Otolith — A High-Gain Turducken

[Aleph Null]

I saw a few YouTube demos of people getting super sludgy, doomy sounds by running a Rat in front of an Acapulco Gold. This design began with asking, "What would happen if you just shoved the LM386 gain stage right inside the Rat where the clipping diodes usually go?" A high-gain turducken! There ended up being more to it than that, but that's where it started.



Bare metal felt like the right look.



This layout should fit in a 1590B. If you were really ambitious, you might be able to cram it in a 1590A.

A short demo:

The "Focus" control grew out of the Ruetz mod for the Rat. The values were changed so that turning the Focus up cuts bass frequencies, but also increases the gain significantly. The clipping diodes help to limit how hard this first gain stage hits the input of the LM386—it can only take so much input signal before it turns to mush—and the asymmetrical arrangement emphasizes 2nd order harmonics, making the guitar sound more nasal and mid-range focused. This feeds the LM386 which is set up the same way we've seen it in the Acapulco Gold, the Smash Drive, the Clari(not) and other designs. Last is an active tone control that creates a high shelf at 720Hz. It's capable of -13dB to +7dB. This design isn't the most versatile, but I think it does very convincingly get into that Sleep/Sunn O))) territory.



..and the layout for anyone that might be interested in building one.

[lrgaraujo]

Nice build! And very nice demo! Some great sounds there!

Whats up with the bias arrangement for the op amp stages? I'm not very used to the 386, does pin 7 provide a Vref?

[Aleph Null]

Nice build! And very nice demo! Some great sounds there!

Whats up with the bias arrangement for the op amp stages? I'm not very used to the 386, does pin 7 provide a Vref?

Thanks! The LM386 creates it's own bias voltage which is accessible on pin 7. I experimented with a separate bias network for the other opamp stages, but didn't find it improved noise performance. Using pin 7 for the bias voltage seemed to tighten up the circuit's response to me—and it saves a couple of resistors!

[jjjimi84]

Very very cool!

[gordo]

I was really expecting this to be a doomy mess but was very pleasantly surprised at the sounds you're getting.  BIG at all times but with nuance too.  Very slick.

[Aleph Null]

I was really expecting this to be a doomy mess but was very pleasantly surprised at the sounds you're getting.  BIG at all times but with nuance too.  Very slick.

Thank you for the compliment! This circuit went through quite a few revisions. Each revision had more and more low-pass filtering to tame fizziness and less and less gain from the initial gain stage. I wanted to be right at "doomy mess" with everything on ten, but be able to dial it back to something more articulate.

[jessenator]

I want to go to there

This is fantastic! I think I might breadboard one up.

[jessenator]

Got this one all breadboarded up and boy oh boy this one's a keeper!

Very well done indeed, good sir!

I'll also have to look up that mod for the Rat you mentioned.

[jimilee]

Got this one all breadboarded up and boy oh boy this one's a keeper!

Very well done indeed, good sir!

I'll also have to look up that mod for the Rat you mentioned.

The Ruetz mod is very nice. Instead of a  straight mod, use a 500r pot if you have one.


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[Aleph Null]

Got this one all breadboarded up and boy oh boy this one's a keeper!

Very well done indeed, good sir!

I'll also have to look up that mod for the Rat you mentioned.

I'm glad you like it! I'd love to see a build report when you get it boxed up!

The Ruetz mod is included in the Slow Loris as the "Sweep" knob. The "Focus" knob on the Otolith does essentially the same thing, but the values are tuned differently and the knob works in the opposite direction, cutting lows as you turn it up instead of increasing them.

If you're looking to get further into "doomy mess" territory, you could try decreasing the 47k resistor that connects to pin 3 of the LM386. It forms a voltage divider with a 50k resistor inside the chip, so reducing this value will cause the LM386 input to clip sooner.  You could also increase the saturation of the first stage by increasing the 15k resistor in the feedback loop. If you go this route, you may need to decrease the 1nF capacitor that's in parallel; the low pass filter is already pretty aggressive to keep things from getting fizzy.

Cheers!

[culturejam]

That's a fantastic idea! 

[jwin615]

Awesome execution! Bet this would be great parallel to the right fuzz as well.
Almost hits some Homme tones at times as well.
I've never cared for the RAT circuit much, and honestly avoided it, because of what I can only call it's "nasal" tone. It feels like it's nagging at me. But this seems to add overtones to the nagging 2k and is much more palatable.

[Aleph Null]

Awesome execution! Bet this would be great parallel to the right fuzz as well.
Almost hits some Homme tones at times as well.
I've never cared for the RAT circuit much, and honestly avoided it, because of what I can only call it's "nasal" tone. It feels like it's nagging at me. But this seems to add overtones to the nagging 2k and is much more palatable.

The Rat is quite nasal! That works well in some situations, but it's nice to be able to dial that back. I have a dual/parallel fuzz in the pipeline, but that'll be based loosely on a fuzz face and harmonic percolator.