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Tuning pAiA 9700 VCOs/modding VCF cutoff
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Author Tuning pAiA 9700 VCOs/modding VCF cutoff
meatballfulton
Just took delivery on my fourth 9700s system and getting ready to do the calibration/scaling.

On my previous ones that I could never get the tuning pots for the two VCOs to work so that fully CW or CCW they would actually be in tune with each other. Seems like they were about a semitone apart, possibly a limitation of the circuit. Is there a way to correct this?

The VCF cutoff sweep available with CV is wide but the cutoff pot only controls a small portion of this and cannot fully open the filter (I usually use 100% key tracking to get around this). Has anyone ever modded their VCFs to widen the range of this pot?
Cat-A-Tonic
Tolerance of pots might cause the tuning difference. seriously, i just don't get it
Kyhotay
I think there is an error in the tuning procedure outlined in the manual. Check out this link: http://www.paia.com/talk/viewtopic.php?f=22&t=250. I run two of the 9720s off a Voyager XL and every VCO is in tune over the keyboard range.
nikmis
here is the section of the schematic of one of the filters. The pot is wired to ground through one leg and the negative supply on the other leg through an 18k resistor. The cutoff pot is a 10k.


Most filter schematics I see tend to do positive supply on one leg, negative supply on the other. That goes through either resistors, 33k on the MFOS SVF, or in the case of the CGS Synthacon Steiner-Parker VCF, it is directly on the positive supply and ground.

So I suppose you could just connect pin 3 of the cutoff knobs and connect them to V+ instead of SG. That might give you too much range. You would also probably run into the other problem people have had with the PAIA, that squelching that can happen when CV opens the filter wide
Virons
I was always under the impression that I had done my build incorrect with the Paia 9700 Vco(s), but after building a few I noticed a common trend. Calibration has been the biggest issue, which ended up leading to breaking away from the keys I guess.. Though it would be nice to have them in tune properly. Blacet works fine, but the instability of the Paia VCO's and the poor design of the VCF have drawn me away finally. Great introduction to modular though.
unease
Well yes, stability was never a strong point of the p9700. I always tune the oscs by ear and don't expect them to be in tune when the pots are pointing the same way. Now that I have added some eurorack to my modular setup I can sort of appreciate the organic and irregular qualities of both the oscs and filters :-)
Kyhotay
There was an error in the instructions for tuning the 9720 VCO which has been changed on the PAiATalk site. It was kinda half-ass backwards. I have two of the dual VCOs and mine track perfectly along with my Voyager XL. 7 sawtooths of analog bliss! 8 if I throw in my Blacet VCO. I think it's also important that the CV source is accurate be it from a MIDI>CV or a CV out from another keyboard.

The 9730 VCF reminds me of the Diode Filters that cost twice as much and are prized for their unpredictability and ability to destroy tweeters. I have a Blue Lantern Euro and it is NASTY. It and the 9730 are my "industrial" filters; they are not very good for ambient new age tones!
RoyG
I've built the PAiA VCF just recently. The panel frequency controls only have a range of four octaves. I changed R47 and R48 to 10k, and disconnected the clockwise ends of Freq A and Freq B, instead wiring them to +12 with another pair of 10k resistors. Fortunately there is a handy source of +12 nearby at the clockwise ends of the Q controls.

This extends the panel frequency controls to have a range of 8 octaves, which is more useful in my opinion.
monstrinho
RoyG wrote:
I've built the PAiA VCF just recently. The panel frequency controls only have a range of four octaves. I changed R47 and R48 to 10k, and disconnected the clockwise ends of Freq A and Freq B, instead wiring them to +12 with another pair of 10k resistors. Fortunately there is a handy source of +12 nearby at the clockwise ends of the Q controls.

This extends the panel frequency controls to have a range of 8 octaves, which is more useful in my opinion.


Great first post! thumbs up
Kyhotay
RoyG wrote:
I've built the PAiA VCF just recently. The panel frequency controls only have a range of four octaves. I changed R47 and R48 to 10k, and disconnected the clockwise ends of Freq A and Freq B, instead wiring them to +12 with another pair of 10k resistors. Fortunately there is a handy source of +12 nearby at the clockwise ends of the Q controls.

This extends the panel frequency controls to have a range of 8 octaves, which is more useful in my opinion.


I ran this mod by Scott Lee to get his take on it: I am not certain why the panel control wasn't originally set-up for more voltage range, but maybe it was just a matter of John having balanced the range of control between it, the cv inputs, and the on-board modulator. It then works more like fine adjust, of offset on external or on-board envelope or cyclic modulation. There is not a problem with modifying the panel adjustment to be full range, as long as it is considered that the sum of control is more easily swamped.

I'm adding the mod to my to do list!
stevebryson
Hi - I'm a relative electronics newbee just about to build the Paia 9730 VCF module. RoyG's mod sounds like a Very Good Idea, but I'm not sure I understand it.

"disconnected the clockwise ends of Freq A and Freq B" sounds like disconnecting all the grounds from lug 3 of R86 and R88 and connecting them through 10K to lug 3 of R85/R87. Then I'd have to route the ground wire to keep all other grounds intact. This means no grounds into R86 and R88. Is that correct?

Thanks and happy new year!
RoyG
Yes, that's right. You have to re-route the ground so it doesn't pass through R86 or R88, then add the two 10k resistors. It's a good idea to put sleeving over the resistor wires to be certain they don't accidentally touch something.
bgf
Are the PAIA modules still in active production?
monstrinho
bgf wrote:
Are the PAIA modules still in active production?


Yes, all of them.
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