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| Elliott Sound Products | Project 29 |
Tremolo is one of those simple effects that has just lasted forever (well, almost). The circuit shown here has wide range, and a very controlled and musical modulation characteristic, and should keep the guitarists happy for minutes at a time.
The project is simple to build, and can even be housed in a pedal if desired. If the pedal option is used, don't try to run it from batteries, as they will not last very long due to the LED current. It needs a ±15V supply as shown in the circuit to operate properly.
Note that this type of circuit is often called 'vibrato', but that's incorrect. True vibrato implies that there is a change of pitch. which may or may not include amplitude modulation as a side effect. Tremolo refers to an amplitude modulated signal, exactly what this circuit does. There is no pitch change, even though the characteristics of human hearing are such that you might hear a small pitch change as well. This is a psycho-acoustic phenomenon - this circuit does not alter the frequency at all.
A simple way to build your own LED/ LDR optoisolator is shown in Project 200.
The unit is simple to build, and does not need really low noise opamps, since they only act as a modulator oscillator. I suggest the TL072, which is more than good enough. The transistors can be any low noise NPN type, and they are simply buffers, ensuring a high input impedance and low output impedance. You can use opamps as buffers instead of the transistors Q1 & Q2 if you prefer. A single TL072 with both halves wired as unity-gain non-inverting buffers will work fine, and may allow you to eliminate C1, C2 and C3 (in the Figure 1 circuit).
If the unit is to be built into an amplifier, it may well be possible to leave out the input transistor, since a low impedance drive circuit is probably already available from an existing opamp. It may also be possible to leave out the second transistor if a high impedance input is available at the insertion point. This is somewhat unlikely, since a common place to have the modulator is before the tone controls which need a low impedance source.
Figure 1 - Tremolo Unit Circuit
The opamp power supply pins are: Pin 4, -ve and Pin 8, +ve. This is the same on virtually all dual opamps. The value of C3 might need to be changed if the load impedance is less than about 20k Ohms. In some cases C3 can be omitted if the following stage is already capacitor coupled. Be warned that there will be about -1V DC at the output if C3 is omitted.
The oscillator is a simple opamp feedback type, and produces a triangle wave across the capacitor (C4). This is amplified and buffered, and fed to the LED in the opto-coupler. If you are unable to obtain this device (made by Vactrol), use a high quality Light Dependent Resistor (LDR) with a high-efficiency LED in a light-proof encapsulation - heat-shrink tubing is good, but you will probably need two layers to ensure it is completely sealed against light getting in. Use the highest output LED you can get, and make sure that the LED and LDR are properly aligned for maximum sensitivity.
The second LED is used as a panel indicator, and can be any colour you choose. When tremolo is switched off, the LEDs will both be off, and when on the panel LED flashes at the selected rate. It might be necessary to reduce the value of R10 to ensure that there is enough drive to the LEDs to get the full modulation. If the modulation is too intense, increase the value of R10 (R9 in Figure 2).
The frequency range is from about 2.5Hz to 14Hz with the values as shown, but this can be changed to suit your needs. This is generally a good range, and will be more than wide enough for most users. Increasing the value of C4 (Fig. 1) or C2 (Fig. 2) will reduce the frequency and vice versa. Reduce the value of R7/R6 (Figures 1 & 2 respectively) to get a greater frequency range. LDRs are fairly slow, so don't expect to get much modulation at frequencies much over 15Hz.
Amplitude modulation can be varied from none at all, to full modulation with the signal varying from fully on to fully off. The frequency range that can be covered with full modulation is dependent on the speed of the LDR. Most of the commonly available ones are fast enough to give a good modulation depth at even the highest frequency - at least to 15Hz or so as noted above.
Figure 2 - Tremolo Unit Circuit (All Opamp)
If you do want to use opamps instead of emitter followers, use the circuit shown above. The cost difference is surprisingly small, but there are no issues with DC offsets, output impedance is much lower as is distortion. There is no longer any need to filter the incoming ±15V, because the opamps are almost immune from power supply disturbances.
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