RAT dist modifications and a gain calculating question.

[jrc4558]

I am currently on a quest to turn the RAT into an ultimate heavy metal distortion. I got one for free, so, why the hell.

Let's use this beautiful and clear schematic courtesy of GGG as a reference.

http://www.generalguitargadgets.com/diagrams/rodentsc.gif

Currently I'm playing with the values of R4 and R5, as well as with those of the correcponding capacitors. The reason - too much distortion in the high freq range. I tried a cap in parallel with the diodes, but it doesn't give the desired result.

There's this schematic, which looks almost identical in topology:
http://hem.passagen.se/robin2/im/lead2.gif
Robin claims it to be based of FAT CAT by Ibanez. The values of resistors in question are different, as well as cap values and, ultimately, the frequency curves and gates to these corner frequency cut-off stages.

So far I decreased the input cap to 0.01 and the bias resistor on pin3 to 56K. Some low-end flabbiness has disappeared, howerer, I lack the punch sufficiently. The clipping diodes are a pair of 1N4001s in series with a pair of 1N60s. The distortion sound is sharp and heavy, but it doesn't sound fantastic (i mean, the way a DIY pedal MUST sound), when I mute the strings with my palm.

Anyway, I assume not many people fancy reading the observations, so:

I will shurely post the results when I arrive at an optimal value chart for a stratocater with single coils, but...
Is this the right fomula to calculate gain of non-inverting op-amp gain stage?
R1 - resistor to Vbias
R2 - resistor in the NFB loop
gain= (R1=R2)/R1

Correct me if I'm wrong, plese.
Knowingthe correct calculation formula will allow to fine-tune the gain factors, and hopefully get rid of ultra harsh high end harmonics as well as carefully balancing the bass content. That's what I'm aming at anyway.
Thank you all in advance.

[cd]

The formula is 1+(R2/R1) for a non-inverting (input to + pin) stage.  R1 is the resistor from the - input to ground (or bias), and R2 is the resistor from the output to the - input.

However you've got another issue to deal with - low frequency rolloff in the non-inverting stage.  Roll off too much low end and you lose distortion,just like a Tubescreamer.  The formula for calculating the rolloff frequency is 1/(2*pi*R*C), where R is R1 as above, and C is the cap attached in series with R1 above.

[WGTP]

There are lots of Rat mods around here.  You might try looking at the AMZ distortion, there is a rat in the middle of it.

C4 30pf is part of the Mojo of the Rat.  It controls the slew rate of the op amp  and the hi end response.  Might try increasing that to 100pf or what ever works.  I assume you already increased the 100pf to 220pf.  Might also increase the pot to 250k for ultra metal.  You can also increase the C3 to .0047uf for more treble cut with the tone control.  Might also try some of the notch filters around here.  Hope that helps.  

[Mark Hammer]

The Rat is capable of ridiculous gain levels.  Using the gain pot maxed (100k) and the 47R leg of the resistor/cap pairs, we see a gain of well over 2000 in the above 1500hz range.  Using the 560R leg, we see a gain of about 180 in the audio range overall.  If you want to go nuts with the gain, you can simply stick a 47k resistor in series with the 100k pot.  That'll increase gain pretty substantially (enough that maybe a 33k resistor is called for instead).  The 100pf cap probably doesn't need to be changed at all.

I echo WG's comments regarding use of a notch filter for nailing the tone you seem to be after.

[jrc4558]

I do EQing with different cirquit, it's esentially a buffered Bassman tonestack recalculated using the TDSL software. So EQ is not the issue (the chinaman is not the issue here! (c) Walter Sobchak, The Big Lebowski).
All suggestions are wonderfull and question-propelling, as usual.

Can someone explain a little better as to what is the function of the 30pf cap and how does it set theslew rate?

Thank you Mark, cd & WGThickPresence!

[Phorhas]

Using the gain pot maxed (100k) and the 47R leg of the resistor/cap pairs, we see a gain of well over 2000 in the above 1500hz range. Using the 560R leg, we see a gain of about 180 in the audio range overall.

How does one calculate the RAT's gain?

[strungout]

Oy.

I messed around with my diy rat, on top of the diode (soft clipping kinda sucks in this pedal tho...) mods from Tonda.
I also wanted more gain. I ended up raising the gain pot to 1M, with a 100pf cap in parallel, and a 5.6k/.22uf and 470/.47uf in the "arm" cause I didn't have the schem values (the gain would've been higher). That, along with some changes to the input (namely a 4.7nf input cap, 100k bias res, and 5.6nf cap to ground, gave me a nice palm mute crunch. Seems that comes with high freq distortion. Oh, and I did use a NE5532, so that already affects the sound.

I lost most of the rat's "clang" unless I turn my guitar knob down.

Ciao.