piezo preamp and buffer report

8/10/2019 Piezo Preamp and Buffer Report

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2013

3C6AAnalogue DesignProject

GROUP C2PIEZO PICKUP SYSTEMS

STEPHEN BRENNAN


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Contents1. Introduction.................. ............. ............. ............. ............. ............. ............. ............. .............. ............. ............. ............. ............. ............. 2

2. The Common Emitter (CE) Amplifier ............ ............. .............. ............. ............. ............. ............. ............. .............. ............ ............. ....... 3

2.1: Introduction to the CE Amp .................................................................................................................................................................. 3

2.2: Design of the CE Amp ............................................................................................................................................................................ 3

Biasing .................................................................................................................................................................................................... 3

Designing the CE Amp............................................................................................................................................................................. 3

2.3: The CE Amplifier Lab ............................................................................................................................................................................. 5

Finding the Circuit Parameters ............................................................................................................................................................... 5

Testing the Amp ..................................................................................................................................................................................... 6

3. The Two Stage Amplifier ............ ............. ............. ............. ............. ............. .............. ............. ............. ............. ............. ............. ............. 9

3.1. The Two Stage Amplifier Introduction ............ .............. ............. ............. ............. ............. ............. .............. ............. ............ ....... 9

3.2. The 2 Stage Amplifier Lab ............ ............. ............. ............. ............. ............. .............. ............. ............. ............. ............. ........... 10

Results from MultiSIM: ......................................................................................................................................................................... 10

Results from the Lab ............................................................................................................................................................................. 11

AC Analysis ........................................................................................................................................................................................... 11

Discussion of Results ............................................................................................................................................................................ 11

4. The Instrumentation Amplifier & PCB Building ............. ............. ............. ............. ............. ............. .............. ............. ............. ............. .. 12

4.1. Introduction to the Instrumentation Amplifier ............. ............. ............. ............. ............. ............. .............. ............ ............. ..... 12

4.2. Simulation & Building ............ ............. ............ .............. ............. ............. ............. ............. ............. .............. ............. ............. .... 13

Simulation ............................................................................................................................................................................................ 13

Building the Circuit Results ................................................................................................................................................................ 14

Discussion of Results ............................................................................................................................................................................ 15

5. Piezo Pickup Systems for Acoustic Archtop Guitars ............ ............. .............. ............. ............. ............. ............. ............. .............. ........ 15

5.1. Introduction ............. .............. ............ ............. .............. ............. ............. ............. ............. ............. .............. ............. ............ ..... 15

What is a Piezo Transducer? ................................................................................................................................................................. 15

Piezo Preamps A Simple Solution ...................................................................................................................................................... 16

5.2. Selecting a Design ............. ............. ............. ............. ............. .............. ............. ............. ............. ............. ............. .............. ........ 17

The Tillman Preamp .............................................................................................................................................................................. 17

Martin NawrathUniversity Of Cologne Lab3 Piezo Disk Preamp ....................................................................................................... 18

L.R. Baggs Godin Solidac Piezo Preamp with blend control for magnetic systems ............................................................................... 19

Scott Thelmke Mint Box Piezo Buffer ................................................................................................................................................. 20

Mongrel Dog Audio Piezo Pickup Preamp for musical instruments ...................................................................................................... 20

Selection Process .................................................................................................................................................................................. 20

5.3. Design and Testing ............ ............. ............. ............. ............. .............. ............. ............. ............. ............. ............. .............. ........ 21

Scott Thelmke Mint Box Buffer ............................................................................................................................................................. 21

Mongrel Dog Audio Piezo Preamp ........................................................................................................................................................ 24

Testing with the ArchtopA Brief Comparison.................................................................................................................................... 26

6. Conclusions................... ............. ............. ............. ............. ............. ............. ............. .............. ............. ............. ............. ............. ........... 26


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1. IntroductionThe aim of this report is to acquaint the reader with the theory and workings of transistor

and Op Amp based amplifier circuits. In addition to this it will act as a review of the work that was

completed by the C2 group over the course of the first semester. The aim of the module was to

develop the practical knowledge of design, simulation, implementation and testing of transistor orOp Amp based electronic circuits. The course structure was balanced over a 12 week period. This

consisted of an introduction to electronic amplifiers, labs that focused on simulating and building

different kinds of amplifier circuits, a lab about PCB design and printing and a 4 week period over

which the group picked a project and designed a circuit to solve a problem or fulfil a particular role

that would be of some use in the real world.

Prior to the execution of the main part of the module, 3 different types of amplifier were

designed, simulated, constructed and tested in the labs. The purpose of this was to give the groups a

deep understanding to both the theory behind how the amps worked and an insight into the

construction of the amps on a breadboard or PCB. In the first 3 weeks of the module the group

simulated, tested and built a common emitter amp; a basic single transistor circuit which addedsmall gain and a 90 degree phase shift to an input signal (thus displaying the amplifying and inverting

properties of a BJT). The next challenge was a two stage audio amp which corrected the phase shift

of the original common emitter amplifier and added sufficient gain for driving an audio source into a

set of speakers. In the final weeks before reading week an instrumentation amplifier circuit which

used three Op Amps as opposed to two Bipolar Junction Transistors was tested and constructed, this

circuit provided a larger amount of gain than the two stage audio amp while being neater to

construct. The testing of the circuits consisted of determining parameters like output gain and

input/output resistance in MultiSIM and then comparing these parameters to measured quantities

recorded in the lab. The six weeks of testing provided the group with a solid foundation in both

amplifier theory and breadboard construction. It was after this that the group learned how to designthe PCB boards (using EAGLE software) on which the vast majority of these electronics are built.

After designing the board in EAGLE, the group soldered an Op Amp based audio amplifier circuit to a

PCB board and tests were taken to determine gain and the 3dB point.

The largest component of the 3C6a module was the 4 week period following reading week.

Here the objective of the group was to design and simulate an amplifier circuit that incorporated the

knowledge that was obtained in the earlier half of the course. The group picked Piezo Pickup

Systems for Archtop Guitarsas their primary objective as it was agreed that given the timeframe, it

would be possible to simulate, construct and test multiple systems and comment on the

characteristics of the various components. In addition to this, a good piezo pickup system can solve

or reduce some of the problems that come with using a piezo as a contact microphone (this will bediscussed in section 5) and the circuits related to this subject use many of the concepts that were

covered in the introductory labs in weeks 2-6. At the time of the demonstration, two systems had

been fully constructed and tested on a guitar equipped with a piezo pickup. The first system

(henceforth referred to as system 1)that was constructed was a single transistor piezo buffer based

on the Scott Thelmke Mint Box Buffer, a discrete FET design piezo preamp. This system was

simulated and some changes were made to the original design before being built onto a breadboard.

Following this, the circuit was soldered onto strip board and tested in a live-band situation to see

how RF signals would affect the performance of the buffer. After using the system live, alterations

were noted that would improve the system and the circuit was redesigned and retested again,

unfortunately there was not enough time to implement these changes. The second system (system2) was designed around 2 TL062 Op Amps and consisted of two stages; a buffer and a preamp. This


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system, unlike System 1 delivered an output gain and included a high

pass filter which acted as a tone and gain control for the piezo pickup,

thus adding to the versatility of the system for the end user. The

schematic that was used was based off a pickup system for a double

bass, so in order to ensure that the device would work well with the

guitar, certain components were replaced in order to better replicate

the projected sound of the instrument; these changes will be outlined

in detail in section 5. Following the testing session it was decided that

although System 2 provided a more accurate representation of the

sound of the guitar, System 1 was a better choice of system for the

user because it could be packed onto a smaller space, it used less

power and due to the JFET transistor the tone produced from the

circuit was analogous to the tone obtained by musicians from the

1920s-1950s which is a desirable property for the guitar type that was

being tested.

2.The Common Emitter (CE) Amplifier

2.1: Introduction to the CE Amp

The common emitter amplifier (seen in figure 2i) is a

one transistor circuit which consists of an NPN bipolar

junction transistor, an input capacitance () andVoltage Divider Biasing. This amp is most commonly

used as a voltage amplifier. It was the task of the group

in week 3 to simulate and build an amplifier similar to

this in Multisim and record the circuit parameters of the

amp. Prior to disclosing these results however it is

important to discuss the theory of the CE amplifier and

how, as electronic engineers, this device can be

designed and built to perform at its maximum capacity.

2.2: Design of the CE Amp

Biasing

Biasing is a term that describes the process of setting the voltage or current at points in a circuit in

order to establish the optimal operating conditions for the electronic component. In the case of the

Common Emitter Amplifier, the Biasing is performed by a Voltage Divider; an arrangement of two

resistors that act as a potential divider across the supply. The centre point of the two resistors

provides the biased base voltage for transistor operation.

Designing the CE Amp

In order to obtain the best values for and , a suitable bias voltage must be found, however inorder to do this, a value must be given for the load resistance. In the lab, a resistor was usedand a voltage drop of 1.4V was desired over the emitter resistor. From this, the max collector

current can be calculated:

Figure 1: An acoustic Archtop

guitar made by Luthier Bill Collings

Figure 2: A schematic of the Common Emitter

Amplifier


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2.3: The CE Amplifier Lab

The CE Amplifier circuit that was used in the lab is very similar to the circuit that was designed

above, very similar resistances were used, but with the exception of more capacitors being

employed. The schematic of the amp is shown in figure 3 below:

Figure 3: Schematic of CE Amp connected to an oscilloscope

Finding the Circuit Parameters

Once the above schematic was built in MultiSIM, it was the task of the group to find the mid band

gain, the upper and lower cut off frequencies and the input and output resistance. In order to

calculate the mid band gain, the ratio of the input and output voltages of the circuit were recorded

using the measurement probe in MultiSIM:

In order to obtain the gain in decibelswas then used in the logarithmic equation:

Calculating the input resistance of the circuit was performed by taking the equivalent resistance

from the base resistance and :


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was then found as the equivalent resistance over and

So:

= Calculating the upper and lower cut off frequencies of the amp were performed using the following

equations:

where the smaller ofandrepresent the cut off.It was found after calculating this, thatgoverned the lower cut off of the amplifier with afrequency of 22.3Hz. This result is significant as it represents a frequency that is just within the

threshold of the human ear. This result also shows that capacitors C2 and C3 are responsible for

governing the lower cut off frequency.

The upper cut off frequency was determined using the following equations:

( ) (

)

Where:

From this, it was determined that the upper cut off frequency was 16.93kHz, just below thefrequency of the highest pitch the human ear can hear.

Testing the Amp

The amp was built and tested in the lab using a breadboard, a function generator and an

oscilloscope. The setup of the Oscilloscope and function generator are shown in figure 5.

Figure 4: Diagram showing how the signal generator and Oscilloscope were configured


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It can be seen from figure 5 that the signal generator is connected to channel 1 of the

oscilloscope and to the input of the circuit, this allows for the viewing of the original signal

juxtaposed with the output signal of the circuit, which is connected to channel 2 of the op amp. This

is the setup the group used throughout the course of the module.

After setting up the circuit on thebreadboard the group did not have time

to take all of the measurements from

abovehowever a mid band gain of 5

was obtained from the output of the

oscilloscope and the phase of the signal

was shifted 90 degrees, showing the

inverting property of the BJT amplifier. In

addition to this, some clipping was noted

to be occurring on the positive peaks of

the output signal. All of these features

can be seen in Figure 5, which was the

output signal when a 1Vpk, 1kHz input

source was used.

The final part of the lab

was testing the output

response of the amp given

a 200mV input at varyingfrequencies. The aim of

this part of the lab was to

determine in practice the

upper and lower cut off

frequencies of the amp. It

is known that the

frequency response of an

amplifier looks like the

graph shown in figure 6,

where and representthe lower and upper cut offfrequencies.

In order to construct a frequency response graph for the CE amplifier that was built in the lab, the

group set the function generator to various frequencies and recorded the output of the amp. Figure

7 shows the outcome, after plotting these results onto a graph (note, x axis is logarithmic).

Figure 5: Oscilloscope output of circuit from MultiSIM

Figure 6: Frequency response of an amplifier


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From Figure 7 it can be seen that the lower cut off frequency is approximately 20Hz and the

upper cut of frequency is approximately 20kHz, which was in line with the theoretical calculated

result. In order to attain the characteristic curve seen in Figure 6, more results would need to be

taken. In order to do this however, it would be possible to use MultiSIM to carry out the frequency

response test using the AC Analyses tool:

Figure 8: MultiSIM Frequency Response for CE Amp

0

2

4

6

8

10

12

14

16

1 10 100 1000 10000 100000

Output(Volts)

Frequency (Hz)

Graph Showing Output Voltage of the CE Amp vs input

Frequency

Output

Figure 7: Graph showing CE Output Voltage vs Input Frequency


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The above figure gives a much larger lower cut of frequency in the range of around 100Hz as

opposed to the 20Hz lower cut off frequency obtained in the lab. However, this discrepancy could be

corrected by taking more measurements although if we examine the CE amp as a component of a

two stage audio amplifier, a lower cut off frequency of 100Hz would lead to an amp that has a very

poor bass frequency replication.

3.The Two Stage Amplifier

3.1.

The Two Stage Amplifier Introduction

Although it was clear from the experiments performed with the CE Amplifier that gain was being

produced from the circuit, the 8dB of gain that were being attained were by no means large enough

to effectively drive an audio source (8dB is just within the threshold of human hearing, about as loud

as a pin drop). In order to rectify this problem, a second stage was added to the CE amplifier in order

to boost the gain so that it could be used to drive audio equipment.

The second stage of the two stage amp is very similar to the first in that the amp is biased

using a voltage divider and at the input a capacitor is used to remove the DC offset. The output

signal from the amplifier is seen to be in phase with the input source, this is due to the second

inversion of the signal which occurs at the second stage of the amp.

It can be seen in figure 9 that the signal is phase shifted 90 degrees at the PNP stage (1st

stage) and

again at the NPN stage (second stage) to bring the input into phase with the output.

One of the key differences in the amplifier lies in the first stage of amplification, unlike the

second stage or the CE amplifier discussed earlier in this report, the first stage contains a PNP

transistor instead of an NPN transistor in order to source current to the second stage. Replacing the

PNP transistor with an NPN transistor resulted in an attenuation of the signal, rather than

amplification, so in order for a second NPN transistor to be used in place of the PNP transistor and

still obtain amplification the circuit would need to be redesigned.

Figure 9:Diagram showing input signal being inverted twice


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3.2.

The 2 Stage Amplifier Lab

The lab for the 2 Stage amplifier consisted of simulating the circuit shown in Figure 10 and

calculating the voltage gains across the two stages and then calculating the overall gain of the

circuit based on the voltage measurements. Following this, the circuit was built in on a

breadboard and the circuit parameters were measured and compared to the simulated values.

Figure 10: 2 Stage Amplifier

Results from MultiSIM:

STAGE THEORETICAL GAIN ACTUAL GAIN

STAGE 1 22 21

STAGE 2 30 29.64

STAGE 2 (WITH LOAD) 17.77STAGE 1 AND 2 115.38


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Results from the Lab

AC Analysis

AC

Parameter

Computed

Value

Re (Q1) 66.988

Re (Q2) 14.021

Rout(Q1) 21.997k

Rin(Q2) 14.995k

Av(NL)(Q1) 20.606

Av(NL)(Q2) 28.963

Discussion of Results

The obtained values for two-stage gain show that adding a second stage to the CE amplifier tackles

the problem of insufficient gain produced by the single-stage amp. The two-stage gain of the amp

was recorded to be 214.922 which, when converted into decibels provides a 23dB gain. This gain is

sufficient to amplify a microphone or MP3 into a set of powered speakers. The amplifier successfully

managed to amplify the input so it could be heard through the speakers but there were prevalent

problems in terms of the quality of the audio being reproduced. This may have been due to RF

signals causing noise as none of the components were shielded. In addition to this, the replication of

the audio lacked low frequencies and the sound was described as tinny. Following this discovery,

the frequency response of the system was obtained in MultiSIM and it was found that the lower cutoff frequency was about 620Hz, this can be seen in figure 11. This frequency is approximately equal

Resistor Listed

Value

Measured

Value

R12 100k 99.51k

R11 2k 2.0459k

R1 330k 299.7k

R2 330k 300.07k

R4 33k 36.08k

R5 1k 1.0005k

R3 22k 21.997k

R6 47k 46.99k

R7 22k 22.033k

R10 4.7k 4.684k

R9 220 217.55

R8 6.8k 6.707

R13 10k 10.999

DC

Parameter

Computed

Value

Measured

Value

Q1

VB 0.462V 0.757V

VE 1.162V 1.389V

IE 0.3732mA

VC -6.859V -6.96V

VCE 8.0206V 8.37V

Q2

VB -5.5545V -5.5V

VE -6.2615V -6.15V

IE 1.783mA

VC 3.0971V 3.055V

VCE -9.3586V -9.2V

AC

Parameter

Computed

Value

Measured

Value

Av 214.922 150

Rin(Q1) 69.4007k 88.8k

Rout(Q2) 6.707k 6.5k

Vin(Q1) 10mV

Vout(Q2) 206.06mV 150mV


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to that of an E5 on a piano. This large lower cut off frequency could explain why the sound produced

from the amp was so poor.

Figure 11: Frequency Response of the 2 stage audio preamp

In addition to the large lower cut off frequency, it is evident from the results obtained from the

multimeter that the wrong resistors were used in the voltage divider at the base of the first stage of

the amp. While in the schematic these resistors are listed as being , the test with themultimeter would suggest that the group inadvertently used resistors. Furthermore therewas some ambiguity in the lab as to whether electrolytic capacitors should be used for C1 and C3

(there were two different versions of the schematic in the lab).

4.The Instrumentation Amplifier & PCB Building

4.1.

Introduction to the Instrumentation Amplifier

The instrumentation Amplifier, or Op Amp is a device that amplifies the difference in voltage

between the positive and negative input terminals. In practice, the instrumentation amplifier is

found at the heart of many ECG and Neural Signal Processing devices, this is due to the devices

ability to amplify small signals that may be riding on larger common mode voltages, for instance, in

an ECG, electrical signals from the body might cause unwanted results so by using an

instrumentation amplifier, these unwanted signals can be truncated.

In the lab, an Op Amp circuit with 3 741C IC Op Amps was created and simulated in MultiSIM

before being built and tested in the lab. It was found over the course of the testing that the Op Amp

based circuit provided a much higher gain than the discrete 2 stage audio preamp built in the weeks

prior to this. However, debugging problems that arose when building the Op Amp onto a breadboard was a much greater task than debugging the discrete component circuits.

Following the building of the Op Amp circuit onto breadboard, the group used EAGLE software

to design a PCB Board layout for a single 8 pin IC amplifier. The design considerations of such a

device are hinged upon minimising the space used by the circuit, as most PCB circuit manufacturers

charge per square cm of material. In addition to this, the ease of assembling circuits on a PCB board

were discovered in the lab, where the circuit was soldered onto a pre-etched board.


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4.2.

Simulation & Building

Simulation

Figure 13 shows the schematic for the

Op Amp based amplifier constructed in

week 6 of the module (Note: green wires

connect the op amps to VCC and black wires

connect the op amp to VEE, these were not

displayed on the original schematic). In

order to successfully simulate the model in

MultiSIM, the group had to deduce which of

the 8 pins on the Op Amp represented +V

andV, Figure 12 shows the pin layout of the 741C Op

Amp IC.

Figure 13: Instrumentation Amplifier Circuit

Before the Simulation took place, the theoretical Closed Loop Gain and Voltage Output were

obtained:

where When the closed loop gain was obtained in MultiSIM using the Oscilloscope (shown in Figure 13), the

gain was found to be 45.01632, slightly higher than the theoretical value.

Following this, the circuit was rebuilt and made so that the input voltage was identical to both U1

and U3. This is the differential-mode input signal. The rebuilt circuit is shown in figure 14:

Figure 12: 741C Op Amp Pin layout


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Figure 14: Second Op Amp Circuit

The output from this circuit was much lower in amplitude than the previous rendition as U2 is acting

as a differential amplifier for 2 nearly identical signals. It was found that by adjusting the

potentiometer labelled in figure 14 to 65%, or the minimum output value was obtained.This output value of 29mV peak, produced a common mode gain of: .The Common-Mode Rejection ratio, a measurement of the rejection of the input signals could then

be calculated using the following formula:

( )

Building the CircuitResults

Resistor Listed

Value

Measured

Value

R1 10k 9.74k

R2 10k 9.81k

RG 470 462

R3 10k 9.91k

R4 10k 9.80k

R5 10k 9.90k

R6 8.2k 8.16k

R8 100k 99.3k

R9 100k 98.7k

Parameter Computed

Value

Measured

Value

Differential input Voltage

Vin(d)

330mV 151mV

Differential Gain, Av(d) 43.316 43.046

Differential Output Voltage,

Vout(d)

14.294 6.5V

Common-mode input Voltage,

Vin(cm)

10V 9.98V

Common-mode input Voltage

Av(cm)

0.09

Common-mode input Voltage,

Vout(cm)

73mV

Common-mode rejection ratio

(CMRR)

53.594


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Discussion of Results

It is clear from the results above that this circuit is acting as a differential amplifier. In figure

13, where the two input voltages differ from the source with a value that is equal to the source the

circuit acts as a voltage amplifier, similar to the 2 stage and CE amplifier. However when the input

voltages are equal, the signal produced is very small in amplitude and can be assumed to be the

noise produced from the circuit as there is no difference between the two signals.

There are certain advantages and disadvantages to using an IC in place of discrete

components in an amplifier circuit. An Op Amp is constructed from many transistors and delivers

consistent performance whereas transistor amplifiers can be temperature sensitive. In addition to

this an Op Amp does not need biasing to be turned on (instead it uses the differential signal), this is

evident from the lack of voltage divider biasing seen in figure 13 and 14. However, an Op Amp

always registers as being On and thus always draws power but a transistor will only draw power in

the linear and saturation regions (as it is turned offin the cut-off region). This is a key design

consideration for the group as the Piezo Pickup Systems that the group will design will be run from

batteries rather than a DC outlet.

5.Piezo Pickup Systems for Acoustic Archtop Guitars

5.1.

Introduction

The group decided that a good project to follow would be the design and development of Piezo

pickup systems. These simple preamp and buffer circuits are extremely useful and sought after in

the world of live acoustic instrument amplification as they are a cheap solution to the problem of

amplifying musical instruments which are not traditionally sold with pickup systems.

What is a Piezo Transducer?

A Piezo transducer (or Piezo pickup) is a device that transforms

mechanical stress into an electric charge (the storage of charge from

mechanical stress is known as the piezoelectric effect). In this

module, the piezo element was being used as a contact microphone;

a microphone that only picks up structure-borne sound. The contact

microphone properties of Piezo pickups are not shared with ribbon,

dynamic and condenser microphones, all of which use vibrations in

the air to reproduce sound, nor is it similar to the magnetic pickup

(used in most electric guitars and basses) as it does not create a

magnetic field.

The amplification method of acoustic instruments ishotly debated by many sound engineers and live

stage technicians as each of the methods of

amplification mentioned above have distinct

advantages and disadvantages. Condenser mics for

instance, provide the best replication of sound but

are expensive and restrict the movement of the

musician, in addition to this they can be prone to

feedback on some acoustic instruments. Magnetic

pickups are not prone to feedback and are mounted

on the instrument so mobility is not a problem,however they are limited in that a lot of the wood-

Figure 15: Piezo Element

Figure 16: Diagram showing the operation of a

magnetic pickup


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characteristic tone is cut and the resulting signal lacks the acoustic character produced by the

instrument. Piezo pickups are extremely cheap (in bulk they can be purchased for approximately 15

cents each) and they replicate some degree of the acoustic tone of the instrument, but they too are

prone to feedback and produce a midrange peak which is known as the piezo quack, this is caused

by the impedance mismatch between the piezo (high output impedance) and the amp or PA system

input (also high impedance, but relatively low compared to the piezo).

Piezo PreampsA Simple Solution

As was mentioned above, Piezo transducers

although providing reasonable sound

replication at an extremely low cost, suffer

from some problems that can ruin the user

experience. In the sound engineering industry

many companies have chosen to leave piezo

pickups behind entirely and focus on small

condenser mics which sit inside the body of

the instrument, however these products are

extremely costly for the end user. The other

solution to this is the creation of Piezo

Preamps and buffers, which minimise the flaws

inherent in the piezo pickup and provide a well-rounded and useable sound. The most notable of

these preamps is the Tillman preamp (Figure 16ii), a circuit originally designed as a booster for the

magnetic pickup systems found in the electric guitar. The

Tillman preamp is based around a J201 discrete Field Effect

Transistor (FET) which offers a very high input impedance

(3MegaOhms) with very low noise levels (a characteristic of the

JFET transistor). The capacitor C1 is used for output coupling and

the capacitor C2 is used to obtain maximum transistor gain. The

Tillman Preamp delivers about 3dB of gain.

A similar concept to the Tillman Preamp is the Piezo

Buffer, a circuit which does not amplify the input signal but instead acts as an impedance matcher

between the piezo and the amplifier or PA system. This resolves the problem of midrange quack

and also makes lower frequencies louder increasing the headroom of the signal. The design in

Figure 17 is known as the Mint Tin

Piezo Buffer as it is usually built into

a tin of Altoids mints which acts as aFaraday cage. The circuit has a very

high input impedance and a gain

switch which allows the user to

decrease the gain by approximately

3dB. This Gain Switch is useful in

instruments like Double Bass, where

the piezo signal is prone to clipping

at the input. Unlike the Tillman amp,

the output of the Preamp Buffer is in

phase with the input.

Figure 17: The Tillman Preamp

Figure 18:Piezo Buffer Circuit


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The final type of preamp that was researched was a two

stage piezo buffer and preamp created by Mongrel Dog Audio. This

two stage circuit makes use of the two Op Amps within the OPA

2134 IC (Figure 18). The aim of this device is to buffer the signal in

the first stage and amplify the signal in the second. The value of the

gain obtained is dependent on the value of R9 which is acting as a

high pass filter. The original circuit designer for this schematic built

the design on PCB and housed it in an aluminium box, again using

the case as a Faraday cage to block RF noise.

5.2.

Selecting a Design

After the different kinds of piezo preamp and buffer circuits were researched, it was the task of the

group to propose schematics for circuits that would be modified, simulated and tested. These

circuits were then built in MultiSIM before the group chose two designs to test and compare. The

initial selections of circuits were:

The Tillman Preamp

Discussed above, the Tillman preamp is the

quintessential single transistor preamp for guitars; it

is a single discrete JFET preamp, delivering a gain of

approximately 3dB to the input signal.

Pros: Small and easy to construct and simulate using

MultiSIMveroboard layouts are readily available

online.

Cons: The Tillman preamp is designed as a signal booster for electric guitars, not a piezo pickup

buffer so the piezo quack will not have been addressed in the construction of this circuit.

Figure 19:Pin label for OPA2134 IC

Figure 20: Schematic for Mongrel Dog Audio Piezo Preamp with 'Tone' control


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Martin NawrathUniversity Of Cologne Lab3 Piezo Disk Preamp

Figure 21: Martin Nawrath Piezo Disk Preamp Schematic

The Martin Nawrath Piezo Disk Preamp is similar to the Mongrel Dog preamp in that there is a two

stage buffer-amplification action happening on one 8 pin IC consisting of 2 Op Amps. This circuit is

used in an electronics lab in the University of Cologne to show how Piezo elements can be used as

contact mics, or as force sensors/accelerometers

Pros: Gain Pot present. 2 stage buffering and amplification, the builder is knowledgeable in

electronics.

Cons: The system is not tailored specifically for Piezo Transducers- the missing high input impedance

resistor, and also building this circuit in MultiSIM was problematic and no output signal could be

obtained.


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L.R. Baggs Godin Solidac Piezo Preamp with blend control for magnetic systems

iii

LR Baggs is a name well known in the music industry for

building high quality acoustic guitar pickups and

microphones, this schematic is a preamp that is

currently used in the Godin Solidac range of electric

guitars. These guitars feature both a traditional

magnetic pickup system and a bridge which contains 6

under saddle piezo elements (6 individual piezo pickups,

one underneath each string of the guitar).

Pros:Professional level sound, crystal clear acoustic

reproduction.

Cons: Designed for a blend of magnetic and piezo pickups. The group are using solely piezo disks

whereas LR Baggs use piezo saddles (shown in Figure 21), these saddles may have different resonant

frequencies and will not respond in a manner that is analogous with the piezo disk. Schematic is not

from an official source.

Figure 22: LR Baggs bridge, circles areas show piezo

elements


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Scott Thelmke Mint Box Piezo Buffer

The Scott Thelmke was the first buffer circuit to be examined by the group, mentioned above, it has

a very high input impedance and a Gain switch that allows the user to cut 3dB of gain off the input

signal.

Pros: The circuit is very compact and easy to construct. The circuit was designed with the groups

exact requirement in mind, Gain switch is very useful for the end user.

Cons: Scott Thelmke is an unknown builder and the design method for the circuit was not disclosed

on the website so his level of experience comes under questionwill the circuit perform as it

should?

Mongrel Dog Audio Piezo Pickup Preamp for musical instruments

This two stage Op Amp based Piezo Preamp wasdiscussed in section 5.1. The builder of this circuit is

known on the web as a professional Boutique Valve

amp designer.

Pros: Experienced Builder, circuit matches the purpose

of the group. Easy to construct schematic.

Cons: The amp needs a larger power supply (18V DC

2 9V batteries) than any of the other schematics on

this list. If this amp were to be used in a gig situation,

the power consumption would be a serious consideration to the user, 9 volt batteries are expensive!In addition to this, there is a space consideration to be taken into account as well, ideally, the

preamp should be able to be mounted on a belt buckle.

Selection Process

The group built all of the above schematics in MultiSIM and tested them with a 50mV 600Hz

AC input signal (The peak output obtained from the piezo when mounted onto the Archtop guitar

was approximately 50mV and 600Hz represents the frequency of the D string on the instrument).

When deciding which circuits would be built on breadboard the group took numerous factors into

account, ideally the group wanted a discrete based circuit and an Op Amp based circuit in order to

compare the results from both. In addition to this, as the group only had 4 weeks to carry out

building and testing, the circuits had to be quick to construct and easy to debug. With these limits in


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place, the Scott Thelmke Mint Box Buffer and the Mongrel Dog Audio Preamp were selected as the

two circuits that would be focused on.

5.3.

Design and Testing

Scott Thelmke Mint Box Buffer

The design of the Scott Thelmke buffer can be broken up into two stages, the first stageaddresses the problem of input impedance; two 10MOhm resistors and a 1pF capacitor. A capacitor is also present at the input in order to act as a remover of DC offset (an input blocker).

Figure 23: First stage of the Piezo Buffer

The second stage of the buffer is the JFET transistor is acting as a common drain amplifier,

buffering the voltage and transforming impedances. Although there is no voltage gain from the

common drain amplifier, there is current gain. The transformation of impedances is what will restore

the bass frequencies of the piezo pickup and reduce the effects of the piezo quack.

Figure 24: Second stage of the Piezo Buffer


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Testing the amp in MultiSIM showed that the input signal was indeed being buffered (Figure 24)

green is the output, blue is the input.

However it is evident that there is some clipping occurring on the lower peaks of the output signal.With this in mind the group set out to improve the design. The main issue with the Mint Box Buffer

on inspection of the schematic is that there is no transistor Biasing occurring, a solution to this

problem is to add a second resistor running to the power rail above the 10MOhm resistor to act as a

voltage divider. However, with this voltage divider it became necessary to change the value of the

capacitor to a capacitor:Another modification that was made to the circuit is altering the

value of the 220K resistor to a 1k LOG pot. The LOG pot will act

as a volume control for the circuit which will greatly benefit the

end user as the dynamic of the music may call for different

volume levels. Logarithmic pots are used because the human

ear is somewhat logarithmic by nature.

After adding these modifications in MultiSIM the clipping on the

lower peaks had gone, suggesting that an improperly biased

transistor was the reason for the clipping in the first place.

Figure 25: Output from Log Pot

Figure 26: Modified Mint Box Buffer

Figure 27: Output shows no clipping


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The testing of the Piezo Buffer was carried out in

the lab using a signal generator and an oscilloscope.

Following this the circuit was built onto Vero board and a

battery was added as a power source, the buffer was then

placed into a plastic box and tested at 3 different concerts

using a Piezo equipped Archtop Guitar. The sound

produced from the Piezo buffer is an improvement over a

piezo pickup by itself and reduces the piezo quack while

increasing the bass frequencies. However some listeners at

the gig reported that the sound lacked high end frequencies

and that the resultant tone was similar to that of a

telephone. Although this is not necessarily a bad thing (on

records from the 20s guitars often sounded like thisiv) the

system could still be improved. Here are two suggestions

that were put forward:

Reposition the piezo:The position of the Piezo Pickup on

the soundboard of the guitar has a huge tonal effect on the

output, on a standard Dreadnought acoustic instrument the

piezo should rest right underneath the bridge of the guitar,

but through numerous tests it was found that on Archtops

the best signal was obtained when the piezo was placed

near the lower F-hole (Shown in Figure 27).

Use Capacitors Intended for Audio/Guitar: A less

significant improvement would be to replace the capacitors

that carry the audio signal with capacitors that are designedfor use in audio applications. An example of this would be

the Sprague Orange Drop Capacitors, or even more ideally,

Paper in Oil capacitors, both of which are hugely popular

with guitarists. The original circuit builder recommended

Mustard Capacitors as they are very popular with guitar effects pedal builders but these are

expensive and hard to come by so Sprague is probably the best brand to follow.

Figure 29: Capacitors for audio (from left to right) Sprague Orange Drops, Mustard, Paper In Oil 'Bumblebee' caps

Figure 28: Archtop Guitar with Bridge and

Position of Piezo Marked


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Mongrel Dog Audio Piezo Preamp

As stated above, the Mongrel Dog Audio Preamp has two stages; Buffer and Amplifier. The first stage

acts as a buffer (seen in Figure 29). By now it should be clear that one of the characteristic features

of any Piezo Buffer circuit is the high input impedance. This is seen here in R8, with a 10M input

impedance. If the gain of the amp were to be changed this value could be edited to a 4.7M resistor

(similar to that of the Mint Box buffer) but due to the large gain that is being added to the signal

from the second stage, additional gain at the input will not be necessary.

Figure 30: Buffer Stage of Mongrel Dog Audio Preamp

Figure 31: Input and output signals are both plotted here but due to buffering the two individual waves are exactly the

same phase and amplitude.

The second stage of the amp delivers the gain to the circuit. This gain is controlled by R9, a 25K

potentiometer. If the inputs of the TL072P Op Amp are examined it can be seen that the differential

input varies depending on the position of R9, if R9 is set 100% then the gain will be maximised as the

inputs will be the input from the buffer and ground (so the difference is ground). Whereas if R9 is set

to 0% then the input at pin 6 of the Op Amp is equal to the output which is equal to the buffer signal,so the signal will lose significant gain. If the area around R9 is examined it can be shown that as well

as a gain control, R9, R11 and C4 are acting as a variable High Pass Filter. This means that as R9 is

increased, the frequencies below the lower cut off frequency are attenuated.


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Figure 32: R9 at 100% Figure 33: R9 at 0%

As seen in Figure 31 and 32, R9 has a large impact on the gain of the circuit, and when the circuit was

built in the lab on a breadboard it was found that when R9 was turned up to 100% the piezo pickup

became microphonic, this means that the piezo stopped performing like a contact mic and began to

pick up vibrations in the air. This also caused the piezo to produce a loud high pitched feedbackshriek. It was clear from this problem that the variable resistor at R9 had to be replaced. The gain

being outputted from the circuit when R9 was equal to 100% was approximately 13dB, which is very

large considering that the preamp will be run into another amplifier before the signal is sent to the

speakers, so the decision was made to remove R9 completely and instead just use the 3dB gain that

was produced when R9 was set to 0%.

Although there was not sufficient time to build this system onto Veroboard and test it in a live Gig

situation, the group used Lochmaster to create a Veroboard layout, this layout is shown in figure 33.

Figure 34: Veroboard layout for Mongrel Dog Audio Preamp


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Testing with the ArchtopA Brief Comparison

Both amps were tested using a Hofner Congress Archtop Guitar, which was then plugged

into the preamp/buffer and then plugged into a 1000W PA system through the line in port. The

sound from the Mongrel Dog Audio system was deemed to be clearer and more acoustic guitar like

than the Buffer circuit, which still retained characteristics of the vintage record guitar tone. However

the Mongrel Dog Audio system produced clear improvements to the bass frequencies while

maintaining the high frequencies and the piezo Quack was not occurring at all. Although the sound

quality from the Mongrel Dog Audio system was superior to that of the Buffer circuit, the buffer

circuit has a much longer battery life and only required a single 9v battery as opposed to 2 9v

batteries. In addition to this, the music being played on the Archtop guitar is contemporary to the

1920s-1930s so the vintage tone produced from the buffer is not unfavourable. So even though the

Mongrel Dog Audio has the advantage of producing a purer tone, the Buffer circuit fits the purpose

of the instrument better.

6.ConclusionsOver the course of the module much has been learnt about amplifier theory and both discrete

and integrated circuits. The theory and function of Op Amps and Discrete Circuit amplifiers were

discovered through simulation, design and hardware testing both in the lab and MultiSIM. The

course provided a practical insight to electronics that could not have been attained in the lecture

theatre and as a result much was learned by all. The first 7 weeks of the course provided the group

with both the theoretical knowledge and practical build skills necessary to read a circuit schematics

and improve the designs of some circuits found online. The building of the CE Amplifier and the two

stage amplifier taught the group about biasing transistors and how to calculate resistance values for

a discrete transistor amplifier circuit. The experiments performed with the Op Amp circuit taught the

group about differential amplifiers and how they can be used in a variety of situations from audio

and hi-fi to Biomedical equipment. The work carried out in EAGLE gave the group the knowledgerequired to design PCB boards upon which circuits could be easily made. Throughout the course of

the final 4 weeks of the project a lot was learned about piezo pickup systems and how electronics

can provide a workable solution to the problems suffered by musicians who use piezo pickups as a

form of amplification. Although many systems were simulated in MultiSIM, only two systems were

built in the lab. It was found that the Op Amp System, despite delivering a sound that encompassed

a larger frequency range did not provide a functional tone for the guitar being used. However the

buffer provided this tone at the expense of a lower volume being produced from the output.

Improvements were made to both the schematics found online, in the buffer circuit, the transistor

was properly biased and in the Op amp circuit, the gain pot was removed to stop the piezo from

turning microphonic.

The main insight into Electronic engineering that was gained from this module was the ability to

read circuit schematics and build these schematics on a bread board. This was not a skill I was very

good at before taking the module and I feel it will be hugely beneficial in the future. In addition to

this my circuit analysis skills have gotten better as well.

My favourite part of the project was investigating the piezo preamp circuits, building them in

MultiSIM and seeing the circuits work in the lab. The practical aspects of the project granted a lot of

insight into how the circuits work and give some context to the lectures in 3C2.

The most challenging part of the project was understanding the theory in the first few weeks of

the module. I often felt that we were not getting the correct results from our calculations so more

time explaining the theory would have been very beneficial.


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If we had more time for the final project

then further tests could have been

carried out on the Op Amp Based circuit,

it would have been great to build the Op

Amp circuit onto Veroboard (as was the

original plan) and test it in a live situation

to see how the sound of the Op Amp

circuit blended in a live band situation. In

addition to this I wanted to add a

Baxandall tone control and a gain

potentiometer to the Buffer circuit in

order to maximise its versatility. Either

this or a 3 band EQ would really make the

buffer a very useful tool for any guitarist

with a piezo loaded guitar.

The project as a whole covered a large amount of material, I dont think that given the

timeframe of the project any more material could be added without the risk of sacrificing some

other areas of the project. However if this was a 10 credit module I would have really liked to see

even one class on vacuum tubes as they are not covered in any modules.

iSource of image (CE Amp schematic):http://www.electronics-tutorials.ws/amplifier/amp_2.html

iihttp://www.till.com/articles/GuitarPreamp/

iiihttp://www.ssguitar.com/index.php?topic=2306.0

Figure 35: Baxandall Tone Control
http://www.electronics-tutorials.ws/amplifier/amp_2.htmlhttp://www.electronics-tutorials.ws/amplifier/amp_2.htmlhttp://www.till.com/articles/GuitarPreamp/http://www.till.com/articles/GuitarPreamp/http://www.till.com/articles/GuitarPreamp/http://www.till.com/articles/GuitarPreamp/http://www.ssguitar.com/index.php?topic=2306.0http://www.ssguitar.com/index.php?topic=2306.0http://www.ssguitar.com/index.php?topic=2306.0http://www.ssguitar.com/index.php?topic=2306.0http://www.till.com/articles/GuitarPreamp/http://www.electronics-tutorials.ws/amplifier/amp_2.html

piezo preamp and buffer report

Documents