From Analogman:

The high cut only effects the ECHO sound, the DRY sound is still the same, nice and clear. It is a 3 position switch. The center setting is the original bright echo sound, one position is a medium high cut, and the other position a heavier cut. This makes the effect sound quite analog and you can distinguish the repeats from the original sound. This mod on the DD-6 sounds a little better than the same mod on the DD-5, very smooth. The high cut functions on OUTPUT A only

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