Posted by & filed under Elka, Filters, OB-Xa, Oberheim, Pro-One, Sequential, Synth DIY, Synthex.

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

CEM3320LowpassFilter

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

CEM3320HighpassFilter

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

CEM3320BandpassFilter

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.

Sequential Pro-One

ProOneCEM3320Filter

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.

Oberheim OB-Xa

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

 

OBXa2PoleCEM3320Filter

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.

SynthexMultimodeFilter

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.

SynthexBlockDiagram

This structure can provide various responses, of which the four below are implemented in the Synthex.

SynthexFilterConfigs

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.

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6 Responses to “CEM3320 Filter designs”

  1. Tony Barbetta

    Quoting your sentence above; ‘The datasheet makes what is actually a pretty straightforward circuit look really complicated.’

    So true. I’m coming to a point where I just re-draw some circuits before even trying to understand them. You can only set up so many memory buffers in your mind, and after that it becomes inhuman to go for more, and possibly even counter-productive.

    Reply
  2. Godric

    Thank you for this explanation. Can I clarify something?

    The switches above are labelled A, B, C, or D.

    Is it correct to assume that the three switches marked A operate together, the two switches marked B operate together and switches C and D operate separately from one another and switches B and A?

    So the truth-table is like this:

    A B C D
    24dB low pass 1 1 1 0
    6dB band pass 0 1 0 1
    12dB band pass 0 0 1 0
    12dB high pass 0 0 0 1

    Reply
    • Tom Wiltshire

      Yes, there are four control signals, A, B, C and D. A and B operate multiple switches. C and D only operate a single one.
      Your truth table looks pretty good to me. The only real query is switch C. It’s not clear when that is used. I imagine if one builds the circuit, it becomes fairly obvious since it allows a level boost to the output.

      Good luck!
      Tom

      Reply
  3. Sylvain Poitras

    The datasheet tells us that max voltage between Frequency Control and GND pins is ±6V and that max voltage between Resonance Control and GND pins is +2V, -18V. Great, I can keep things within those limits, but I can’t quite make out what would be the voltage range for a full sweep of the cutoff frequency (for instance in the four-pole LPF from the datasheet).

    0V to 5V?
    -5V to +5V?
    -6V to +6V?

    What about the resonance CV?

    Reply
    • Sylvain Poitras

      Ok, reading the CEM datasheet cleared things up a bit. To control cutoff with a 0-5V CV, I’d need to attenuate it down to 0-155mV (75K and 2.4K voltage divider should do).

      Reply

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