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Sustain Unit for electric guitars Are you playing electric guitar without a sustain pedal? What? In this day of electronics and technology, you don’t have a sustain pedal? Add one now and have a more fulfilling musical life and set out on the path to fame and fortune. By JOHN CLARKE If you’re a guitar player without a sustain pedal you must be feeling pretty deprived. But now you can fix all that by building this new design. It can give you really long sustain notes and will help make the sound much more live. 18  Silicon Chip Sustain is just one of many effects that can enhance the sound from a guitar. Some effects produce deliber­ ate distortion – eg, distortion pedals (previously called fuzz pedals) – so that they sound harsh, while others are more subtle and add in frequency response shaping or envelope modula­ tion. The sustain effect works by con­ trolling the signal from your guitar so as to maintain a constant level of sound. When you pluck the string on your guitar, it initially produces a high level of oscillation which ultimately dies away to nothing. This is reflected in the volume of the note – quite loud when initially plucked and then decay­ ing away quite fast. A sustain circuit modifies this natural decay so that the volume remains fairly constant as the sound from the string itself dies away to nothing. Some guitar sustain pedals are rel­ atively crude and provide the sustain function by amplifying the guitar signal and clipping the waveform Fig.1: some guitar sustain pedals work simply by clipping the waveform when the level becomes too high. The top waveform is a sinewave signal with 12dB higher bursts occurring every 125ms. The lower trace is the waveform with a clipping circuit added. This degree of clipping can sound pretty awful. Main features •  Low distortion •  Separate Attack and Decay controls •  Sustain/Bypass (In/Out) switch •   Matched in and out levels (adjustable) when the level becomes too high. The oscilloscope waveforms of Fig.1 show this type of sustain. The top waveform is a sinewave signal with 12dB higher bursts occur­ ring every 125ms. The lower trace is the waveform with a clipping circuit added. Note how the higher signal bursts are clipped hard to provide flat top limiting which con­ stitutes considerable distortion – it can sound pretty awful. Such a clipping circuit cannot be considered to be a pure sustain unit since it adds in very high distortion. If distortion is wanted, this can be added in with a distortion pedal. The Sustain pedal circuit described here produces much lower distortion than the clipping circuit but still maintains the output level over a wide signal range. The oscilloscope wave­ Fig.2: these waveforms show the response of the sustain circuit with the same input waveform as in Fig.1. Note how the lower waveform remains sinusoidal over its full duration. forms of Fig.2 show the response of the sustain unit under the same type of pulse waveform as the clipping cir­cuit referred to in Fig.1. The upper trace is the pulsed input. Note how the lower waveform remains sinusoidal over its full dura­ tion. Note that the initial attack of the waveform is more or less preserved and that the waveform does still decay away eventually. These times can be adjusted with the Attack and Decay controls. How it works Fig.3 shows the block diagram of the circuit. The guitar signal is applied to a gain-controlled amplifier which provides the output signal. The sig­ nal produced at the amplifier output is full-wave rectified and filtered to produce a DC level which is depend­ ent on the signal level at the gain controlled amplifier output. This DC level is compared against a reference level set by VR3 in the error amplifier IC2a. The error signal is then fed back to the gain controlled amplifier in order to maintain a constant output. Fig.4 shows the full circuit. It comprises six op amps, four in IC1 (a TL074) and two in IC2 (an LM358). Q1 is a 2N5484 JFET which provides the variable gain feature for IC1. Fig.3: the block diagram of the circuit. The guitar signal is applied to a gain-controlled amplifier which acts to provide a more constant output signal. March 1998  19 Fig.4: the circuit employs op amp IC1a and Mosfet Q1 as the gain controlled amplifier while IC1b, IC1c and diodes D2 & D3 act as a precision full-wave rectifier. Signal input from the guitar is AC-coupled via a 1µF ca­pacitor to pin 3, the non-inverting input of op amp IC1a. The 22kΩ resistor biases pin 3 to +5V while the 10Ω series resistor acts as a “stopper” to reduce the possibility of RF breakthrough. JFET Q1 is used to dynamically vary the gain of op amp IC1a; this is the gain-controlled amplifier referred to earlier in Fig.3. The gain of IC1a is set by the 10kΩ feedback resistor between pins 1 & 2, in conjunction with the drain-source resist­ance of JFET Q1 and the 100Ω source resis­ tor. If the JFET is biased on hard, the drain source resistance is low and the corre­sponding gain is high. Note that the JFET does not pass DC because of the asso­ciated 47µF capacitor. As well as blocking DC and effectively setting the DC gain of the circuit to unity, the 47µF ca­ pacitor also sets the low frequency 20  Silicon Chip rolloff of the circuit. High frequen­cy rolloff above about 16kHz is provid­ ed by the .001µF capaci­tor between pins 1 & 2. Rectifier & envelope control Op amps IC1b & IC1c, plus diodes D2 & D3 and associated resistors form the full wave rectifier. When the sig­ nal from IC1a goes negative, the out­ put of IC1b goes high, forward biasing D3. The gain for negative signals is set by the 20kΩ input and 20kΩ feedback resistors to a value of -1. The signal at the cathode of D3 is coupled to the inverting input, pin 9, of IC1c via the 10kΩ resistor. Gain for IC1c is set at -10 by this 10kΩ input resistor and the 100kΩ feedback resistor. Overall gain for the input signal is therefore -1 x -10 = +10. However, there is an extra path for the input signal via the 20kΩ resistor to pin 9 of IC1c. This path gives a positive signal at the output of IC1c with a gain of -5. Adding the two gains gives us +5. So when the input signal is negative, the output at pin 8 of IC1c is negative. For positive input signals diode D2 conducts and clamps the output of IC1b to +5V. Signal then passes via the 20kΩ resistor connecting to pin 9 of IC1c. IC1c inverts the signal and provides a gain of -5. Therefore, posi­ tive input signals result in a nega­tive output at pin 8 of IC1c. So, regardless of whether the input signal swings negative or positive, the output at pin 8 of IC1c always swings negative. Thus we have a full-wave rectifier. The 10pF capacitor across the 20kΩ feedback resistor for IC1b prevents instability while the 0.1µF capacitor across the 10kΩ feedback resistor of IC1c provides a measure of filtering. The full-wave rectified signal is filtered using D4, VR1, VR2, the 10kΩ resistor and the 10µF capacitor. Diode D4 allows the 10µF capacitor to be charged via VR1 but only discharged via VR2 and the series 10kΩ resistor. This allows separate control over the attack and decay times. Error amplifier IC2a is the error amplifier. It compares the rectified signal from D4 with the DC voltage (Vadj) at its pin 2 and it amplifies the difference between these two signals by a factor of 5.7, as set by the 10kΩ input and 47kΩ feedback resistors. Reference. Vadj, the DC reference fed to the error amplifier, comes from op amp IC2b and is set using trimpot VR3. The error amplifier drives the gate of JFET Q1 via a 10kΩ resistor and switch S2. The 10kΩ resistor between the gates and drain of Q1 has the effect of linearising the signal and thereby reducing distortion. Slide switch S2 is used to select Sustain (in) or Bypass modes (out). When Sustain is selected, the voltage from pin 1 of IC2a controls Q1’s gate. When S2 is in the out position, the gate is held at a voltage set by VR4 and the 10kΩ resistor between gate and drain. In this mode, the drainsource resistance of Q1 is constant and so the gain does not vary. In use, trimpot VR4 is adjusted so that the same volume is experienced whether the switch is in or out as the guitar string is first plucked. Power for the circuit is derived from a 12V DC source which will usu­ ally be a plugpack. Diode D1 protects against reverse polarity connection, while the 100µF capacitor decouples the supply. LED1 indicates power when S1 is switched on. Most of the op amps are biased from a 5V DC supply. This is de­ Fig.5: all the components mount on the PC board, including the input and output jack sockets. Note that IC1 and IC2 are oriented in different directions. rived with zener diode ZD1 which is supplied via a 1kΩ resistor from the 11.4V rail, following D1. The resulting regu­ lated voltage across ZD1 is filtered with a 10µF capacitor and then buffered with op amp IC1c. Construction Typically, a guitar sustain circuit such as this would be pedal operated and perhaps the electronics would all be mounted in the pedal housing itself. However, some guitar players would be just as happy mounting the circuit board in a simple plastic utility case and with simple switches to operate it instead of a pedal. With those thoughts in mind, we are presenting this project in the simplest possible form, as a PC board with all circuit components mounted on it. The PC board measures 105 x 60mm and is coded 01302981. It has been designed to fit into a standard UB3 plastic utility case measuring 130 x 67 x 43mm (Altronics Cat. H-0153 or equivalent). Fig.5 shows the component layout. Before you install any parts on the board, check it thoroughly against the PC artwork shown in Fig.6 and make sure that all holes have been drilled. That done, install the two Capacitor Codes ❏ Value IEC Code EIA Code ❏ 0.47µF   470n   474 ❏ 0.001µF   1n0   102 ❏ 10pF   10p   10 Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  3 ❏  3 ❏  6 ❏  1 ❏  1 ❏  2 ❏  1 Value 100kΩ 47kΩ 22kΩ 20kΩ 10kΩ 2.2kΩ 1kΩ 100Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown red red orange brown red black orange brown brown black orange brown red red red brown brown black red brown brown black brown brown brown black black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown red red black red brown red black black red brown brown black black red brown red red black brown brown brown black black brown brown brown black black black brown brown black black gold brown March 1998  21 Parts List Specifications 1 PC board, code 01302981, 105 x 60mm 2 DPDT slider switches (S1,S2) 2 6.35mm PC mount mono unswitched sockets 1 10kΩ linear PC mount pot (VR1) 1 100kΩ linear PC mount pot (VR2) 2 knobs 1 5mm red LED (LED1) 2 PC stakes 1 40mm length of 0.8mm tinned copper wire Total harmonic distortion (1kHz) ................... 0.7% at 10mV input, 2% <at> 20mV, 0.02% <at> 200mV Semiconductors 1 TL074, LF347 quad op amp (IC1) 1 LM358 dual op amp (IC2) 1 2N5484 N-channel JFET (Q1) 1 1N4004 1A 400V diode (D1) 3 1N914, 1N4148 signal diodes (D2-D4) 1 5.1V 400mW zener diode (ZD1) Signal to noise ratio at maximum gain (with respect to 100mV)............................-60dB with 20Hz to 20kHz filter (better noise figure at lower gains) Output level versus input level............................. flat from about 10mV to 170mV input Attack time ................................................................................. 5ms (max) Decay time ................................................................................ 25ms (min) Maximum gain ...............................................................................18 times Frequency response .....................................-1dB at 20Hz; -3dB at 16kHz Capacitors 1 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 2 1µF 16VW PC electrolytic 1 0.47µF 63V MKT polyester 2 .001µF 63V MKT polyester 1 10pF ceramic Resistors (0.25W, 1%) 1 100kΩ 1 2.2kΩ 1 47kΩ 1 1kΩ 3 22kΩ 2 100Ω 3 20kΩ 1 10Ω 6 10kΩ PC stakes, two links, resistors and diodes, followed by the capacitors, the LED and the two trimpots. Make sure that the diodes and electrolytic capacitors are installed with correct polarity. Next, install the two ICs and note that they are oriented differently. Pin 1 of IC1 is adjacent to the two jack sockets while pin 1 of IC2 faces the other end of the board. Slide switches S1 and S2 are in­ stalled by inserting the switch pins into the PC board and soldering in place. If the pins are difficult to in­ sert, crimp them with pliers first or use tinned copper wire through the 22  Silicon Chip Fig.6 actual size artwork for the PC board. It has been designed to slip into a standard plastic utility box (UB3). switch pins which then insert into the PC board. The JFET (Q1) is mount­ed with its package oriented as shown on Fig.5. The Attack and Decay potentiome­ ters, VR1 and VR2, are PC types and are soldered directly into the board. Note that they have different values so don’t get them swapped around by mistake. Finally, mount the two 6.35mm PC sockets in position. They are PC types too and solder directly into the board. Testing, testing Connect up a 12V DC power supply to the PC stakes on the board and check that there is about +11.4V at pin 4 of IC1 and pin 8 of IC2 when S1 is on. Pin 7, pin 10 and pin 12 of IC1 should have about +5V present. You are now ready to test the sus­ tain unit with your gui­tar. Switch S2 to the Sustain setting (towards S1) and play a few notes. Adjust VR3 for best effect on the sustain. You may also need to adjust the volume level from your guitar to suit the input range of the sustain unit which operates best between 10mV and 200mV. Adjust the Attack control to set the rate at which the note is reduced in volume when the string is first plucked. Then adjust the Delay control to ensure that the note’s volume is main­tained as much as possible. SC