From Scott Lee @ Paia

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Vocoder Noise Reduction Modification

This mod for the 6710 Vocoder virtually eliminates an otherwise objectionable
amount of hiss from the mono and stereo outputs.  A voltage follower op-amp
circuit inserts at the input to each filter, isolating the IN1 and IN2 level
controls from the eight filters they feed.  Each follower drives its own load
instead of the variable resistance of the level controls driving all eight. 
This means more signal with a flatter response gets to the output of the bands,
there's more dynamic range, and the signal to noise ratio is improved. 
Additionally, the summing resistors to the mono/stereo output amps can be larger
which lowers the gain of these stages contributing to lower noise.

A Component PC Board (general purpose) available at Radio Shack (276-168b) makes
it easy to add the four ICs and wiring for the mod.  The board is full of holes
and soldering points/grids optimized for wiring IC circuits.  Sockets for the
four ICs can be soldered and connections between the output pin to the inverting
(-) input pin is easily made with a wire jumper.  Two busses facilitate common
circuit connections and the IN1 and IN2 taps can distribute to IC pins on
these.  The IN1 and IN2 taps, the buffered outputs, and the V+/V- power supplies
connect using #22 insulated, stranded wire (about a foot for each connection
with the mod board situated behind and at about the middle of the vocoder
circuit board).

Notice on the vocoder tht the IN1 and IN2 level controls wire to circuits from
their #2 terminals (via wire P for IN1 and wire Y for IN2) and these two
circuits each have eight 4700 ohm (yellow-violet-red-gold) spaced across the
board which are the inputs to the band filters.  Remove and discard the P and Y
wires; they'll be replaced with new ones to the mod board.  Desolder and lift
the ends of the 4700 ohm filter input resistors from the circuits that tie them
to the P and Y wiring points; they become the connecting points for the mod
board buffer outputs.

The summing resistors which set the mix of the bands should be replaced before
the mod board and its wiring are added.  Remove resistors R31, R32, R3, R4, R33,
R34, R5, and R35.  Install 33k (orange-orange-orange-gold) resistors at all
these locations except R5 and R35.  Install a 22k (red-red-orange-gold) at R5
and a 68k (blue-gray-orange-gold) at R35.  When making the connections to the
filter input resistors with the wires from the buffers, a piece of tubing can be
slipped over the wire before it's soldered to the end of the resistor so the
joint can be covered/insulated.

Finally, wire the mod board to the points on the vocoder.  When making the
connections to the filter input resistors with the wires from the buffers, a
piece of tubing can be slipped over the wire before it's soldered to the end of
the resistor so the joint can be covered/insulated.  Make the V+ and V-
connections from the mod board to the vocoder power supply (there is no ground
circuit to the mod board).  Test operation.

If it didn't look like you had a plate of spaghetti in front of you before, it
should now!  Enjoy.

Scott Lee - Paia tech

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
appx. 20 ft. #22 ins. stranded wire


Example for one band (note: i'm doing the best i can ascii-izing this :) -
justin)

               ________
              6|-\    |   R8      
               |  >7--+---vvv----C30, 31
R26-2  -------5|+/ 
IN1 level         IC13 

               ________
              6|-\    |   R9
               |  >7--+---vvv----C32, 33
R40-2  -------5|+/ 
IN2 level         IC11





       _______
 R11-,=|     |=,-R13     IC 11
     '=|     |='
R40-2-=|     |=-R40-2
   V+-=|TL084|=-V-
R40-2-=|     |=-R40-2
    _,=|     |=,_
  R9 '=|     |=' R15
       -------

       _______
 R19-,=|     |=,-R21     IC 12
     '=|     |='
R40-2-=|     |=-R40-2
   V+-=|TL084|=-V-
R40-2-=|     |=-R40-2
    _,=|     |=,_
 R17 '=|     |=' R23
       -------

       _______
 R10-,=|     |=,-R12     IC 13
     '=|     |='
R26-2-=|     |=-R26-2
   V+-=|TL084|=-V-
R26-2-=|     |=-R26-2
    _,=|     |=,_
  R8 '=|     |=' R14
       -------

       _______
 R18-,=|     |=,-R20     IC 14
     '=|     |='
R26-2-=|     |=-R26-2
   V+-=|TL084|=-V-
R26-2-=|     |=-R26-2
    _,=|     |=,_
 R16 '=|     |=' R22
       -------