a 4558 type RAT .... does this make sense ?

[Melanhead]

Well I've been toying around with making a Rat with a 4558 type chip and using the second half as a buffer ... I drew this up, but before I attempt a build and tweak it to death, does this make sense ? Too may ideas, not enough knowledge, hope I'm not getting too carried away with the questions lately! 

[PenPen]

Not sure if its intentional, but the first thing I noticed was that R3 and C4 should be connected to ground instead of Vbias. Least it is in the original Rat. Otherwise it looks nice.

[Melanhead]

Not sure if its intentional, but the first thing I noticed was that R3 and C4 should be connected to ground instead of Vbias. Least it is in the original Rat. Otherwise it looks nice.

Yup! ... you're right, I made this from a tubescreamer type schematic I drew up ... ... I'll change that right now. ... done and thanks!

[Mark Hammer]

Almost 30 years ago to the day, I was sharing a house in St. John's Nfld with 8 other people.  One of them made what she called a "pizza".  It had a whole wheat crust, broccoli, some slices of tomato, and no cheese whatsoever.  Vegetables on a large whole wheat "thing" that didn't rise.  Yes, it was flat and round, and there were tomatoes associated with it, but that was pretty much where the resemblance to pizza ended.

What you've got there is clearly a clipper which Will work, has a similar tone control, and uses the feedback resistance as the gain adjustment....pretty much like a Rat.  But that's where the resemblance kind of stops.

Fairly critical to the (IMHO) somewhat unique tone of the Rat are a couple of things.  First is the use of the dual ground legs in the gain stage.  This provides a selective boost in certain parts of the frequency range.  Second is the use of a compensated op-amp.  Whether the 308 is absolutely critical, is another thing, but the cap in the compensation loop partly contributes to the tone by limiting the gain in certain ways.  Indeed, the compensating cap and the extra gain introduced at higher frequencies seem to be working at deliberately cross purposes.  Third, of course, is the use of asymmetrical clipping.  Fourth is the absence of FET buffers, and whatever distortion the output buffer in particular provides.  Like the pizza, no single one of these presents a radical enough change that one could say it has changed from being a Rat.  But lump them all together and it turns into a different beast, I think.  Not a bad or unreasonable beast, mind you, just a different one.

[Melanhead]

  .... geesh, well I guess you're getting to know me ... The idea was to be like a RAT but to be different enough from a RAT to not be considered a RAT clone ... I guess I'm looking for a high gain rat-like tone but tweaked. 

I built one from the Tonepad PCB, as I had one as a kid and really liked it ... I modded it to use a 5534 and added the assymetrical clipping. I'm rather fond of it now. I'll build this new beast and compare the two for fun. It'll be interesting to see how different they sound.

I recently bought Jack Orman's Rat/BigMuff which is a great read and gives some great insight into the Rat ... ( and of course the Big Muff )

ahhh St John's, I plan to visit NFLD this summer ... I work with a bunch of Newfoundlanders and my wife's from Cornerbrook  ....

P.S. ...  that's NOT a pizza 

Thanks!

[PenPen]

I remember about 10 years ago, I wanted a dist pedal and I was 14 so I had little money, so I bought the cheapest thing at the local store. It was an Ibanez "Slam-Punk" pedal, really stupid name but it sounded ok. My band's inspirations were The Germs, Babes in Toyland and the Melvins, so I wanted something harsh and nasty. I still remember the day we walked in the store a little later, and they had a used, beat up Rat. Knowing that both Babes and Melvins used it, we were in awe. Once I plugged it in to try, I knew I had to have it. I have used that pedal ever since, nothing sounds like or can replace it. My tastes have changed somewhat, but even now I have a soft spot for that harsh clipping. I am also working on making a 'clone', but I want it to sound different, mostly in the freq range of it. I'm personally going to stick with the LM308 at the heart of the design. When you have finished your design, please post up some clips if possible, I'm really interested in hearing how yours turns out.

[hank reynolds 3rd]

look for the schematic of the bixonics expandora, i think this is rattish but is responsive to picking dynamics via an led/ldr or a phototransistor...but also uses a 4558 (aswell as the 308 IC)
just thought it may be of use for ideas..

Cheers

Sam

[Melanhead]

I remember about 10 years ago, I wanted a dist pedal and I was 14 so I had little money, so I bought the cheapest thing at the local store. It was an Ibanez "Slam-Punk" pedal, really stupid name but it sounded ok. My band's inspirations were The Germs, Babes in Toyland and the Melvins, so I wanted something harsh and nasty. I still remember the day we walked in the store a little later, and they had a used, beat up Rat. Knowing that both Babes and Melvins used it, we were in awe. Once I plugged it in to try, I knew I had to have it. I have used that pedal ever since, nothing sounds like or can replace it. My tastes have changed somewhat, but even now I have a soft spot for that harsh clipping. I am also working on making a 'clone', but I want it to sound different, mostly in the freq range of it. I'm personally going to stick with the LM308 at the heart of the design. When you have finished your design, please post up some clips if possible, I'm really interested in hearing how yours turns out.

I will, probabably take a while before I build it though ...

[Melanhead]

look for the schematic of the bixonics expandora, i think this is rattish but is responsive to picking dynamics via an led/ldr or a phototransistor...but also uses a 4558 (aswell as the 308 IC)
just thought it may be of use for ideas..

Cheers

Sam

thanks for the heads up ... I'll take a peak at that.

[WGTP]

IMHO you can make a Rat clone with the only difference being the buffer at the end.  I'm not sure how much difference that will make.  Using LED's as the diodes would send more signal to the buffer for potential distortion.

The Expandora has some other circuitry and still uses the LM308 for the Distortion part, but does have an op amp output buffer.

As some of the threads point out, discrete components/op amps have different pro's and con's for buffering.   

[wampcat1]

Figured I'd post my findings here too.
http://www.indyguitarist.com/soundclips2/rat_vs_4558-rat-emu.mp3

I'm using the exact schem as the rat except using a rc4558 and using the second half of the opamp as the buffer.

[Mark Hammer]

A)  Sounds great.  Nice sample, nice tone.

B) There is stuff the Rat does that any diode clipper can do, but some stuff that perhaps the LM308 is critical too.  The high gain imposed on the chip, together with the compensation cap that limits gain/bandwidth product, results in an interesting kind of low whistling tone underneath, almost like a tracking oscillator.  Of course, this is the sort of surplus tone that requires sustained single notes to be audible, and maybe even only single notes on the unwound strings.  But it's there and is part of the distinctiveness of the Rat compared to other diode clippers.

C)  None of any of that matters if what you use it for is nice crunchy power chording as Brian demonstrates.  I guess it is a bit like having two sports cars of equal comfort and acceleration but one of these has a terrific cigarette lighter system, and a special spot in the trunk for holding grocery bags in place.  If you're a smoker and use your car for grocery shopping that may be a real perk, but if you aren't why would it matter?

[WGTP]

Thanks Mark, I've never hear the slew rate limiting described that way.  So that's what it is.  Common slew rates.

Slew rate LM308 - 0.3us
             RC4558 - 1.7us
             NE5532 - 9.0us
             TL070 - 13.0us externally compensated commonly used for Rats single op amp

[Mark Hammer]

In Art Thompson's "Stompbox" book there is an interview with the developer of the Rat.  He describes how a misread of a resistor value resulted in setting the circuit up for way too much gain, and this "woooooooooo" tone underneath that he thought was kinda cool.

For the uninitiated, that should be 0.3V/usec, 1.7V/us, etc.

Slew rate is a measure of the kind of speed with which a device can respond to instantaneous demands for voltage swing.  If the amount of gain applied is minimal, then devices with lower slew rates have little problem swinging up and down at fast speeds (higher frequencies).  As you may or may not know, op-amps are described in terms of their "open loop gain/bandwidth product".  In other words, if there is no feedback loop and no negative feedback applied from the output to the input, this is the maximum gain the device could achieve.  Since it can't apply that gain out to infinity, what you will see in spec sheets is a tapering off of maximum gain as you reach higher and higher frequencies.  The maximum value of gain-times-frequency is this gain-bandwidth product. 

Devices with higher slew rates generally have a higher gain-bandwidth product, meaning that they can be asked to produce lots of gain and still respond fast enough (which is why slew rate is in volts per microsecond) that this maximum open-loop gain can be realized at frequencies well above human hearing sometimes.  Very often, the compensation loop (generaly found between pins 1 and 8 on a single 8-pin op-amp) is used to alter the gain-bandwidth product.

In the case of the Rat, the tiny resistance values going to ground demand so much gain from the 308 that, with the compensation cap, it is simply pushed to its limits at higher gains.  For instance the 47R path provides for a maximum gain well over two thousand for frequencies above around 1.5khz.  When you consider how many pedals don't really apply more than a gain of x500 to get searing leads, that's something.

Of course there are many gain settings below x2000 (2128 to be exact) to be gotten on the Rat, such that the slew-limiting inherent in the Rat setup is not all that pertinent.  Again, I draw your attention to when slew-rate is and isn't important.  At low gains and low bandwidth, the slew rates of many medium to high-quality chips are moot when it comes to guitar.  Crank the gain and expected bandwidth up a notch, and slew rate starts to matter.  I would imagine there are plenty of medium-gain settings, where lots of people are going to say they like this op-amp or that one in the Rat circuit.  The Rat seems to be one of those designs, however (not unlike the Doctor Q and Bass Balls), where the inherent weaknesses of a "lesser" chip are used to advantage to produce a particular outcome.  In this case it is the way the chip responds to excessive gain demands when its hands are tied via a compensation cap.  But one has to emphasize that this synergistic outcome is only for some types of settings.  Depending on how you like to use your Rat, YMMV.