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Musical Instrument Amplifier

Project
Proposal

Developers Steven
Ward & Udit Jalan

Introduction:

High
power musical instrument amplifiers are typically expensive and heavy. 
The objective of this design project is to make a low cost, lightweight
and flexible amplifier that competes with currently available products
on the market.  This amplifier would offer the following benefits:

Less than half the weight of conventional amplifiers
Higher efficiency
for less power consumption
Lower cost
Smaller package

Features:

500 Watts
R.M.S output power
DSP preamplifier
section

Block Diagram:

Block Diagram Description:

Power Supply Unit:

The power supply unit
is a switch-mode power supply delivering all of the necessary voltages
to the amplifier and the DSP units.  The input is standard 120VAC
at less than 15A (standard U.S. outlet).  The power supply outputs
are all isolated from each other. There is a dedicated +5VDC for the
DSP and its associated hardware.  The amplifier requires multiple
power inputs: +/-5VDC, +/-70VDC (at 10A), +5VDC and +12VDC.  Because
the power supply needs to give multiple isolated outputs, the flyback
or forward types are the best choice.  The decision between flyback
and forward converter will be made based on further research on their
suitability to our application.

Amplifier:

The amplifier is class-D
type.  Its basic function is to amplify the analog input signal
(1V RMS) to drive a loudspeaker (50V RMS).  The amplifier can work
from any standard line-level analog signal input.  The amplifier
does not depend on a switching power supply to operate, so a standard
60hz transformer-based power supply will work as a substitution.

DSP Preamplifier:

This section takes in
an analog signal from the musical instrument, performs filtering processes
in the digital domain, and then converts the processed signal to an
analog output to the amplifier.  Potentiometers will be used to
set the parameter values for the filters.  The use of potentiometers
(as opposed to encoders) allows the DSP to keep the same filter settings
between power cycling.  External A/D and D/A converters will be
necessary as the dsPIC chip does not offer high quality converters for
audio use.

Performance Requirements:

Total weight
less than 20lbs.
Output power
: 500W into a 2 ohm load
T.H.D. less
than 0.1% at 50% rated output (amplifier only, not DSP)
Frequency
Response: 20Hz to 20kHz +/-3dB
Propagation
delay < 1mS (DSP and amplifier)

Verification:

Testing procedures:

Amplifier
Output Power:  Use a 2 ohm power resistor and apply full output
across it, while measuring the RMS voltage across the resistor. 
This test may be limited by the resistors ability to dissipate 500W!
THD can be
measured using an audio analyzer connected to the output of the amplifier. 
The analyzer can generate distortion vs. frequency plots.  Many
sweeps would be performed at various output power levels.
Frequency
response of the amplifier alone can be measured using an audio analyzer
connected to the output of the amplifier.  The audio analyzer can
generate frequency response plots.
Frequency
response of the DSP can also be measured with the audio analyzer to
ensure the filters are working properly.  An oscilloscope, DVM
and function generator can easily replace the use of the audio analyzer
if we cant get access to it for some reason.  The response would
be plotted by hand, then.
Propagation
delay can be measured with an oscilloscope by probing the input to the
DSP and the output from the amplifier and measuring the time delay.
The power
supply will first be tested at lower than 120VAC input voltages using
a variac and isolation transformer for safety reasons.  This also
allows us to scope the high voltage (120V) components without grounding
out 120V power!  The variac will also allow us to test the regulation
of the output over a large input voltage range (0 to 140VAC).

Tolerance Analysis:

Within the amplifier there
will be an integrator.  This integrator is based around an op-amp
and some resistors and a capacitor.  From experiences with control
systems, the performance of the integrator can have a large impact on
the amplifiers performance.  The key component of the integrator
is a high quality capacitor in the op-amps feedback loop.  If
this capacitor has significant leakage resistance then the circuit becomes
a leaky integrator and its benefits to the precise control of the
amplifier may be lost.  As a test, we can artificially increase
the leakage current of the capacitor by parallel connection of a resistor. 
By changing this resistance we can identify some worst-case performance
specifications of this sub-circuit and identify its impact on the amplifiers
THD performance.

Costs:

Parts cost:

Part Description

Cost

Preamp

 

dsPIC30F4013

$3.93

Potentiometers (qty 10)

$0.50*10 = $5.00

Printed Circuit Board

$15.00

Misc. Resistors, Capacitors,
ICs.

$5.00

Connectors

$2.50

D/A and A/D converters

$5.00

Amplifier

 

Power MOSFETS (qty 2)

$3*2 = $6.00

Power Inductor (qty 1)

$4.00

Electrolytic Capacitors (qty
4)

$3*4 = $12.00

Heatsink

$4.00

TLC082 Op amp (qty. 3)

$0.70*3 = $2.10