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Stereo Echo & Reverb Unit By NICHOLAS VINEN Based on the Stereo Audio Delay featured in the November 2013 issue, this modified unit can be used to provide adjustable echo or reverberation for recording or public address (PA) work. By using revised software and slight changes to the circuitry, we show how the same hardware can provide these different functions. We’ll also describe some extra features that can be useful in either mode. I N ESSENCE, the Stereo Audio Delay described in the November 2013 issue consists of an analog-to-digital converter (ADC) and a digital-toanalog converter (DAC), with a PIC32 microcontroller processing the digital audio stream between the two. This microcontroller has a large internal RAM (128KB) which, together with an optional external 1MB SRAM chip, can be used for buffering and manipulating the audio data stream. By controlling how much of this memory is used for buffering, the PIC32 can delay the audio by a variable amount. But it can also process the audio data and perform some sort of filtering, for example. In fact, providing an adjustable echo effect requires only a small amount of additional processing compared 28  Silicon Chip to what’s needed for audio delay; we simply need to mix a proportion of the delayed audio back into the input signal. This simulates a real (acoustic) echo, whereby sound waves travel a significant distance, resulting in a time delay (since sound travels at around 340m/s at sea level). The attenuation of expanding sound waves travelling through a significant volume of air, along with the losses inherent in reflections off less-than-perfect surfaces, result in the volume of the echo being lower than that of the original sound. The echo itself has an echo, so that a single transient sound has a number of echoes, spaced equally apart in time and with a decaying sound level. This aspect of echo is also simulated by the above simple method. That’s because by mixing an attenuated version of the delayed signal back into the input signal, that echo itself is delayed and attenuated, and so on ad infinitum until the volume has decayed so far that it is no longer audible (see Fig.1). Fig.3 shows the circuit of the Stereo Echo/Reverberation Unit. It’s basically just the Stereo Audio Delay described in November 2013 with various optional extra bits added on (plus the revised software for the micro). Provision was made on the original PCB to accept these extra bits, so you don’t have to start from scratch with a new board. Instead, it’s just a matter of building the PCB has originally described and adding the extra parts. Enabling echo mode As it stands, the November 2013 siliconchip.com.au Features & Specifications •  Adjustable stereo echo or reverb with interval of 0-640ms •  Echo delay and attenuation adjustable via front panel knobs •  Optional defeat switch connection for foot pedal; can configured as normally on or normally off •  •  Signal-to-noise ratio: typically >76dB (line inputs/outputs) •  Optimal line input signal range: 0.5-2V RMS •  Line output signal: 1V RMS •  Input impedance: 4-6kΩ (line input), 8.2kΩ (microphone input) •  Power supply: 7.5-12V DC or 3.8-6.5V DC, depending on configuration; current drain THD+N: <0.03% (typically <0.02%), 20Hz-20kHz (20Hz-22kHz bandwidth) 60-80mA •  Microphone input: 20-50mV input for full scale output, signal-to-noise ratio 67dB. •  Headphone output: drives 8-32Ω at up to 50mW, THD+N 0.05% <at> 10mW/32Ω, volume adjustable in 1dB steps The new unit is built on the same PCB as the Stereo Audio Delay Unit described in November 2013. It’s just a matter of adding a few extra parts and using revised software. unit can be switched from delay mode to echo mode by placing a shorting jumper across pins 3 & 4 of CON7, the ICSP header. When the unit is powered up, the software briefly attempts to pull pin 4 high and checks its state. With no jumper plugged into CON7, this pin will be sensed as high and so the unit will perform its default task which is to provide audio delay. However if pins 3 & 4 are shorted, pin 4 will remain low despite the pull-up and so echo mode is activated. As explained last November, pins 4 & 5 of CON7 are normally PWM signals generated by the microcontroller which can be measured in order to determine the configured delay in milliseconds. But if the software detects that pin 4 is shorted to ground at start-up, it disables this PWM output in order to avoid driving this short circuit. You can measure the echo Fig.1: this scope grab shows the input (yellow) and output (green) signals when the unit is set to echo mode with a delay of approximately 70ms and an attenuation of around 6dB. The initial burst is output immediately at a somewhat reduced level, followed by echoes, of which the first two are shown. Each is lower in amplitude compared to the previous echo. siliconchip.com.au delay at pin 5; in echo mode, the unit will only operate in stereo so there is only one delay to measure. The other reason that echo will only operate in stereo is that in this mode, VR1 (or VR3) is used to set the echo delay while VR2 (or VR4), if present, sets the echo attenuation. If neither VR2 nor VR4 are installed, then the attenuation is set to 12dB. As with the regular delay mode, a delay of up to 600ms is available without the external SRAM chip IC3 fitted or up to six seconds with IC3 in place. But 600ms is quite a long delay and should be sufficient for most echo effects. Fig.2: this scope grab shows the same waveforms as in Fig.1 but this time with reverb mode enabled and using a similar delay. In this case, the echoes are even lower in amplitude but they are followed almost immediately by a further series of ‘sub-echoes’ which themselves decay fairly rapidly. This makes for a more complex echo effect with greater ‘depth’. February 2014  29 4.7Ω 1k 2x 100nF 2x 100 µF 1000 µF 1 µF MMC 20 1 µF MMC VR5 5k 19 1k CON1 18 17 1nF MMC 14 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 AUX1 (PIN 7, CON5) 6 1 µF Rmic MMC 2 1 OPTIONAL MIC INPUT 2x 100nF FB1 MMC 8 1nF INPUT +3.3V 3 MIC 680Ω BIAS CON9 2 X1 12MHz MICBIAS XTO ADCLRC DACDAT ADCDAT SCLK BCLK SDIN CSB CLKOUT VMID HPGND AGND DGND 16 47k 220pF 33pF 33pF 5 DACLRC 15 11 4 24 23 22 28 100nF 22 µF MMC L1 100 µH +3.3V 100nF NO (S2) 19 39 40 50 51 42 55 54 48 53 52 21 49 NC DEFEAT SWITCH VR3 10k (ALT TO VR1) DELAY VR4 10k (ALT TO VR2) DEPTH AUX4 (PIN 1, CON5) POT1 MCS AUX1 RD WR 11 33 34 36 37 VBUSON USBID VBUS D– D+ POT2 35 100nF FB2 ANALOG GND 60 61 62 63 64 1 2 3 D7 D6 D5 D4 D3 D2 D1 D0 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 A19 A18 A17 A16 A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0 41 D2 1N4004 7.5 – 12V DC INPUT POWER V+ D1 1N4004 A K 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 STEREO ECHO/REVERBERATION UNIT 30  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 100Ω 1 µF MMC OUTPUT 100Ω 1 µF MMC CON2 47k 47k +3.3V 4x 100nF 10k 2 3 PGED 4 PGEC 5 CON7 EXPAND 2 1 2 3 CON6 EXPAND 1 1 AUX4 2 SDO 3 SCK 4 V+ 5 +5V 6 +3.3V 7 PGED 8 PGEC 9 10 CON5 EXPANSION SOCKETS LED1 K A 1N4004 A LM317T K OUT ADJ OUT siliconchip.com.au IN So really it’s just a matter of building the unit as described in the November 2013 issue, with VR3 & VR4 fitted, installing the jumper on pins 3 & 4 of CON7, powering it up and then adjusting the knobs until you get an echo effect that you are happy with. We have produced new front and rear panel labels (Fig.5) with positions marked to drill the extra holes for VR3 & VR4. Positions are also marked for a headphone volume control and output socket which we’ll explain later. These panels can either be copied or downloaded from the SILICON CHIP website. Defeat switch ICSP SKT 1 AUX1 Fig.3: the Stereo Echo/Reverberation Unit circuit. It’s basically the same as the Stereo Audio Delay Unit published in November 2013 but with the added circuitry highlighted with a red background (plus revised software for IC1). If you are building this as an echo effects unit for musical performances, you will need a way to switch it on and off. To shut off the echo, we simply pull input RD7 (pin 55) of IC1 low; it is internally biased high by a weak current source. This pin is labelled as AUX4 on the circuit diagram (Fig.3) and is wired to a pad on the PCB at the top, near the middle (see Fig.4). These pads are designed to suit an SPDT right-angle pushbutton switch (eg, Altronics S1498) but for musical performances, having a button on the unit isn’t very practical. Instead, we suggest fitting a 3.5mm phono jack socket to the rear panel of the unit and wiring it to the two switch connections on the PCB via a 3-way header (ie, middle-pin unused). A foot switch can then be plugged in via a length of cable fitted with a 3.5mm jack plug. Foot switches generally have a double-throw switch, with three terminals: COM, NC (normally closed) and NO (normally open). If you wire the plug tip and sleeve to the COM and NC terminals, pushing on the foot switch will enable the echo effect and it will stop when you lift your foot off. This is the most logical way to wire it. However, you could also wire the plug to the COM and NO terminals and then the echo effect will be disabled by pressing on the switch and re-enabled by lifting off. The wiring arrangement for the foot switch socket is shown in Fig.4, along with some extra wiring we’ll describe later. This is also shown on the circuit diagram (Fig.3). Our revised rear panel artwork includes a hole position marked for the foot switch socket and an associated label. Note that the position shown has been chosen to avoid interference between the panel-mounted socket and trimpots VR5 & VR6. Reverb Echo is basically a simple form of reverb (or reverberation). In a space such as a concert hall, there won’t be just a single echo duration for sounds originating on the stage and being heard by people sitting in the audience. Instead, there will be many different paths that the sound can take. The direct path is the shortest and gives the least attenuation but sounds also bounce off various surfaces before reaching the listener and each path will have its own delay and (probably frequency-dependent) attenuation. Professional reverberation units can provide many different options, to simulate halls of various different configurations. In this unit, we’ve stuck with a simple approach which gives a more complex (and audibly distinct) response than a simple echo without being terribly processor intensive or having a lot of parameters to tweak. Essentially, to obtain the reverb effect, we add an echo with a short time delay to the sound, then take the resulting sound and process that with a much longer delay. This simulates a large space where there are multiple paths for the sound to bounce around, each with a slightly different length, and thus the echoes arrive at slightly different times. To enable reverb mode, pins 3 & 5 of CON7 must be shorted. Pin 4 should be left open and can be used to measure the set delay. Note that since pins 3 & 5 aren’t adjacent, you can’t use a shorting block to do this. The trick is to use a 3-pin female header and solder a short length of wire between the two outside pins and then plug this into CON7. Of course, you could solder a wire directly to pins 3 & 5 of CON7 but then it’s harder to disconnect. As with echo, reverb mode only operates in stereo and the adjustments are February 2014  31 Fig.4: 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 CON5 CON2 100 µF 22 µF 1nF 100nF PIC32MX470F 1 CON7 ICSP + 120Ω 200Ω 5k 10 µF 100nF CON3 DC 7 .5 –12V 5k R R T S identical. The long delay is adjusted as for echo mode while the short delay is automatically set to be 1/8th as long. So if you select a 200ms long delay, the short delay will be 25ms. The same attenuation setting is used for both short and long delays and as with echo, this can be adjusted with VR3/VR4; otherwise it’s set to 12dB by default. Echo/reverb switching An SPDT switch can be used if you want to be able to switch between echo and reverb modes. To do this, first connect its common terminal to pin 3 of CON7 (or another ground connection point) via a series 1kΩ resistor. The two remaining switch terminals then go to pins 4 & 5 of CON7. The 1kΩ resistor is necessary to prevent a dead short to the PWM output if switching is done while the unit is on. Note that since the unit only checks the state of these pins at power-up, you 32  Silicon Chip 4004 ECHO MODE: SHORT PINS 3 & 4 OF CON7 REVERB MODE: SHORT PINS 3 & 5 OF CON7 T NOTE: IC3 & THE THREE CAPACITORS LABELLED IN PURPLE NOT NEEDED FOR ECHO/REVERB BUT CAN BE LEFT IN-CIRCUIT IF ALREADY FITTED + 100 µF 1k INPUT 1000 µF 100 µF 1k CON1 D2 4004 10k 100nF IC1 1nF 10k 3.3Ω 100nF IC3 R1LV0808ASB 100nF D1 D3 4004 1 + VR5 OUTPUT 100nF 100nF 2x 1 µF 33pF REG1 LM317 5 L1 100nF 47k 47k + 1000 µF POWER 100 µH 100nF 220 µF 100Ω 100Ω 47k 47k + 100nF 100 µF + X1 33pF S1 VR6 FB2 100 µF 10k 4.7Ω 220 µF 100nF + 100 µF + K A + R PHONES CON8 100 µF+ + 100nF 1 µF 1 µF 1 µF + GND 01110131 Stereo Audio Delay/ DSP Board 24bit/96kHz FB1 L S2 DEFEAT IC2 WM8731L MIC CON9 DEPTH 0Ω 47k + 220pF VR4 DELAY 680Ω VR3 MICROPHONE INPUT S TO FOOT SWITCH would then have to switch the unit off and then on again to complete the changeover. Headphone & mic support In the November 2013 issue, the following pins of CODEC IC2 were unconnected: LHPOUT, RHPOUT, MICIN and MICBIAS. These pins can be used for a microphone input and/or a stereo headphone output – see Fig.3. This allows you to take advantage of the headphone amplifier with digital volume control and the microphone amplifier with electret bias built into the IC. When a microphone is connected, the microcontroller detects this and automatically switches from sampling the line inputs to sampling the microphone input. By the way, the microphone amplifier in IC2 is quite a bit noisier than a good external microphone amplifier (which could be connected to the line inputs) but you may find it suitable for some uses (see the spec. panel). To add a headphone output, it’s just a matter of installing the additional parts labelled in green on the PCB layout (Fig.4). This consists of two 220µF DC-blocking electrolytic capacitors and their associated 47kΩ resistors, plus 3-pin header CON8. A panel-mounted 3.5mm phono socket is then wired back to this header. We’ve shown a 2-core shielded cable for this connection but it doesn’t really need to be shielded. You also need to wire up an extra potentiometer (VR7) to allow the volume to be controlled. This volume pot is also mounted on the front panel, next to the other two pots, and wired to 10-way pin header CON5 (which must also be installed) and to a single pin soldered to a pad just below and to the left of IC1. By wiring the pot this way, we’re connecting in a virtually the same manner as VR3 and VR4, ie, across the 3.3V siliconchip.com.au supply rail. The wiper is connected to pin 11 of IC1 via the single pin connection shown, which is the only remaining free ADC-capable input of IC1. This connection is also indicated on the circuit diagram (Fig.3). The revised software (0111013B. hex) for microcontroller IC1 auto­ matically detects when this pot is present and if it is, constantly samples the voltage at pin 11. If this voltage changes, IC1 sends a command to CODEC IC2 to adjust the headphone output volume. Below: this photo shows the completed PCB with all the echo/reverb, microphone and headphone parts all in place. The SRAM chip (IC3) isn’t necessary for echo/ reverb but can be left in circuit if already fitted. Microphone input Above: the fully-assembled PCB mounted inside the case and with the wiring completed. Use cable ties to secure the wiring as shown. HEADPHONES SILICON CHIP POWER VOLUME STEREO ECHO/REVERB DEPTH The extra circuitry required to hook up a microphone is also quite simple. As shown on Fig.3, the signal from the microphone is fed in via a 1µF non-polarised capacitor. The resistor labelled “Rmic” is normally 0Ω which sets the microphone gain to 26dB. However, if this is too much gain, you can reduce it somewhat by using a higher-value resistor. A 39kΩ resistor for Rmic will reduce the gain to 20dB, while a 15kΩ resistor will give a gain of approximately 23dB. For maximum gain, if you don’t have a 0Ω resistor, use a wire link instead. If using an unpowered electret microphone, it will require a small bias current to operate. In this case, the 680Ω resistor should be fitted and the bias current will come from IC2’s MICBIAS output which is enabled by default when a microphone is plugged in. The 47kΩ resistor to ground provides 0V DC bias for the microphone when there is no bias current, while a 220pF capacitor provides a small amount of RF filtering. The micro detects when a microphone is plugged in by monitoring input pin RC14 (pin 48) which is connected to a track labelled “AUX1” via pin 7 of 10-way pin header CON5. As explained previously, this header DELAY www.siliconchip.com.au www.siliconchip.com.au . siliconchip.com.au AUDIO INPUT MICROPHONE INPUT FOOT SWITCH + AUDIO OUTPUT Fig.5: these two artworks can be copied and used as drilling templates for the front & rear panels. They can also be downloaded as a PDF file from the SILICON CHIP website. 7.5-12V DC February 2014  33 The additional 3.5mm microphone input and foot switch jack sockets are mounted on the rear panel. A 7.5-12V DC plugpack powers the unit. Parts List 1 double-sided PCB, coded 01110131, 148 x 80mm 1 ABS plastic instrument case, 155 x 86 x 30mm (Altronics H0377) 1 set front and rear panel labels 4 No.4 x 6mm self-tapping screws 1 12MHz HC-49 crystal (X1) 1 100µH axial RF inductor (L1) 1 10kΩ multi-turn vertical trimpot (VR1) OR 1 x 10kΩ 9mm horizontal potentiometer (VR3) 2 5kΩ horizontal mini trimpots (VR5,VR6) 2 6.35mm PCB-mount stereo switched jack sockets (CON1,CON2) (Jaycar PS0195, Altronics P0099 or P0073) 1 5-way pin header, 2.54mm pitch (CON7) 1 PCB-mount SPDT right-angle toggle switch (Altronics S1320) 1 DC plugpack, 7.5-12V, 100mA+ 2 4mm ferrite suppression beads 1 PCB-mount switched DC socket to suit plugpack 1 M3 x 6mm machine screw and nut Semiconductors 1 PIC32MX470F512H-I/PT 32-bit microcontroller programmed with 0111013B.hex (IC1) (available from SILICON CHIP Online Shop) 1 WM8731SEDS or TLV320AIC23BIPW 24-bit 96kHz stereo CODEC (IC2) (element14 1776264) 1 LM317T adjustable regulator (REG1) 34  Silicon Chip 1 3mm blue LED (LED1) 3 1N4004 diodes (D1-D3) Capacitors 2 1000µF 25V electrolytic 6 100µF 16V electrolytic 1 22µF 16V electrolytic 1 10µF 6.3V 0805 SMD ceramic 4 1µF 50V monolithic ceramic 11 100nF 6.3V 0805 SMD ceramic 2 1nF MKT 2 33pF ceramic disc Resistors (0.25W, 1%) 2 47kΩ 1 120Ω 3 10kΩ 2 100Ω 2 1kΩ 1 4.7Ω 0.5W 5% 1 200Ω 1 3.3Ω 0.5W 5% Add-on Features For echo/reverb: 1 3.5mm panel-mount stereo jack socket (eg, Jaycar PS0133, Altronics P0092) 1 3-way pin header 1 jumper shunt 1 100mm length 2-strand ribbon cable 1 2-core cable with 3.5mm jack plug at one end (length as required) 1 foot switch For headphone output: 1 3.5mm panel-mount stereo jack socket (eg, Jaycar PS0133, Altronics P0092) 1 10kΩ 9mm panel-mount linear potentiometer (Jaycar RP8510) 1 small knob to suit 2 220µF 10V electrolytic capacitors 2 47kΩ 0.25W resistors 1 100mm length 2-core shielded cable or 3-strand ribbon cable 1 100mm length 3-strand ribbon cable 1 14-way (or more) snappable pin header For microphone input: 1 3.5mm panel-mount stereo jack socket (eg, Jaycar PS0133, Altronics P0092) 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 100mm length shielded cable 1 100mm length ribbon cable strand or light-duty hookup wire For low-voltage supply: 1 LM3940IT-3.3 or TS2940-3.3 low-dropout 3.3V regulator (REG2) (Jaycar ZV1565, Altronics Z0599) 1 1N5819 1A Schottky diode (D1) 1 470µF 10V electrolytic capacitor 1 100µF 16V electrolytic capacitor Delete REG1 & associated parts Note: microcontroller IC1 must be programmed with revised software (ie, 0111013B.hex) for echo/reverb and the other add-on features to work. siliconchip.com.au 47k 47k + FB2 100nF 100 µF + + 100nF 2x 1 µF 100 µF X1 100nF 33pF 1nF 100 µH 5819 10k 3.3Ω 100nF CON7 ICSP D1 L1 33pF + 22 µF IC3 R1LV0808ASB 100 µF REG1 LM317 100nF 100nF IC1 1nF 10k 4.7Ω 220 µF 100 µF + + 100nF 100nF + Fig.6: the unit can be powered from a 4-6V DC supply by replacing + REG1 with an LM3940IT-3.3 lowR 220 µF 1000 µF dropout regulator as PHONES shown here. GND 100 µF+ FB1 OUT S1 POWER PIC32MX470F 1 1 L 1 µF 01110131 Stereo Audio Delay/ DSP Board 24bit/96kHz 100nF GND MIC IN GND VR1 VR2 10k 5V SUPPLY OPTION DELAY 2 DELAY 1 100nF + 220pF DEFEAT 1 µF 1 µF 100 µF 47k 47k GND K A VR4 100nF VR3 0Ω 470 µF OUT 47k IN LED1 POWER +3.3V IC2 WM8731L (REPLACES REG1) REG3 LM3940IT-3.3 680Ω +4-6V 10 µF Link Link siliconchip.com.au VR6 VR5 100Ω 100Ω + 100nF was intended at the time for future 1k 470 µF REG3 CON2 CON1 expansion. The micro enables a weak CON3 DC LM3940IT-3.3 1k 4-6V internal pull-up on this pin which is OUTPUT INPUT + connected to the ring terminal of the 5k 5k microphone input. 100 µF Since the microphone input is mono, ALTERNATIVE SUPPLY ARRANGEMENT FOR 4-6V DC when a mono plug is inserted, this will short the ring and sleeve terminals. The Fig.7: follow this PCB parts layout diagram to install the parts for the 4-6V power sleeve is connected to ground and so supply option. Note that D1 must be changed to a 1N5819 Schottky type. AUX1 is pulled low. The micro mutes the input for half a second when this diode (D1) is replaced with a 1N5819 viding a delay and operating in echo/ input changes state. If, after this period, Schottky diode since the latter has a reverb mode. So the same hardware the input is low then the microphone much lower forward voltage. can fulfill either role. input is selected. Otherwise, the line This gives a minimum operating Either way, you will need to program input is used. voltage of around 3.6V, so if you the micro with the revised firmware Thus if a microphone is plugged power the unit from a Li-ion or Li-Po (ie, 0111013B.hex) which can be in, the unit automatically switches to cell, the cell will be pretty much fully downloaded (free for subscribers) from that as the signal source and when it discharged before the circuit ceases the SILICON CHIP website. If you buy is removed, it automatically switches normal operation (in practice, it will a pre-programmed chip, it will come back to the line inputs. Because the probably operate down to at least 3.3V with this version. microphone input is mono, the same but without supply regulation). Basically, it’s just a matter of first signal is sent to both audio output Note that while this arrangement building the PCB as described in the channels. allows the unit to run off lower input November 2013 article. You then voltages, damage may occur if more simply add the extra parts to the PCB 5V/ battery operation than 7V is applied, even briefly. So for the microphone and/or headphone The power supply for the unit is if using a plugpack with this new options and complete the wiring as based on a series polarity protection arrangement, be sure to measure its shown in Fig.4. diode (D1) and an LM317 regulator actual unloaded output voltage rather Don’t forget to link the appropriate (REG1) configured to provide a 3.3V than relying on its nominal rating. A pins on CON7 to enable echo or reverb output. This requires an input voltage 5V unregulated plugpack could easily mode. Link pins 3 & 4 for echo mode of 6V or more (preferably 7.5-12V) to put out more than 7V at light load. (use a shorting jumper), or pins 3 & 5 ensure proper output regulation. for reverb mode (use a 3-way female However, as stated in November Building it header with the outside terminals 2013, it’s possible to reconfigure the The Stereo Echo & Reverb Unit is linked). unit to run from 3.8-6.5V. This makes built on the same PCB as the DualIf you want to run the unit from a it suitable for use with USB power Channel Audio Delay (November 4-6V supply, then build the power sup(4.25-5.5V), a single Li-ion or Li-Po 2013), which is available from the ply section as shown above in Fig.7. cell, or four standard cells (alkaline SILICON CHIP online shop. We can also Don’t forget that diode D1 (near switch or rechargeable). supply all the SMD parts including S1) must be a 1N5819 Schottky type. This alternative power supply ar- the pre-programmed microcontroller, Fig.5 shows the revised front and rangement is shown in Fig.6 and the CODEC and ceramic chip capacitors. rear panels and these can be used as parts layout diagram of Fig.7. BasicalNote that the some of the new fea- drilling templates for the extra holes ly, an LM3940IT-3.3 fixed low-dropout tures, such as the microphone input required for the pots and the stereo regulator (REG3) is used instead of the option and headphone output, can also jack sockets. Note that the headphone LM317T, along with a couple of wire be used for the delay function. volume pot (VR7) is mounted directly links to get power to it. In addition, In fact, once the unit is completed, on the front panel rather than on the SC the 1N4004 reverse polarity protection it can be easily switched between pro- PCB. February 2014  35