[sdiy] Thomas Henry XR-2206 VCO Page Up

[scottnoanh at peoplepc.com]

Oh, wait a minute, Karl,  - I missed that.  I was thinking for a bit that R5
(which and been assigned to a different function in a previous rev) had been
eliminated from the design, but not the parts list.  Silly me.  Didn't spot
it hanging off of pin 8 there.  I really am punchy right now....

I happened to just have an email exchange with Thomas (he just happened to
send me some samples to post on the page) and I asked him about that.  He
mentioned that, with no sync signal applied, Q3 is turned off, and the
control current at pin 7 of the XR is used.  Pin 8 is disabled in this case,
and so the 1K has no impact.

Speaking of the sync input, it is an interesting way of getting sync - sync
via FSK =-D.  Notice it's not referred to as hard sync or soft sync.  It's
certainly different to any other sync I've used.  Well, if you go to the
page, you'll hear it (a couple of Thomas' samples use the sync).

There are also a couple of skew samples there, much more demonstrative than
my liver-fingered sample of before.

They're here:

http://mypeoplepc.com/members/scottnoanh/birthofasynth/id20.html

Oh, yes, he'd tried the 1n in the servo (no difference), but mentioned he
wished he would have thought of Ian's suggestion about the cap in the
summer, and asked me to give it a go, since I've still got mine on
breadboard.  I'll try that out with a TL072 and see how that works.  But
after I get some sleep =0)

Cheers all,
Scott





----- Original Message -----
From: "karl dalen" <dalenkarl at yahoo.se>
To: "Scott Stites" <scottnoanh at peoplepc.com>; "Ian Fritz"
<ijfritz at comcast.net>; "SDIY" <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, June 29, 2006 8:02 PM
Subject: SV: Re: [sdiy] Thomas Henry XR-2206 VCO Page Up


> The reason for the lack of low range might be R5,
> it causes an offset  to the freq of the XR2206 VCO!
>
> I suggest removing it and adjust Iref in the expo a bit
> and also as Rene mentioned use a larger cap if needed.
>
> KD
>
> --- Scott Stites <scottnoanh at peoplepc.com> skrev:
>
> > Thanks, Ian!
> >
> > Yes, that lower octave was the heartbreaker.  Tuning for perfect
flatness
> > down that low would shift things enough on the high end where the
tracking
> > just wasn't terribly good beyond five octaves.
> >
> > I guess the main reason one would want to worry about the low end like
that
> > is that the ramp wave (from the skew) is double the frequency, so with
the
> > wave skewed to the octave up, it would still have the same amount of
error.
> >
> > Of course I think it's possible to get a ramp wave at the fundamental
> > frequency, but that takes a bit more circuitry than Tom was willing to
put
> > in.
> >
> > Cheers,
> > Scott
> >
> >
> > -----Original Message-----
> > >From: Ian Fritz <ijfritz at comcast.net>
> > >Sent: Jun 29, 2006 11:40 AM
> > >To: Scott Stites <scottnoanh at peoplepc.com>, SDIY
> > <synth-diy at dropmix.xs4all.nl>
> > >Subject: Re: [sdiy] Thomas Henry XR-2206 VCO Page Up
> > >
> > >At 10:02 AM 6/29/2006, Scott Stites wrote:
> > >
> > >>We were able to get around 5 musically useful octaves out of it - it
would
> > >>be much better if the low end would have cooperated more, but we think
> > >>that's all the XR-2206 would provide without running afoul of the law
of
> > >>diminishing returns - IE, it would start to increase in parts
> > >>count/circuit complexity, which Thomas reasoned would defeat the
original
> > >>intent of the circuit (a very simple, easy to build VCO with some neat
> > >>features).
> > >
> > >
> > >Hmmm... looks better than 5 octaves to me.  Within 0.1% from 1V to 8V.
I'd
> > >call it 7 octaves.
> > >
> > >Nice work!
> > >
> > >   Ian
> > >
> >
> >
> > ________________________________________
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> >
>