[sdiy] AR Envelope buffer?

[synth at oldmail.charlielamm.com]

I had the same sort of issues with my AR/VCA combo when I started out.

In the process of sorting all this out, I learned many things....

--Check the obvious.  If you have a scope, plug it into the output of the 
EG and make sure you really have C/V there.  Or, get an analog VOM and do 
the same.  Don't use one module to check another if you can help it.  I 
can't tell you how often I've fooled myself on stuff like this!

--Make sure the VCA works at all.  Take some known good source of C/V (i
have a 5V DC wall wart wired up to a pot, acting as a voltage divider,
just for this) and plug it into the CV of the VCA.  Now take a known good
audio signal, say a CD player, and a known good amp, say a guitar practice
amp, and plug that in as well.  When you crank the CV up and down does the
audio signal go up and down?

Doing these tests, you will have isolated the problem to either the VCA or
the EG.  It probably isn't both.

Assuming the EG is really at fault:

a) For the VCA to sound good with a RC type EG (like yours) you need a
linear response VCA.  I am not sure what VCA you're using.  If you have an
exponential response design, your A/R controls will sound like they're
attacking and decaying too fast even when set to long A/R times.  Been 
there and done that!

b)Using an EG design like yours, I never had much luck sourcing the
current needed to charge up the cap directly from my trigger source.  I
ran into latch ups and all sorts of undesireable behavior--my assumption 
was that the trigger source didn't source enough current to effectively 
do what I was trying to do. In the end I used a few transistors to buffer 
the trigger input of the EG, 
I have not had any issues since.

Another minor point is that I usually try to follow a 741 type buffer as
you use here with a 1K resistor between the op amp output and your jack.  
That way if something shorts out the CV signal to ground you won't be
asking the 741 to drive as much current as it can into ground; the 1K
resistor serves as to protect against this.  Your design will work without 
it, and I am not a big subscriber to protecting against any blunder I 
might create down the road with more parts, but, I usually do put in this 
one.

My site has info on the EG and VCA I used. After much mucking around I got
it to work, and it was well worth the time as far as learning how to build
and troubleshoot modules.  Hang in there and have fun, because if I can
get this to work, you definitely can!!!!

AR:
http://www.charlielamm.com/synth/miniar.php

VCA:
http://www.charlielamm.com/synth/duallinearvca.php

On Fri, 25 Jun 2004, Charles Brodeur wrote:

> Hello All,
>  I built this circuit, an AR generator, and I thought it worked.
> However, just because the envelope light comes one doesn't mean it's
> going to work with a VCA.
> 
> Today I built a VCA, and wanted to test it out. So I plugged the output
> of the envelope into the VCA. It will turn on when given a trigger, but
> it will not do the attack or release stage.
> 
> The schematic is here:
> 
> http://www.sdiy.org/cbrodeur/ar.gif
> 
> I was wondering if a buffer would be the solution. I know the envelope
> is being formed becuase it will light an LED in such a fashion. Could a
> simple buffer made out of an op-amp be the solution?
> 
> I hope this all makes since, my emal composing skills are subpar
> tonight. Thanks for any insight you all can give.
> 
> -Charles
> 
> 
> 		
> __________________________________
> Do you Yahoo!?
> Yahoo! Mail is new and improved - Check it out!
> http://promotions.yahoo.com/new_mail
>