SSM2044 LP Filter designs
This page is a look at how various synths implemented the SSM2044 filter, with the datasheet design as a reference standard. I've redrawn all the schematics to make comparisons very easy.
First, the datasheet design.
The datasheet suggests using the two differential inputs (+audio and -audio) as individual oscillator inputs for a two-oscillator synth (or synth voice). It also suggests that these two inputs should be provided with slightly different resistor values (68K Vs 100K) so that the oscillators don't cancel completely when beating against each other.
Korg used the SSM 2044 in various synths. The Polysix is quite an interesting case, as it runs the chip on +/-5V. This probably entails some of the other component changes. The Polysix only has a single oscillator, so it uses the second input to cancel any DC offset on the output.
Although the MonoPoly has four oscillators, it also only uses one of the signal inputs on the chip, instead using an op-amp mixer (not shown) to mix the oscillator signals. The circuit is very similar to the Poly6, except the MonoPoly uses the more common +/-15V supply. Note the 100nF supply decoupling around the chip, too.
I read a forum recently where someone claimed that SSM2044 filters sound better or worse depending what circuit they're in, and said the filter from the Trident was much better than the one in the Polysix. “Hum! How interesting!“, I thought, and went off to find the circuit diagram. Here it is, and it's virtually identical to the Polysix design. There is no way on God's earth that this sounds markedly different from the Polysix filter. Moral of the story: Don't believe every “expert” opinion you read on a web forum! (but you knew that already, right?)
PPG Wave 2.3
I don't know whether the earlier versions of the PPG Wave used this same filter circuit or not. This one was copied from the service notes for the 2.3. Note that it includes an apparently optional trimmer for output offset voltage, called ‘Klick A’.
Siel Opera 6
Siel's design uses a slightly different arrangement for the Frequency CV input (pin 13). I haven't shown the CV mixer. It also uses pin 1 (+signal input) as a mixer for the oscillator and noise signals. The design also uses supply decoupling, like the MonoPoly.