Using a Capacitor To prolong a pulse
You've left us a lot of guesswork due to lack of schematic.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Rectified amplifier output and smoothing capacitor.

How it works:


D1 rectifies the amplifier output.
C1 (1000 uF in the schematic - use whatever big ones you've got) stores charge between pulses from the amplifier.
R1 and D2 you should have already.


How well it works depends on the amplifier and the size of the capacitor. 

If there's an output capacitor on your amplifier you'll need the circuit shown in Figure 2.







simulate this circuit

Figure 2. Modified circuit for capacitor coupled output.


D3 is required in this case to "recharge" C2 on negative half-cycles