[sdiy] VCO circuits using the 555 timer?

[Scott Gravenhorst]

music.maker at gte.net wrote:
>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. 

Oops.  I forgot to add that Q3 and Q4 are also part of the current source...

>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.
>
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>

-- 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.