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The Roland Filter (IR3109 / BA662)

To keep the focus of this article more direct, only the IR3109 / 80017a / BA662 style filters will be discussed.

The development of IR3109 was in direct relation to simplify the process of building a resonant filter with discreet BA662 ICs (which had to be hand-picked, tested for matching, organized into specific groups, etc). The amount of labour it took to build the first revision of Jupiter-4 voice cards was staggering, and IR3109 helped reduce that as no matching was necessary and the four discreet ICs became one. Later IR3109 would be reduced further in an integrated surface mount package (80017a).

The million dollar question is how different are the sonic changes between: four BA662 ICs, IR3109, and the later surface mount shrunk 80017a.

IR3109-based Models

Discreet BA662-based Models

I'm not certain if there is a sonic change using a BA662 A over a BA662 B. I'm also not certain if there's a sonic change when BA662 is used for buffers in conjunction with the FET transistors, or FET transistors exclusively.

Filter Differences

This section will be pending. I intend to begin comparing a Jupiter-4 (IR3109) with a SH-09 (BA662 A w/ Hybrid FET/BA662 buffers). And then if I can get in contact with someone who has SH-2 or BA662-based Jupiter-4 to continue doing further comparisons. Once that's accomplished I would like to compare IR3109 from a JUNO-60 with 80017a on a JUNO-106 (preferably original ICs 'just to be safe').

BA662 Replacement Solutions

Due to the amount of classic Roland synthesizers that have had a bit of a resurgence, many of the discontinued ICs have been cloned or reverse engineered.

It's important to note that if you are going to replace BA662 *filter designated* ICs on an earlier Roland synthesizer, you actually have to buy a large batch of them and transistor match them just like Roland had to do in the late 70's/early 80's. Failure to do so will prevent oscillation at certain frequencies... and possibly cause problems with portamento.

IR3109 Replacement Solutions

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