Evaluation / Conclusion

Conclusion

In terms of its primary aim the project has been a complete success in  that a wide range of astonishing and idiosyncratic distortion timbres can be achieved from a single analogue circuit. Considering that most manufacturers of audio electronics employ a handful of experienced and properly qualified engineers for each specific role such as PCB design and noise control, it is unsurprising that this device has a few downfalls. The fact that it requires a manual for use contradicts the terms of reference but the writer is certain that, given the chance to revise the design as suggested in the recommendations below, an enhanced, euphonic and truly marketable product would be achieved as a result

The fundamental lesson learned from this project is that, no matter how innovative and complex an analogue circuit might be, the transfer of electrical signals using hand-soldered, non-ideal components will always be outdone by a cheap and reliable processor where fault finding normally involves altering a few lines of code rather than spending many frustrating hours with a soldering iron and an oscilloscope probe. In this case the unique tone provided one definite advantage, as a concluding thought one might consider whether this harmonic distortion could be duplicated using DSP through precise analysis of the outcome in terms of frequency content, miniscule inherent delays and global frequency response in order to maintain enthusiasm for this archaic passion for years to come.

 Recommendations

The malleable potential of this circuit is mostly thanks to a complex and electronically immersed user interface, more effort would have to go into designing an easy to use product in order for it to have an advantage in this area over similar DSP devices.

The supply of power to the op amps should be re-designed to utilise biased unipolar battery power in order to apply a +4.5V DC offset onto the signal as suggested by  Salminen (2000) so that no negative supply is required as shown below. This would incur a reduced maximum output before clipping but would avoid the grounding issues found during this investigation.

A gain of 101 via a 100kΩ pot would be useful for extending into levels of harmonics and noise present in genres such as heavy metal. It is possible that the excessive output impedance value hinders the clipping by almost depleting the transfer of power to the load, this issue could be solved by using a non-inverting op amp with variable feedback to control the output level thus creating a reduced output impedance and Johnson noise. 

Even in a laboratory environment the noise levels present during operation are unacceptable for audio equipment, a smaller PCB layout and shielded enclosure to reduce exposure to EMI and more concern to cross talk over adjacent PCB tracks would reduce said noise to an acceptable level. Furthermore, following the general practice associated with using unbalanced lines more closely would virtually eradicate noise but only with increase in total costs.

Practical Finale

In order to find out why the circuit was having issues as of yesterday I connected the unit up to a dual rail power supply, used my guitar as the input and monitored the audio as described earlier in my log book. Upon doing so I found that everything was working perfectly, this information concludes that the unit doesn’t operate correctly when power is supplied by the Boss PSU230. Retrospectively I realise that this is due to the fact that my circuit requires a ground which floats half way between ±9V rather than the potential of any signals present in the building’s ground circuit, this is not particularly useful with respect to the nature of guitar effects pedals as it does not meet the criteria of my project brief as such. It does, however, mean that I have plenty to reflect on and am able to provide suggested improvements when writing my project report.

Another task completed today was creating some reference as to whether the finished product would be useful in a piece of music, Eoin Hyde arrived at the electronics lab with a laptop and soundcard into which the signal could be routed and saved. A selection of guitar solo’s were recorded over backing tracks, different clipping transients were used to alter the sound such that the final cuts could be included in my project report for reference. I also collected a Marantz flash recorder and an SM58 to record the effect of each diode, these may also be included for reference in the project report if necessary.

To find the input and output impedances I measured the voltage and current of each and used ohms law to find the resistance, a signal generator was used to provide a stable input. The calculations used are shown below:

Input: 0.243V/0.000004 ≈ 714kΩ

Output: 6/0.011 ≈ 545kΩ

The input impedance is sufficiently high as it was taken into consideration during the design stage. Unfortunately the output impedance is also high due to the many cables and transfer between mediums the signal has to endure, to worsen this matter it also varies with the volume control position due to the inherent configuration. In retrospect I realise that I should have used a non-inverting op amp to control the output level, this would have provided a high output impedance and introduced far less johnson noise than being forced to flow through a 100kΩ resistor as is currently the case.

The final conclusion today was that this distortion unit is almost unusable due to the excessive amount of noise present when connected to an output device, when measured by the Neutrik test set with gain and volume level at their maximum the THD was 23.5% and this only increased by 16.5% when clipping was added. Reasons for why this is so equate to a combination of the following:

• Aforementioned high output impedance
• Exposure of signal to large amounts of electronic noise
• Cross talk between signal and ground due to poor PCB layout methods

Project Build – Day 6

Today was dedicated to finishing off the distortion unit after a few days making good progress on my project report, the first task was to break the ground loop in a few places and join the separate sections at a single point. In order to do this I used a pen knife to break the track and then joined each ground section to a strip of veroboard and insulated it to avoid short circuits(shown below), from here the signal was routed to each 1/4″ jack shield in order to enable a current flow.

Unfortunately this was an unsuccessful attempt as the oscillation still occured, the most likely cause in this case is a lack of concern to cross-talk between tracks and cables which I should have considered upon designing the PCB. Unfortunately time constraints for the project hand in mean that this artifact will have to stay as it is for the time being.

Other circuit alterations included replacing the 680pF capacitor for a 1nF to raise the cut off frequency for one of the High Pass filters and installing yet another toggle switch to control the cut off frequency of the Low Pass filter. The existing LP filter option was removed from the DIP switch unit and altered so that a toggle switch was able to determine the cut off frequency by choosing between two values of capacitor. An image of this is shown below where the purple and orange cables are inputs from the toggle switch and the red cable routes the output to ground.

The resultant circuit frequency responses measured with the Neutrik are shown below, the first represents the 160Hz HP filter created by the 1nF capacitor in a potential divider with R15 and a 7.2kHz LP filter created by the 22nF capacitor and R6. The equations used to calculate these cut off frequencies are:

1/ (2 * π * 1000000 * 0.000000001) ≈ 160Hz

1/ (2 * π * 1000 * 0.000000022) ≈ 7230Hz

The second bode plot demonstrates the effects of applying the 80Hz HP filter rather than the 160Hz and the third is proof to the fact that the 3.4kHz LP filter still operates as it did prior to these modifications. Upon first glance it appears that the 7.2kHz filter has a steeper slope after the cut-off frequency, close inspection will reveal that the value representing each vertical division varies by 3dBu between the two thus causing this illusion.

After routing ±9V through the SPDT switches and connecting ground to the shield and signal to the tip of each 1/4″ jack socket  the unit was ready to be tested in an entirely different electrical environment, once supplied with power and connected to an amplifier a very strange thing occurred. When strumming gently almost no audio was heard even with the gain and volume turned up full, a more aggressive strum led to a ‘burst’ of distortion around 50dB louder than usual. My thoughts are that a component or track has been short circuited or there is a slight possibility that the op-amp has been damaged somehow, tomorrow will entail an investigation into this and failing that I will use the existing and quite conclusive information gathered prior to this issue.

Project Build – Day 5

For today’s practical session I packed my guitar and an unbalanced cable with a 1/4 TRS connection on one end and bare terminals on the other to provide an input to the circuit, just as I did in the prototype stage. The system used to amplify and transduce the output was identical also, amplifying the signal with a ‘flying mole’  and sending this signal to a ‘Gale Mini Monitor’ for inefficient conversion into sound pressure waves.

Upon initial testing of the system I noticed that the resultant sound was lacking in harmonic distortion regardless of the pre-amp gain setting, to counteract this problem I exchanged the 1kΩ static feedback resistor for a 510Ω, effectively doubling the relative gain. In a search for yet more distortion I decided to exchange this for a 200Ω resistor, much to my dismay this forced the op amp into oscillation and the 510Ω resistor was determined to be a safe compromise.

My project tutor and I spent a while analysing the schematic and PCB layout in search of the sweep middle mystery, we came to the conclusion that the positive and negative terminals of Op-Amp 2 had been reversed at some point during the design process. To solve this without removing the IC holder I bent out the corresponding pins (5 & 6) and strategically formed two insulated solid core wires into a ‘Z’ shape so that the signals could be reversed to their correct operation.

 

This very nearly solved the problem, attenuation of the selected frequency range appeared to work perfectly but excessive boosting of these frequencies sent the entire TL074 into a low frequency oscillation. It was at this point that I realised the 3rd op amp had not been stabilised, to do this I joined the positive input to ground with a short length of insulated wire and tied the negative input and output pins together.

 

Unfortunately the same oscillation continues to occur, National Semiconductor (2011)  mention that this could be due to a poor ground structure.A star-ground layout is formed when the ground reference for different sections of the circuit are joined to a single point rather than forming a large loop as I have done, luckily I did place various ground references so splitting the ground track in a few places and fulfilling the above  may solve this problem. However, as much as I would like to infatuate myself in perfecting this system I feel that it would be useful for me to write up recent progressions in my project report and use any remaining time to carry out the former.

Regardless of this I decided to retrieve some more bode plots for the circuit due to the major circuit alteration, contrary to yesterday’s results I found that the 2.2nF capacitor now provided an 80Hz HP filter and the 680nF provided an all pass filter. This is because, theoretically, the cut off frequency for this filter is

1/(2 x π x 0.0000068 x 1000000) ≈ 0.23Hz

The LP filter now operates almost exactly as was simulated, as stated before this will lead to a ‘smoother’ distortion tone although I would like to install a toggle switch which provides a higher cut off point if enough time remains. The associated bode plots can be found below for reference, note that a slight boost to the low frequencies was difficult to remove due to the unstable EQ gain control.

To demonstrate what the sweep middle is capable of the bode plot below shows a full cut at around 250Hz.

The final task for today was to carry out a critical listening test for each variation of clipping, the results were as follows:

1N4148

• Dual Asymmetric – Controlled overdrive, good for power chords. Similar to tube amp overdrive
• Single Asymmetric – More high frequency harmonics
• Dual Symmetric – A good compromise between the two

DO204

• Dual Asymmetric – Harsh harmonics, bluesy overdrive. Good for solo’s
• Single Asymmetric – Similar but with less HF harmonics. Warm, fuzzy low frequencies
• Dual Symmetric – Not so pleasant, sounds messy and harsh.

MOSFET

• Dual Asymmetric – A more controlled version of the DO204, even tone and smooth distortion
• Single Asymmetric – An almost clean tone, a subtle amount of overdrive can be heard at full volume
• Dual Symmetric – Very similar to asymmetric, high frequencies are emphasised
• Single Asymmetric & 0A47 – A euphonic distortion tone. Sustain is long, lows buzz nicely and odd harmonics are emphasised

0A47

• Single Asymmetric – A controlled tube overdrive tone, similar to the 1N4148 but more confined
• Single Symmetric – A more rocky distortion, although volume is limited as a result of excessive clipping.

All things considered this is a very successful distortion unit, I would like to get the parametric EQ working properly but the variety of tones and musical styles you can achieve is already quite impressive considering the build time. I will connect up the DC power and TRS sockets at some point to check these although I was really trying to avoid it whilst there are still problems with the circuit as it makes access much more difficult.

References

National Semiconductor (2011) Answers/FAQ’s – Audio. National Semiconductor. [Online] [Accessed 28/03/2012] Available at: http://www.national.com/kbase/category/Audio.html

Project Build – Day 4

The first task for today was to re-test the circuit in case of any inconsistencies, I connected power and an input signal and monitored the output waveform, just as described in the Project Build – Day 3 post.

When checking the DIP switches for associated clipping I noticed that DIP 4, the negative clipping schottky diodes, provided an output waveform which clipped both polarities of the signal at around 100mV. Although this is not what I expected no further investigation was taken into why this was so, such a low clipping voltage will give tremendous tonal effects which I am happy to investigate.

DIP 5 & 6 which activate the MOSFET clipping sub-circuits imposed no clipping or modifaction of the signal whatsoever, swapping the upper IRF-510 with an IRF-9540 on each sub circuit completely resolved this problem. These new MOSFETs deliver a much lower Source-Drain resistance for minimal Johnson noise and a bias voltage of -1.5V for more noticeable clipping, having just found out that the body diode in this component is reversed I will rotate one of these MOSFETS in its socket to ensure a valuable outcome. Swapping the MOSFET clip on DIP 6 for an 0A47 diode operated as planned, determined by a SPDT toggle switch.

Regardless of any problems encountered each DIP switch now provides a  unique signal clipping function, to prove this I took a picture of each resultant waveform where the caption underneath states the associated component arrangement.

1N4148 - Single Asymmetric

1N4148 - Dual Asymmetrical

DO204 Schottky - Single Asymmetrical

Do204 Schottky - Dual Symmetrical

MOSFET - Asymmetrical

0A47 - Single Asymmetrical

MOSFET + 0A47 - Asymmetrical

These images and the variety between each modified waveform is further proof to the potential malleability of this circuit, by analysing the waveform shapes I have made the following predictions:

• The 1N4148 diode will provide a hard clip, the high bias voltage will incur a relatively subtle distortion if pre-amp gain is applied sparingly.
• The D0204 Schottky diode will provide a slightly softer clip although the low bias voltage may counteract this.
• The IRF-9540/IRF-510 MOSFETs will provide distortion similar to that of the 1N4148 with somewhat softer clipping.
• The 0A47 diode will provide the most subtle distortion if gain is applied sparingly, when combined with the MOSFETs a pleasant middle-ground will be achieved.

I also exchanged the 680pF capacitor for a 680nF as mentioned, this should provide a much more useful cut off frequency which I may decide to alter a little more once the circuit bode plots have been finalised. I would have carried out this task today but I still am unable to get the sweep middle EQ to operate correctly, I was very hopeful once I’d noticed that two of the resistors had somehow swapped component labels when transferring to the PCB layout but putting this right made no noticeable difference to the controls.

I spent many frustrating hours trying to unveil the problem and have gotten nowhere, until I can get additional help or advice from my tutor I intend to carry out some listening tests and update my project report with recent events and knowledge.

Project Build – Day 3

The main task after completing the enclosure build was to route all the cables to the correct component terminals. Firstly I routed all of the DIP switch connections, but only after checking continuity between the PCB and wire terminal. I kept the cables in place with a crocodile clip for easy soldering and to reduce the chances of short circuiting veroboard tracks, this is shown in the image below where a strip of red PVC was present on each right hand cable to indicate input to the switch.

A diagram of the DIP switch unit and a table identifying the use for each SPST switch is shown below.

Wiring the SPDT toggle switches was carried out by cross-referencing to the PCB/Schematic diagrams in order to identify which cable and terminal should be joined together. The potentiometers were slightly more difficult to set up, I used my multimeter to determine the nature of each terminal and then cross referenced with the PCB layout to provide a duplicate of this circuit.

In the case of the EQ gain, terminals one and two, the extents of the resistor, were swapped over so that a clockwise rotation incurred a frequency dependant increase in gain. Turning a dial anticlockwise to make something louder seems counter intuitive, carrying out this simple task avoided it completely.

Once I all the components had been connected appropriately I decided to carry out an electrical test,  this was intentionally carried out before the power/signal sockets had been connected for easy manipulation if necessary. A dual tracking DC power supply provided independent ±9V for the TL074, circuit ground was connected to 0V. A signal generator set to output a 600mV p-p sine wave was used as the input and a digital oscilloscope was used to analyse the resultant output.

As is to be expected with a complex system arrangement such as this, there were many problems encountered before everything worked as it was designed to. The main issues and how I went about solving them were as follows:

• Contrary to my predictions, the gain and volume controls increased with anticlockwise rotation. Swapping terminals 1 & 2 gave a quick fix for this.
• Very few of the diodes were achieving any form of clipping when activated on the DIP switch, after some close inspection I noticed that there were some deep scratches stretching across the ground track and in various other places around the PCB. To solve this I carefully spread solder across the affected areas to re-gain contact, as a result of which most of the diodes performed their clipping duties.
• One of the 0A47 feedback diodes wasn’t responding to the DIP switch control, I realised after a short while that this was due to the way in which I had organised the schematic. This was such that the -ve clipping diode could only be put to use if the +ve switch was closed too, altering the PCB layout slightly by soldering solid core wire between the two inputs meant that I was able to control each one individually.
• The upper IRF-510 set to dip switch 6 was unresponsive, unfortunately there are none of these in stock at the University but hopefully I will be able to retrieve one from a prototype or get a few ordered ASAP.
• The sweep middle EQ was and still is completely unresponsive, I will continue the investigation as to what is causing this tomorrow.

These issues and the time it took to discover them lasted for the majority of the day, I did, however, manage to use the NEUTRIK test set to get a frequency response for the high pass and low pass filters formed by simple series/parallel capacitors as described in an earlier post.

The first frequency response graph (bode plot) illustrates the effects of the what I designed to be a 80Hz HP filter combined with a 3.4kHz LP filter, the frequency response shown here suggests otherwise. The cut off here is approximately 500Hz, this could have been altered by any combination of the following:

• Signal generator output impedance
• Op-Amp Input Impedance
• Inherent resistance & capacitance from the PCB track/signal cable

Nonetheless I have decided to keep the first filter response in the circuit as it will represent a scratchy, rhythmic alternative. The low pass filter shown by the slow fall in HF level gave results as predicted which I am pleased about.

The second bode plot further emphasises the LF attenuation, imitating what appears to be a 1.5kHz HP filter which is not much use.  Time permitting I  intend to install a larger value capacitor in place of the 680pF to better this, although this will be somewhat dependent upon the capacitor lead spacing and dimensions.

Before any of this is carried out I will attempt to work out why the sweep middle is unresponsive and replace the IRF-510, if this is successful I can then go on to collect screenshots of a variety of clipped signals, analyse the tonal qualities with a guitar and perform some final bode plots confirming the potential EQ variation.

Project Build – Day 2

Before I could make any progress with the circuit the next stage was to design the layout for the physical/electrical interface and install this accordingly, I decided that a simple and accurate way of doing this would be to use AutoCAD software. I proceeded to measure the panel dimensions and transferred these to an A4 template, after retrieving the component dimensions from their data sheets I placed these in a symmetrical and logical fashion.

The gain, volume and bypass controls are grouped at the top left, the sweep middle EQ and LP filter controls are grouped below this. The DIP switch has been placed centrally for easy access and toggle controls for component bypass are grouped on the right. A screenshot of the final design using the “paper space” view is shown below.

I printed this design and stuck it to the enclosure, ready for drilling. The centre point of each circle was drilled with a 6mm bit as illustrated below, in retrospect I should have drawn the exact centre point with AutoCAD for a more accurate control position although these slight discrepancies were not evident in the final product.

The toggle switches fitted perfectly through these holes, the potentiometers required a 10mm bit but this was a simple case of using the pillar drill to extend the width.The 1/4″ TRS sockets also required a 10mm passage, these were placed towards the back on each side of the enclosure so that the PCB could easily be lifted in and out  after installation. The DC power socket with a similar thread width was marked up, drilled and installed in the centre of the back panel, a SPDT switch was placed at each side of the socket for individual +/- 9V cut off. The power switch, evidently, was the one component which I forgot to include in the order but these toggle switches serve as an excellent replacement since the idea is both practical and unique. The dip switch opening was crafted by drilling a sequence of small holes and then using a file to extend these until it sat comfortably within.

Since I had ordered PCB DIP switch units rather than panel mount the next task was to find an alternative means of affixing these. I found a small piece of veroboard and cut the tracks half way along, soldering the DIP switch unit across these breaks as shown below provided an improved solder contact for each cable. Since the DIP switch fits so tightly into the opening and regular removal would be necessary a few small strips of double sided sticky tape were placed on the component side of the veroboard in preparation for semi-permanent enclosure sealing in the future.

Project Build – Day 1

After having the PCB layout checked over by a qualified technician it was etched and returned to me within 24 hours, allowing me to begin the main practical section of this project.

The first task was to drill out every solder contact point with a 0.8mm bit, the zener diodes required a 1.2mm bit and the schottky diodes required a 1.5mm for their leads to fit through but overall this task took only a few hours. Before placing any componenents I used a short piece of solid core wire to connect both sides of the via, using the continuity test on my multimeter to check the signal flow between the two sides after soldering. This was shortly followed by cleaning the PCB tracks with a non-metallic scrub block for improved solder contact.

When installing the 14 pin IC socket for the TL074 I made sure that there was a big enough gap to make a connection to the component side tracks without destroying the socket, I realise now that I could have avoided this problem by ensuring that all connections to this IC were solder side.

Using the PCB layout as a guide I continued to solder in the components, ensuring that any component side tracks were soldered to the component lead. After a short while I had installed all of the components, as shown below, this left the task of installing a length of wire to each test point for soldering to subsequently installed hardware such as switches and potentiometers as planned.

Carrying out this task consumed the rest of the hours in the day but by the end of it the circuit was fully prepared for connection to the hardware, a snapshot of the vibrant wiring scheme is shown below.

PCB Design

The final component necessary for a working electronic project is the PCB, initially I had planned to carry out the design of this using the National Instruments Ultiboard software which intertwines with Multisim. Conversely, as a result of my unfamiliarity with the software and an abundance of ‘virtual’ components which I had unknowingly added to the circuit, I decided to duplicate the schematic in EdWin and proceed to use its simple and effective PCB design tools which  I had become confident in using as of another electronic project.

The complete schematic after I had unpacked all the components ready for layout on the PCB is shown below, some design changes which were essential for correct operation are noted below the image.

The most obvious design change here is the replacement of switches and potentiometers with multiple ‘Test Points’ (TP), this provided an easy way of creating large contact points so that each component solder tag can be routed to the correct part of the circuit via multi-threaded cable. A test point has also been inserted for ±9V and ground, making the connection to the power socket a much easier task.

To ensure that the distance between component through-holes was correct I examined all of the components and found a similar match. For instance, I found that the 33nF & 47nF capacitors had 5mm lead spacing and the 680pF had 2.5mm lead spacing. Finding these was a simple task of searching through the available descriptions until I found a suitable match, the ceramic caps with respective lead space of 0.2″ and 0.1″ were found to be ideal. The 0A47 was replaced by the 0A79,  this was slightly longer as shown below but causes no issues due to the inherent lead length.

The DO-204 schottky diode was replaced by a 1N5822 after cross referencing with its diode package as also shown below, this turned out to be of very similar length and provided an ideal alternative.

The IRF510 MOSFET was another component unavailable in the library, after finding that this component shared the TO-220 package with the IRF830 as shown below these were immediately interchanged.

After unpacking all the components, which transfers the component and Net information to the PCB layout, I began to place my components in a similar layout to that of the schematic in order to avoid later confusion resulting in the screenshot below, each important detail will be described underneath.

• The PCB size was set to 8″ x 4.8″ in order to fit snugly into the enclosure, although this seems excessive a smaller enclosure would have meant a cramped panel layout and increased risk of parasitic capacitance/inductance through cross talk.
• Test points have been placed strategically, each switch has its poles grouped together for reference when carrying out the physical build.
• Power test points were placed close to the IC to reduce interference.
• Input/output points were placed close to the edge where the 1/4″ jack sockets will be for similar reasons.
• Multiple ground references have been inserted in order to make potential fault finding a little more simple.
• Via’s, and therefore component side traces, have been avoided as much as possible to reduce build time.
• Solder contacts for elevated components were confirmed to be on the solder side.
• Traces were widened for increased reliability and current handling, if the traces were particularly long this might incur parasitic resistance but considering the nature size of the circuit I don’t feel that this will be an issue.

This layout has now been sent off for checking and etching, in a few days time I will yield all the necessary parts to begin the build stage of this project.

Semi-Final Circuit Design & Component Order

With the deadline for component orders drawing ever closer I decided that today I would finalise my circuit design by adding an active tone control and output stage, following this I spent a large part of the day searching the internet for components and accessories which would combine to form the final product.

The sweep middle EQ circuit which I found was originally designed by Self (2010, pp.279), this circuit is a variation of the Wien Middle EQ which creates a parametric Equalizer with ≈ ±15dB cut/boost using relatively few components. Self mentions that his circuit demonstrates “somewhat greater interaction between boost/cut, Q and frequency” as a result of its simplified design, for me this is not a problem in this context as the centre frequency and Boost/Cut can be controlled using single ganged potentiometers which are both cheaper and more readily available. To provide a variable output voltage I used a 100kΩ potentiometer where the potential difference between the cold terminal (connected to ground) and wiper connection would provide the output.

The circuit diagram below shows the improvements I have made, the power supply and clipping circuits have been replaced by sub-circuits with labeled inputs and outputs for a more compact and easy to understand image.

At this stage I realised that using an internally regulated battery power supply would only detriment the audio quality and, since the overall design was no longer ‘stompable’, was completely unnecessary. As an alternative I decided to use a BOSS PSU-230 which provides a fully regulated ±9V, 200mA supply via a 2.1mm DC connector.

As a result the final circuit diagram shown below contains nodes for connecting ±9V and ground to a DC power socket, although easier on the eyes the simplified schematic above would not be very useful when advancing to PCB design.

 

In order to identify the frequency and phase response of the initial and extended circuits I took advantage of the the AC Analysis within Multisim, placing a resistor across the output to provide a point of reference. The images below are individual screen shots and their respective circuit details, the first of which illustrates the completely flat phase response which was maintained throughout each simulation regardless of filter settings.

Phase Response With Inherent 1st Order, 80Hz, High Pass Filter

 

80Hz High Pass Combined with 3.4kHz Low Pass

 

234Hz High Pass

 

Sweep Middle (Max Cut-Off Frequency, Max Boost)

 

Sweep Middle (Max Cut-Off Frequency, Max Cut)

 

Sweep Middle (Min Cut-Off Frequency, Max Boost)

 

Sweep Middle (Min Cut-Off Frequency, Max Cut)

 

As a result of these ideal responses I was more than happy to accept this circuit as a final design, this decision led to countless hours of searching the internet for components and accessories necessary for the completion of this project. The one thing which I didn’t manage to find was a 100kΩ potentiometer with reverse logarithmic taper (C taper), this would have been especially useful since the majority of control in the middle sweeps cut off frequency is in the lower 20%. To save time and money I decided to stick with a linear taper which would still provide a good level of control if operated by careful hands.

After adding high quality potentiometers, a large amount of switches and an appropriate encasement to the component order form my budget soon became limited. After some electronic bargain hunting the total came to £ 65.74, considering that a “Boss OD-3” has an RRP of £90 this is quite reasonable.

 

Self, D. (2010) Small Signal Audio Design. Oxford: Focal Press. pp.279