Possible dumb question: transistors instead of op-amp for Rat (slow loris)?

[greysun]

Okay, okay, okay - but hear me out... lol...

As I get my pedal board narrowed and finessed a bit (I've built over 3 dozen pedals at this point), I'm finding that of the muffs I've built, I prefer the 4 transistor versions (mudbunnies) over the op-amp versions (pig butt, and an actual EH big muff). Part of it is that Brian's docs encouraged experimentation and I was able to make something that sounded a bit fatter and louder than the IC ones with a bit more character...

Then I started looking at my slow loris - I have what I think is the 2012 version, which I had to dig for the documentation on - the new version has switching to different clipping diodes (mine is a board-mounted switch, no clipping center, LEDs up, diodes down - I almost always use the LEDs up). It's a great pedal, but can be finicky in the mix. I also built 2 Runts, but they don't have the same feel of the Slow Loris.

Then I thought: ya know - this uses an op-amp (lm308, I think?)... can one make a transistor version that might have different characteristics? So I went to the internet, and found more articles about how nobody knows the original circuits or conflicting information and blah blah blah... nothing specific enough for my question.

So I come to you, madbean forum, to see if anyone's tried it - I don't know enough about circuits to know where the op-amp sits in terms of affecting the character of the pedal, but in the spirit of experimentation, why not?

Any info is appreciated. :-)

[jimilee]

Okay, okay, okay - but hear me out... lol...

As I get my pedal board narrowed and finessed a bit (I've built over 3 dozen pedals at this point), I'm finding that of the muffs I've built, I prefer the 4 transistor versions (mudbunnies) over the op-amp versions (pig butt, and an actual EH big muff). Part of it is that Brian's docs encouraged experimentation and I was able to make something that sounded a bit fatter and louder than the IC ones with a bit more character...

Then I started looking at my slow loris - I have what I think is the 2012 version, which I had to dig for the documentation on - the new version has switching to different clipping diodes (mine is a board-mounted switch, no clipping center, LEDs up, diodes down - I almost always use the LEDs up). It's a great pedal, but can be finicky in the mix. I also built 2 Runts, but they don't have the same feel of the Slow Loris.

Then I thought: ya know - this uses an op-amp (lm308, I think?)... can one make a transistor version that might have different characteristics? So I went to the internet, and found more articles about how nobody knows the original circuits or conflicting information and blah blah blah... nothing specific enough for my question.

So I come to you, madbean forum, to see if anyone's tried it - I don't know enough about circuits to know where the op-amp sits in terms of affecting the character of the pedal, but in the spirit of experimentation, why not?

Any info is appreciated. :-)

That fat rat has a CMOS transistor. I have a Rat PCB with 8 different clipping options including CMOS transistors.

[Aleph Null]

A big port of the Rat's character is it's pre-clipping EQ. It has a very distinct, aggresssive EQ going into the diodes. If you replicate that, it should sound similar. Most transistors won't anywhere near the amount of gain available as an opamp (even the crappy one the Rat uses), but that may not matter, depending on the clipping diodes you use.

Has anyone done this before? Is there a schematic floating around somewhere? Not that I know of.

[greysun]

A big port of the Rat's character is it's pre-clipping EQ. It has a very distinct, aggresssive EQ going into the diodes. If you replicate that, it should sound similar. Most transistors won't anywhere near the amount of gain available as an opamp (even the crappy one the Rat uses), but that may not matter, depending on the clipping diodes you use.

Has anyone done this before? Is there a schematic floating around somewhere? Not that I know of.

The more I read, I'm finding most folks trying to find LM308 replacements because it's not made anymore - I have one or 2 extra of those, so that's not my goal necessarily... not knowing enough about these circuits, the gain is a mystery to me (and I'm sure far many before me have tried this route as 308s became hard to come by) - but maybe since the 308 is a single op-amp, there's a dual, or even triple NPN transistor combo that could approximate to the gain level?

That fat rat has a CMOS transistor. I have a Rat PCB with 8 different clipping options including CMOS transistors.

I'm not familiar with the fat rat schematic - do you have more info or a schematic to check out?

Also, thank you both for the replies so far. :-)

[BricksnBeatles]

Most transistors won't anywhere near the amount of gain available as an opamp

Darlington?

[thomasha]

What about discrete OpAmps?
https://hackaday.io/project/176860-homemade-operational-amplifier
It seems there is a 4558 tube screamer kind of pedal
https://www.parasitstudio.se/stripboard-layouts/discrete-tube-reamer

[Aleph Null]

Most transistors won't anywhere near the amount of gain available as an opamp

Darlington?

A darlington could get you there, but honestly, it might not be necessary. I've never seen any one run a Rat with the gain all the way up. The other approach would be to select diodes with a forward voltage that has been reduced by the same percentage as the total gain. That would reduce the total output, but you could fix that by changing the JFET buffer into a recovery gain stage. The output impedance won't be quite as low, but that shouldn't matter too much.

[Bio77]

Maybe take a look at the LM308 schematic in the data sheet for some clues. Theres a lot in there that isn't used in the Rat.

https://www.analog.com/media/en/technical-documentation/data-sheets/lt0108.pdf

[jwin615]

The Katzenkönig is kinda that.  See the Aion Poseidon
https://aionfx.com/project/poseidon-silicon-fuzz/

[jimilee]

That fat rat has a CMOS transistor. I have a Rat PCB with 8 different clipping options including CMOS transistors.

I'm not familiar with the fat rat schematic - do you have more info or a schematic to check out?

Also, thank you both for the replies so far. :-)
[/quote]

Here's a great thread over at DIYSB. https://www.freestompboxes.org/viewtopic.php?t=27632

Here is my implementation

[greysun]

Thanks, Jimilee! man, that looks like a fun pedal. I'll read that thread shortly... Are there particular clipping settings you prefer with that pedal? Is it, in fact, fatter/louder overall?

It's kinda fun to dig into this a bit... I had no idea RAT and subsequent clones/iterations had such lore. I'm intrigued by a darlington transistor (thank you, Bricksnbeatles), but have no idea where to start... if you or anyone knows, I'm game to breadboard something up!

I guess after looking at the data sheets (thank you BIO77), I'm still a bit intrigued at how the IC muff came about - from what I'm reading, it uses 3 gain stages using 2 ICs (as opposed to 4 gain stages using 4 transistors for the originals - someone correct me if that's not right). Fully realizing that the op-amp and original muffs do *not* sound the same (probably because the ICs produce more gain, BUT...), they still have a similar flavor.

I guess what I'm after isn't an exact RAT sound, but everything online that I'm finding is trying to mimic the lm308 or play with the clipping diodes (which I also want to do! hehe, but I'd love to go further).

Great stuff here - I'll do some sleuthing, but keep it coming if you have knowledge on it...

[jimilee]

I'm digging the opamp sound and the turbo rat. Look up the deucetone rat. I have a PCB for that as well.


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[matmosphere]

Maybe you could find some answers here as well. Even if not it's an interesting read

http://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html?m=1

I would probably start by taking a transistor fuzz design I liked the sound of and start slicing the two circuits together.  Maybe even start with the Electra distortion with the tone control and output section of the rat.

[greysun]

super cool... always fun to look under the hood at these things from time to time - maybe I should try and get a can version of the 308 just to A/B test. Either way, this has been super helpful - thank you, all!

Before I start slicing and mixing schematics, I might start out trying to see what the different op-amps would do to the sound (op07, 741, 071, 301, 3130 - I read through this thread ( https://www.diystompboxes.com/smfforum/index.php?topic=127757.0 ) and am curious to see what it does), and for those ICs with external slew adjustment I might adjust the ceramic cap (which may cut off wanted, or accentuate unwanted, frequencies, but all in the name of science, right?).

I never use the ruetz mod - I'm not looking for subtlety when I use this pedal, lol - so I might do away with the sweep pot altogether in favor of something like jimilee's 8-way switch with the clipping options (might not fit into a 1590B this way, but maybe a 125b is possible? I mean, if the Runt can go into a 1590A... Might need to cut the number of options down, find a smaller rotary switch, or get creative with several toggles, but that's a "later" problem) and then use the FatRat schematic since it seems like those mods can be switched off, as well.

TO THE BREADBOARD!!! Well, after I order a few parts... Thanks, everyone! If anyone has more info, always open to it! :-)

[cspar]

Here's a thread that works towards a transistor based approach towards a Rat.

https://www.diystompboxes.com/smfforum/index.php?topic=129355.40