DSP Implementation of a 1961 Fender Champ Amplifier James Siegle Advisor: Dr. Thomas L. Stewart...

DSP Implementation of a1961 Fender Champ

AmplifierJames SiegleAdvisor: Dr. Thomas L.

StewartDecember 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

• 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.

Background

Solid-State Amplifiers

Summary

• 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.

Background

Solid-State Amplifiers

Summary

• This particular harmonic distortion can be seen below for the basic transistor amplifier configuration.

Background

Solid-State Amplifiers

(Input: Analog Oscillator with2V rms@1(kHz)

output)

Reference: Barbour, Eric. "The Cool Sound of Tubes.” Ed., Michael J. Riezenman. IEEE Spectrum August 1998.

Summary

• These harmonics are unpleasant to the ear.

• This inferior sound is the reason why vacuum tubes remain most popular in the guitar amplifier market.

Background

Solid-State Amplifiers

Summary

• 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.

Background

Tube Amplifiers

Summary

• This characteristic frequency response results in the lower harmonics having the most presence and thus producing a louder sound than solid-state amplifiers at high volumes.

• This behavior can be seen on the next slide for basic pentode and triode tube amplifier configurations.

Background

Tube Amplifiers

SummaryBackground

Tube Amplifiers

(Input: Analog Oscillator with2V rms@1(kHz)

output)

Reference: Barbour, Eric. "The Cool Sound of Tubes.” Ed., Michael J. Riezenman. IEEE Spectrum August 1998.

SummaryBackground

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)

SummaryProject 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.

SummaryObjectives

• 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.

SummaryObjectives

SummaryResults

• 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 WorkPatents

• 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 WorkPatents

• 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 WorkPatents

• 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 WorkStandards

• 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

Volume Selection from

Hardware/Software

DSP with C/C++ or Assembly Digital Filters

Analog Audio Signal from Guitar or File

Functional Description

Audio Output withTube Amplifier Sound

Overall Block Diagram

Inputs/Outputs

• 12 volume settings similar to those provided with the 12-volume switch on the 1961 Fender Champ

• echo, reverberation, fuzz, and vibrato

Modes of Operation

• 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

Project DescriptionBlock Diagram

Analog Audio Signal Input from Guitar or File

External Volume Selection

Summer

Equivalent Tube AmplifierSignal Output

Mode of Operation (Software)

BP ...BP BP BP BP BP

Parallel Bandpass FIR Filter Approach

Project DescriptionBlock Diagram

FFT Parallel Filter Network Approach

Analog Audio Signal Input from Guitar or File

External Volume Selection

Summer

Equivalent Tube AmplifierSignal Output

Mode of Operation (Software)

BP ...

FFT

IFFT

BP BP BP BP BP

Project DescriptionBlock Diagram

Multirate Signal Processing Approach

Analog Audio Signal Inputfrom Guitar or File

External Volume Selection

Mode of Operation (Software)

LP

HP

2

2

LP

HP

LP

HP

2

2

2

2

2

2

2

...

...

...

...

2 2

2

Equivalent Tube AmplifierSignal Output

LP

LP

HP

HP

LP

HP

Reference: Digital Signal Processing: Principles, Algorithms, and Applications. John G. Proakis, Dimitris G. Manolakis. Third Edition. 1996. pp. 832-834.

Project DescriptionOperation Test

Operation Test• Output will be recorded in laboratory and correlated

with Fender Champ output data in MATLAB.

• 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.

Datasheet

Project DescriptionLab Work

• Conduct several tests to determine Champ’s distinct output in time and frequency domains.

Approach

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 DescriptionLab Work

Reference: http://home.pacbell.net/vaughn44/m-3.music.notes.6.pdf

Project DescriptionLab Work

PSPICE Simulations

12AX7 Triode Amplifier

Input (above), Output (below)

Project DescriptionLab Work

PSPICE Simulations

12AX7 Triode Amplifier

Project DescriptionLab Work

PSPICE Simulations

6V6GT Pentode Amplifier

Input (above), Output (below)

Project DescriptionLab Work

PSPICE Simulations

6V6GT Pentode Amplifier

Project DescriptionLab 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 DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender Champ

Project DescriptionLab Work

16-bit Audio Output of 1961 Fender ChampSummar

y• ‘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 DescriptionLab Work

Transformer Frequency Response

Reference: Barbour, Eric. "The Cool Sound of Tubes.” Ed., Michael J. Riezenman. IEEE Spectrum August 1998.

Project DescriptionLab Work

Transformer Frequency Response

Non-Ideal TransformerTransformer 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 DescriptionLab 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 DescriptionLab Work

6V6GT Output of 1961 Fender Champ

Volume ‘3’

Project DescriptionLab Work

6V6GT Output of 1961 Fender Champ

Volume ‘6’

Project DescriptionLab Work

6V6GT Output of 1961 Fender Champ

Volume ‘12’

Project DescriptionLab Work

6V6GT Output of 1961 Fender ChampSummar

y• 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 DescriptionLab Work

Fender Champ Response to 1952 Fender Telecaster

Project DescriptionParts 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 DescriptionEE 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

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