High Power Audio Bass Amplifier
University of Illinois Urbana-Champaign ECE 345 (now ECE 445)
Instructors Award Winner Spring 2002
In the spring semester of 2002, Steve Lapen, Nithin Cherian and I developed
a switchmode bass guitar amplifier to investigate the implementation
of a high power, high efficiency low frequency audio amplifier. I was
primarily responsible for the PWM conversion stage of the amplifier,
along with playing a very active role in integration, debugging, and
testing.
 In
the oscilloscope plot to the right the output current at an amplitude
of 22.6 A(p-p) into a 2.2 Ohm load for an output power of 140W RMS /
280W peak is shown as the top trace, while the high-side gate drive
signal is shown in the bottom trace (notice the noise from the switching
currents in the power FETs corrupting the gate drive signals - this
is the limiting factor of high power operation).
The abstract below is excerpted from our final project documentation,
available online in its entirety at https://courses.ece.uiuc.edu/ece445/cgi-bin/view_project.pl?spring2002_2
(link opens in new browser window).
HIGH POWER AUDIO BASS AMPLIFIER
GROUP 2:
NITHIN CHERIAN
STEVE LAPEN
NICK SEARS
TA: DAVE CROWE
APRIL 30, 2002
PROJECT#: 2
ECE 345 — SENIOR DESIGN LABORATORY
UNIVERSITY OF ILLINOIS URBANA-CHAMPAIGN
ABSTRACT
The most common audio amplifiers available today use biased power transistors
to amplify the audio signal. Maintaining this bias wastes a large amount
of power, and efficiency is typically below fifty percent at normal
listening volume, introducing the need for heavy heat sinks and noisy
cooling fans. The goal of this project was to build a more efficient
bass amp by using switching power processing techniques which eliminate
the need for biasing in the amplifier circuit. This technology is relatively
new in the audio realm because of the comparatively recent arrival of
high-power semiconductor devices fast enough to accommodate the switching
requirements.
The topology of the circuit created in this project includes a linear
preamplifier and equalizer, an analog-to-digital signal processing section,
an isolation subcircuit, a half-bridge switching section, and an output
filter. The switching scheme is open-loop pulse-width modulation. The
final implementation is capable of driving 280 W of peak power (140
W time-average) into a 2 Ohm load with flat gain (+/- 3dB) from 20 Hz
to 5 kHz. The limiting factor in power output is disruption of the digital
gate-drive signals by switching noise. Preliminary efficiency measurements
are above 40% for output power from 30 W to 280 W peak. It is expected
that efficiency and power output will both be improved significantly
if noise-induced gate drive disruption can be remedied.
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