PLAN B analog blog

This is really well put Peter. 
Most people do not understand the difference between the Model 15 and the Moog Style Oscillator utilizing the Octave Switch. You may consider including some of this explanation on the site somewhere (perhaps you have, and I just have not seen it).

Across several octaves, 4 cent pitch tracking variances are very acceptable as analog VCOs are concerned. 1 cent variances should just be considered DEAD ON.



Tony


> To: PLAN_B_analog_blog@yahoogroups.com
> From: peter@buzzclick-music.com
> Date: Mon, 4 Feb 2008 06:48:12 +0000
> Subject: [PLAN_B_analog_blog] Re: Big entertainment (and commentary on the M15 tracking)
>
> My opinion (doesn't make it right, just makes it mine):
>
> What Hex said was fine. He was frustrated because he had problems
> with his Model 15's tracking, which he called 'intonation'. Basically
> he would set it up, cal it and then it would go out a few minutes
> later. In order for me to comment on this I'd have to see the VCO to
> determine what the problem was. I never received the unit back, so I
> can't verify or discredit his results.
>
> More on M15 tracking in a second.
>
> In reply to his initial comment, Dan Levey (ReWire) went ahead and did
> our cal procedure on his M15 and then recorded a sound file which
> plays six octaves. To that, Hex put it through a tuner and found
> variances, the worst of which was 4 cents off dead on, with most only
> 1 cent off. One actually was spot on.
>
> To his results, I was elated. This type of resolution is really good
> for an analog VCO. Hex however made a comparison to a out of tune
> singer. This really isn't fair, at best a gross exaggeration given
> that most people can't hear something out of tune until it's in the 20
> cent range. 5 cents is about the peak of human ability, and this is
> considered the best of the best of those with perfect pitch.
>
> His second comparison to an MFB VCO was also a bit unfair in that it's
> digital.
>
> OK fine no harm done, no fowl, right? The bottom line is my VCO will
> not nail tracking perfectly...but none will. Some will do better than
> others (read: ours) and there are reasons for this, most of which have
> to do with the incorporation of a Moog's octave switch which limits
> the range of the VCO to within what's called the 'linear region' but
> is able to offset that to higher or lower registers. In this way, the
> circuit is responsible for tracking a much smaller distance, but it
> can move that gap higher and lower over the audio band. If you want a
> VCO which demands keyboard tracking, this is the type you should stick
> to because this is the only config which will give you large-range
> keyboard tracking.
>
> So why didn't I do that?
>
> The Expo converter circuit used in the M15 is about 80-90% responsible
> for it's sound - it's reaction to audio bandwidth FM. It's a discrete
> driver - no op amps in the expo circuit whatsoever. Sounds great, but
> tracking is tougher than using other methods. 6.5 to 7 octaves, no
> problem. 10? It'll never happen. Maybe for a day. Most VCO's drive
> their expo with either an op amp or an transconductance amp such as
> the 13700. I can do this and tracking improves - but the instrument
> loses it's sound and I am not willing to make that trade off.
> Unfortunately, even when matching the discrete semi's in the expo
> drive circuit (not the expo itself, that's always been very tightly
> matched via a Linear Systems matched transistor pair) - it's hard to
> center into what's called the 'linear zone' of the expo converter's
> response. That's what gives you the steller tracking.
>
> If your type music requires a large tracking range - you may want to
> look into one of the other Eurorack VCOs on the market. There are many
> Excellent ones to choose from. Actually, the Aries VCO is about the
> best in the world for tracking if you can find one. If you want the
> Buchla 200 sound, then either pay $2k for a 258, or get a Model 15
> which tracks a lot better than the 258 ever did. That's what the M15
> was designed to do.
>
> OK, that's why.
>
> Over the course of this company's existence, I'm proud to say that I
> do look into people's comments and I have made changes to circuits in
> reply. The M15's sync, the soon-to-be M10's retriggering capability
> to name a few. So I tell you what...let me look into this one. If
> there's a way to improve the tracking without sacrificing the audio
> qualities and without changing the design of the expo converter
> circuit as it is I'll gladly do that. It may be as simple as
> replacing the zener diode in the expo circuit with a 6.9 volt voltage
> reference and making a few resistor value changes to compensate for
> that delta (7.5 volts to 6.9).
>
> Added to the list and I'll update you all as I've had an op to test
> it. In the meanwhile, I hope this helps to clear things up. The M15 is
> a great VCO...for what it does. Like anything else, it's a crap VCO
> for things it can't do. It is however what it is...!
>
> - P
>
> P.S. Matrix: please don't post, thanks!
>
>
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