Hi All,
I measured the current control and frequency response of this LM13600 VCF. If you're interested in data and graphs, check out:
http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects.htmlAt the end of the post, I do a simple mod to force the filter to be way open. Man, is sure does make the sawtooth buzz. I wouldn't call it a sweet sound, but it sure is lively and invigorating!
Chip
--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Thanks for the added insight!
>
> Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
>
> I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
>
> Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
>
> If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
>
> Chip
>
>
> --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
> >
> > The circuit is a dBx-style noise shaper: the more amplitude the dry
> > signal provides, the wider the response of the filter. This squelches
> > MN3005 noise at low levels and allows more high frequency content at
> > higher levels.
> >
> > The 2SA798 is a standard V/I converter that is "locked on" (filter
> > wide open) if the "OFF" signal voltage appears.
> >
> > The timbre change is of course due to the VCF changing vs. amplitude.
> >
> > Crow
> > /∗∗/
> >
> > On 2/25/2013 11:10 AM, chipaudette wrote:
> > > Hi Folks,
> > >
> > > Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> > >
> > > Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> > >
> > > http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> > >
> > > Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> > >
> > > I'd love your thoughts.
> > >
> > > Chip
> > >
> >
>