aion Arcturus Manual - Página 6

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BUILD NOTES

Germanium diode selection

Based on the factory schematic, the original ODR-S seemed to use AA-112 germanium diodes. However,

according to the designer, this was just a placeholder from the schematic software and not the actual

type used in production. He does mention that Nobels eventually switched to a type of diode that he

found to be inferior because they weren't able to find enough supply of his preferred type, so at least

according to him, the type makes a difference.

If you're curious, Kai Tachibana has written an article about the

development of the ODR-S

that's a very

interesting read.

IC2A bias

IC2A has an odd bias arrangement in the original ODR-S circuit, getting its bias voltage through the

one of the germanium diodes instead of from a bias resistor as you'd expect. This may have been

an intentional choice and it's possible that it leads to a desirable effect—but from an engineering

standpoint, it looks like a mistake.

We've corrected this by adding a capacitor (C8) between the diodes and the IC2A input and adding a

separate bias resistor (R10) at the input. We chose 43k because it's the value used in the ODR-1. If you

want to omit this, just jumper C8 by running a wire between the pads and omit R10 (leave it empty).

In the ODR-CS, this whole stage has been redesigned so that the midrange control can cut as well as

boost, so we can't infer whether the designer would see this as a mistake today upon revisiting the

circuit. However, the fact that the IC no longer gets its bias from the diode would indicate that he does

not see it as critical to the sound.

Be aware that if you want to put the hard-clipping diodes on a switch, the op-amp will not bias if they're

switched out unless you include our C8/R10 fix.

Midrange pot value

The midrange pot is 30kC on the original ODR-S. This value is basically impossible to find without

custom-ordering in large quantities, so just use 25kC or even 20kC.

ARCTURUS NATURAL OVERDRIVE