DIY: Micromoog/Multimoog Oscillator "fault" correction

Haible Juergen Juergen.Haible at nbgm.siemens.de
Mon Nov 22 13:07:37 CET 1999


		>:::This should work for Multimoogs since they use the same
board as the Micro.
		>:::
		>:::Sounds similar to what Juergen found in his OB-8...
		>
	>Does this mean the manufacturers deliberately weakened the bass
response
	>in some model to make other mdodels more preferable (higher price ,
too)?
	> [...]
	>m.c.

No, I don't expect Oberheim to make the discontinued OB-Xa more desirable
than the
OB-8 they were just introducing (;->).

I've written a few mails about this which you should find in the archives,
containing
some speculation of mine. (It's not more than speculation from what I found
in the
old documents - I never talked to Tom Oberheim himself.)

In short, this is what I think:

(1) There is *not* much sonical difference due to the altered lower cutoff
frequency.
(speaking for the Obies here; don't know about the Moogs). Yes, I've
published
a modification to restore the OB-Xa-like bass response for the OB-8, but I
also
tell everybody don't bother to make this mod unless you're looking for a
very subtle
difference in sound. (Build my Multimode filter mod instead, it's much more 
rewarding !)
The big surprise for me was that the waveforms looked so distorted on the
scope,
due to the coupling capacitors, but I think the ear is not so sensitive
here. (Much
of the altered waveforms might be due to the phase response of the coupling,
and
only a tiny amount might be from the amplitude response.)

(2) I found a *possible* reason for decreasing these capacitors, when I
soldered
larger values in parallel: The autotune routine would occasionally stop
working.
Why is that ? Larger coupling capacitors also mean a longer step response,
so
it takes more time until the signal has lost it's transient from switching
and is in the
optimal voltage range for the autotune comparator threshold. (Mind you,
switching
between VCOs and Voices in autotune mode is most probably just done by
setting the PW below 0% for 15 of the 16 VCOs at a time, so this should
produce some transients !)
A fast Autotune operation was a big selling argument back then, so make the 
caps as small as possible. (The compromise they found was not a bad one !)

(3) Tom Oberheim has once said in an interwiew that the OB-8 does sound
different
than the OB-Xa because of the anvanced autotune, and because of the
generally
advanced circuit design. (I don't remember the exact wording.)
Right he is (of course - this interview must have been long after they had
stolen
the company from him ...), and in fact you can make a OB-8 much more
OB-Xa-sounding
if you avoid the autotune button. When you use it in the studio with fairly
the same
temperature, just wait for 10 minutes and the OB-8 will show the same tuning
it had when you switched it off last time. (At least mine does this.)
As for advanced circuit design, much was done just to reduce cost (like
reconfiguring
one CEM3320 chip for two filter modes, rather than just switching between
two
parallel filters). The VCA of each voice was actually "upgraded" to a Curtis
chip
on early OB-8 models, but later models have the cheap CA3080 again (like the
OB-Xa,
and like the SEM's).

JH.



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