Like my SSM2044 page, this page is a look at how various synths implemented the CEM3320 filter, with the datasheet design as a reference standard. Whereas the SSM2044 is a dedicated lowpass filter, the CEM3320 is just a set of filter building blocks. In this respect, it is more like the earlier SSM2040. The CEM equivalent of the SSM2044 is probably the CEM3328, which is basically the same as the 3320, but wired up as a dedicated lowpass filter, thereby saving a few pins.
First, the datasheet designs.
Datasheet CEM3320 Lowpass Filter
The datasheet makes what is actually a pretty straightforward circuit look really complicated by showing the chip with its pins numbered in order. This makes the whole thing a tangle, as the layout of the separate stages inside the filter is not at all logical. I’ve put the stages in order here and just noted the pin numbers to make it clearer.
Datasheet CEM3320 Highpass Filter
This is a slightly simpler circuit than the lowpass design, and it is fairly easy to see the way that the “R”s and “C”s have been swapped around to turn a lowpass filter into a highpass. This filter is a fairly rare beast, a 24dB/Oct highpass.
Datasheet CEM3320 Bandpass Filter
This circuit is simply a combination of the two above. The first two stages are set up as highpass filters, and the last two as lowpass filters. This gives a 12dB/Oct bandpass response overall.
The Sequential Pro-One uses the filter in a circuit which is quite close to the datasheet. They’ve added a buffer op-amp on the output, which also affects the resonance path. They also use the virtual ground node at Pin 1 to sum signals from the two oscillators and the noise source. The filter capacitors have also been halved in value, which is going to raise the basic cutoff frequency of the filter by an octave.
Update: What is that buffer for, anyway?
It seems from experiments that the plain datasheet lowpass circuit has a couple of characteristics you might want to avoid. One of these is that the signal level when the filter is oscillating is much less at lower frequencies than at high frequencies. This also affects the filter’s ability to start oscillation at these frequencies. Adding a buffer like in the Pro-One (notice it has a gain of x3.4) solves this problem and gives the filter a more even response.
The Oberheim OB-Xa is a quite extraordinary synth in that it has two filters, but only lets you use one at once, which means one filter is always entirely wasted. Obviously CEM chips didn’t cost then what they cost now!
The idea of this was to provide a switchable response between 2 pole, 12dB/Oct lowpass and 4 pole, 24dB/Oct lowpass. There is one filter for each response, and you can switch between the two.
Oberheim OB-Xa 4-pole Lowpass Filter
The 24dB/Oct lowpass filter is a fairly standard design, although it uses a +15V/-5V supply. Otherwise, it is much as the datasheet suggests. I don’t understand why they specified 1% resistors for the Frequency CV input, and then went and put a 10K trimmer in – seems daft to me. Perhaps the 1% resistors had better thermal characteristics or something.
Oberheim OB-Xa 2-pole Lowpass Filter
The 12dB/Oct lowpass filter is a completely different story! This is nothing like the datasheet! Instead, Oberheim have used the chip to implement a 2 pole state-variable filter. Stage 1 is only used to mix the signals. This is usually done with a differential op-amp, but they’ve saved one amp by doing it like this. Stages 2 and 3 are the actual filter poles, and Stage 4 is unused. I think the fact that they haven’t got a proper differential amp accounts for the extra-complicated feedback and resonance paths, but I’m not sure. Given that this is a state-variable filter, it should also produce a highpass and a bandpass response, but these aren’t used here. It’s a pity, as that would have set the OB-Xa apart from the competition.
Elka Synthex Multimode Filter (LP/BP/HP)
The Elka Synthex has probably the most sophisticated filter circuit built on the CEM3320 to ever have been used commercially. It uses analogue switches to reconfigure the circuit and provide a variety of responses. The Oberheim OB-8 that followed the OB-Xa did something similar, but only provided the 12dB and 24dB lowpass reponses that the OB-Xa had.
I’ve left out some of the Freq CV input circuitry, which includes a CV mixer and some other components to allow an inverted filter envelope.
The filter looks pretty complicated at first sight, but once you break it up into its consituent units, it isn’t so bad. The first two stages both include a pair of analogue switches to enable them to switch between lowpass and highpass responses. For each stage, with the switches open, it’s highpass, closed it’s lowpass. The other switches select one of two output points, either after the first two stages, or after all four. Three switches are used for this to allow some adjustment of output and feedback resistances. The overall structure is shown below.
This structure can provide various responses, of which the four below are implemented in the Synthex.
Note that other configurations are possible that the Synthex doesn’t use. A 12dB lowpass filter could also be provided, as could a 18dB lowpass, combined with a 6dB highpass. Only the Oberheim Xpander/Matrix followed this direction to its logical conclusion.
Building these designs
Since I originally wrote this article, the CEM3320 VCF chip has been cloned as the AS3320. We have the AS3320 voltage-controlled filter chip available in the shop.