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CIRCUIT DESIGN NOTES

Power supply design

Like most solid-state preamplifiers of the era, the L5 ran on a bipolar +/-15V supply. This voltage can't be

supplied by an external adapter, and the current draw of the circuit is too high to use a charge pump.

When developing the original Lab Series L5 Preamp, we adapted a supply scheme from Alesis rack units

in the early 1990s that involved a 9VAC adapter and an AC voltage tripler. This was then rectified to

bipolar +/-19V DC and regulated down to 15V on each rail.

This solution used cheap and readily-available parts, and it has worked very well for several years since

the L5 Preamp was first developed. But the power adapter requirement has always been the major flaw.

A 9VAC adapter will destroy most other pedals if it's plugged in, and if you own one, there's an infinitely

higher chance that it'll be mistaken for a 9VDC adapter and plugged into the wrong pedal at some point.

Because of this, when developing the

IVP Preamp

project in 2021, we set out to find a reliable way to

supply +/-15V from a standard DC adapter. Fortunately, there are a few more options available today

than there were in 2015 when the L5 Preamp was originally developed, and a high-quality DC-DC

converter module will give us exactly what we need.

They're not cheap (USD$9-15 each), but once you account for the fact that you no longer need a

specialized power adapter, the total cost is about the same. We have begun using these DC converters

in all of our preamp projects going forward, including this new version of the original L5 project that

started it all.

See the build notes on page 11 for more information on the specific DC-DC converters that are

recommended for use in this project.

Multifilter

The multifilter is a

patented Moog invention

that uses a set of six fixed-frequency resonant filters to add

a unique harmonic sheen to an instrument signal, sometimes described by Lab Series owners as making

their electric guitar sound more like an acoustic.

On the original amps, the multifilter was hard-wired to take its input signal from channel 2 and had no

function when channel 1 was used. However, in developing this updated version of the L5 project, we

experimented with taking the input from channel 1 instead, and the results were stellar.

There's a bit of a switch pop when moving the multifilter from one channel to the other, so it's not

something you'll want to adjust in a live environment. Also beware that depending on the EQ settings,

there may be feedback on the upper end of the multifilter since it's operating in a way it wasn't originally

designed to. Just turn down the multifilter level slightly and it will go away.

L5 PREAMP