Here is someone's heroic effort in reverse engineering the SID chip from decapped die shots, containing almost complete schematics of the entire chip [0], both

Here is someone's heroic effort in reverse engineering the SID chip from decapped die shots, containing almost complete schematics of the entire chip [0], both the digital parts (oscillators, control logic) and the analog parts (filter, envelope, mixer). Unfortunately, re-creating a SID chip is almost impossible given that we don't have 1970s era NMOS semiconductor manufacturing facilities available. It is very difficult to build analog circuitry with similar characteristics from discrete components or modern IC technology. For example the filter section (see page ~9 of the forum post) is a dead simple state variable filter with 4 operational amplifier (opamp) stages and just a handful of resistors and capacitors. But the opamps in the original 6581 were not real opamps, but something hacked together from 3 MOSFET transistors due to silicon area constraints. The later revision SID (8581) had something more like an opamp, but SID enthusiasts claim it "doesn't sound as good" (and it definitely sounds different). For a hobby synth project, I re-created the filter section with a standard quad opamp chip (TL084 or similar), and sure enough it behaves like a filter, but it doesn't sound like the original (the resonant peak is way too sharp). In particular, the original filter changes frequency response with the amplitude of the incoming signal. If you look around the web, you can see several attempts at re-creating the circuitry (the filter in particular), but it's darn difficult to re-create it faithfully. And the original design is not a great musical filter design, its character is almost entirely due to the characteristics of the manufacturing process and the "bad" ompamp circuitry. For creating analog filters, the Moog transistor ladder and other classic synth circuits easier to get good sounds out of. [0] http://forum.6502.org/viewtopic.php?f=8&t=4150 http://forum.6502.org/viewtopic.php?f=8&t=4150