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BUILD NOTES

LM308 and OP07

The Expandora was based on the Pro Co RAT, and like the RAT, it originally 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, and likely the Expandora as well by extension.

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

changed over to the OP07. Bixonic followed suit in later versions of the Expandora.

The RAT is plagued with decades of heated debates about whether the LM308 sounds better than the

OP07. However, 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.

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!

C2 and C7 compensation capacitors

The OP07 has an internal slew-rate compensation capacitor while the LM308 does not. C2 and C7 (both

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

Optocoupler selection

The original Expandora used a Sharp PC419 photo-FET optocoupler. They're now obsolete, but not

terribly hard to find (e.g. from

Small Bear

Electronics). However, they were only available in SMD format.

The H11F1M is an exact substitute with the same current-to-resistance curves, and it's available in

through-hole format, so it's a better choice all around. Note that it's a DIP-6 package, so if you can't

source a 6-pin socket, you'll need to cut down an 8-pin socket or use two SIP snap-apart strips.

The JHS Kilt uses the H11F3, which is similar to the F1 but with slightly higher resistance at the same

currents. We haven't traced a Kilt to know whether there are any other circuit changes, but if you want

to experiment, you can try this optocoupler as an alternate to see if you like it any better.

Drive knob

The original Expandora had a set of DIP switches that selected between three different fixed resistances

in parallel with the optocoupler, plus a fourth to engage "Forbidden mode" by removing the parallel

resistors and using the optocoupler to control the gain directly.

In version 4, this was converted into a potentiometer called "Drive" along with a minimum-value

resistor. This is a significant improvement in usability, so we've back-ported this change to our version of

the circuit, which is otherwise largely based on V3.

Using 240R for R5 (the minimum-value resistor) and a 1kB pot for the Drive control, the original three

settings can be found at approximately 13% rotation (371 ohms), 32% rotation (560 ohms), and 86%

rotation (1100 ohms). It can go slightly lower and higher than the stock range, as well as having all the

in-between settings that were otherwise unavailable.

DYNAMO REACTIVE DISTORTION