[sdiy] VCA venting

[Scott Gravenhorst]

Just a guess, but maybe the current source isn't sourcing current?

One thing you can try is to connect the top end of the 15K resistor feeding
Iabc to the 3080 to a 10K pot to ground.  The 10K pot is used as a variable
series resistor.  Twisting the pot should control the VCA somewhat.  If it
does, the problem is in the current source.

I'm sure you can measure it.  My guess is that the Iabc pin should be more
positive than the negative rail if there's any Iabc current flowing.

"Pat Kammerer" <spivkurl at wearerecords.com> wrote:
>Hi,
>
>This is the one I'm working on right now;
>http://www.oldcrows.net/~jhaible/tonline_stuff/hjvcaod.gif
>
>The drive part works fine, I think, but it's the only way I can get a decent
>signal level at the output.  With drive turned down, the signal is virtually
>non existent.
>
>I did find one mistake, which was a wrong value capacitor, but it didn't
>really make a difference.  BTW, am I right that the capacitor in the
>feedback of the 2nd op amp is 10p as in picofarads?  It seems so small.  I
>couldn't track one of these down so I had to use a 12p one, would that make
>a difference.
>
>But, I've check over my work and reflowed solder points with no effect.  The
>only way I can get a decent CV control of level is to wire a trim from the
>CV input to the collector of the transistor
>
>No matter what, the two pots other than drive don't seem to do anything.  Or
>maybe I just can't hear what they are doing ... ; )
>
>Here is the other, or so I think, I can't get to Yves pages anymore : (
>http://www-timc.imag.fr/Yves.Usson/personnel/SDIY/Modular/index.html
>
>This one just has no control, and little gain.  I haven't looked at it in a
>little while.
>
>Thanks,
>Pat
>----- Original Message -----
>From: Scott Gravenhorst <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Monday, February 07, 2005 11:36 AM
>Subject: Re: [sdiy] VCA venting
>
>
>> Can you link us to schematics?
>>
>> "Pat Kammerer" <spivkurl at wearerecords.com> wrote:
>> >Hi,
>> >
>> >I just had to vent...  When I got into Synth DIY I had this strange
>> >assumption that building VCAs would be easier than the other main
>modules.
>> >My recent experience has made this seem not the case.
>> >
>> >I've built two different VCAs on perfboard, and both of them seem to have
>> >the same problem.  The CV does nothing.  I can get a signal, but no
>matter
>> >how I trim it I can get the CV to work.  I can however get the CV to
>effect
>> >gain by sending it into a different point in the circuits.  This doesn't
>> >make any sense to me...
>> >
>> >If anyone has any good general advise, then I would appreciate it,
>otherwise
>> >just ignore me.   : )
>> >
>> >Thanks,
>> >Pat
>> >
>>
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>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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>>
>>
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.