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Vactrol selection
The Luna sounds best with a VTL5C3 workalike. Since the original VTL5C3 is no longer made, you'll
likely need to find a substitute. Xvive and CoolAudio make their own version under the same part
number, VTL5C3, and both are available from Small Bear Electronics as well as several international
retailers. Alternately, Small Bear Electronics sells the Macron MI1210CLF-R which works just as well.
It's also possible to make your own photocoupler by using an LED and an LDR. However, this is outside
the scope of this documentation and you'll have to ask for help on a forum if you're not sure what to do.
Note: Do not use a low "on" resistance photocoupler such as the NSL-32. The resistance (500 ohms to
1k) is so low that the gain ratio will be thrown off balance in the following gain stage. The volume control
will be unusable, and the combined high-pass filter with C3 will cut the bass significantly at the higher-
volume end of the cycle.
Input buffer
The original Tremulus Lune does not have an input buffer. Since the first op-amp stage is in an inverting
configuration, this means the source impedance directly affects the gain of the effect, and you will have
drastically different behavior if the pedal is connected straight to a guitar vs. coming after a buffer.
The Luna adds an input buffer to counteract this. The default configuration from the parts list & layout is
to use a JFET. However, due to the scarcity of TO-92 JFETs, the buffer can be configured to instead use a
more common NPN bipolar transistor such as a 2N5088 or 2N3094 with only one resistor change.
To use a BJT, omit R2 and use 470k for RX1. Then, insert the transistor as shown below:
Assuming E-B-C pinout, you just shift it to the left by 1 pin from where the JFET would normally go.
JFET selection
The 2N5457 is a great general-purpose JFET, but anything with a high V
value (also known as V
GS(off)
P
or pinch-off voltage) will work well here. Other options include 2N5458, MPF102, PN4303, PN4304,
BF244A, BF245A among many more.
LUNA OPTICAL TREMOLO
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