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Digital effects processor for guitars & musical instruments By NICHOLAS VINEN This deceptively simple unit provides 10 different musical instrument effects, including echo, reverb, tremolo, fuzz, compression, flanging and phasing. Each effect is adjustable and can be defeated with a foot pedal switch. It’s designed for use with electric guitars but will work with other instruments and vocals too. W ANT TO SPICE up your guitar performances? Build this Digital Effects Processor into a guitar amplifier and you will get many different effects to play with, without needing to lug around and wire up many different effects pedals. It can apply the majority of common effects to a line-level signal and you can adjust them to suit your needs. We can’t promise that this will replace all your effects units but it certainly gives a lot of different options which suit a variety of instruments, 58  Silicon Chip performers and musical styles. The idea is to build it into a guitar amplifier by connecting it between the preamplifier and amplifier sections. It can be powered directly from an amplifier supply rail, assuming a suitable DC voltage is available, or the supply rail can be derived, creating one very convenient package! But it is not just intended for use with guitars. It is suitable for use with a large variety of other musical instruments, whether they are keyboards or instruments with pickups. And they can be used to enhance vocals as well. Basically, if you want to add some pizazz to your performance, this Digital Effects Processor is a great way to do it. Digital effects The available effects are shown in Table 1. For each effect, there are two parameters which can be set using potentiometers VR3 and VR4. Those parameters are also listed in the table and described in the list of effects below. Note that when one of the enabled siliconchip.com.au effects causes a reduction in signal level (eg, echo or reverb), the level for all effects is reduced, as well as the level when no effect is selected, in order to prevent noticeable changes in signal when switching between them. The available effects are as follows: •  Echo: delays and attenuates the incoming signal, then mixes it back in for the output. VR3 adjusts the delay between (nearly) 0ms and 1200ms with an exponential curve, to make it less sensitive at the shorter end, which is more useful. VR4 adjusts the amount of attenuation; at higher settings, the echo is louder. Note that as the echo becomes louder, the original signal must become quieter to prevent overload. •  Reverb:  the same as echo, except that many extra short echoes are added to simulate reflections from multiple hard surfaces in close proximity. •  Tremolo: the output volume is modulated by a sinusoidal waveform. VR3 adjusts the amount of modulation (ie, ‘depth’) while VR4 changes the frequency. •  Vibrato: the output frequency is modulated by a sinusoidal waveform. VR3 adjusts the amount of modulation (ie, ‘depth’) while VR4 changes the frequency. Note that this is performed by slightly speeding up and slowing down the audio signal although the change in delay that this causes should be imperceptible. •  Overdrive:  this provides adjustable clipping for the signal. VR3 adjusts the gain applied to the signal and once the amplitude is high enough, it clips. VR4 adjusts how progressively the clipping occurs; at minimum setting it is hard, resulting in a square wave while at higher settings for VR4, the clipping is more progressive and the waveform becomes rounded. •  Fuzz:  the same as overdrive except that the gain is applied asymmetrically, in order to inject extra distortion into the signal. •  Compression: the gain is slowly increased until the output reaches 90% of maximum. If the output exceeds this 90% level, the gain is decreased. VR3 sets the rate of increase while VR4 sets the rate of decrease. The scale for VR4 is different for VR3 as the rate of decrease is normally much higher. •  Noise gate:  similar to but not quite the opposite of compression. When the input signal is below the threshold, there is no output. When the input goes above the threshold, it is sent to siliconchip.com.au Features & Specifications •  10 effects to choose from: Echo, Reverb, Tremolo, Vibrato, Overdrive, Fuzz, •  •  •  •  •  •  •  •  •  •  •  Compression, Noise Gate, Flanger and Phaser Each effect has two adjustable parameters Maximum echo/reverb delay: 1.2 seconds Four-position switch selects between three effects and no effect with seamless transitions Optional defeat switch (eg, foot pedal) Low noise and distortion: THD+N typically <0.02%, signal-to-noise ratio >76dB Two power supply options: 3.5-6V DC or 7.5-12V DC; current drain 60-80mA Optimal input signal range: 0.5-2V RMS Line output signal: typically 1V RMS Input impedance: 4-6kΩ Optional headphone output Optional microphone preamplifier the output. VR3 adjusts the threshold while VR4 adjusts the hysteresis, to prevent the output from fluctuating on and off with a signal near the threshold. •  Flanger:  this mixes the input signal with a version of the signal that has slight vibrato applied, causing a distinctive ‘comb filter’ Doppler effect. •  Phaser:  similar to flanger but mixes the signal with a version that has a modulated phase shift, causing a ‘rippling’ effect which makes the sound seem artificial. Modes The effects are selected using a 4position rotary switch (a slide switch could also be used). The second position selects no effect and the other three positions can each select one of the 10 effects listed above. We’ve made the second position the ‘off’ position to make it easy to switch between two commonly used effects and none. A pushbutton is used to change which effect is selected by each switch position and is also used when adjusting the two knobs, so that different settings can be used for each effect and they don’t have to be reset each time a different effect is used. The same effect can be used with different settings for each switch position. If you don’t need three effects, the switch can be limited to fewer positions. There is also a simple fall-back mode available; if the pushbutton is permanently shorted out (say, on the PCB) then the three effects selected by the rotary switch are always echo, reverb and tremolo and VR3 & VR4 can be adjusted at any time. A defeat switch (eg, a foot switch) can be added and this has the same effect as switching the rotary switch to the ‘off’ position as long as it is held down. Or you can wire it the other way around, so that effects are only applied when the switch is held down. Options This Digital Effects Processor uses Table 1: Effects Controls # 1 2 3 4 5 6 7 8 9 10 Effect Echo Reverb Tremolo Vibrato Overdrive Fuzz Compression Noise Gate Flanger Phaser VR3 Echo Delay Reverb Delay Amplitude Amplitude Gain Gain Attack Threshold Amplitude Amplitude VR4 Echo Fall-Off Reverb Fall-Off Rate Rate Softness Softness Decay Hysteresis Rate Rate October 2014  59 4.7Ω 2x 100nF 2x 100 µF 1000 µF MMC 14 8 20 1 µF MMC 19 1k 18 17 1nF HPVdd AVdd LLINEIN 2x 100 µF 10k 27 1 DBVdd DCVdd 21 MODE 9 LHPOUT RLINEIN LOUT MICIN 12 10 RHPOUT IC3 WM8731 13 25 XTI/MCLK ROUT CODEC VR6 5k 26 7 6 1 µF TO PIN 62, IC1 Rmic MMC 2 1 OPTIONAL MIC INPUT 2x 100nF FB1 MMC INPUT CON1 +3.3V 3 MIC 680Ω BIAS CON9 2 X1 12MHz MICBIAS ADCLRC DACDAT ADCDAT SCLK BCLK SDIN CSB CLKOUT VMID HPGND AGND DGND 16 47k 220pF 33pF 33pF 5 DACLRC XTO 15 11 4 24 23 22 28 100nF 22 µF MMC L1 100 µH +3.3V NO 100nF S4 19 39 40 50 51 42 55 54 48 53 52 21 49 NC DEFEAT SWITCH DELAY AUX4 (PIN 1, CON5) POT1 VR3 10k AUX1 DEPTH VR4 10k 11 33 34 36 37 POT2 35 100nF 60 61 62 63 64 1 2 3 TO OPTIONAL MIC INPUT FB2 ANALOG GND DIGITAL GND 56 26 10 AVdd Vdd CLKI/RC12 CLKO/RC15 SCK1/RD2 RPD3/RD3 RD8 RD7 RD6 RC14 PMRD/RD5 PMWR/RD4 AN8/RB8 AN24/RD1 VBUSON USBID VBUS D– D+ VUSB3V3 PMD0/RE0 PMD1/RE1 PMD2/RE2 PMD3/RE3 PMD4/RE4 PMD5/RE5 PMD6/RE6 PMD7/RE7 Vcap 10 µF AVss 20 Vdd 57 38 Vdd Vdd MCLR RF1 PGED2 PGEC2 RD0 RC13 RF0/RPF0 RD9/RPD9 RB4 RB3 RB2 RB1 IC1 PIC3 2 MX470- RB9/PMA7 PIC32MX470F512H RB10/PMA13 RB11/PMA12 RB12/PMA11 RB13/PMA10 RB14/PMA1 RB15/PMA0 RD11/PMA14 RD10/PMA15 RF5/PMA8 RF4/PMA9 RB0/PMA6 RG9/PMA2 RG8/PMA3 RG7/PMA4 RG6/PMA5 Vss Vss Vss 9 25 7 59 18 17 46 47 58 43 12 13 14 15 22 23 24 27 28 29 30 45 44 32 31 16 8 6 5 4 41 D2 1N4004 7.5 – 12V DC INPUT POWER K V+ D1 1N4004 A K REG1 LM317 3.3Ω IN S1 CON3 LED1 OUT ADJ 10k POWER A A K 120Ω A 1000 µF λ +3.3V D3 1N4004 200Ω 100 µF 100 µF K SC 20 1 4 DIGITAL EFFECTS PROCESSOR 60  Silicon Chip siliconchip.com.au +3.3V HEADPHONES 220 µF 10V 1 47k 2 3 220 µF 10V CON8 47k +3.3V TO IC1 PIN 11 VR7 OPTIONAL STEREO HEADPHONE OUTPUT OUTPUT 100Ω 1 µF MMC CON2 47k +3.3V 4x 100nF 10k ICSP SKT 1 2 3 PGED 4 PGEC 5 CON7 S2 CHANGE EFFECT 1 AUX4 2 SDO NOT MOUNTED ON PCB 3 SCK 4 V+ D4 & D5 1N4148 5 +5V 6 +3.3V 7 EFF. 3 PGED 8 EFFECT 2 AUX1 S3 MODE OFF PGEC 9 10 EFFECT 1 CON5 EXPANSION SOCKET LED1 1N4004 A K A K LM317T 1N4148 A K siliconchip.com.au OUT ADJ OUT IN Fig.1: the basic Digital Effects Processor circuit. The incoming audio analog signal at CON1 is digitised by CODEC IC2 and then fed to IC1 where it is processed and then sent back across the same digital audio bus to IC2. A DAC in IC3 then converts it back into an analog signal which is fed to the output (CON2). the same hardware as the Stereo Echo & Reverb Unit (February 2014) and the Dual Channel Audio Delay (November 2013). However, we have removed a number of components which aren’t needed. For example, most musical instruments are not stereo so components are only fitted for one channel (and indeed, the software only supports one channel). As with those earlier designs, it is possible to add extra components to provide a microphone input or stereo headphone output. The processed mono signal is sent simultaneously to both headphone output channels. The headphone output could be useful for monitoring purposes. It’s up to you whether you want to install the few extra components required which are shown in the circuit diagram at upper-right and on the overlay diagram, labelled in green. The microphone input is less useful as its signal-to-noise ratio is only average. For a musical performance, you would be better off using an external microphone preamplifier such as our High-Performance Microphone Preamplifier from the September 2010 issue, which can run from the same DC voltage source as the Digital Effects Processor unit. Software In adding these new effects to the software, we have made some other changes at the same time. By making it process only a mono signal, this doubles the maximum echo to 1.2 seconds without needing an external RAM chip. This is more than long enough for instrumental work and so we haven’t even bothered to provide the option of extra RAM in the software. We’ve also gone to some effort to make changes between effects and changes in effect settings ‘seamless’ so that clicks and pops are not generated during a performance, even if settings such as echo delay are adjusted live. Circuit description The circuit diagram of the Digital Effects Processor is shown in Fig.1. As stated earlier, this is a simplified version of the circuit for the Stereo Echo & Reverberation Unit from the February 2014 issue, with unnecessary components removed. That’s why there are so many unconnected pins on IC1; those originally used for interfacing with the unused SRAM chip and USB socket are not connected to anything. A line level signal, from a guitar preamp, mic preamp etc, is fed into CON1 (connector tip). RF signals that may have been picked up are rejected by a low-pass filter comprising a 1kΩ series resistor and 1nF capacitor to ground, while 5kΩ trimpot VR6 is used to reduce the level to no more than 1V RMS, the limit of what the CODEC can handle. The signal is then AC-coupled to the right channel input of the CODEC (IC3) via a 1µF DC-blocking capacitor. A half-supply (~1.65V) DC bias for this input is provided by the IC itself Alternatively, a microphone signal can be applied to a 3.5mm jack socket connected to pin header CON9 and this is coupled to IC3’s microphone input pin (pin 18) via a 1µF capacitor and optional series resistor (Rmic) which reduces the amount of gain if fitted; otherwise it is linked out. IC3 can supply a bias current for electret microphones, and this is fed via a 680Ω series resistor. The associated 220pF capacitor provides some RF filtering for the microphone signal. The microphone input is selected when the RE2 input of IC1 (pin 62) is pulled low. This is wired to the microphone socket so that the sleeve of the mono jack plug shorts it to ground when it is inserted. When this line is open-circuit, the line input is the active input. If the microphone input is not needed, the components in the pink box at left do not need to be installed. CODEC operation Whichever signal is selected, it is digitised by IC3 with a sampling rate of around 40kHz and the resulting PCM digital audio signal is transmitted to PIC32 microcontroller IC1 via an I2S bus. This appears at pins 3, 5 & 6 of IC3 which are the serial bit clock, sample clock and serial data line respectively. These connect to the audio CODEC compatible SPI peripheral in IC1. IC1 reads the digital audio data from the CODEC, processes it to add the seOctober 2014  61 Fig.2: follow this layout diagram to build the PCB and complete the wiring. The parts labelled in blue & green are for the optional microphone and headphone features. VR7 HEADPHONE VOLUME S HEADPHONE OUTPUT T R LED1 POWER OUTPUT 100Ω 100 µF 1nF 10k 3.3Ω 1nF CON7 ICSP + 10 µF 100nF 4 1000 µF 100 µF EFFECT 1 2 1 D4 A 5 6 S3 1k CON1 + 48 120Ω 200Ω OFF EFFECT 2 3 100 µF 1 PIC32MX470F 1 D2 4004 48 IC1 D3 4004 10k 100nF 100nF 100nF 100nF D1 41 1 µF 33pF + 22 µF 5 L1 100nF 100nF 100 µH 100nF + 1000 µF CON2 100nF 100 µF + X1 10k 220 µF FB2 33pF 1 µF + 100 µF + 4.7Ω 100nF 220 µF 47k 47k 47k R PHONES CON8 100 µF+ REG1 LM317 + + + GND 1 µF FB1 L POWER 01110131 Stereo Audio Delay/ DSP Board 24bit/96kHz 41 DEPTH S1 4004 DELAY K A S2 DEFEAT IC3 WM8731L MIC CON9 VR4 0Ω 47k + 220pF 680Ω CON5 VR3 S2 CHANGE EFFECT 7 8 12 11 D5 10 9 CON3 DC 7.5 –12V INPUT VR6 5k T R R S T S MICROPHONE INPUT lected effect (depending on the mode) and also stores it within its 128KB RAM, for the echo and reverb effects. Processed audio data is sent back over the same I2S bus, this time to pin 4 of IC3 but timed using the same clock lines. The CODEC then converts this digital stream back to analog audio data which it transmits from its line out (pin 13) and headphone out (pins 9 & 10). These signals are all AC-coupled to the respective output connectors, to remove the 1.65V DC bias, via a 1µF capacitor for the line output and 220µF capacitors for the headphone outputs. The reason the headphone output needs much larger capacitors is that the headphones will have a much lower impedance than the line input of other equipment; 8-600Ω for headphones compared to several kilohms for a line input. The line output also includes a 100Ω series resistor, both to prevent cable capacitance from causing instability in 62  Silicon Chip TO FOOT SWITCH the output drivers of IC3 and to protect IC3 against a shorted output. IC3 also contains a digital volume control which adjusts the headphone amplifier output. If VR7 is fitted to the board, IC1 detects this and sends commands to IC3 so set the headphone volume depending on the voltage at VR7’s wiper. If VR7 is not fitted, the headphone outputs are disabled and in that case, the other components in the pink box may be omitted. Controls & power supply Pots VR3 and VR4 are used to change the effect parameters. These form voltage dividers across the 3.3V supply rail and the wiper voltage is read by IC1 using its internal analog-to-digital converter (ADC). The power supply is quite simple. D1 provides reverse polarity protection while REG1 drops the incoming 7.512V rail to a regulated 3.3V, as required by IC1 & IC3. LED1 indicates when power is applied. IC1 and IC3 have 100nF bypass capacitors for each pair of supply pins, plus a 10µF capacitor for IC1’s internal core regulator (on pin 56, Vcap). CODEC IC3 also has 100µF bypass capacitors for each supply pin to ensure a low supply impedance and thus good performance. Its analog and digital grounds are separated by ferrite bead FB2 to minimise digital noise coupled into the analog ground, where it could otherwise reach signal paths. A 4.7Ω series resistor also provides audio-frequency low-pass filtering for the analog supply, in conjunction with the 1200µF of capacitance on the analog supply rail (ie, 1000µF plus 2 x 100µF). 5V operation As with the Echo & Reverb unit, you can change some components to operate the unit from a 5V supply such as is available from a USB port. This siliconchip.com.au right-handed, it’s easiest to start with the top pad on the right side or if lefthanded, with the top pad on the left side. Avoid getting any solder on the adjacent pad. Now, pick up the part with a finetipped pair of angled tweezers and while heating this pad, gently slide the IC into place. Check the part’s alignment under a magnifying lamp. All the pins must be centred fairly accurately over their respective pads. If they aren’t, don’t panic, it’s just a matter of re-melting the solder on that one joint and carefully nudging the IC in the required direction, then reinspecting it. It may take a few attempts to get it correct. Care and patience are a virtue here, the goal being to eventually get it properly aligned without spreading solder onto any more pins or pads and without heating the PCB or IC enough to damage them. Once the part is in place, solder the diagonally opposite pin, then re-check the alignment under magnification as it may have moved slightly. If it has, you can reheat this second pad and gently twist the IC back into alignment. Once you’re happy, proceed to solder the remaining pins without worrying too much about bridging them (it’s hard to avoid). Remember to refresh that first pin you soldered. Now spread a thin layer of flux paste along all the pins and gently press down on them with solder wick and a hot iron to suck up the excess solder. If done correctly, this will leave you with neatly soldered pins and no solder bridges. Go over all the pins once with the solder wick, then check under a magnifier for any remaining bridges. If there are any, add a dab of flux paste and go back over them with the solder wick. With all the joints looking good, you can install the other SMD IC using the same procedure. Note that a hot-air This photo shows the completed PCB with the rotary Mode switch (S3) and the Change Effect switch (S2) wired to CON5. arrangement is shown in Fig.3. Basically, REG1 and its associated components are deleted and an LM3940 low-dropout 3.3V linear regulator is substituted. This is necessary because the LM317 used for higher voltage supplies drops too much voltage and can’t operate from 5V. Also D1 is replaced with a 1N5819 Schottky diode which has a much lower forward voltage. Construction Fig.2 shows the parts layout on the PCB (code 01110131). If building the 5V-powered version, refer also to Fig.4 for the necessary changes to fit the different regulator and Schottky diode (D1). Start by fitting SMDs IC1 and IC3 (IC2 is left out). In each case, place the IC alongside its pads, right-side up and identify pin 1 (there should be a depression in one corner but magnification may be required to spot it). A pin 1 dot is also shown on the overlay diagram and PCB. Apply a very small amount of solder to one of the corner pads. If you are Table 1: Resistor Colour Codes   o o o o o o o o o siliconchip.com.au No.   1   5   1   1   1   1   1   1 Value 47kΩ 10kΩ 1kΩ 200Ω 120Ω 100Ω 4.7Ω 3.3Ω 4-Band Code (1%) yellow violet orange brown brown black orange brown brown black red brown red black brown brown brown red brown brown brown black brown brown yellow violet gold brown orange orange gold brown 5-Band Code (1%) yellow violet black red brown brown black black red brown brown black black brown brown red black black black brown brown red black black brown brown black black black brown yellow violet black silver brown orange orange black silver brown October 2014  63 4-6V DC INPUT POWER V+ D1 1N5819 A REG3 LM3940IT-3.3 3.3Ω K IN S1 CON3 GND 10k POWER LED1 +3.3V OUT A 470 µF 100 µF λ LED1 K K SC  20 1 4 A Through-hole parts K A 1N5819 DIGITAL EFFECTS PROCESSOR 5V POWER SUPPLY OPTION LM3940 GND IN GND OUT Fig.3: the unit can be powered from a 4-6V DC supply by replacing REG1 with an LM3940IT-3.3 low-dropout regulator and changing D1 to a 1N5819. gun/toaster oven and solder paste can also be used for these ICs. Once you’ve checked that the ICs are all soldered properly, follow with the SMD ceramic capacitors, using a similar procedure; ie, add solder to one pad, heat and slide the part into place, then solder the other pad and refresh the initial joint. Don’t get the 10µF capacitor mixed up with the others. especially if solder has taken to the other pad too. So take it slowly and be careful not to short any of the adjacent IC pins when soldering the pads; the capacitors have been placed quite close for performance reasons. A fine soldering iron tip will make this easier. You do need to be careful to wait about 10 seconds after soldering one side of a capacitor before applying solder to the other side though. The capacitors are so small that the solder joint can remain molten for quite some time. If you try to solder the opposite pad too early, the capacitor will move out of alignment and it’s frustrating to re-align capacitors when this happens, Proceed now with the low-profile components such as resistors and diodes – remember to slip a ferrite bead over the 4.7Ω resistor lead before soldering it in place. It’s best to check each resistor value with a DMM before fitting it, as the colour bands can be difficult to read. The diodes are all the same type and all have their cathode bands facing to the top or right edge of the board. For FB2, slip another bead over a resistor lead off-cut and then solder it to the board. You can also mount axial inductor L1 at this point. Then fit REG1 or REG3 (depending on supply voltage); bend its leads down about 6mm from its body, feed them through the holes, screw its tab to the PCB tightly Parts List 1 double-sided PCB, coded 01110131, 148 x 80mm 1 12MHz HC-49 crystal (X1) 1 100µH axial RF inductor (L1) 2 10kΩ 9mm horizontal potentiometer (VR3,VR4) 1 5kΩ mini horizontal trimpot (VR6) 2 6.35mm PCB-mount stereo switched jack sockets (CON1,CON2) (Jaycar PS0195, Altronics P0099 or P0073) 1 10-way pin header, 2.54mm pitch (CON5) 1 5-way pin header, 2.54mm pitch (CON7) (optional) 1 PCB-mount SPDT right-angle toggle switch (S1) (Altronics S1320) 1 chassis-mount NO momentary pushbutton switch (S2) 1 4-position rotary or slide switch (S3) 1 3-way pin header, 2.54mm pitch (for S4) 1 foot switch with cable (S4, optional) 1 DC plugpack, 7.5-12V, 100mA+ 1 PCB-mount switched DC socket to suit plugpack 2 4mm ferrite suppression beads 64  Silicon Chip 9 M3 x 6mm machine screws 1 M3 nut 4 tapped spacers 1 metal case (optional) Light duty hook-up wire/ribbon cable Resistors (0.25W, 1%) 1 47kΩ 1 120Ω 5 10kΩ 1 100Ω 1 1kΩ 1 4.7Ω 0.5W 5% 1 200Ω 1 3.3Ω 0.5W 5% Semiconductors 1 PIC32MX470F512H-I/PT 32-bit microcontroller programmed with 0120914A.hex (IC1) (available from SILICON CHIP Online Shop) 1 WM8731SEDS 24-bit 96kHz stereo CODEC (IC3) (element14 1776264) 3 1N4004 diodes (D1-D3) 2 1N4148 diodes (D4,D5) 1 LM317T adjustable regulator (REG1) (refer to text for parts required for 5V DC operation) 1 3mm blue LED (LED1) Extra parts for headphone output Capacitors 2 1000µF 25V electrolytic 6 100µF 16V electrolytic 1 22µF 16V electrolytic 1 10µF 6.3V 0805 SMD ceramic 1 1µF 50V monolithic ceramic 11 100nF 6.3V 0805 SMD ceramic 1 1nF MKT 2 33pF ceramic disc 1 panel-mount stereo jack socket 1 10kΩ linear potentiometer, panel mount (VR7) 1 small knob to suit 2 220µF 10V electrolytic capacitors 2 47kΩ 0.25W resistors 1 3-way pin header 1 100mm length 2-core shielded cable or 3-strand ribbon cable 1 100mm length 3-strand ribbon cable Extra parts for microphone input 1 3.5mm panel-mount stereo jack socket 1 1µF multi-layer ceramic capacitor 1 220pF ceramic capacitor 1 47kΩ 0.25W resistor 1 680Ω 0.25W resistor 1 2-way pin header 1 length shielded cable 1 length light-duty hookup wire siliconchip.com.au 5819 10k 3.3Ω 1 100nF 10k 100nF 100nF 1 µF 10k IC3 WM8731L 47k 100nF 0Ω 47k 47k 47k + GND 680Ω and then solder and trim the leads. Horizontal trimpot VR6 can go in LED1 next, followed by the ceramic capaciPOWER K A S1 tors (disc and monolithic multilayer) VR3 VR4 and then pin headers CON5 and CON7, POWER DEFEAT DELAY 2 plus the one for S4. You willDELAY also1 01110131 have to fit CON8/CON9 if you are usStereo Audio Delay/ 220pF ing those optional features. Note that + DSP Board 24bit/96kHz 1 µF D1 CON7 is not required if you have a MIC 100 µF+ FB1 100 µ F pre-programmed microcontroller. 33pF + 100 µH 4.7Ω X1 + Now solder the DC socket in place, 220 µF L1 L FB2 100nF followed by VR3 and VR4. Note that 100nF 100nF 33pF 100 µF + + 100nF you could mount all these components 1 + IC1 off-board (eg, chassis mount them) and R CON7 + PIC32MX470F PHONES ICSP µF 1000 µF run them back to the pads via220 flying 100nF 22 µF leads, if that suits your application. 1 µF 100 µF 1nF 1nF 10 µF Link Link This would be the way to fit the unit 100nF into a guitar amplifier, for example. 1k 470 µF REG3 CON2the CON1 CON3 DC You can then fit crystal X1 and LM3940IT-3.3 4-6V electrolytic capacitors, of which there OUTPUT INPUT + are three different values (four, if using VR6 5k the headphone outputs). As usual, the 100 µF longer lead is positive and this should ALTERNATIVE SUPPLY ARRANGEMENT FOR 4-6V DC go in the hole marked with the ‘+’ symbol on the overlay, ie, towards the Fig.4: follow this PCB parts layout diagram to install the parts for the 5V power top edge of the board. supply option. Note that D1 must be changed to a 1N5819 Schottky type. Next, fit power switch S1 and the power LED. The latter should have programming the chip. Use the firm- of amplifier. You should hear clear, its lead bent at right angles 4mm from ware for the Digital Effects Processor undistorted audio with no effects. You the base of the lens and then soldered which is named “0120914A.hex”. can then try out the effects to check so that the centre of the lens (and thus If you don’t have a PICkit3, you that they operate as expected. this short lead section) is 6.5mm above will need to power the unit from a the top surface of the PCB. This aligns DC plugpack for testing. In this case, Using effects the centre LED with the centre of the connect a voltmeter across the 3.3Ω Initially, the effect for switch posiswitch. When bending the leads, pay resistor next to D1. Small alligator clip tion #1 is echo, position #3 is reverb attention to the “A” and “K” mark- leads (or other test probe clips) are and position #4 is tremolo, so you ings on the PCB as the longer (anode) very useful for this purpose as you can can easily try these out. To adjust the lead of LED1 must be soldered to the switch the unit on while watching the parameters, hold down S2 and then roanode pad. meter reading and switch it off quickly tate VR3 and/or VR4. Once you release S1 and the power LED could also be should the voltage across this resistor S2, turning VR3 and VR4 will have no chassis-mounted if you wish. rise too high. effect, so you can’t accidentally change The PCB assembly can now be comExpect a reading in the range of the settings. pleted by soldering jack sockets CON1 0.2-0.3V, depending on the exact To assign a different effect to one and CON2 in place. You will also need resistor value and how you have con- of these switch positions, select that to wire up a rotary or slide switch (S3) figured the unit. Much less than 0.2V position and then give S2 a brief press and a momentary pushbutton switch indicates that there is an open circuit without turning either VR3 or VR4. (S2) to a pin header socket, as shown somewhere while much more than The unit will switch to the next effect in Fig.2. 0.3V indicates a likely short circuit. and will emit a series of ‘pips’ from If using the foot (defeat) switch, If the reading is outside the expected the audio output; one pip for effect #1 headphone or microphone options, range, switch off immediately and (echo), two for effect #2 (reverb) and wire them up too. We’ve shown the check for faults. so on. If you press S2 when effect #10 foot switch connected via a 3.5mm The most likely faults would be pins (phaser) is selected, it will switch back phono socket but you could use a on the SMD chips bridged to an adja- to #1 (echo). 6.35mm socket or some other connec- cent pin or not properly soldered to the The settings are remembered even tor instead. PCB pad, followed by incorrect device when power is removed; they’re stored orientation (primarily ICs, diodes and in flash memory. If you press S2 in Checking it out electrolytic capacitors) or poor/bridged order to adjust VR3/VR4 and then If you used a blank PIC32 chip, through-hole solder joints. decide against it, hold S2 down for program it now. The circuit can be Assuming all is OK, connect S3, set a short period before releasing it to powered from a PICkit3 at 3.3V. In fact it to position 2 (no effect) and feed a prevent an undesired change in the the whole unit will operate normally signal into the input; if it’s a stereo selected effect. Any press longer than from this supply so you can test the plug, the left channel will be shorted about half a second will not cause the SC audio signal path immediately after out. Connect the output to some sort selected effect to change. 100Ω + siliconchip.com.au October 2014  65