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.



I've been experimenting with different solutions to off-board wring. I'm please with how neat the bypass board makes things.

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 as the "Focus" control is turned up. 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 may not be the most versatile, but I think it does very convincingly get into that Sleep/Sunn O))) territory.

If anyone is interested, I have extra PCBs. Just shoot me a DM.

[ElectricDruid]

What, no Vbias for the +ve inputs of the 4558? I've not seen pin 7 of the LM386 used to provide the bias voltage like that before. What voltage is it at? <curious>

Otherwise, seems good. Sounds good. 

[Aleph Null]

What, no Vbias for the +ve inputs of the 4558? I've not seen pin 7 of the LM386 used to provide the bias voltage like that before. What voltage is it at? <curious>

Otherwise, seems good. Sounds good. 

The voltage on Pin 7 of the LM386 is it's own internal bias voltage: it sits halfway between the rails, just like you'd want. I tried a separate bias network for the 4558, but I think the response is a little tighter this way...and it saves a couple resistors.

[PRR]

Pin 7 is nominally half supply.





I suspect the actual values are not exact equal, but IAC it is not a precision chip. Stealing pin 7 to bias other stages invites howl-around but Aleph's values seem good.

[ElectricDruid]

The voltage on Pin 7 of the LM386 is it's own internal bias voltage: it sits halfway between the rails, just like you'd want. I tried a separate bias network for the 4558, but I think the response is a little tighter this way...and it saves a couple resistors.

Nice trick. I'm always learning.

[Aleph Null]

Nice trick. I'm always learning.

Same!

I got curious and remeasured the voltages in my build. With 9.39V going in I see:

LM386	Pin 7	4.78v
4558	Pin 3	4.37v
	Pin 5	4.38v

So, close enough for rock and roll!