MightySmallElectrons
Active member
I’m getting ready to build my 3rd Muroidea pedal and am curious if you have a mod that you think makes this classic even better and why?
Favorite Rat Mods?
- Thread starter MightySmallElectrons
- Start date
Nostradoomus
Well-known member
I usually lower the 4.7uf cap down to 1uf to get rid of the farty compression sound but other than that I love them stock or Turbo spec.
Chuck D. Bones
Circuit Wizard
There are two 4.7uF caps. Are you talking about C2 or C7?
I like the BASS mod and increasing C8 so the entire range of the FILTER control is usable.
MightySmallElectrons
Active member
What’s your preferred value for C8?
Nostradoomus
Well-known member
Sorry C2 tied to the 560 ohm resistor and DIST pot.
Chuck D. Bones
Circuit Wizard
6.8nF or 10nF
Nostradoomus
Well-known member
I’ll play with that to make up some of the pre opamp bass signal lost by lowering C2. I don’t really notice too much oomph gone with the corner frequency moved up to 260-ish Hz. It definitely doesn’t bottom out anymore and that was my only real issue.
Put a 1k or 500ohm pot between R5 and C3 as a “voice control” like a zendrive has. Maybe experiment with removing R3/C2 with this mod. R5/C3 cuts bass below 1.5kHz. The voice knob allows you to choose a lower frequency (though you lose some gain when you turn the voice knob down for lower frequencies). On the rat, R3/C2 adds some bass back in, but at a much lower gain than R5/C3. A bass knob like chuck mentioned will also allow some similar results.
I did this voice mod on my DRV (in built reports), after trying a bass knob first. Chuck added lots of great info to that thread, including some simulation graphs of the different mods. The DRV circuit is very different from a rat, but the voice knob should still work on the rat. For example, instead of 47R/2.2uF, the DRV has 470R/220nF. Same 1.5kHz cutoff, but 10x less gain at this stage (because the DRV has the boost circuit first).
I did this voice mod on my DRV (in built reports), after trying a bass knob first. Chuck added lots of great info to that thread, including some simulation graphs of the different mods. The DRV circuit is very different from a rat, but the voice knob should still work on the rat. For example, instead of 47R/2.2uF, the DRV has 470R/220nF. Same 1.5kHz cutoff, but 10x less gain at this stage (because the DRV has the boost circuit first).
benny_profane
Well-known member
What is the ‘BASS mod’ ... I’m guessing it’s in addition / complementary to increasing C8?
I've just modelled the Rat in LTSpice to learn about the circuit before I build it. I noticed that the distortion pot seems way too big as you are clipping the op amp before you get close to half travel even on a 100k log pot (~10k). So I think you are right. Of course the diodes then clip again.
What pot size did you settle for?
Another mod I've modelled - and it seems to work- is J201 as the buffer JFET instead of 2N5458 because I have a few spare J201s but no 5458s.
Chuck D. Bones
Circuit Wizard
The Rat has two tone ranges: opamp clipping and diode clipping. Some people only like the latter and in that case, limiting the max gain is a good idea. Referring to the Muroidea schematic, the max gain can be reduced by either reducing the DIST pot or by increasing the impedances of C2, C3, R3 & R5. If you reduce the DIST pot, you might want to increase C5.
The J201's Vp is too low for the output buffer unless you bias the gate up. There are plenty of JFETs that are good subs for 2N5458 in the Rat. J201 is not one of them. Look at Q1's drain current waveform in LTSpice. Does it go to zero during part of the waveform?
The J201's Vp is too low for the output buffer unless you bias the gate up. There are plenty of JFETs that are good subs for 2N5458 in the Rat. J201 is not one of them. Look at Q1's drain current waveform in LTSpice. Does it go to zero during part of the waveform?
I like to add soft clipping diodes on a switch (red or blue LED) to prevent op amp clipping. This has a way of tightening up or focusing the sound (op amp clipping sounds muddy to me).
This is messing with a beloved circuit so put it on a switch h so you can compare to classic rat tones.
This is messing with a beloved circuit so put it on a switch h so you can compare to classic rat tones.
With the 2N5457 (don't have the -58 model) the drain current is 45uA-165uA, and with the J201 it falls to 15uA-130uA. So, not zero but close.
I'll order the right JFET as I also need to order a Op07. I looked at using the TL071 (which I have) but noticed the different slew rate raises the peak from 1kHz to 2kHz so it would sound different.
Chuck D. Bones
Circuit Wizard
Yup. For the true Rat tone, you need an LM308. OP07 is is close, but no cigar. TL071 is not even close. Once you get the right opamp in there, you might change your mind about opamp clipping. OTOH, maybe a different pedal might suit you better. The BJFe Honey Bee and its derivatives have both soft and hard clipping and are very smoooooooooth.
I've built a few Rats for people with the CA3130 as favoured by Bjørn and they have sounded excellent. Authentic? Close enough for me! Just use a 100pF compensation cap instead of the 30pF.
A friend of mine has a 13 yr old son who has had a rough time lately with health issues. He's a really lovely fella and a mad keen guitar head so I asked his dad what sort of music he liked. It was the usual suspects for a kid of that age so I built him a Rat with a genuine metal can LM308N and the bass pot. I stoked because apparently he loves it - he sent me a card to say thank you. My friend says that if he is playing guitar he's playing that pedal.
A friend of mine has a 13 yr old son who has had a rough time lately with health issues. He's a really lovely fella and a mad keen guitar head so I asked his dad what sort of music he liked. It was the usual suspects for a kid of that age so I built him a Rat with a genuine metal can LM308N and the bass pot. I stoked because apparently he loves it - he sent me a card to say thank you. My friend says that if he is playing guitar he's playing that pedal.
