21st century scizoid rat

[Fancy Lime]

Hi there,

it has long struck me how few pedals make use of the basic Proco Rat topology and expand upon it instead of just plain cloning it. In fact the IdiotBox Blower Box is the only one that comes to mind off hand. So I did a bit of exper'mn'tn to explore some of the more obvious mods. This is the preliminary result:



Still a basic rat circuit with some extra bells and whistles. I'll try to explain the reasoning behind the more important changes.

Switching: I used a CD4053 for bypass switching as per R.G.s instructions here: http://www.geofex.com/Article_Folders/cd4053/cd4053.htm Highly recommended. This switching type does not absolutely need but greatly benefits from a buffer, therefore:

Input buffer IC1a: Well, just a buffer doin' some bufferin'. Using a dual opamp here gave me a second half for:

Bass boost stage IC1b: I want to use this thing with bass and the stock Rat is a bit on the flat chested side for bass use. This stage lets you bring back the bass "lost" in the gain stage. Plenty of it. The thing is designed to tolerate almost any dual opamp for the IC1 position. I like me my NJM2068, but NE5532, NJM4556, JRC4458, TL062, TL072, TL082, TLC2262 and many others will perform almost identically here.

Gain opamp IC2: I only tested the classic LM308, the NE5534 and the LM301 in this position. I ordered some OP07's but they have not arrived yet. In this position, the opamp does make a tonal difference, especially if you use it without the clipping diodes D1 and D2. I found the LM301 to sound better in all settings than the LM308. It is also lower noise. The NE5534 sounds great at high gain but breaks up quite harshly and somewhat brittle at low gains. If you intend to use this mainly as a high gain distortion or ultra-fuzz, try the NE5534. But in that case the copensation cap goes between pins 5 and 8 instead of 1 and 8 as shown in the schematic.

Buzz control: Controls the bass gain, allowing to make the sound more fuzzy or more distortiony. Very useful, highly recommended, makes the good old rat much more versatile with bass as well as guitar.

Compensation cap / slew rate switching S1: This is in essence a treble cut inside the gain stage. Switching on the higher values makes the sound a bit milder in low gain settings and fatter (more old-school fuzz like) in high gain settings. Pretty useful, check it you like it.

Clipping mode switch S2: Lets you switch between 1) "normal mode" with clipping diodes (I used asymmetric blue and red LEDs but you can take whatever sounds best to you or even add another switch to select different types, which is a fairly popular mod); 2) "no diodes" mode, with just the natural distortion of the overdriven opamp plus overdrive from Q1 (if it does overdrive, see below); 3) "Big Muff style" feedback clippers. This last one is very fat and tubey, completely different from the normal rat sound. Not bad for blues guitar. Not sure if that is what people want to have a rat for so... I think the "no diodes" mode stands head and shoulders above the others, at least with the LM301. Very fat, dynamic and pleasant. Not too aggressive, not to mild. Give the mode switch a try if you want a very versatile pedal.

2SK117 as Q1: Using a low V(GS) type like the 2SK117 or J201 here gives greater headroom and makes this stage less prone to additional clipping. I went with that way because I wanted the  unabashed LM301 clipping to shine through. If you use a higher V(GS) type like a 2N5457/8 or MPF102 you would probably overdrive it in this position. I did not try that but would be willing to bet a modest sum that that would also sound interesting and possibly even great.

The rest is just some tuning of the frequency shaping and leaving out unnecessary parts.


Suggested further mods:
IC1a can easily be converted into a booster that you can switch to unity gain (i.e. back to buffer mode) using one of the two extra switches in the CD4053 in the feedback loop and have a second foot switch for it, too. I tried that with a full range booster with a maximum gain of 10. That may be a bit much, gain of 5 is probably plenty. You can also make it a treble booster instead. Or, hear me out: a tube screamer clipping stage. Go nuts.

Hope you enjoy,
Andy

[Fancy Lime]

I just realized now that if you like mixing a clean signal with the distorted, you could change the second opamp half into an inverting mixer and blend the buffered clean with the distorted path. Not my cut of tea but many people seem to like that sort of thing or even consider it essential for bass use (especially guitar mag writers).

Andy

[pinkjimiphoton]

to me, the bixonic expandora was probably the best in the rat school.

rats are great sounding pedals, but not necessarily the best in live use for some forms of music. i find 'em to not cut as well, tho i love 'em for filthy crunchy rhythm tones or creamy solos on heavier stuff.

[Fancy Lime]

So I finally got my OP07C delivery and tried them in the rat-ish thing. I was surprised to find that it sounded quite bad. Had a strange high pitched buzzing when overdriving the opamp. In extreme settings it got a bit 8-bit fuzzy, but the lower drive settings were really nowhere near as good sounding as the LM301. So, lesson learned: If you build a rat now that the LM308 is long obsolete, don't go with the OP07C as Proco did. Get the old LM308 or if you can't, use the LM301 for a slightly different but arguably better sound.

Andy

[pinkjimiphoton]

i love the 301 too. just a sweet sounding chip. almost sounds like germanium to my jaded faded deaf old ears.

i've had that issue with similar circuits, could there be any chance the chip is oscillating? maybe check to see if ya need a decoupling cap somewhere to tame it down a little?

just had that issue earlier today on another project, made me think of it.

rock on dude!