There is alternate for the design of the Guitar Sustain. This version is designed specifically for operation from ±15V supplies (±12V supplies can also be used with no changes). This version can be used as a full range limiter for music, and might typically be applied in an amp rack to prevent amps from being overdriven (and distorting). While the Figure 1 version can also be used, it is less convenient because of the single supply, and the rectifier is not quite as good as the unit shown here. This is the figure of the circuit.
Because of the arrangement of the rectifier, it is much easier to add a cap (C1) to increase the attack and decay time. Note that using a value much higher than that shown will not help much, because the opamps cannot supply enough current to charge a higher value, and the results can be less than satisfactory.
There are many
alternatives, but when wired as shown, the circuit gives a very good account of
itself. Operation is unchanged from that described above, despite the
re-arrangement of the LDR wiring. If you need more input gain (to
increase the limiting effect), simply increase the value of R3. I don't
recommend that it be greater than around 47k though. Likewise, increased output
level is obtained by increasing the value of R6, and again, I suggest a maximum
of 47k.
