BETA PREAMP
2
TABLE OF CONTENTS
1
Project Overview
9
Drill Template
2
Introduction & Usage
10
Enclosure Layout
3-6
Parts List
11
Wiring Diagram
5-6
Usage
12
Licensing
7
Build Notes
12
Document Revisions
8
Schematic
INTRODUCTION
The Beta Preamp is based on the Sunn® Beta Bass and Beta Lead amplifiers, first released in 1979.
The guitar and bass models were identical except for a handful value changes that made the EQ more
suitable for the frequency range of each instrument.
The Beta amps were unique in that they used overdriven CMOS gain stages for their drive tone. This
had been previously pioneered by Craig Anderton for his
pedal in 1978 with a similar topology, but the Beta was the first time
this method had been used in an amplifier.
The Beta Preamp is a single-channel adaptation of the original amplifier. It can be built in either Bass or
Lead configurations by swapping out a few components. While it can be used as a drive pedal, it works
best as a true preamp driving a power amp directly.
The original Beta amplifiers ran on +/-15V, but in this adaptation the supply voltage has been reduced to
+/-9V so it can be powered conveniently by a standard 9V supply. In the original amp, the drive tone is
generated by CMOS hex inverters run on a reduced 8V single-supply, so the heart of the distortion tone
remains exactly like in the original amplifier. All of the other active components will perform the same on
the slightly reduced voltage since there is a great deal of of headroom either way.
USAGE
The Beta Preamp has five controls:
•
Drive
controls the amount of gain going into the overdriven CMOS stages.
•
Treble
,
Mid
and
Bass
form a 3-band active tone stack. The Lead and Bass amplifiers have different
resistor & capacitor values in this section to optimize the tone stack for the frequencies of the
namesake instruments.
•
Level
sets the overall output of the preamp.
