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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
5