[sdiy] Oberheim SEM module

[Magnus Danielson]

From: "john mahoney" <jmahoney at gate.net>
Subject: Re: [sdiy] Oberheim SEM module
Date: Tue, 6 Apr 2004 09:48:54 -0400
Message-ID: <08a701c41bdd$e7490060$6400a8c0 at BABYUTEST>

> I'd suggest that your friend look at the OB-X voice card schematics to see
> what came after the SEM. It's "the same but different," as some people
> say -- the next version of the same basic circuits. For example, the SEM
> uses 741 op amps paired with FETs whereas the OB-X uses TL081s or TL082s
> (which have built-in FET buffers, if I've got my facts straight).

Not quite, the TL081 and TL082 has JFET input diffrential pair rather than JFET
buffered NPN diffrential pair. From a functional point of view (high-Z input)
it's about the same, but the distorsion characteristics are not the same. Which
is better or worse I leave to you to decide, but a colleague said when he heard
my SEM that he thought the sound was tube-like. JFET-buffer characteristics?

If I would do it now, I would do it with JFET op-amp like everybody else I
guess.

> The OB-X has CA3080s all over the place to provide for programmability, but
> they also present additional VC possibilities. These 3080s are used like
> voltage controlled resistors.
> 
> The OB-X uses Curtis CEM3310 envelope generator chips for the ADSRs.
> 
> 24dB filters didn't appear on Oberheims until the OB-Xa or OB-8, which used
> Curtis VCF chips.

The OB-8 is a virtual Curtis app! Dual CEM3310 and Dual CEM3340 per voice!

BTW, anyone got spare OB-8 voiceboards?

Cheers,
Magnus