Available as an assembled Module and as a DIY project.
This Module is currently available.
CV Echo
The ECHO (formerly known as EKO) is a very unique echo using the PT2399 IC featuring Voltage Controlled Rate. Manual controls for Rate, Feedback, and Mix. Beautiful and bizarre, CV modulation of the rate brings in ghostly pitch bent sounds to metallic reverbs. An effects module that everyone needs in their rig.
http://www.synthrotek.com/products/modular-circuits/echo/echo-schematic/
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These merchants probably sell this module. Huh?