If you are guitar player or want to have wonderful sound of a guitar, where the note just hangs there seemingly forever, you need a sustain booster for your guitar audio outputs. Sustain can be obtained by turning the amp up full, but the rest of the band will just kill you - they need to be able to hear themselves too! This little project is best used in the effects loop of a guitar amp (if it has one - not all do). It can be used direct from the guitar, but the effect is not as good, since it is designed for relatively high levels (around 1 Volt). Here is a design for guitar and bass sustain schematic diagram. This is the figure of the circuit.
There is not a lot to it, but the LED and LDR
(Light Dependent Resistor) are critical - they must be completely enclosed in a
light proof enclosure of some kind. Vactrol make some very nice little
LDR opto-isolators, but unfortunately they are not easy to get and are fairly
expensive. The next best thing is an LED, an LDR and a couple of pieces
of black heatshrink tubing. The LED and LDR must be as close to each
other as possible, and a flat topped LED is recommended if you can get one. Note the rather unusual
earth (ground) connection. This is not a mistake in the drawing.
U2A is used to buffer the 1/2 supply voltage created by R3 and R4, and instead
of using the 12V supply negative as earth, the output of U2A is used
instead. This gives a balanced supply from a single voltage source.
Note that the AC/DC adapter (plug pack or wall wart - select the term you are
most comfortable with :-) must not be used to power other equipment as well,
since this may cause problems. If you wish, a conventional +/-15V supply
may be used instead (see below).
All resistors
are 1/4 or 1/2 Watt, and may be 1% or 5%. R1 and R2 should be metal film
for lowest noise. Although the TL072 is suggested for the audio path,
other opamps may be used as well. Likewise, the LM1458 can also be
substituted if you like. Caps are 16V types, but higher voltage units can
be used if desired. D7 is a power on indicator, and D6 is there to
prevent damage to the circuit if the polarity of the applied 12V DC is
incorrect. Be warned that the AC/ DC adapter will be damaged if the
polarity is wrong, and it is left connected for any length of time. VR1 is a
simple volume control, and is used to set the output level. VR2 is the
limiting threshold control - as it is adjusted to a higher setting, the volume
will decrease. You may wish to wire the pot 'backwards', so that maximum
output is obtained when VR2 is set to the fully clockwise position.
U1A is the gain control stage. Maximum gain as shown is
unity, but R2 can be increased if you find that the gain is too low. When
the signal level is high enough for D1 - D4 to conduct, the LED illuminates,
and reduces the gain of the input stage. Any further increase of input
voltage will just cause the LED to glow brighter, which reduces the gain
further. In this way, a constant output level is maintained, since as the
input signal reduces, so does the LED brightness and the stage gain increases
again. The connections shown will be fine for most purposes, but some LDRs may
give distortion at low frequencies. A 100µF cap in parallel with the LED
will probably help if this is a problem. LDRs typically have a slow
release time. After illumination, they take some time to return to their
full dark resistance. This characteristic is exploited here, to allow a
very simple circuit with an almost perfect attack and release time for musical
instrument use.
