Piezo Preamp and Buffer Report [PDF]

Zitiervorschau

2013

3C6A – Analogue Design Project GROUP C2 – PIEZO PICKUP SYSTEMS STEPHEN BRENNAN

Contents 1.

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 Nawrath – University 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 Archtop – A Brief Comparison.................................................................................................................................... 26 6.

Conclusions........................................................................................................................................................................................... 26

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1. Introduction The 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 or Op 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 added small 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 design the 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 Guitars’ as 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 be discussed 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 (“system 2”) was designed around 2 TL062 Op Amps and consisted of two stages; a buffer and a preamp. This 2|Page

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 Figure 1: An acoustic Archtop guitar made by Luthier Bill Collings

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 ( ) and Voltage 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

Figure 2: A schematic of the Common Emitter Amplifier

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 in order to do this, a value must be given for the load resistance. In the lab, a resistor was used and a voltage drop of 1.4V was desired over the emitter resistor. From this, the max collector current can be calculated: (

)

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Taking a gain value of 400 (as was the case with the transistor from the lab; a BC107BP), the base current is found to be: Can then be calculated using the base emitter voltage and the voltage drop across

In this case,

is fixed at 0.7 volts for a silicon NPN transistor.

Working out a value for

is as follows: (

)

Finally, a value for the emitter resistor can be calculated using Ohms Law. Taking the emitter current to be the sum of the base and collector current, the following is obtained:

So to summarise, using these calculations it has been found that with a transistor beta value of 400, the following resistances are suitable:

If the above amplifier were to be constructed in a lab, the values of resistors used would be: R1 R2 RL RE

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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 decibels

was 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 of

and

represent the cut off.

It was found after calculating this, that governed the lower cut off of the amplifier with a frequency 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 the frequency 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 the breadboard the group did not have time to take all of the measurements from above – however 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. Figure 5: Oscilloscope output of circuit from MultiSIM

The final part of the lab was testing the output response of the amp given a 200mV input at varying frequencies. 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 represent Figure 6: Frequency response of an amplifier the lower and upper cut off frequencies. 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).

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Graph Showing Output Voltage of the CE Amp vs input Frequency 16 14

Output (Volts)

12 10 8 Output

6 4 2 0 1

10

100

1000

10000

100000

Frequency (Hz) Figure 7: Graph showing CE Output Voltage vs Input Frequency

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

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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.

Figure 9:Diagram showing input signal being inverted twice

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.

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

THEORETICAL GAIN 22 30

ACTUAL GAIN 21 29.64 17.77 115.38

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Results from the Lab Resistor

Listed Measured Value 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 Computed Measured Parameter Value 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 Analysis AC Computed Parameter 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

AC Computed Measured Parameter Value 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

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 cut off frequency was about 620Hz, this can be seen in figure 11. This frequency is approximately equal 11 | P a g e

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 the multimeter would suggest that the group inadvertently used resistors. Furthermore there was 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 bread board 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 and –V, Figure 12 shows the pin layout of the 741C Op Figure 12: 741C Op Amp Pin layout 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:

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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 Circuit – Results Resistor R1 R2 RG R3 R4 R5 R6 R8 R9

Listed Value 10k 10k 470 10k 10k 10k 8.2k 100k 100k

Measured Value 9.74k 9.81k 462 9.91k 9.80k 9.90k 8.16k 99.3k 98.7k

Parameter Differential input Voltage Vin(d) Differential Gain, Av(d) Differential Output Voltage, Vout(d) Common-mode input Voltage, Vin(cm) Common-mode input Voltage Av(cm) Common-mode input Voltage, Vout(cm) Common-mode rejection ratio (CMRR)

Computed Value 330mV

Measured Value 151mV

43.316

43.046

14.294

6.5V

10V

9.98V 0.09 73mV 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 ‘off’ in 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.

Figure 16: Diagram showing the operation of a magnetic pickup

Figure 15: Piezo Element

The amplification method of acoustic instruments is hotly 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 wood15 | P a g e

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 Preamps – A 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. Figure 17: The Tillman Preamp

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 a Faraday 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 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.

Figure 19:Pin label for OPA2134 IC

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

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 MultiSIM – veroboard 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.

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Martin Nawrath – University 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.

Figure 22: LR Baggs bridge, circles areas show piezo elements

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.

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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 group’s 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 question – will the circuit perform as it should?

Mongrel Dog Audio Piezo Pickup Preamp for musical instruments This two stage Op Amp based Piezo Preamp was discussed 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 stage addresses 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 Figure 25: Output from Log Pot lower peaks had gone, suggesting that an improperly biased transistor was the reason for the clipping in the first place.

Figure 27: Output shows no clipping

Figure 26: Modified Mint Box Buffer

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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 designed for use in audio applications. An example of this would be the Sprague Orange Drop Capacitors, or even more ideally, Figure 28: Archtop Guitar with Bridge and Position of Piezo Marked 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

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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 feedback shriek. 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 Archtop – A 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. Conclusions Over 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 knowledge required 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. 26 | P a g e

Figure 35: Baxandall Tone Control

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 don’t 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.

i

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

ii

http://www.till.com/articles/GuitarPreamp/ http://www.ssguitar.com/index.php?topic=2306.0

iii

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