Tremolo sound effect is basically an amplitude modulator. Some people might get confused with vibrato effect, but vibrato effect is a frequency modulator, big difference. When you apply a tremolo effect on electric guitar for example, you’ll hear something similar with turning the guitar’s volume knob up and down, while vibrato effect will sound like pitch bending. With electronic circuit, a tremolo effect will be much simpler than a vibrato effect, below is a schematic diagram of such circuit:
Using a LM324 and speed pot, the rate of phase shift oscillator can be set to 5-10 Hz. The two 1N914 diodes are used as voltage controlled attenuators. The resistance of two 1N914 diodes is modulated by a potion of the oscillator output which is taken from the depth pot. A undesirable clipping will occur if the incoming signal level is not restricted to less than 0.6 V pk-pk.