2
REVERB AND EFFECTS LOOP SECTIONS
The signal from the preamp is fed into IC3A which is configured as a boot strapped voltage
follower. Z1 and Z2 have been added to prevent any nasty spikes from damaging the opamp,
this should in practise never happen.
The output of IC3a is fed to the reverb and effects loop sections.
C12 and R22 roll off a lot of the lower frequencies before the signal gets sent to IC3B and
IC1A. These opamps are configured for current gain, the actual gain being dependant on the
impedance/frequency curve of the reverb tray. Because of this it is crucial to the correct
operation of the reverb that the right reverb tray is used. This should be an Accutronics
8CA3B1B.
The output of the reverb tray goes into IC1B. This is configured as a differential amplifier as a
way of reducing any hum that may be picked up on the sensitive reverb return leads.
RV3 controls the level of the reverb. Across pins 1 and 2 is TR4, a J175 FET. When the ring
connection on SK3 is shorted out, i.e. by a footswitch, TR4 is also effectively shorted which
results in the reverb being turned off.
The reverb signal is then split and sent to the two mixer stages in IC4, however one side goes
through a unity gain inverting transistor stage, TR6, just before the mixer stage. This obviously
puts one of the reverb returns out off phase which produces the ‘surround sound’ reverb effect.
The signal from the output of IC3a is split two other ways for the effects loop. One goes to the
series/parallel switch, SW1A, the other goes to the SEND socket. R34 and R1 are a potential
divider to reduce the nominal send level to about -20dBu.
SK6 and SK7 are the LEFT/MONO and RIGHT RETURN sockets respectively. Each of these
are fed into the two sections of IC2 which are both configured as non-inverting gain stages.
The outputs of these then go into RL2. This relay basically turns the effects loop on or off.
Without a jack is inserted into SK6, LEFT/MONO RETURN, the ‘LOOP_MUTE’ line is held
down at 0V, this keeps TR5 turned off, which turns on TR3 and effectively shorts R70 to 0V.
This turns RL2 which means the effects loop is bypassed and cannot be switched on. When a
jack
is
inserted into SK6 the reverse happens, TR5 is turned on, TR3 is turned off and the
effects loop is activated. In this mode, shorting out SK4, Footswitch 2, will turn the relay on
and again bypass the effects loop. TR5 and TR3 were needed to isolate the ‘LOOP_MUTE’
line from the relay circuit to reduce switching noise.
MIXER SECTIONS
The two halves of IC4 are used as standard virtual earth mixer sections. Here the dry, effects
loop and reverb signals are mixed together depending on the settings of the series/parallel
switch and RL2. The outputs from each half of IC4 then provide the left and right signals sent
to the two power stages.
