Tuesday, June 9, 2020

PAIA 2720-3B Filters: Gasboss and Unleaded

You can read part I of this post here....

Jumping back in: my first synthesizer was a PAIA, and this time, let's finish up a PAIA inspired filter from E-M guru Gasboss775; it's his take on a the classic PAIA bandpass 2720-3B filter from a bygone era.

I add details about building a 2720-3B "stock" clone as well.

You can read more about the Gasboss design on the E-M forum here. Again part I of this post is here. You might want to skim it.

A sound demo for the filters is  here, Go to my website (here) to can get schems, Gerbers, etc. etc. for the two filters discussed in this post.

Build notes: From the last post the Gasboss version was a single PCB, but why make things easy?

For the Gasboss module I added a "jacks board" to make this into a SKIFF setup and used PCB material (post here) and "Mr. Label" (here) to create the one-off front panel you see above.

Sandwich it all together with 100 mil headers and 4x40 hardware.

Worked--the Q pot turns the wrong way (so fully CCW is full Q), but, anyway, that will do:

I also added more input buffering and "BOWAL" (Bowal? ??? Post for Bowal is here) capabilities to the "jacks" PCB as well....bias offsets seem to help these filters deal with different CV sources so that's in.

My PAIA/Gasboss filter jacks board uses tall trimmers and here I learned something that should have been obvious: you can't hold a skiff front panel in place without 9mm threaded pots at the top or some damn thing to screw the front panel to. So, the front panel was wiggling all over. I ended up epoxying screws at the top so the front panel didn't move around; kludgey but it worked:

Son of a 2720-3B!! Just for S-n-G's, as long as I was fabbing Gasboss' filter, I figured why not send out a gerber at the same time that is a direct lift from the original PAIA schema, but modified very slightly for +/- 15V (the original required +18V and GND).


Orig 2720 schem--it's all over the Internet so I assume it's OK here?

Poof! Done, board is back, took about an hour to build?

That was easy, the original PAIA circuit is super simple and I think I successfully guessed what values are needed for the lower voltage rail by 3V.  Sounds ok and no smoke! I buffered the outputs, giving the world one goes outta at +6DB and one at +12db, because you KNOW you're going to need to crank up the output.

The 0db output isn't op amp buffered. Wasn't on the original. I guess 741s were expensive back then?

For the "stock" 2720 front panel I used Lazertran (how-to post here).  Decal graphics came out a bit crappy so I might redo it.

Some build photos--sorry.

2720-B clone, but +/- 15V.  I added buffers, otherwise no need for V--.

Used a 5x jack sub-board and 2x buffer non inverting buffer boards--one of two shown here.

Using the Gasboss filter and 2720-3B clone: For normal bandpass filtering uses, the 2720 is, well, subtle?  But! Right off--it's super easy to get either of these filters to distort. So you'll just crank up the input signal and pin that puppy right?  Nope. I noticed for both filters the band pass frequency sweep is less noticeable if you overload the filter at input. I don't know why--I need to think about that. To get the distortion and wah-wah action simultaneously, best to crank up the resonance--as you would with an MS20 filter.

I found myself attenuating the signal going into the filter and then cranking it back up at output. It got noisy and in some cases I got a 60 cycle hum when I really pushed up the output gain. But that's actually kinda cool--I don't have anything else in my rack that does that. If I need 60 cycle hum for some specific sound design need, well, now I know how to get it.

Another interesting use case is to put audio signals into the CV input. You get odd amplitude modulation effects doing that. I remember the 2700 PAIA had different jacks for audio and control (banana and 3.5" I think), so when we were youngsters we never thought about patching audio signals to control inputs; and it didn't dawn on us that we could make patch cables with different connectors on each end.

I remember doing a lot of wind/drum sounds with the PAIA 2700 back in the day and my old PAIA 2720 bandpass module was a big part of that. The 2720's are really good filters for processing pink noise. The Taiko sounds you hear in the sound clip demo (again--here) was created by using an aconitum noise circuit with the 2720 bandpass in series and the Gabbar EG--that was it. That got put into a BitBox 1010 and captured with Expert Sleeper modules back into Ableton.

Yeh, it's a pretty crappy sound demo--you can hear the filters at work but it's not my best compositional moment.  Oh well.

All the other sounds you hear in the demo utilized the filter primarily as a distortion unit, giving me grunge and grit which changed timbre as the filter swept. That's what the 2720 repop is best for as far as I can tell--for a VCF that only goes wah-wah you have a lot better choices.

On the demo everything you hear touches the 2720's somehow, the exception is the Roland D50-ish "big lead pad"--that didn't utilize this filter, sorry, the 2720-3B isn't right for everything.

I was a bit perplexed by the fact that these two filters never fully remove all input source sound, as you can with say a 24db/oct lowpass VCF. But then I found the other BP filters in my studio, including expensive fancy ones: they didn't either. Didn't know that about my bandpass filters.

Another thing to check out--use audio signals into these filters' CV inputs, in fact, try that with any VC filter. here, sticking audio or AM signals into the CV inputs, you get some really wild and strange distortion. Hours of fun.

Finished the PIE-Ah? So will the 2720 get a lot of use in my rack?  I don't know.  I kinda doubt it really. It's not a good filter for everything--it's not a good filter for a lot of things--but maybe no filter is. It does add another vehicle for me to get at distortion but I also have good DAW plugins for that (wait--DAW Plugins?) and get more precise control of the crunch.  That said, the 2720 has its own sort of distortion.

Nevertheless it was fun building these modules and remembering the good old PAIA days, when we wanted to sound like Rick Wakeman and didn't/couldn't; where the PAIA VCOs never were even close to being in tune, and DIY protogeek toys in our Central-Valley-Ranch-Home-Basement were the boss. Aloha!

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