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

[David Ingebretsen]

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