Why is this please???? Was: RE: [sdiy] MFB SEQ-02 question

[Oren Leavitt]

Excellent.
As a more permanent solution, you (and MFB) may want to employ an output 
buffer design with that small resistor within the feedback loop of the 
buffer. That will isolate capacitive loading without the resistor 
introducing error when the CV output is loaded.

- Oren

David Ingebretsen wrote:
> Oren,
> 
> Bulls-eye. I found a 2k resistor in the bin, put it in series with the
> output jack and a shielded patch cable and the ripple disappeared. It was
> absolutely a capacitive loading effect.
> 
> Thank you.
> 
> David
> 
> 
> ~~ -----Original Message-----
> ~~ From: Oren Leavitt [mailto:obl64 at ix.netcom.com]
> ~~ Sent: Tuesday, January 05, 2010 11:41 PM
> ~~ To: David Ingebretsen
> ~~ Cc: synth-diy at dropmix.xs4all.nl
> ~~ Subject: Re: Why is this please???? Was: RE: [sdiy] MFB SEQ-02 question
> ~~ 
> ~~ David Ingebretsen wrote:
> ~~ > Alright. I've pinned down exactly what causes the ripple in the SEQ-02.
> ~~ >
> ~~ > In essence, if I use a patch cord made from shielded cable longer than
> ~~ about
> ~~ > 10 cm, regardless whether or not I connect the shield at either end, it
> ~~ > creates this ripple when I plug it into the CV output. On the bench, in
> ~~ the
> ~~ > rack, regardless. It always does.
> ~~ >
> ~~ > If I use a single conductor wire, regardless the length within reason,
> ~~ to
> ~~ > patch the SEQ-02 anywhere else, all is good, with one exception. If I
> ~~ plug
> ~~ > the SEQ-02 into a multiple and plug a shielded cable into the multiple,
> ~~ I
> ~~ > get the 2 volt peak-to-peak 400 kHz ripple again.
> ~~ >
> ~~ > So, what magic is the shield doing to introduce this ripple regardless
> ~~ > whether or not it is connected to ground?
> ~~ >
> ~~ > The easy fix is to make up a bunch of single conductor patch cables,
> but
> ~~ I'd
> ~~ > love to understand why.
> ~~ >
> ~~ > Thanks
> ~~ > David
> ~~ >
> ~~ 
> ~~ 
> ~~ 
> ~~ David,
> ~~ Sounds like the CV output is direct coupled to an op-amp that does not
> ~~ like the capacitive load the shielded cable may be imposing on it.
> ~~ As a quick test, to isolate the issue, you can try placing a 100 ohm (or
> ~~ more) resistor between the internal SEQ-02 output and the CV out jack.
> ~~ See if that stops the problem.
> ~~ My guess..
> ~~ - Oren
> 
>