søndag 27. januar 2019

OTA filter cell

Fun fact - after looking for an explanation for the OTA used in the juno for a couple of years, I found it in my bookshelf because someone mentioned a circuit in the Musical Applications for Microprocessors book by Hal Chamberlin.

When trying to understand a circuit, I often look for visually similar circuits, but even though I have found several OTA filters, they are always different from the Juno one. But here is the clue: These are similar:

The typical Roland filter cell to the left, its equivalent to the right.
The left one is what you find in the Roland schematics, the right one is found in papers and OTA filter explanations elsewhere. It is explained in some detail in the section on voltage controlled filters, page 201 and onwards in 'Musical Applications for Microprocessors'.

The cell is really quite simple - the resistors, 68k and 560Ohm function as voltage dividers to reduce the input signal to the OTA. This is true for both the 68k on the input and the 68k in the feedback loop.

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