PAiA 9720 VCO - Multi-turn pots??
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Author:  silverboxmusic [ Sun Apr 23, 2017 6:26 pm ]
Post subject:  PAiA 9720 VCO - Multi-turn pots??

I apologize in advance if this has been posted before.

I'm considering installing 1K-ohm multi-turn potentiometers in place of the single-turn scale trimmers (R27 and R58) in my 9720 VCO. (This would also encompass installing multi-turn 10k symmetry trimmers in place of R87 and R5 as well). Anyway, I'm wondering if this would perhaps making scaling more accurate? Has anyone else done this? Being that the resistance values are the same, would it cause any issues with the operation of the oscillator circuit?

Thanks for your time and help!


Author:  Kyhotay [ Mon Apr 24, 2017 7:10 pm ]
Post subject:  Re: PAiA 9720 VCO - Multi-turn pots??

I haven't tried it with my 9720 VCO but I have with my 4720 VCO and it works great!

Author:  silverboxmusic [ Wed Apr 26, 2017 3:24 am ]
Post subject:  Re: PAiA 9720 VCO - Multi-turn pots??

Wow..that's great news! Thanks for the reply. I will go ahead and order some 10-turn trimmers and install them. I'll post back here on this thread once I have the VCO calibrated and see how it works. I guess if there's any issue, I can always just re-install the original trimmers.

Thanks again!


Author:  silverboxmusic [ Wed May 17, 2017 11:52 am ]
Post subject:  Re: PAiA 9720 VCO - Multi-turn pots??

Hello again!

I just wanted to update anyone who is interested in this topic of adding multi-turn trimmer resistors to the 9720 VCO. I ordered some 12-turn trimmer pots from Mouser...and replaced the original single-turn pots for Scale A and Scale B. (I decided not to replace the Symmetry trimmers, as I didn't feel it was necessary). The result of being able to accurately scale the oscillators was quite surprising! They are now much better than before!!

As per the 0V to 4V scaling PAiA instructions, to scale the oscillators over 4 still didn't really turn out was closer than before, but not perfect. I decided to try a new approach to scaling. Previously, I was only able to get approximately 3 octaves of in-tune oscillators..but with scaling over 4 octaves combined with the old trimmers, even getting 3 in-tune octaves wasn't always the case. So I decided to scale over 3 octaves (1V to 4V) which, combined with the new multi-turn pots yielded a very in-tune set of oscillators that are also in tune with each other over the entire 3 octaves, and I also have about half an octave on either end (high and low).

Another thing I did while scaling, was I didn't use the B oscillator as a reference while scaling the A oscillator. I used an oscillator from my Arp Odyssey and used the same note of the Odyssey to scale both the A and B oscillators. Using the PAiA instructions, it explains to use a reference, but no specific note. I picked the A below middle C as my reference tone for both oscillators and I feel that that helped tremendously to scale them more accurately in relation to each other.

All in all, while switching to multi-turn trimmers didn't change my life, I very much recommend anyone who has a 9720 VCO to perform this upgrade and you'll definitely be satisfied with the results! If anyone needs any clarification or more information on the scaling process, please don't hesitate to ask!


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