[sdiy] VCO circuits using the 555 timer?

[Scott Gravenhorst]

David Betz <dbetz at xlisper.com> wrote:
>I bought a few 7555 and 7556 chips recently and would like to  
>experiment with building a VCO. I got the idea for using the 555 from  
>the fact that they use it in the PAiA FatMan but I've been unable to  
>find any other VCO circuits that use the 555. Can someone point me to  
>some examples of using it to build a VCO? My first idea was to  
>duplicate the VCO part of the FatMan circuit but I'm not sure I  
>really understand how they are using the 555 in that circuit. It  
>doesn't match any of the examples I've seen for how to use the 555 in  
>astable mode. For example, the timing capacitor is wired differently  
>than the example circuits I've found. Can anyone point me to  
>resources of using a 555 for musical applications?

Hi David,

The reason for the different topology is that in order for the VCO to be linear, it
must charge it's timing capacitor with a current source, not a simple resistor.  If
you're doing an expo VCO, you'll want to replace the linear current source with an
expo one.  Looking at VCO1 in the FatMan schematics, the current source is made up
of opamp IC10:C, R43 and R37.  Current is supplied to the cap via R44 (which really
has no function other than a disaster preventer should things get hooked up
incredibly wrong).  The voltage input to control the current source is R40 and R36.
 This is the piece that would have to be replaced with an expo thing (which I can't
speak to).  The reason for the VCO being powered from -12V and 0V is to facilitate
the proper operation of the current source, the output of which actually sits at 0V
and sources current through the capacitor to the -12V rail.

One reason I think that the _bipolar_ 555 wasn't very popular is because of a
problem in it's output circuit which causes both of the totem pole output
transistors to be on simultaneously for an instant while the device switches from
one output state to the other.  These two transistors effectively connect the power
supply to ground for that instant.  This current pulse will cause problems with
other things connected to the power supply, namely soft synchronization.  The CMOS
version, abbreviated as 7555, doesn't have this problem and makes for a much better
VCO in the FatMan.  Even though it is CMOS instead of bipolar, at least in a FatMan,
it's a drop in replacement and drastically reduces soft synch in that synth.  So if
you want to do this, I strongly suggest the CMOS part for less headaches.

-- ScottG

-------------------------------------------------------------

-- Scott Gravenhorst
-- GateMan II - Xilinx Spartan-3E Based MIDI Synthesizer with SVF
-- PolyDaWG/8 - 8 Voice FPGA Polyphonic MIDI Synthesizer
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.