[sdiy] SEM filter questions

[Tom Wiltshire]

On 22 Apr 2012, at 09:15, Neil Johnson wrote:

> Magnus Danielson wrote:
>>> Nah. While Oberheim did use some CEM and/or SSM chips for things like
>>> ADSRs and so on the filters were 2-pole SVFs based on CA3080s.
>> 
>> Oh really? For what synth are you talking about? It really changes over
>> the years.
> 
> I was focussed on SEMs.  It may well be that later OBs used more esoteric chips.
> 
>> SEMs use really standard stuff. The CA3080 is as special as it gets.
> 
> I did look at an OB-X a few years ago, and I'm pretty sure that had similar 3080-based filters.
> 
>> OB-X is really SEM like filters, but when you get to OB-8 it is all
>> modernized.
> 
> I couldn't possibly say, having neither owned an OB-8 nor looked at its schematics.

The OB-X is about the point at which it changes over. It retains the CA3080 SEM filter, but the oscillators and envelopes are already CEM chips. The OB-Xa and OB-8 both use CEM throughout. The OB-Xa uses two separate CEM3320s, with one implementing a 2-pole SVF, and the other a 4-pole LPF. The OB-8 uses a single CEM3320 with analog switches to alter the circuit and provide different responses.

Tom