DSP Implementation of a
1961 Fender Champ Amplifier
James Siegle
Advisor: Dr. Thomas L. Stewart
December 10, 2002
Outline
• Summary
• Background
• Project Justification
• Objectives
• Result
• Previous Work
• Patents
• Standards
• Project Description
• Functional Description
• Block Diagram
• Operation Test
• Lab Work
• Parts List
• Proposed EE452 Schedule
Summary
Background
Solid-State Amplifiers
• As solid-state technology has become
more advanced in recent years, devices,
such as transistors and ICs, are
increasingly available to be used to design
inexpensive guitar amplifiers.
• However, these analog solid-state designs
require much feedback to improve their
linear transfer characteristic.
Summary
Background
Solid-State Amplifiers
• This heavy feedback results in a sharp
clipping characteristic that produces
successive harmonics with high amplitudes
when the configuration is driven at a high
volume.
• There are several other disadvantages to
transistor-based guitar amplifiers, but the
above effect is most prevalent.
Summary
Background
Solid-State Amplifiers
• This particular harmonic distortion can be
seen below for the basic transistor
amplifier configuration.
(Input: Analog
Oscillator with
2V rms@1(kHz)
output)
Reference: Barbour, Eric. "The Cool Sound of Tubes.”
Ed., Michael J. Riezenman. IEEE Spectrum August 1998.
Summary
Background
Solid-State Amplifiers
• These harmonics are unpleasant to the ear.
• This inferior sound is the reason why
vacuum tubes remain most popular in the
guitar amplifier market.
Summary
Background
Tube Amplifiers
• There are several theories to explain the tube
guitar amplifier’s superior sound as compared to
the solid-state amplifier’s sound.
• Overall, the tube amplifier configurations result
in a frequency response with a dominant 1st
harmonic component, followed by a 2nd harmonic
component that is around half the magnitude of
the 1st harmonic, and higher harmonics with
decreasing amplitudes.
Summary
Background
Tube Amplifiers
• This characteristic frequency response results in
the lower harmonics having the most presence
and thus producing a louder sound than solidstate amplifiers at high volumes.
• This behavior can be seen on the next slide for
basic pentode and triode tube amplifier
configurations.
Summary
Background
Tube Amplifiers
(Input: Analog
Oscillator with
2V rms@1(kHz)
output)
Reference: Barbour, Eric. "The Cool Sound of Tubes.”
Ed., Michael J. Riezenman. IEEE Spectrum August 1998.
Summary
Background
Tube Amplifiers
• Tube disadvantages:
•
•
•
•
•
•
short life time
fragility
storage inconvenience (bulky size)
high power and heat dissipation
high voltage operation
high impedances requiring matching
transformers
• high cost (Fender Champ cost = +$1,000)
Summary
Project Justification
• The disadvantages of tube guitar
amplifiers can be resolved by emulating
tube distortion characteristics with a
device that already emulates other
guitar-related characteristics (i.e.
reverb, vibrato, etc.).
• Low-cost digital signal processors (DSP’s)
have been used to create several guitar
effect standards.
Summary
Objectives
• The goal of the project is to reproduce
the output characteristics of a 1961
Fender Champ with a DSP nonlinear
modeling algorithm from a guitar input.
• The Champ has been chosen due to its
popularity among vintage tube amps and its
simple design as seen in the next slide.
• As a result, more time can be spent on
improving the DSP algorithm.
Summary
Objectives
Summary
Results
• When the project is completed, the output
from speakers interfaced to the DSP
board should match the sound of the
Champ and resemble the characteristics of
the data recorded in lab.
• The result will be a superior tube amplifier
sound implemented with a low-cost DSP.
• Other tube amplifiers may be modeled
from this specific DSP algorithm.
Previous Work
Patents
• Two patents pertaining to this project
were found at U.S. Patent and Trademark
Office website (http://www.uspto.gov)
from “DSP” AND “guitar amplifiers”
keywords.
Previous Work
Patents
• PAT. NO. 5,789,689 - Tube modeling
programmable digital guitar amplification
system
• Does not specifically model the 1961 Fender Champ
• Uses a sampling rate conversion algorithm to model the
nonlinear transfer function of the tube circuitry.
Later slides indicate that this method is not planned
for the project.
Previous Work
Patents
• PAT. NO. 6,350,943 - Electric instrument
amplifier
• Does not specifically model the 1961 Fender Champ
• Uses a similar DSP algorithm to model the tube
nonlinearities, but also includes a tube configuration on
the D/A output of the DSP board and a solid-state
power circuit. Purpose of the project is to model the
tube amplifier WITHOUT tubes or solid-state
devices.
Previous Work
Standards
• No applicable standards to this project
other than avoiding a >5 (V) input into the
board.
• If the amplifier is successfully modeled,
other linear effects will be added to the
design including reverb, vibrato, etc. in
which there are several resources or
standard techniques to produce these
effects.
Project Description
Functional Description
Analog Audio Signal
from Guitar or File
DSP with C/C++
or Assembly
Digital Filters
Audio Output with
Tube Amplifier Sound
Volume Selection from
Hardware/Software
Overall Block Diagram
Inputs/Outputs
•
Inputs - analog audio signal from either a guitar A/D interface or a saved audio file
and software or hardware based volume selection will regulate the filters’ behavior
• Output - audio signal with tube amplifier effect
Modes of Operation
•
•
12 volume settings similar to those provided with the 12-volume switch on the 1961
Fender Champ
echo, reverberation, fuzz, and vibrato
Project Description
Block Diagram
Analog Audio Signal Input
from Guitar or File
External Volume Selection
Mode of Operation (Software)
BP
BP
BP
BP
BP
...
BP
Summer
Equivalent Tube Amplifier
Signal Output
Parallel Bandpass FIR Filter Approach
Project Description
Block Diagram
Analog Audio Signal Input
from Guitar or File
External Volume Selection
Mode of Operation (Software)
FFT Parallel Filter
Network Approach
FFT
BP
BP
BP
BP
BP
Summer
IFFT
Equivalent Tube Amplifier
Signal Output
...
BP
Project Description
Block Diagram
LP
Analog Audio Signal Input
from Guitar or File
LP
2
2
HP
2
LP
2
Mode of Operation (Software)
HP
External Volume Selection
2
HP
...
...
...
...
2
LP
2
HP
2
LP
2
2
LP
Equivalent Tube Amplifier
Signal Output
2
HP
HP
Multirate Signal Processing Approach
Reference: Digital Signal Processing: Principles, Algorithms, and Applications.
John G. Proakis, Dimitris G. Manolakis. Third Edition. 1996. pp. 832-834.
2
Project Description
Operation Test
Operation Test
• Output will be recorded in laboratory and correlated
with Fender Champ output data in MATLAB.
Datasheet
• This project involves modeling a tube amplifier
response with a DSP algorithm and does not require a
datasheet.
• All necessary operation data for the Texas
Instruments DSP board can be referenced in the
provided manuals.
Project Description
Lab Work
Approach
• Conduct several tests to determine Champ’s distinct
output in time and frequency domains.
Tests/Measurements
• PSPICE simulations of basic tube amplifier
configurations for 12AX7 triode and 6V6GT Pentode
• Measure Fender Champ output with 16-bit audio A/D
Converter with Cool Edit software in Acoustics lab for
several sinusoidal inputs (see next slide)
• Measure single-ended power tube stage output of
Fender Champ before transformer
Project Description
Lab Work
Reference: http://home.pacbell.net/vaughn44/m-3.music.notes.6.pdf
Project Description
Lab Work
PSPICE Simulations
Input (above), Output (below)
12AX7 Triode Amplifier
Project Description
Lab Work
PSPICE Simulations
12AX7 Triode Amplifier
Project Description
Lab Work
PSPICE Simulations
Input (above), Output (below)
6V6GT Pentode Amplifier
Project Description
Lab Work
PSPICE Simulations
6V6GT Pentode Amplifier
Project Description
Lab Work
PSPICE Simulations
• From the previous slides, the output of the triode
amplifier configuration exhibits higher harmonic
components beyond the 1st harmonic than the pentode
configuration output.
• Triode amplifier configuration still exhibits expected
frequency response since the 2nd harmonic magnitude is
slightly greater than half the 1st harmonic magnitude.
• Both outputs of the amplifiers clip at the supply voltage.
• PSPICE simulation of 1961 Fender Champ circuit can
not be run since no transformer winding data is
available, the transformer output on the amplifier is
difficult to access, and the PSPICE transformer model
is too ideal with no capacitive coupling.
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Project Description
Lab Work
16-bit Audio Output of 1961 Fender Champ
Summary
• ‘Ringing’ effect seen from the Fender Champ output for
input frequencies <1046.50 (Hz).
• This ‘ringing’ is most dominant at 1046.50 (Hz) and
results in 2nd and odd harmonic components with higher
amplitudes than the first harmonic.
• Odd harmonics are expected to be higher than even
harmonics as seen in the next slide, but the first
harmonic amplitude should not be smaller than
successive harmonics.
Project Description
Lab Work
Transformer Frequency Response
Reference: Barbour, Eric. "The Cool Sound of Tubes.”
Ed., Michael J. Riezenman. IEEE Spectrum August 1998.
Project Description
Lab Work
Transformer Frequency Response
Non-Ideal Transformer
Transformer Frequency Response
• From all the coupled capacitances, the transformer appears to
exhibit a ‘resonance’ property that has the most effect on the
output for a 1046.50 (Hz) input.
• Champ characterized by poor low frequency response (F < 1046.50
(Hz)) due to the cheap output transformer.
Project Description
Lab Work
6V6GT Output of 1961 Fender Champ
Voltage Divider to Measure Output of Single-Ended Power Stage
Vo = (R2/(R1+R2))*6V6GT output = 0.00662*6V6GT output
PR1 = 1.33 (W), PR2 = 0.00890 (W)
• Since tube circuits are characterized by high output voltages, a
voltage divider connected to the 6V6GT output of the Fender
Champ will allow the oscilloscope to measure the output at lower
amplitudes.
Project Description
Lab Work
6V6GT Output of 1961 Fender Champ
Volume ‘3’
Project Description
Lab Work
6V6GT Output of 1961 Fender Champ
Volume ‘6’
Project Description
Lab Work
6V6GT Output of 1961 Fender Champ
Volume ‘12’
Project Description
Lab Work
6V6GT Output of 1961 Fender Champ
Summary
• Similar output for volume ‘3’ setting as transformer output.
• First harmonic is most dominant over successive harmonics
for both volume ‘6’ and ‘12’ settings on the Fender Champ.
• Volume ‘6’ does not drive the amplifier enough to produce
second harmonic component with higher amplitude than the
odd harmonics.
• Harmonics greater than 10 have not been removed by
transformer as expected.
• More data points in the time domain would improve
harmonic spikes in the frequency domain.
Project Description
Lab Work
Fender Champ Response to
1952 Fender Telecaster
Project Description
Parts List
• The goal of the project is to model the Fender Champ’s
output characteristics.
• If the DSP algorithm is completed and coded for the
DSP board, some interfacing circuitry from a guitar
cable to the ADC and from the DAC to a set of
speakers will be required.
• This interfacing circuitry will only consist of resistors,
capacitors, a potentiometer, and a low voltage power
amplifier, and these parts are available.
Project Description
EE 452 Schedule
• Next semester, the following schedule has been
developed:
• Weeks 1-4: Complete and Simulate model of Fender Champ in
MATLAB from obtained 12AX7 and 6V6GT tube data sheets
• Weeks 5-8: Complete Software to program the actual DSP
board and interface the appropriate hardware to the ADC and
DAC
• Weeks 13-14: Senior 2003 Expo Preparation
• Weeks 15-16: Senior Project Presentation
• There is a 4-week window that is intended to allow for
setbacks