybe a 10nF, so the slew rate goes way way slower than even LM308 speed.
RAT mode is probably the best mode. You can see why it`s such a big deal what opamp is in your RAT. In this mode the pedal sounds fat and balanced and has a nice, spongy feel.
With no cap for a faster slew rate, the pedal is a bit brighter and stiffer feeling. You can hear the pick attack much better. It`s a much clearer sound. A bit harsh. Probably good for black metal.
With it slowed the fuck down with the higher rated cap, it`s a sludge machine. With the tone filter all the way open in this mode I thought it sounded like a really fucked up IC Big Muff. Definitely a cool sound.
So that was my experiment, a Rat with three `modes` that aren`t your usual different clipping diodes or whatever.