This mod to the PAiA 6710 Vocoder increases clarity and virtually eliminates noise by buffering the IN1 and IN2 level controls from the sixteen filters they feed. Each buffer drives its own filter which prevents loading of the level controls. This gets more signal to the filters and eliminates interactions between them. Also, the value of the summing resistors to the mono/stereo output amps are increased to lower the gain of these stages resulting in less noise.
The summing resistors which set the mix of the filters should be replaced before the mod board and its wiring are added. Remove resistors R31, R32, R3, R4, R33, R34, R5, and R35. Replace R5 with a 22k ohm (red-red-orange) resistor and R35 with a 68k ohm value (blue-grey-orange). Replace the remaining resistors with 33k ohm values (orange-orange-orange).
Review the Vocoder manual and notice that the wipers of the IN1 and
IN2 level controls wire to circuits board pads "P" and "Y"
and that these points then each connect to eight 4700 ohm (yellow- violet-red)
resistors spaced across the board. Desolder and lift the ends of these
resistors that connect to "P" and "Y" and solder a
length of wire from the free end of each resistor (slide tubing over the
solder joint for insulation) to the outputs of the new buffers as illustrated.
Remove and discard the two wires between the wipers of the level controls
and circuit board points "P" and "Y" and replace them
with wires from the pots to the buffer inputs. Make the V+ and V- connections
from the mod board to the vocoder power supply (there is no ground circuit
to the mod board).
Mod Parts List:
1 component pc board
4 14 pin IC sockets
4 TL 084 quad op-amps
6 33k resistors
1 22k resistor
1 68k resistor
Approx. 20 ft. #22 ins. stranded wire
For more info, contact Scott Lee.