There's also pretty affordable solution from Doepfer:

The A-119 Ext. In. as input (note the input there is stereo):

https://images.musicstore.de/images/1600/doepfer-a-119-ext-input-envelope-follower_1_SYN0003615-000.jpg

and A-139-2 as the output (this is also stereo):

http://www.schneidersladen.de/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/a/-/a-139-2_front.jpg