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BUILD NOTES

LM308 and OP07

The original RAT used the LM308, an early op-amp with a very low slew rate compared to modern

alternatives. The slew rate is critical to the unique sound of the RAT.

In the mid-1990s, the LM308 supply began to dry up, so after some research & experimentation, Pro Co

changed over to the OP07. Despite insistence from gear enthusiasts that the LM308 sounds better and

the OP07 is inferior, it's been shown on more than one occasion (both by

the DIY community

and more

thoroughly by JHS) that the OP07 performs and sounds identical when used in the same circuit with the

same settings.

Therefore, while it's very possible that one RAT with the LM308 and another with the OP07 may sound

slightly different in A/B tests, this is due to the differences in component tolerances and manufacturing

changes throughout the years, not due to the IC itself.

The LM308 has been obsolete for a long time, and most of what's available today on eBay and

component distributors in Asia are relabeled fakes, so it's not recommended to risk using one. Let the

article from JHS

convince you the LM308 is not special, and use the OP07 with a clear conscience!

C3 compensation capacitor

The OP07 op-amp has an internal slew-rate compensation capacitor while the LM308 does not. C3

(33pF) should be omitted when using the OP07, but it's required for the LM308 to work properly.

Clipping switch

The clipping switch allows you select between different sets of diodes.

The downward position of the toggle switch is the stock RAT clipping arrangement, which has one

silicon diode in each direction. The upward position of the toggle is two diodes in each direction, a higher

clipping threshold with a more open sound. The center position leaves only the LEDs for a more dynamic

and transparent tone.

If you look at the schematic, you'll see that the LEDs, D8 and D9, are always connected. However, in

either the up or down switch positions, the lower-threshold diodes are engaged. The LEDs have no

effect in this mode since there's no signal left for them to clip.

Feel free to experiment, For example, you could jumper D7 for asymmetrical clipping. Or, you could omit

the two clipping LEDs for a diode-lift mode in the center position.

Sweep control

The Sweep knob (also called the "Ruetz mod" after

Philip Ruetz of Ruetz Amplification

who came up

with it) puts some resistance in series with one of the low-end filters in the op-amp gain stage which

changes the EQ and feel of the pedal while reducing some of the gain.

This control is entirely out of the circuit when the knob is turned all the way down (full CCW). However,

if you want to omit it, you can jumper pins 1 and 2 of the pot.

HELIOS CLASSIC DISTORTION