<p>The vibrato is the oscillators being modulated by the 60hZ line voltage from the input. A regulated supply will eliminate that vibrato effect. You can use a regulated supply OR you can install a voltage regulator at the input. In order to do this you will have to make a few small board alterations. There's space on the board for a regulator, but in order to use it you will need to add a diode at the input AND a diode at the output. 1N4001s will work fine.
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