FatMan VCF LFO and better VCO isolation

[Scott Gravenhorst, Synthaholic]

Yesterday, I added the VCF LFO mod (method #1) which is on 
www.paia.com.  It works very well.

                   ALSO

Here is a description of how I was able to get my FatMan's VCOs 
extremely close in frequency without causing soft-sync:

Several months ago I had done the 'LED' modification.  For those 
who don't know of it, it is extremely simple and by itself 
greatly reduces soft sync.  It involves removing R38 and R39 and 
replacing them with (identical) LEDs.  The arrow must point away 
from pin 1 of the 555 to which it is attached.  That's it.  If 
done correctly, the LEDs should light solidly whenever power is 
applied.  

Recently, I added an external input and I needed to be able to 
control each VCO level separately in order to be able to balance 
them against the external input.  Originally, my circuit had two 
level controls that passed their respective signal to the 1/2 mix 
pot.  Since this pot was always centered, it was actually 
redundant.  I then decided to remove it from the circuit and run 
each VCO signal thru it's own 100K summing resistor to the VCF.  
A happy consequence of this was (apparently) better isolation of 
the VCOs, because now, I can get them much closer together 
without locking.  I can now get them within about 1/12 of a hertz 
before they lock.  

The following "ASCII-matic" depicts the FatMan modification to 
further isolate the VCOs and shows where to feed your external 
input.  The external input requires a preamp to provide a fairly 
large signal (8 volts p-p)  I used an op-amp for that.


           +----------/\/\/\/-----+
           |           100K       |
           V  10K (VCO 1 level)   |
A  -----/\/\/\/----+              |
         R46       |              |
                  gnd             |
                                  |
                        100K      |
            +---------/\/\/\/-----+-----+---> to IC17 pin 3
            |                           |
            V  10K (VCO 2 level)        |
B  ------/\/\/\/----+                   |
           R55      |                   |
                   gnd                  |
                                        |
 from output           100K             |
 of external   >------/\/\/\/-----------+
 input preamp

Remove R46, R55 and R57.  Disconnect R56 (1/2 mix pot).  Replace 
R46 and R55 each with a 10K pot.  You can use R56 for one of 
these is desired.  Point A comes from the positive side of C15.  
There is a convenient connection hole left by the removal of R46.  
Point B comes from the positive side of C18.  There is a 
convenient connection point left by the removal of R55.  Add in 
the 2 new 100K resistors as shown above.  Pin 3 of IC 17 may be 
accessed using the convenient connection hole left by the removal 
of R57.  



                                     

-- 
-- Scott G., Synthaholic

There is no 12 step program for synthaholics.  Thank your Superior Being.