              Power Supply Modification for Low Hum
                                                      Mike Sucher 3/14/88 (WMUG)
   
   Power supplies for all those little stomp boxes can get a bit 
expensive if you always by the brand-name version.  A generic power
supply can be 50-70% cheaper or more.  The only problem is that 
these boxes often don't have adequate filtering to suppress the
dreaded 60Hz A.C. hum to an acceptably low level. Following is a
description of how to get around this problem.
   
   Chris (Sysop) initially expressed interest in it at the last
user group meeting, so I wrote it up. Then we realized that others 
could use it too, so here it is.
   
   First of all, I don't recommend trying this on any supply rated 
at less than 500 ma.  I've had bad results with these lower capacity 
supplies.  The current rating should be clearly marked on the unit.
Also, try to avoid the switchable voltage supplies.  What we're 
interested in here is a 9 volt D.C. supply.  With the switchable 
versions you're paying extra for a feature you're unlikely to use.
   
  While I'm at, it let me clear up a common misconception about these
units. A supply rated at a given current value does NOT always put 
out that much current.  They are supposed to put out a constant
VOLTAGE.  The amount of current drawn depends on the load resistance
or the device you are driving.  The current rating of the supply is
just the MAXIMUM amount of current that it can put out while keeping
the voltage constant.
  
   Here's the circuit:
     
  +----------+               ________
  |          |---------v-----|  r1  |------v--------*
  |          | (+)   +---+   --------    +---+     (+)
  |  POWER   |       | c |               | c |   filtered
  |  SUPPLY  |       | 1 |               | 2 |    output
  |          | (-)   +---+               +---+     (-)
  |          |---------^-------------------^--------*
  +----------+
     
   Parts:
      supply   -  9V, 500ma D.C.
      c1 & c2  -  2000 to 5000uf electrolytic, voltage rating 
                  of at least 15V
      r1       -  10 ohms, power rating of at least 2W 
     
   Note that electrolytic capacitors are polarized.  You must attach
the positive side to the power supply positive side, and negative to 
negative for proper operation. 
 
   The values given are nominal. Higher values for the capacitors
will give better filtering, and hence lower hum.  Increasing the 
resistor value will improve filtering, but if it gets too large the
voltage will drop, giving poor performance.
 
   Be sure to use good soldering technique, and insulate all bare
wires.  I find that RTV (the silicone rubber in a tube) works well
for mounting the components on the power supply case.
 
   The mod seems to be quite reliable. I've been using it for several
years with good results. All the parts are available at Radio Shack.
 
   Enjoy!
                             Mike

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