Paia 2720 BPF improvements?
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Author:  patrickvf [ Mon Nov 25, 2013 10:10 pm ]
Post subject:  Paia 2720 BPF improvements?

Hello, I'm trying to explore some possibilities of improving one of my 2720 BPFs. I'm primarily aiming for a wider resonance range, up to near or total self oscillation, and a steeper cutoff range, but I'd love to hear any other suggestions to improve the quality of the filter, or even turn it into state variable filter perhaps? (BPF filters are made up of a low pass and a high pass part, right?) So I'm pretty much all ear for any suggestions.

Author:  ModularMe [ Tue Nov 26, 2013 10:24 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

Longtime PAIA user Valis posted this comment about the band pass filter in the electro-music forum.

You can change the range of the filter to better suit your needs by proportionally altering C3, C4, and C5. Halving the values should double both the high and low end of the filter.

Author:  toneman [ Tue Nov 26, 2013 10:57 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

There was an article about a 2720-3B "Q Enhancer".

I think it was in The Source booklet(?) pg6-7

Basicly, you use a 2720-12 InverterBuffer as variable gain feedback amp
around the 2720-3B. It requires an external pot to control the feedback.
U can get that little filter to really scream!!

I have a scan of it but can't upload it here.
I uploaded a while back to the YahooPaiaGroup.
But, that group is very quiet and I can't get in touch with the owner.


Author:  toneman [ Tue Nov 26, 2013 11:30 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

Just realized that the "Q enhancer" 2-page article is in the 2720-12 manual that is posted at this site in the documents and manuals area!! :shock:

Author:  patrickvf [ Wed Nov 27, 2013 12:38 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

I know about the trick with the inverter/buffer, however it takes it straight to scream plus the feedback/q appears to be affected by the lfo/envelope CV for the Cutoff. What I'm aiming for it so get the filter more fluid/squelchy sounding but the way as it is right now it's more like syrup.. mmmm syrup.. :D

PS: it also came to my attention that when introducing the lfo to the filter, it starts/rests at the original tone, instead of being filtered down at the lowest setting - let me try to explain that with a verbal analogy:

The way it is now it works like: aaaaaaawawawawawaaaaaa..
The way I think it should be is: ...wwaww....wwaww...wwaww...
Hope that helped with explaining what I want to do..

Author:  toneman [ Wed Nov 27, 2013 1:43 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

"sounds" like you want a Limiter on the out of the BPF(?)

Yeh, that's why I said "scream".
U can easily go rail-2-rail.

Most/all modern SVF (except for the 9730) use limiting feedback to turn super high resonance into a sine wave. That's why I invented the diode clamper for for the 9730. It doesn't STOP the square wave, but it limits the amplitude. I don't know what the Scott's "gain mod" sounds like compared to the clamper mod.

U have to remember what kind of "technology" U are dealing with with the 2720-3B. Using a lfo CV in to the filter is going to take it beyond the safe feedback area of the filter. So, you need to either limit the amount of resonance CV U can apply, or limit the output of the filter.

wwwwaaacccaaa woooo

Author:  patrickvf [ Wed Nov 27, 2013 2:45 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

hmmm.. can I hear more about this diode mod you talk about? What about turning the BPF into an SVF? In theory a BPF consists of a LP part and a HP part that cross-filter each other, am I correct? So shouldn't I be able to wire an output jack to the LP part, and one to the HP part? or is there more to it than I think?

On a related note, would it be possible to mod the LPF with an adjustible resonance and cutoff (essentially turn it from a static to an lowpass VCF)? This one is not that important, but would be good to know if there's an easy way (without adding extra op-amps, etc.).

Author:  Kyhotay [ Thu Nov 28, 2013 9:56 am ]
Post subject:  Re: Paia 2720 BPF improvements?

re: PAiA LFO problem. I found this in a thread on Muff Wiggler:
One quirk I didn't like with the LFO module was its unipolar output, it only swung from ground upwards. I AC coupled the output and it was much more useful.

I'm not technical enough to know how to do that but I'm sure someone will chime in with an answer!

Author:  toneman [ Thu Nov 28, 2013 10:48 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

Here's a quick drawing I made showing how to AC-couple the LFO output(s).

The resistors provide a load to drain off any accumilated voltage. ... t=3&page=1

Tote: I HAVE NOT built this.


Author:  patrickvf [ Sun Feb 16, 2014 7:06 pm ]
Post subject:  Re: Paia 2720 BPF improvements?

Getting back at this topic - I have a few more questions about the BPF and LPF:

About the BPF - ModularMe pointed at an Electro-Music post that suggests that replacing capacitors C3, C4 and C5 with lower values would increase the Q of the BPF. I'm curious what would happen if I used unequal values, or replaced one or two caps with resistors or diodes, how would it affect the sound and quality of the filter? Also at least two of the capacitors look like tantalum capacitors, can I replace them with electrolytic ones? I would of course try that on the older filter that most likely need some components replaced anyhow.
Also, is there a way to boost the BPF's low end? It gets very weak when I turn the resonance all the way down. Shouldn't it be the otheer way around (thicker/louder end on low Q, and thinner on high Q)?

About the LPF - Is there an easy way to feed back the LPF to itself, essentially turning it into a sine VCO, and track/sync it with the real VCO? I would love to use that for some FM sounds/effects without having to find another VCO.

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