MATRIXSYNTH: CEM3350 and the Rhodes Chroma

Wednesday, December 29, 2010

CEM3350 and the Rhodes Chroma

This came in on the Rhodes Chroma list via Heinz:

"Here a discription of the CEM3350 what I believe it's working. Note all components red marked in the schematic are always present in the Chroma. All OTAs are LM13700.
What is new is SW 2. If it is set to the right position we got a BP response. Together with SW1 to right postion we got an Allpass. The circiut is not adapted to the Chroma yet. Frequency control is set to somewhat 0.5V/Octave. My problem at the moment is the Q-control. When I'm pulling the FIX input to ground (no pos. feedback) the filter behaves as described. Because I have no audio sweeper I did't know what is the Q when stepping trough from 0-6 with Q-control. My only choice is the pink noise source as a audio source and measure the increase of output level at a fixed frequency i.e. 32. But - the source isn't bad but not the best. So what I need is more measurements from different Chromas to have an average. Ok - the next problem. The LM13700 needs current sources for control. Thus the expo converter is hooked up around a PNP array. Hard to find and all controls have the opposite direction. But the good news. The filter is breadboarded and on my bench. It oscillates from 16kHz to appr.12 Hz (better as in simulation). Because I have no quad PNP array I've glued two 2SA798 together. In other words - the filter works.


Update: thread on here.

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