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Digital FX Unit Part 1 by John Clarke Make like a pro muso with this Digital FX (Effects) Unit. It will produce unique sounds when connected to a variety of instruments . . . like an electric guitar, bass, violin or cello, even the output of a microphone preamp or within the effects loop of an amplifier or mixer. I t’s very common for musicians to add effects to the sound of their musical instruments. These are used to add depth, ambience and tonal qualities and to personalise the sound. Effects can be subtle or extreme, and can be tailored to produce a unique sound. Purely analog audio circuitry can be used for effects units such as in the Overdrive and Distortion Pedal from March 2020 (siliconchip.com.au/Article/12576). But for complex effects, digital signal processing (DSP) is more convenient and flexible. Our Digital FX Unit utilises a digital signal processing integrated circuit (IC) that is designated the SPN1001 FV-1 (or FV-1 for short). This is preprogrammed with eight effects, and while one of these is a test function, the remaining seven provide flange, chorus and tremolo as well as pitch shift and reverb effects. A further eight extra effects are stored within an external EEPROM that connects to the FV-1. These effects have been chosen by us. However, you can change the stored effects patches. The FV-1 has been available for many years, and has been used in many commercially available effects units. 24 Silicon Chip The FV-1 has a somewhat cult following amongst digital effects enthusiasts. This has led to the production of numerous freely-available effects patches and software to enable the writing of your own unique effects. For our Digital FX Pedal, the preprogrammed EEPROM is filled with eight effects that add to the seven usable effects preset within the FV-1. These individual effects are selected using a rotary control knob, while the parameters of each effect are adjusted using up to three rotary controls. Many effects have already been created for the FV-1 IC, and these are free to use. These effects include chorus, echo, flange, phase shift, vibrato, limiter, wah, various reverberation effects, distortion, octave shifts and a ring modulator. For information on what some of these effects are and how they are achieved, see www.spinsemi.com/ knowledge_base/effects.html We will explain some of the basic effects at the end of this article. There is also an assembler and a graphical software package to help you write your own effects if you feel inclined to experiment. The software can then be assembled and programmed into the EEPROM. This requires an EEPROM programmer; we will have more details on where to get effects patches, how to store Australia’s electronics magazine siliconchip.com.au Features • 15 different effects including chorus, echo, flange, vibrato, wah, reverb & distortion • Each effect has up to three adjustable parameters • Provision to experiment by adding new effects • Rugged enclosure, suitable for stage use • Power supply reversed polarity protection • High input impedance to suit piezo pickups etc • Low power consumption • Battery or DC plugpack power • True bypass switch • No signal phase inversion them in EEPROM and how to use the assembler and graphical software later. Presentation Our Digital FX Pedal is designed for live music use, and so is housed in a rugged diecast aluminium case. On the top, it has a footswitch, eight rotary controls plus indicator LEDs. The signal inputs and outputs are two 6.35mm (1/4in) jack sockets at the rear, along with a DC barrel socket for power. The unit can also be powered via an internal 9V battery. Its power is automatically switched on when a jack plug is inserted into the output socket. Operating principle Fifteen different effects are available, with the option to change eight of the effects to your liking. You can choose them from a list of many freely available effects, or create them yourself using freely available tools. The block diagram, Fig.1, shows the signal flow of the Digital FX Pedal. The original signal is applied to CON1, and this is connected to the bypass switch (S2a). When not bypassed, this signal goes to the high input impedance buffer (IC1a) and is then filtered with a 19kHz low-pass filter. This prevents unwanted artifacts in the subsequent digital signal processing (DSP) stage, by removing RF and ultrasonic signals. l Fig.1: the input signal is fed into the SPIN FV-1 effects chip, and the resulting modified signal is mixed with the original signal in a ratio set by the user via potentiometers VR2 & VR3. VR4 adjusts the mixed signal gain and this is then fed to S2 which controls whether CON2 receives the original or modified (mixed) signal. siliconchip.com.au Australia’s electronics magazine April 2021  25 9–12V DC INPUT + D1 1N5819 CON4 A V+ K +3.3V OUT IN (ACTIVATED BY CON2) 9V BATTERY CON3 REG1 LD1117V33C S1 GND A l K IC2: OPA1662 10kW 200 W 100 m F 100 m F +3.3V 1 POWER LED1 10kW 100mF 2 S2 c 2 3 IC2a A 1 l BYPASS LED2 K 4 200 W Vcc/2 Vcc/2 V+ V+ 1MW INPUT 1 S2a FB1 100 W 2 CON1 1 0 0 pF 100nF 3 2 100nF 8 IC1a IC1: OPA1662 10k W 1 4 1.2nF 10kW 5 6 560pF BUFFER 7 IC1b EFFECTS INPUT LEVEL 22 m F VR1 10kW LOW PASS FILTER BYPASS SIGNAL 2N7000 LEDS LD1117V33C 1N5819 K K A SC Ó2021 D G S A O UT GND OUT IN DIGITAL AUDIO EFFECTS UNIT Fig.2: the complete circuit, which expands on what is shown in Fig.1. There are two options for selecting the current effect: the 16-way BCD rotary switch (S3) is the simplest, but could be somewhat hard to get. The alternative is potentiometer VR8, which has its position read by microcontroller IC6 and converted into a binary value to control IC4. IC6 includes hysteresis to avoid unwanted effects changes. V+ Specifications • Supply requirements: 9-12VDC, 100mA (can operate down to 7V on battery) • Current draw: 70mA typical • Maximum input & output signal levels: 2.3V RMS with a 9V supply; 3.3V RMS at 12V • Frequency response: -0.25dB at 20Hz and -2dB at 20kHz for ‘dry’ signal; -2dB at 20Hz, -1dB at 15kHz and -6dB at 18kHz for modified signal • Signal-to-noise ratio (SNR), 1V RMS in/out: 95dB for ‘dry’ signal; 85dB for modified signal The signal from the filter is fed to two separate level controls, VR1 and VR2. VR2 sets the level applied to the signal mixer (more on this later), while VR1 sets the signal level applied to the SPIN FV-1. VR1 is required so that the level can be set below the clipping level for the FV-1 input. The clip LED lights up to indicate signal limiting when the level is too high. The SPIN FV-1 contains a stereo analog-to-digital converter (ADC), a DSP core and stereo digital-to-analog converter (DAC) to produce the output signals. All processing is done using 24-bit digital audio samples. For more information, see www.spinsemi.com/knowledge_base/arch.html Note that while the FV-1 can process stereo signals, the Pedal is a mono device, so we are only using a single channel. There are two versions of the Pedal, where the effects se26 Silicon Chip Vcc/2 BYPASS SIGNAL lection is made using either a rotary switch (S3) or potentiometer (VR8) and associated components – more about this later. The effect parameters are adjusted using potentiometers VR5, VR6 and VR7. The FV-1 also has inputs for the crystal oscillator and EEPROM serial connections. After processing within the FV-1, the output signal goes through a 19kHz low-pass filter, to remove high-frequency noise (DAC step artefacts) and then to the effects level control, VR3. Both the effects signal and the original (or dry) signal from VR2 are combined in the inverting mixer stage, comprising IC3a and IC3b. The mixing allows adjustable portions of the dry and effects signal to be blended to provide the desired result. The mixer can also provide a signal gain of up to five times, adjusted with potentiometer VR4. The resulting sig- Australia’s electronics magazine siliconchip.com.au +3.3V +3.3V +3.3V C VR6 10kW VR5 10k W B K VR7 10k W A S3 CLIP LED3 200 W 6 5 20 LIN LIN LIN l 21 22 1m F 1kW SIG INPUT 1 2 +3.3V 1nF 3 10 m F 100nF 10 X1 40kHz 9 8 3 VCC A2 2 A1 1 7 A0 IC5 24LC32A SDA S CL WP 15 5 14 12 6 15pF VSS 4 1 2 3 4 5 23 DVDD AVDD DVDD REFP CLIP 10 m F POT 1 100nF LED4 l K 100nF A B C D 1 200 W VDD S2 LIN S1 RIN S0 IC4 FV–1 SPN1001 MIDREF T0 18 C 7 17 B 6 16 A 13 D GP1 RA0 IC6 5 RA2 PIC12F1571 2 3 Q1 2N7000 D RA1 RA3 RA5 4 G VSS X1 8 4x 10kW X2 SDA LOUT SCK ROUT T1 REFN GND GND GND GND 4 7 11 19 S 1.2MW 28 27 25 CIRCUITRY INSIDE THIS AREA IS ALTERNATIVE TO USING IC6, VR8, Q1, LED4 AND ASSOCIATED COMPONENTS GND 24 DIGITAL PROCESSOR (CON5) V+ V+ 100nF 10kW 10 m F 10kW 5 6 8 VR4 100kW LIN 20k W VR3 10kW 1 0 0 pF VR2 10k W Vcc/2 4 .7 m F 2 3 5 8 IC3a 1 10m F 4 6 LOW PASS FILTER DRY MIX 10k W 4 .7 m F MIXER 100 m F 10kW EFFECTS MIX 7 IC2b 100nF IC3: OPA1662 OUTPUT LEVEL 1.2nF 560pF SELECT LIN POT 2 6 SELECTED A V R8 10kW BCD ROTARY SWITCH 26 POT 0 EEPROM ICSP (PICKIT) 8 E 100 W SIG OUTPUT A PARAMETER ADJUST IC3b 100 W 100k W 7 20k W 1 S2 b 2 BYPASS OUTPUT CON2 INVERTER AMPLIFIER Vcc/2 BYPASS SIGNAL BYPASS SIGNAL nal is then applied to the bypass switch, S2b. This selects between the original signal from CON1 and the signal with effects, with the selected signal going to the CON2 output. signals can be mixed along with the dry signal to produce the desired effect. How the effects work The full circuit for the Digital FX Pedal is shown in Fig.2. The input signal from CON1 passes through a 100Ω stopper resistor and ferrite bead FB1. In conjunction with the 100pF capacitor, these block RF signals from entering the circuit and causing radio-frequency detection and reception. The 100pF capacitor also provides a suitable load for piezo string pickups. The signal is AC-coupled to pin 3 of IC1a, and is biased to half-supply (Vcc/2 or about 1.65V) via a 1MΩ resistor. This keeps the input impedance reasonably high at 1MΩ, suitable for a piezo pickup. IC1a is connected as a unity-gain While it is difficult to show many of the various effects available, the “octaver” effect can be easily demonstrated. This is where the dry signal is mixed with a signal shifted up or down by one octave. These are harmonically related, at half the frequency and double the frequency, respectively. In Scope1 (overleaf), the top yellow trace (channel 1) shows the dry signal and the lower white trace (Ref A) the up octave signal, produced by doubling the frequency. The middle blue trace (channel 2) is the down-octave signal, at one half the frequency. The up and down octave siliconchip.com.au Circuit description Australia’s electronics magazine April 2021  27 Scope1: the signal being fed into the device is shown at the top, in yellow. Below are the outputs of the ‘octaver’ effect, set for one octave lower (blue) or higher (white). These effects signals can be mixed into the original to create richer harmonics and different sounds. buffer that can drive the following low-pass filter stage. The Vcc/2 voltage is derived using two 10kΩ resistors connected in series across the supply and is bypassed with a 100μF capacitor to remove supply noise, then buffered by unity-gain amplifier IC2a. Note that all the op amps in the circuit have very low noise and distortion figures of 0.00006% at 1kHz at a gain of 1, with a 3V RMS signal level. Therefore, the op amps do not contribute any audible distortion to the signal. The low-pass filter is a Sallen-Key two-pole 19kHz Butterworth type that rolls off at 40dB per decade (12dB per octave). It is included along with further passive filtering to remove any high-frequency signal components above 20kHz. This prevents signal aliasing due to digital sampling at 40kHz. Without the filter, strange audible artifacts could be generated by the ADC. Following this filter, the signal is AC-coupled to the level potentiometer, VR1. This sets the signal level applied to input pin 1 of IC4, the FV-1. IC4 provides an internal half-supply DC bias for this pin, hence the AC coupling. The 1kΩ resistor and 1nF capacitor after the AC-coupling capacitor attenuate any remaining high-frequency noise. The signal fed to IC4 must be lower than about 1V RMS to avoid clipping. Clipping occurs when the signal goes beyond the 0-3.3V supply range of IC4. The clip indicator output, pin 5, goes low and drives LED3 if this happens. VR1 should be adjusted so that this LED does not light. IC4 includes a crystal oscillator amplifier. The typical circuit for the FV-1 depicts the crystal as a 32,768Hz watch type. This is recommended mainly because it is commonly available, but the high-frequency audio response suffers if one is used. Instead, we use a 40kHz crystal, extending the processor’s frequency response from around 16kHz (when using the watch crystal) to just under 20kHz. Per the Nyquist theorem, the highest frequency that an ADC can handle is at half the sampling rate. Effects IC4 requires several supply bypass capacitors. These are 100nF for the analog and digital 3.3V supply 28 Silicon Chip pins, while the half supply bypass at the MID pin (pin 3) is 10µF. As mentioned above, the mid supply is about 1.65V. IC4 also requires positive and negative reference voltages for the ADC at pins 25 and 26. Pin 25 is tied directly to GND (0V), while pin 26 connects to the +3.3V supply via a 100Ω resistor and with a 10µF filter capacitor, to keep supply noise out of the signal path. Effects parameters are varied using potentiometers VR5, VR6 and VR7. These are connected across the 3.3V supply and can provide voltages of 0-3.3V to the POT2, POT1 and POT0 inputs of IC4. The function of each pot depends on the selected effect. Effects are selected by the state of IC4’s digital inputs S0, S1 and S2 (pins 16, 17 and 18) and the voltage level at the T0 input, pin 13. When the T0 input is low, the effects selected by the S0, S1 and S2 inputs are those that are preprogrammed within IC4. If all the S0, S1 and S2 inputs are low, the first effect is selected. Further effects are chosen with different levels at S0, S1 and S2. S0 is the least significant bit, and S2 is the most significant bit of a binary value. The three inputs provide for eight possible selections (23). The effects stored on the EEPROM (IC5) are selected when the T0 input is high (3.3V). Eight further selections are then available using the S0-S2 inputs. IC4 connects to the EEPROM via an I2C serial bus using two pins, the serial clock, SCL and serial data SDA. These connections are also brought out to the ICSP header for in-circuit programming of the EEPROM memory chip (if required). The EEPROM’s supply is bypassed by a 100nF capacitor. The EEPROM is a 32kbit (32,768 bit) memory arranged as 4096 x 8bits (ie, 4k bytes). Effects patches stored within the EEPROM are placed in memory blocks of 512 x 8bit. There are eight 512 x 8bit memory blocks in the full 4k x 8bit memory. Output signal handling The effects signal from the left channel output of IC4 (pin 28) is fed to a low-pass filter comprising IC2b, two 10kΩ resistors plus 560pF and 1.2nF capacitors. This is another Sallen-Key two-pole 19kHz Butterworth low-pass filter. It is included to remove high-frequency DAC switching artifacts from the signal. The output signal from IC2b is applied to the VR3 effects level potentiometer. The signals from the wipers of VR3 and the dry signal potentiometer, VR2, are combined in the inverting mixer stage based on IC3b. The mixer gain is adjusted using VR4, with a maximum gain of negative five times when VR4 is at its maximum resistance of 100kΩ. The following inverter stage, built around IC3a, re-inverts the signal so that the output signal is in-phase with the input. The output of IC3a is fed via a DC blocking capacitor and stopper resistor to the bypass switch, S2b. When in position 1, this signal goes to the CON2 output socket. When bypass is selected (with S2 in position 2), the input signal at CON1 bypasses the effects circuitry, connecting directly to the output at CON2 via the S2b terminals. The S2a terminals tie the input for IC1a to ground. This prevents noise from being picked up and amplified by IC1a in bypass mode. The remaining switch pole, S2c, controls indicator LED2. This bypass LED is lit when the signal is bypassed; the 200Ω resistor from cathode to ground limits the LED current to around 6.5mA. Australia’s electronics magazine siliconchip.com.au Two effects selection options So effectively, a binary value of 0000-1111 (0-15 decimal) is required to select one of the 16 possible effects. This value controls the states of the S0-S2 and T0 inputs of IC4. Our circuit provides two ways to make this selection. The simple way is to use a BCD (binary-coded decimal) switch, which has 16 positions and four outputs that provide the required binary states. However, 4-bit BCD switches can be difficult to obtain, so we offer the alternative option of using a potentiometer instead. So circuit Version 1 uses a potentiometer (VR8) and a microcontroller (IC6) to convert the voltage from the potentiometer’s wiper to a BC (binary-coded) value. VR8 connects across the 3.3V supply and can provide 0-3.3V to the pin 3 analog input of microcontroller IC6. This voltage is internally converted to a digital value. The micro’s digital outputs at RA2, RA1, RA0 and RA5 then generate the required binary (0V or 3.3V) values to feed to the S0, S1, S2 & T0 inputs of IC4 respectively. The resulting binary value varies smoothly from 0-15 decimal as VR8 is rotated from fully anticlockwise to fully clockwise. Hysteresis is included to avoid the binary value flicking between two adjacent values near each voltage threshold. This requires you to rotate the selection pot a little clockwise further than the threshold to select the next higher BC value output, and a little further anticlockwise from the threshold to select the next lower BC value. A change from one effects selection to another is indicated using LED4. The LED flashes off and then on again as the pot is rotated, to indicate a change in the binary value. Typically, an 8-pin PIC microcontroller does not have sufficient pins to handle the analog sensing, 4-bit binary output and the indicator LED drive. We solve this by using the master clear (MCLR) input at pin 4, and task it as a general-purpose input to drive the LED. It might seem that an input cannot be used as an output, but this input includes the option of a selectable pull-up current. While many of the 8-pin microcontrollers include an internal pull-up when the MCLR input is set to operate as a master clear input, there are not many microcontrollers that also allow the pull-up to be switched on or off when this pin is used purely as an input. However, the PIC12F1571 does have that capability. To be used as an output, the internal pull-up current is enabled, so the input will be pulled high near to the 3.3V supply. The input will go low without the pull-up when there is an external pull-down resistor. The pull-down resistor must be sufficiently high in resistance to allow the internal pull-up current to pull the input high enough to switch on the following stage. Using a 1.2MΩ resistor as the pull-down resistance, the minimum pull-up current for that input at 25μA is sufficiently high to swamp the pull-down current from the resistor. Thus, this pin will be quite close to 3.3V with the pull-up engaged. We use a 2N7000 N-channel Mosfet (Q1) to convert the high-impedance drive from this ‘output’ to a low-impedance drive for the indicator LED. It then drives LED4 via the 3.3V supply and 200Ω current-limiting resistor when its gate is high. The second version of the circuit (Version 2) simply uses a 4-bit BCD rotary switch (S3) to select the effect. This requires 10kΩ pull-down resistors at the A, B, C & D siliconchip.com.au Common effect descriptions Reverb Several delayed versions of the original sound are mixed back with the original dry sound, to simulate sound in a room or area where there are sound reflections (a complex form of echo). The ideal reverb period or delay setting depends on the type of sound; for music, it depends on the music’s tempo. As a general rule, longer reverb times are for slow tempo music, while shorter reverb times are suited to faster tempo tunes. Different reverb programs will have their own tonal qualities due to differences in the reverb time of high or low frequencies and differences in the reverb sound’s overall frequency response. Be careful not to apply too much reverb, particularly in the high frequencies, as this will result in an unnatural sound (unless that’s what you want!). Start with reverb level all the way down, then gradually bring the reverb mix up until you can just hear the difference. Any more than this will give an unrealistic sound. Phasing, chorus, and flanging (modulation effects) All of these effects have a portion of the audio signal delayed and then mixed back with the dry signal. The amount of delay is modulated by a low-frequency oscillator (LFO). The delay is quite short compared to the reverb effect. For phasing effects, the delay is less than the period of the signal. This phase difference between the modulated and direct signals causes cancellation at some frequencies and reinforcement at others. It produces a comb filter like effect, where some frequencies are amplified, and others are attenuated across the audio band. It causes a ‘shimmering’ type of sound. Phasing is the subtlest of all these effects, producing a gentle shimmer that can add life to a wide range of sources without being too obtrusive. For chorus and flanging, the signal is delayed by a longer period, up to several milliseconds, with the delay time modulated by an LFO. This also produces a comb-filter effect and a pitch-shift effect after mixing with the dry signal, giving a harmonically rich ‘swirling’ or ‘swishing’ sound. Chorus and flanging effects mainly differ in the amount of delay time and feedback used. Flanging uses longer delay times compared to chorus, and chorus generally uses a more complex delay structure. Chorus is most often used to ‘thicken’ the sound of an instrument, while flanging is usually used to produce other ‘whirling’ sounds. Pitch and octave shifts Australia’s electronics magazine These effects involve altering the frequency of the signal. Pitch varies the frequency by a variable amount, while the octave shift changes the frequency by a factor of 0.5 for octave-down and 2.0 for octave-up. Mixing the octave-shifted signals with the dry signal produces various effects, including making a single instrument sounding fuller, or sounding as though there are multiple instruments. April 2021  29 Fig.4: two versions of the project have been designed, as described in the text. Each uses a slightly different PCB so make sure you order the appropriate board. Note that in the switched version, four resistors are mounted on the PCB underside. switch pins. The common E pin connects to 3.3V, and so pulls a combination of the A-D pins high, depending on the switch’s rotation. Power supply The circuit is powered when microswitch S1 is activated by inserting the output jack plug into CON2. The plug physically raises the socket’s ground connection, lifting the microswitch actuator and activating the switch. While many effects pedals are switched on when a jack plug is inserted, it is usually done by a switch internal to the socket. We are not using a socket that has isolated switching mainly because they are not commonly available. These also have the disadvantage of stressing the PCB connections each time a jack plug is inserted, especially if the jack is moved at an angle to the socket. This eventually causes the solder joints to harden and break. While the sockets we use also solder directly to the PCB, the body is secured to the case at the socket entry as well. That keeps the socket fixed in place against the enclosure side, minimising movement of the solder joints. Power is automatically selected between 9V battery or DC supply. When there is no DC power plug inserted, the DC socket (CON3) will supply battery power via its normally-closed switch connecting, the negative of the battery to ground. When a power plug is inserted, power is via the DC input and the battery negative is disconnected. 30 Silicon Chip Power switch S1 connects power to the rest of the circuit whether via the battery or an external source, while diode D1 provides reverse-polarity protection. REG1 is a low-dropout 3.3V regulator which supplies IC4, IC5 and IC6 (if used). The input and output pins of REG1 are bypassed with 100µF capacitors. Its output drives the power LED (LED1) via a 200Ω resistor. Construction The Digital FX Pedal is built using a double-sided, plated-through-hole PCB measuring 86 x 112mm. The version using the BCD switch is coded 01102212, while the version using potentiometer VR8 is coded 01102211. Either way, it is housed in a diecast enclosure measuring 119 x 94 x 34mm. Figs.3 & 4 are the two PCB overlay diagrams for the different versions. Refer to the appropriate diagram during construction to see which parts go where. Begin by fitting the surface-mount parts, IC1-IC5 (and possibly IC6), on the top side of the PCB. These are not difficult to solder using a fine-tipped soldering iron. Good close up vision is necessary, so you might need to use a magnifying lens or glasses. If you’re using the version with potentiometer VR8, also mount IC6 now. In each case, make sure the chip is orientated correctly before soldering it in place. Make sure that IC1-IC3 are the OPA1662 op amps, IC5 is the 24LC32A and IC6 is the PIC12F1571 (if used). For each device, solder one pad first Australia’s electronics magazine siliconchip.com.au the DC socket, CON3. Switch S1 must be mounted so that the lever is captured under the front sleeve contact of jack socket CON2. We have provided slotted holes so that the switch can be inserted and slid along until the lever slips under the contact. Check that the switch is open-circuit between the two outside pins when there is no jack plug inserted, and closed between the two outer pins when a jack plug is inserted. The lever might need to be bent a little so that the switch works reliably, switching at the centre of the travel between the open and closed position of the CON2 jack contact. Mount foot switch S2 and rotary switch S3 (if used) now. Make sure these are seated fully and not skewed before soldering. Leave the LEDs until later, when the PCB is mounted in the case. The next step is to cut the battery wires to 60mm, then crimp or solder them to the plug pins. Insert these pins into the plug shell, making sure you get the red and black wires in the correct position. When you plug it in, the red wire should go to the terminal marked + on the PCB, adjacent to D1’s anode. It’s necessary for the GND terminal on the board to be connected to the case, to prevent hum injection via the enclosure. Cut a 50mm length of green medium-duty wire, solder a solder lug to one end and the other to the GND terminal on the PCB. It’s a good idea to place some heatshrink tubing over the lug terminal and the GND PC stake. When assembled, the solder lug is secured to the case using an M3 x 6mm screw, star washer and M3 nut. Powering up and testing Same-size photo of the switched version, the version at right opposite. The cutout is for a 9V battery, as shown. and check its alignment. Readjust the component positioning by reheating the solder joint if necessary before soldering the remaining pins. Continue construction by installing the resistors (use your DMM to check their values), followed by the ferrite bead (FB1). Use a resistor lead off-cut to feed through the bead and solder to the board. Push the bead lead fully down so that it sits flush against the PCB before soldering its leads, so it doesn’t rattle later. Diode D1 can be installed next. Take care to orientate it correctly. The MKT and ceramic capacitors can now go in, followed by the electrolytic capacitors. The electrolytics are polarised, so they must be orientated with the correct polarity; the longer lead goes into the hole marked with a + symbol. Install potentiometers VR1-VR7 (and VR8 if used), noting that VR4 is 100kΩ and the remainder are 10kΩ. The 10kΩ potentiometers may be marked as 103, while the 100kΩ pot may be marked 104. Crystal X1 can now be fitted, along with CON5, the 6-way header EEPROM programming connection. Next, mount REG1 with its leads bent over so that the regulator body lies above VR4. Make sure it does not lean so far as to make contact with the metal parts of VR4. A 45° angle to the PCB face will prevent contact with the enclosure and VR4’s body. Also install the PC stake at the GND test point, and the two-way polarised header for the battery lead (CON4) now. Follow by fitting the two jack sockets (CON1 & CON2) and siliconchip.com.au If you are planning to use a battery, connect this now. Alternatively, connect a DC supply (9-12V DC). Plug a jack lead into CON2 to switch on the power. Then, using a multimeter set to read DC volts, connect the negative probe to the GND point and measure the regulator input and output voltages. The input should be about 0.3V below the battery or DC supply voltage. The regulator output should be between 3.267V and 3.333V. If that checks out, you can connect up a signal source and some sort of amplifier, fiddle with the knobs, and check that they appear to be working as intended. Housing The PCB is housed inside a 119 x 94 x 34mm diecast aluminium enclosure. We use the lid as the base, with the controls protruding through the main enclosure body. Use the drilling template, Fig.5, to make the required holes in the base. You can also download this as a PDF from the SILICON CHIP website. The only differences for the two versions are that the board with a potentiometer needs an extra 3mm hole for LED4, and the shaft hole is 6mm rather than 10mm. Cut-outs are also required in the side for the two jack sockets and DC power socket. The template shows the slots required for the jack sockets so they can be slid in place. The resulting gaps in the side of the enclosure, after the jack sockets are inserted, can be filled in. These can be covered with a small blanking piece made from a 45mm x 9mm piece of 1mm thick (or up to 1.5mm) aluminium. You can also glue shaped plastic or aluminium ‘infill’ Australia’s electronics magazine April 2021  31 Fig.5: same-size drilling diagrams for both the mechanical switching version (top left) and the potentiometer switching version (lower left). End drilling and blanking, or infill pieces are the same for both versions. These diagrams can also be downloaded from siliconchip.com.au 32 Silicon Chip Australia’s electronics magazine siliconchip.com.au There are quite a few holes to be drilled in the diecast box – see the drilling template (Fig.5, opposite) for details. Note also the “infill”, or blanking, piece – this helps seal the box after the PCB is placed in it. And speaking of placing the PCB, this photo shows how it’s done Ignore the tacked-on components in our prototype: PCBs have these additions already made. Note, though, the four resistors top left are required in the switched version. pieces to the rectangular backing piece for the neatest possible appearance, as shown in Fig.5. If doing this, cut a piece 31 x 12mm or a little larger, then drill a 12mm diameter hole in the centre. Once carefully filed, the piece will break apart so there will be two pieces that match the gaps in the enclosure. For the enclosure feet, you can stick rubber feet on the ‘lid’. Alternatively, you can replace the original lid securing screws with Nylon M4 screws. The Nylon screw head then acts as the feet. To allow this, the holes in the enclosure for the original mounting screws will need to be drilled out to 3.5mm, and tapped using an M4 thread tap. ink will be between the enclosure and film when affixed. Use projector film suitable for your printer (either inkjet or laser) and affix it using clear neutral-cure silicone. Roof and gutter silicone is suitable. Squeegee out the lumps and air bubbles before the silicone cures. Once cured, cut out the holes through the film with a hobby or craft knife. For more detail on making labels, see siliconchip.com.au/Help/FrontPanels Panel labels The front and side panel label artwork is available for download from our website. The two side panels show the effects available (1-8 & 9-16). These can be affixed to the sides of the enclosure. Note that there are two front panel labels and you need to select the one which suits your build (rotary switch or pot). A rugged front panel can be made using overhead projector film, with the label printed as a mirror-image so the Final assembly Attach the 9mm-long M3 tapped spacers to the underside of the PCB. These are located just behind CON1 and CON2, and between VR5 and VR6. Secure them using an M3 screw from the top of the PCB. The spacer keeps the PCB in place by resting on the lid when the case is assembled. For the version using VR8, there is another 9mm M3 tapped spacer required near VR8. The ground lug mounting position is adjacent to the DC socket. This is secured using an M3 screw, star washer and nut before the PCB is inserted into the case. Have the solder lug orientated so that the wire is closest to the enclosure base, so it does not foul the components on the PCB. Before mounting the PCB in the enclosure, insert the LEDs into the PCB (longer leads to anode pads, marked “A”). Place the Nylon washers for the footswitch onto its shaft before inserting the PCB into its position in the enclosure. Then feed the LEDs into the bezels to capture them. Solder the LED leads from the rear of the PCB and trim them. The battery compartment is the rectangular cut-out on the PCB. The battery can be prevented from moving with some foam packing sandwiched between the end of the battery and the PCB’s edge. If you are not using the battery option, remove or fully insulate the battery clip at CON3 to prevent the contacts shorting onto a part of the circuit. Knobs An upside-down view of the finished project: the box base becomes the front panel (with appropriate label) and the box lid, with four Nylon screws used as feet, becomes the base. Labels fixed to each side make effect selection simple. siliconchip.com.au Since the potentiometer shafts do not protrude much more than 9mm above the panel, standard knobs with a skirt to cover a potentiometer securing nut will not have sufficient internal fluting length to keep the knobs secured. So use knobs that don’t have the skirt, as listed in the parts list. Australia’s electronics magazine April 2021  33 POWER POWER (Jack plug inserted) (Jack plug inserted) Clip . . .. .. . .. .. . . . IN . OUT .+. .9-12VDC . + . . .(centre . . +)+ + + . . . . . POWER . . . . . (Jack plug inserted) Clip . . .. .. . .. .. . .. .. . .. . +. IN . 9-12VDC . .OUT . . . . (centre +) + . . . . . .. . + + + + . POWER . . . . . . . . . . . . . . . (Jack plug inserted) Min. Min. Max. Max. Min. Max. . . Clip . . . . . . . .Dry . . . . + . .Effects . . . .Effects . . . . . input level . . . mix mix . . . . . . . . . + . . + . . + . .. .. + . . . . . . . . . . . . .. . . Min. Max. Effects input level Min. Max. Dry mix . Min. Max. . Effects . + mix . . .Max. Min. .. Output .level . . . + . . . .. . . Min. Min. Max. . + . . Min. Max. . Effects . input level Min. Max. Min. Max. Min. Max. Min. Dry mix . . . . .. .. SILICON CHIPA Digital FX C B Min. Max. Clip .. . . . . . . .Dry Output . . . . . + . . . . . .Effects . . . . . . . input level mix. level . + .. .. + .. .. + .. .. + .. . . . .. . . . . . . . . . .. . . . Min. Max. .Min. Max. . Min. Max. . . . .C . . . . B A . . . . . . . .. . . . Effects parameters + + + . . . . . . . . . . . . . .. .. Min. Max. . Min. Max. + C . Max. Max. Effects mix . . .. . . . .. .. + . . .. . . Min. . . . . .. . . Max. Effects mix .. . Min O le . .. .. .. .Min. Max. . . . . . .Output .. . . level+ . . + . . . . . . . Min. Max. Min. Min . . Max. . .. C B . . parameters + Effects . . . SILICON CHIP Dig B A . Min. . Max. Min. Max. 8. .9 Effects parameters 10 . . .11 P CHIPB Digital FX SILICONBYP CHIP Digital FX56. . SILICON Y .12 A 8. .9 P T + + 10 + 7 . . + + A . . A 13 C 4 7 8 9 10 6. 11 P . B . S 6 11 B S . H P Y 5 12 S 14 A3 . Y . . 12 S 5 4 13 A P P T 2 + + Effects parameters . . 2 ++ . . . 4 3 1 + .13 .14 CH . . . . A S S 7 1A 16 15 S S 16 15 + + Fig.6 (above): front panels for the two Patch Effect Adjustment C Adjustment B versions of the project – on the right 1 Chorus-reverb Chorus mix Chorus rate is the potentiometer-selected version 2 Adjustment Flange-reverb Flange mix rate Patch Effect C Adjustment B Adjustment Flange A while the left panel is for the switch 3 Tremolo-reverb Tremolo mix Tremolo rate 1 Chorus-reverb Chorus mix Chorus rate Reverb mix selected. Once again, this artwork can be 2 Flange-reverb 4 FlangePitch mix shift Flange rate Reverb mix downloaded from siliconchip.com.au 3 Tremolo-reverb Tremolo mix Tremolo rate Reverb mix 4 5 6 7 8 Pitch shift Pitch echo Test Reverb 1 Reverb 2 5 6 7 8 Patch Fig.7 (right): the side labels arePatch identical Effect 9 for both versions and show at a9 glance Octaver 10 Pitch shift glider what the various combinations10achieve. 11 11 Oil can delay Label 1-8 should be fixed to one side and 12 12 Soft clip overdrive label 9-16 to the other side. 13 Bass distortion 13 14 15 16 Pitch echo Test Echo mix Reverb 1 Reverb 2 Low filter Low filter 14 Aliaser Wah 15 Faux phase shifter 16 Effect Silicon Chip Echo delay Adjustment C Adjustment B + 1 14 15 16 T C H Adjustment A Reverb mix Reverb mix Reverb mix Side panels +/- ~4 semitones Pitch shift Reverb time Reverb time Adjustment C Adjustment B octave Adjustment Octaver Down level A Up octave level DownPitch octaveshift levelglider Up octaveGlide level Dry mix Depth Glide Depth Oil can delay FeedbackRate Chorus width Feedback Chorus width Time rate Volume Tone Soft clip overdrive Volume Tone Gain threshold Bass Dry/wet mix Tone Dry/wet mixdistortion Tone Gain Aliaser Sample rate Filter Q Sensitivity Wah Filter Q Reverb Sensitivity Feedback level Faux phase Time shifter FeedbackSpeed levelwidthTime For the PCB version that uses the rotary switch, you will need to cut the switch shaft, leaving sufficient length for the knob to attach securely close to the panel. Also, a flat will need to be filed on the side of the shaft to form a D-shape suitable for the knob. This will need to be carefully filed so it is a tight fit. The knob pointer will also need to be prised off and orientated correctly. Knob pointer orientation is best found during the testing procedure. While 15 of the 16 positions will give an effect, position six is the test position, and the output signal closely matches the input signal. With the knob rotated to this position, adjust the pointer to line up with 6. Another way is to measure the voltage at the A, B, C and D points at pins 16, 17, 18 and 13 of IC4 when powered 34 Echo mix +/- ~4 semitones Echo delay Pitch shift Low filter High filter High filterLow filterReverb timeHigh filter High filter Reverb time 3 2 Adjustment A Dry mix Rate Time rate Gain threshold Gain Sample rate Reverb Speed width up. Position 1 is when all of these are at 0V. Finally, secure the lid in place using either the original screws or Nylon M4 screws, as mentioned previously. Stick rubber feet to the base if you are not using the Nylon screws as ‘feet’. Removing the knobs After installation, the knobs are likely to be difficult to remove. You will need to lever them off; make sure the lever (such as a flat-bladed screwdriver) is against a packing piece placed on the front panel to prevent damage to the panel. Usage Note that some patches available in the default selec- Australia’s electronics magazine siliconchip.com.au S Parts list – Digital FX Unit 1 double-sided PCB coded 01102212, 86 x 112mm* [SILICON CHIP ONLINE SHOP 01102212] 3 panel labels (one front, two sides – see opposite) 1 diecast aluminium enclosure 119 x 94 x 34mm [Jaycar HB5067] 2 6.35mm PCB-mount jack sockets (CON1,CON2) [Jaycar PS0195] 1 PC-mount barrel socket, 2.1mm or 2.5mm ID (CON3) [Jaycar PS0520, Altronics P0621A] 1 2-pin vertical polarised header, 2.54mm spacing (CON4) [Jaycar HM3412, Altronics P5492] 1 2-pin polarised plug (CON4) [Jaycar HM3402, Altronics P5472 and 2 x P5470A pins] 1 6-way pin header with 2.54mm spacings (CON5) 1 C&K ZMA03A150L30PC microswitch or equivalent (S1) [eg Jaycar SM1036] 1 3PDT footswitch (S2) [Jaycar SP0766, Altronics S1155] 1 Lorlin BCK1001 16-way 4-bit binary-coded switch* (S3) [RS Components 655-3162] 6 B10kΩ linear pots (VR1-VR3,VR5-VR7) [Altronics R1946] 1 B100kΩ linear pot (VR4) [Altronics R1948] 7 11.5mm-diameter 18 tooth spline (6mm) knobs (see text for special requirements) [Altronics H6560, RS Components 299-4783] 1 13mm-diameter D-shaft knob* [Jaycar HK7717] 1 ferrite RF suppression bead 4mm OD x 5mm (FB1) [Altronics L5250A, Jaycar LF1250] 1 40kHz crystal (X1) [Citizen CFV-20640000AZFB or similar; RS components 1849668] 1 9V battery clip lead (optional) 1 9V battery (optional) 1 PC stake (GND point) 1 solder lug (for grounding the enclosure) 4 M4 x 10mm Nylon screws or stick-on rubber feet (see text) 2 9mm-long M3 tapped Nylon standoffs (support for PCB rear) 3 M3 x 6mm panhead machine screws (for solder lug and standoffs) 1 M3 nut and star washer (for solder lug) 1 50mm length of medium-duty green hookup wire 1 6.3mm mono jack plug or jack-to-jack lead (for testing) Semiconductors 3 OPA1662AID dual op amps, SOIC-8 (IC1-IC3) [RS Components 825-8424] 1 SPN1001-FV1 digital FX processor, wide SOIC-28 (IC4) [www.profusionplc.com/parts/spn1001-fv1] tions use the A, B and C parameter adjustments while other patches only use adjustment A. Also, some effects give you control over the effect/dry mix while others do not. See the side panel labels (opposite) for details. When the effects parameters include a mix control, the main dry mix control should be set fully anticlockwise, the effects mix control set fully clockwise, and the mixing done with the parameter mix control(s). Where an effect has no mixing control, the dry mix level adjustment provided can be used instead. When connecting to an amplifier, it should have a switch siliconchip.com.au 1 24LC32A-I/SN EEPROM, SOIC-8, programmed with 0110221A.hex (IC5) 1 1N5819 1A schottky diode (D1) 1 LD1117V33C 3.3V low-dropout regulator (REG1) [RS Components 6869767] 1 3mm high-intensity green LEDs (LED1) 2 3mm high-intensity red LEDs (LED2, LED3) Capacitors 4 100µF 16V PC electrolytic 1 22µF 16V PC electrolytic 4 10µF 16V PC electrolytic 2 4.7µF 16V PC electrolytic 1 1µF 16V PC electrolytic 5 100nF MKT polyester 2 1.2nF MKT polyester 1 1nF MKT polyester 2 560pF ceramic 2 100pF NP0/C0G ceramic 1 15pF NP0/C0G ceramic Resistors (all 1/4W, 1% metal film axial) 1 1MΩ (Code brown black black yellow brown) 1 100kΩ (Code brown black black orange brown) 2 20kΩ (Code red black black red brown) 12 10kΩ* (Code brown black black red brown) 1 1kΩ (Code brown black black brown brown) 3 200Ω (Code red black black black brown) 3 100Ω (Code brown black black red brown) Parts for version using a potentiometer for effects selection (delete items marked * above) 1 double-sided, plated-through PCB coded 01102211, measuring 86 x 112mm 1 B10kΩ linear potentiometer (VR8) [Altronics R1946] 1 11.5mm-diameter 18 tooth spline (6mm) knob (see text for special requirements) [Altronics H6560, RS Components 299-4783] 1 9mm-long M3 tapped Nylon standoff (support for rear of PCB) 1 M3 x 6mm panhead machine screw (for standoff) 1 PIC12F1571-I/SN 8-bit microcontroller programmed with 0110221A.hex, SOIC-8 (IC6) 1 2N7000 N-channel small-signal Mosfet (Q1) 1 3mm high-intensity red LED (LED4) 2 100nF MKT polyester capacitors 1 1.2MΩ 1/4W 5% carbon axial resistor 8 10kΩ 1/4W 1% metal film axial resistors 1 200Ω 1/4W 1% metal film axial resistor that allows the jack’s shield connection to be either Earthed or floating. A guitar with piezo pickups should have less hum when the switch is selected to connect to Earth. Next month We’ll have a follow-up article next month that describes how to create and load your own effects into the EEPROM chip, changing the nature of effect selections 8-15. This can be done using freely available software and a Microchip PICkit 2 or PICkit 3 programmer. SC Australia’s electronics magazine April 2021  35