DIY Portamento for ARP 2600

Recently I encountered a few issues with the 3604P keyboard. Also, there was some pitch difference when I jumped between the keyboard and the MIDI-CV in and I was never able to get it in sync. I got tired of it and decided to use a MIDI Keyboard which is sharing a Midi CV-Interface with the Computer (sometimes it's function before original look. ;-) ) and stored the 3604P away for now.

Storing away the keyboard means losing the Portamento. So I was looking around for some schematics to build a substitute and found a glide circuit that looked interesting, additionally I was using some ideas from the original circuit which keeps the capacitor loaded during bypass instead of cutting it completely off the circuit like many other schematics do. With help from a friend I came up with a smooth gliding substitute.

The module is designed for +15V/-15V. Since the original keyboard is not in use anymore the power could be used from the Arp, but I decided to use a dedicated power supply as I plan to add more modules in the future and do not want to tax an old vintage machine too much. The MIDI-CV goes into the portamento module and the portamento output is plugged into the KBD-CV Output on the Arp2600.

 

In case you are ever in need of a Portamento for the Arp2600, maybe the schematics below is of some use for you (do not forget those decoupling capacitors). Make sure not to use a LM358 or similar BJTs as the tracking will slightly fall apart which is a no-go for CV/Oct. I tried to make it work for too long, in the end, using the TL072 cured all issues. Furthermore, I experimented with different capacitor values, but found the 1uF the most applicable one. The Portamento works pretty well here.