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Jazz up your music with this TREMOLO TREMOLO Add another popular effect to your musicmaking with this easy-to-build Tremolo unit. It features low distortion and extremely low noise, thus providing a very clean sound. By JOHN CLARKE Musicians and particularly guitarists often have a myriad of effects units attached to their equipment. They can switch these in and out at will, so as to add various effects while playing a particular section in the music. This low-cost Tremolo unit will add another effect to your repertoire and can be used simultaneously with other effects units. 50  Silicon Chip The Tremolo effect is one which has been with us for a long time and is easily implemented using electronic circuitry. It is achieved simply by rapidly varying the volume over time. If these volume changes occur at a reasonably fast rate (eg, 5Hz), then the effect is quite noticeable. The amount of volume change (or depth) also determines the degree of the effect. Effects units usually include several controls so that you can tailor the sound to suit your requirements. This unit has two controls – one to adjust the modulation depth and the other to adjust the rate of modulation or the frequency. You can also switch the effect in or out at will, using either a front panel switch or an external foot switch which plugs into a jack socket on the rear panel. The unit is housed in a compact case with the input and output jack sockets at the rear. The front panel controls are quite simple and include an on/off switch, the Rate and Depth potentiometers and an In/Out switch. A front-panel LED which flashes in sympathy with the volume modulation indicates the Tremolo rate, while Fig.1: the block diagram of the Tremolo Unit. It has two distinct sections – a signal path section (via IC1a, LDR1 & IC1b) and a control section consisting of a sinewave oscillator (IC2a, IC2b, LED2 & LED3) and buffer stage (Q1 & LED4). The control section continuously varies the resistance of the LDR to modulate the signal to produce the tremolo effect. a second LED provides power on/off indication. The unit is powered by a 12VDC plugpack supply. Block diagram Fig.1 shows the general arrangement of the Tremolo Unit. It has two distinct sections, one being the signal path and the other the control section. As shown, the incoming signal is first amplified by IC1a and then fed to a gain element stage (LDR1). This stage varies the signal level at its output in response to a signal from the control section before feeding it to an output buffer stage (IC1b). Normally, with no tremolo, the gain element provides a small amount of attenuation. When the tremolo effect is switched in, the gain of this stage is continuously varied, so that the signal is constantly boosted and cut. The gain of IC1a is such that the signal level remains constant when the tremolo effect is switched off. The gain element itself is nothing more than a light de­pendent resistor (LDR) which varies its resistance according to the light that falls on it. In this circuit, we use a high-brightness red LED to control the LDR and this is driven by a sinewave signal that’s generated by the control circuit. The control circuit is basically a sinewave oscillator and consists of Main Features • • • • • • Adjustable tremolo rate Adjustable tremolo depth Tremolo rate indicator LED In/out switch on front panel and socket for a foot switch Compact size Operates from a 12VDC plugpack supply a high-Q filter stage (IC2a), op amp IC2b and a “clamping” stage. Also included are the depth pot (VR2) and the In/Out switch (S2). The sinewave output is buffered by transistor Q1 which in turn drives LED4 to control the amount of light falling on the LDR. The oscillator operates by amplifying the signal from the high-Q filter, clamping this to produce a square wave and then reapplying the signal back to the filter via a positive feedback path. The high-Q filter produces a very clean sinewave at its output while the level is set by the square wave level (ie, the feedback signal), which in turn is set by depth pot VR2. Circuit details Refer now to Fig.2 for the full circuit details. It’s rela­tively simple and is based on five op amp stages. Specifications Total harmonic distortion ...........................................0.1% at 100mV in and <at> 1kHz Signal to noise ratio .............108dB with respect to 1V input and 1kΩ input loading; 112dB A weighted Maximum input before clipping ..............................1.2V RMS (12VDC input supply) Frequency response ................................................... -0.1dB at 20Hz; -3dB at 34kHz Signal gain ........................................... 1V in for 1V out with no tremolo modulation Tremolo frequency range ........................................................................2Hz to 17Hz Tremolo modulation depth .......................................from 0% up to 80% modulation Average output level change for 0-50% modulation ....... -0.4dB at 50% modulation April 2001  51 Fig.2: the complete circuit of the Tremolo unit. IC2a & IC2b form the heart of a sinewave oscillator and this drives LED4 via buffer transistor Q1. LED4 in turn is optocoupled to LDR1 and modulates its resistance to vary the signal gain. Op amp IC1a, LDR1 and IC1b make up the signal path. As shown, the input signal comes in via a 47µF capacitor and a 100Ω resistor and is applied to the pin 5 (non-inverting) input of IC1a. The 47µF capacitor is needed to provide AC coupling because IC1a is biased at half supply (6V), as are all the other op amps in the circuit. It is also much larger than necessary to ensure that IC1a sees a very low 52  Silicon Chip source impedance, to minimise noise. The 100Ω resistor and 10pF capacitor on pin 5 are there to filter out any radio frequency (RF) signals at the op amp input. IC1a operates with a gain of 2.8, as set by the 18kΩ feed­back resistor between pins 6 & 7 and the 10kΩ resistor connected between pin 6 and the half-supply rail. This gain compensates for any signal losses in the following LDR1 and 1.5kΩ attenuator circuit. When the tremolo modulation signal is off, LDR1 receives a constant amount of light from LED4 and has a resistance of about 2.7kΩ. As a result, the signal is attenuated by a factor of 2.8 before being applied to unity gain buffer stage IC1b. IC1b then drives the output socket via a 10µF coupling capacitor. Note the 150Ω resistor in series with the output. This isolates IC1b from any capacitive loads which may be connected to the output socket and prevents oscillation. Another two op amps are used in the control circuit, with IC2a providing the high-Q filter section. This op amp has a “T-filter” circuit connected into its negative feedback loop (between pins 1 & 2). The filter components include resistors R1 & R2, capacitors C1 & C2 and the Rate pot (VR1). The frequency of the filter is set by the value of VR1 according to the following formula: f = 1/2πC1√((R1 + VR1) x R2). Substituting the relevant values into this formula gives a frequency range of 2Hz to 17Hz (VR1 = 0-100kΩ). The output signal from IC2a appears at pin 1 and drives transistor Q1 via a 10kΩ base resistor. Q1 in turn drives LED4, which is optocoupled to LDR1. IC2a’s output also drives inverting op amp stage IC2b, which operates with a gain of about 21 (ie, 47kΩ/2.2kΩ). Its output signal appears on pin 7 and the level is clamped at about 1.8V above and below 6V (ie, half supply) using LEDs 2 & 3. This gives a square wave signal which swings between 4.2V and 7.8V (ie, 3.6V p-p). This signal is applied to Depth pot VR2 and the signal on its wiper then applied back to the T-filter stage via a 220kΩ resistor. It is this positive feedback that makes the circuit oscillate. As mentioned before, the amplitude of the sinewave signal from IC2a is set by the Depth pot (VR2). This sinewave signal swings above and below the 6V level (ie, 1/2Vcc). As the signal voltage from IC2a rises, it drives LED4 harder and so its light output increases. This reduces the resistance of LDR1 and so the audio signal output level increases. Fig.3: here’s how to build the unit. Note that LED4 and LDR1 are enclosed in a light tunnel made from heatshrink tubing – see photo. Take care with component orientation during the board assembly. Conversely, as the signal swings down, the resistance of LDR1 increases and the audio output level is attenuated. As a result, the audio output level varies continuously. VR1 sets the Rate at which the audio output level varies, while VR2 set the Depth (or range) of the level variation. Table 1: Resistor Colour Codes  No.   2   2   1   1   5   1   3   1   1   1   1 Value 1MΩ 220kΩ 47kΩ 18kΩ 10kΩ 4.7kΩ 2.2kΩ 1.8kΩ 1.5kΩ 150Ω 100Ω 4-Band Code (1%) brown black green brown red red yellow brown yellow violet orange brown brown grey orange brown brown black orange brown yellow violet red brown red red red brown brown grey red brown brown green red brown brown green brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown red red black orange brown yellow violet black red brown brown grey black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown green black brown brown brown green black black brown brown black black black brown April 2001  53 Table 2: Capacitor Codes      Value IEC Code EIA Code 0.22µF   224   220n 560pF   561   560p 330pF   331   330p 10pF    10   10p the resistance of LDR1 (and thus the audio output level) remains constant. Because LEDs 2 & 3 are wired as voltage clamps, they flash on and off whenever the circuit is oscillating. We have put this to good use by having LED2 protrude through the front panel of the case, to give a visual indication of the oscillator rate. Of course, once you get above about 10Hz, the LED will appear flicker quite rapidly. Note that the LED will be off when the Depth control is set to minimum and the oscillator stops, or when switch S2 is closed. Power supply This view shows how LED4 and LDR1 are enclosed in the heatshrink tube light tunnel. Don’t shrink the tubing down too far – it should be shrunk down just enough to firmly grip the two components. When S2 is switched to the “Out” position (ie, tremolo off), VR2’s wiper is held at Vcc/2 and so there is no positive feedback signal. As a result, the circuit stops oscillating and IC2a’s output sits at a constant 6V. This drives LED 4 (via Q1) with a constant amount of current and so Fig.4: the top trace shows the audio input signal to the Tremolo unit while the lower trace is the modulated output signal that produces the tremolo ef­fect. 54  Silicon Chip Power for the circuit is derived from a 12VDC plugpack. This is applied via reverse polarity protection diode D1 and filtered using 100µF and 10µF electrolytic capacitors. S1 is the on/off switch, while LED1 provides power on indication. Op amp stage IC3 is used to provide a 6V (Vcc/2) supply rail with a low source impedance. This op amp is wired as a voltage follower and has its pin 3 input biased to Vcc/2 (6V) by two 10kΩ resistors. A 10µF Fig.5: the top trace in this scope shot is the sinewave output at pin1 of IC2a. Notice how the lower waveform (ie, the output signal) follows the sinewave shape. Parts List 1 PC board, code 01104011, 117 x 102mm 1 front panel artwork, 130 x 29mm 1 rear panel artwork, 130 x 29mm 1 ABS instrument case, 140 x 110 x 35mm 3 6.35mm mono PC-mount jack sockets 1 2.5mm DC power socket 2 mini SP rocker switches (S1,S2) 1 100kΩ 16mm linear pot (VR1) 1 10kΩ 16mm linear pot (VR2) 2 16mm diameter knobs 1 LDR (LDR1) (Jaycar RD-3480 or equivalent) 4 M3 x 6mm screws 1 20mm length of 6mm black heatshrink tubing 1 60mm length of 0.8mm tinned copper wire 1 100mm length of twin light-duty hookup wire 6 PC stakes Semiconductors 2 TL072, LF353 dual op amps (IC1,IC2) 1 TL071, LF351 op amp (IC3) 1 BC548 NPN transistor (Q1) 3 5mm red LEDs (LED1-LED3) 1 3000mcd red LED (LED4) 1 1N4004 1A diode (D1) The PC board fits neatly into a compact low-profile instrument case. You can switch the tremolo effect in or out using either the front-panel switch or an external foot-operated switch capacitor decouples this bias voltage to minimise noise. In operation, IC3 adjusts its output at pin 6 so that pin 2 is kept at the same voltage as pin 3 (ie, Vcc/2, or 6V). The 100Ω resistor provides short-circuit protection for IC3, while the 10µF capacitor at pin 2 prevents the IC from oscillating. Construction Building it is easy since virtually all the parts are in­stalled on a PC board coded 01104011 (117 x 102mm). This is housed in an ABS instrument case measuring just 140 x 110 x 35mm, to make a really compact unit. As usual, check your etched PC board against the published pattern to ensure there are no defects (eg, shorts between tracks or breaks in the copper pattern). You should also check the hole sizes – the pots and jack sockets require 1.5mm holes, while the four corner mounting holes should be drilled to 3mm. Fig.3 shows what you have to do to build the unit. Begin the board assembly by installing the resistors and wire links. Table 1 shows the resistor colour codes but we suggest that you check the values using a digital multimeter as well – just to make sure. The three ICs, diode D1 and transistor Q1 can all go in next, making sure that IC3 is the TL071. Take care Capacitors 1 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 5 10µF 16VW PC electrolytic 2 0.22µF MKT polyester 1 560pF ceramic 1 330pF ceramic 1 10pF ceramic Resistors (0.25W 1%) 2 1MΩ 3 2.2kΩ 2 220kΩ 1 1.8kΩ 1 47kΩ 1 1.5kΩ 1 18kΩ 1 150Ω 5 10kΩ 2 100Ω 1 4.7kΩ to ensure that these parts are correctly orientated. This done, you can install all the capacitors but again watch the polarity of the electroly­tic types. Table 2 shows the codes for the low-value ca­pacitors. The two potentiometers can now be installed (don’t mix them up), followed by the jack sockets the LEDs and the LDR. LEDs 1 & 2 should be April 2001  55 Fig.6: this is the full-size etching pattern for the PC board. mounted at full lead length, so that they can later be bent over and pushed into their respective holes on the front panel. LED3 should be mounted about 5mm clear of the PC board, while LED4 and LDR1 should both be about 12mm clear of the board. Important: LED4 is the high-brightness LED. Once these parts are in, LED4 and LDR1 should be bent over at right angles so that they face each other. These two devices are then pushed into a light tunnel made from 6mmdia. heatshrink tubing (about 20mm long), so that only the LED light falls on the LDR – see photo. Shrink the tubing slightly using a hot-air gun, so that the devices are properly sealed. Finally, complete the board assembly by installing PC stakes at the external wiring points. There are six stakes in all – two each for switches S1 & S2 and two for the DC socket. Final assembly The next step is to drill the necessary holes in the front and rear panels, to accept the various hardware items. You can use the full-size artworks published with this article as tem­ plates to do this job. For the larger holes, it’s best to drill a small pilot holes first and then carefully enlarge 56  Silicon Chip them using a tapered reamer. The switch mounting holes can be made by drilling a series of small holes around the inside perimeter and then knocking out the centre piece and filing to a smooth finish. Once you’ve drilled the holes, attach the front and rear panel la­bels, then clip the switches into the front panel and secure the two pots. You will need to fit two nuts to each of the bushes on the pots – one on either side of the panel. LEDs 1 & 2 on the PC board can then be bent over through 90° and pushed into their front panel holes. The PC board mounts on four integral pillars on the base of the case and is secured using self-tapping M3 screws. Note that it will be necessary to first remove the unused pillars on the base using a pair of side cutters, to prevent them fouling the PC board. Finally, complete the assembly by wiring up the switches and the DC socket, as shown on Fig.3. The smoke test Well, there won’t really be any smoke – or at least, we hope not! To test the unit, apply power, switch on and check that there is 12V between pins 8 & 4 of both IC1 and IC2. Similarly, there should be 12V Fig.7: these full-size artworks can be used as drilling templates for the front and rear panels. between pins 7 & 4 of IC3, while pin 2 of IC3 should be at about 6V. Take care not to short out any of the IC pins while making these checks. In fact, it’s generally best not to probe the IC pins directly. Instead, you can connect the negative lead of your DMM to an earth point (eg, at the DC socket) and connect the positive lead to points on the circuit that directly connect to the relevant IC pins. Now check that LEDs 2 and 3 light alternately at an increas­ing rate as the Rate pot is wound up. Note that the Depth pot must also be turned up for these to operate, while S2 must be switched to the “In” position. Finally, you can check that the unit operates normally by connecting it to an amplifier and feeding in an input signal. The Tremolo effect should become quite prominent as the Depth control is wound up and you should be able to vary the rate from about 2Hz SC to 17Hz using the Rate control.