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 Forum index » DIY Hardware and Software » PAIA Equipment
PAIA2720-3B VC Bandpass Filter_Augmentations
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cslammy



Joined: Apr 27, 2018
Posts: 202
Location: USA
Audio files: 1

PostPosted: Thu Mar 05, 2020 6:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK I have linear current version board back and populated it. It worked first time, and is a good sounding filter (some of the settings sound pretty good really) but for me anyway this is a sensitive circuit.

Also, for my prototype anyway, there is an output amplitude loss--sort of like a ladder filter but a lot worse.

I did a few revisions w/ the resistor values to try to moderate the distortion a bit (wait, you may want distortion? Very Happy ), but I think overall the best solution here is to add an attenuator to the audio input, a variable gain stage for the CV (beyond what the CV control pot can do--you need more control) and then a gain stage (adjustable) for the output that can add 0 to maybe 12db. So that's adding 3 more pots....oh well.

Anyone have other ideas?

Blog post for the project so far: http://audiodiwhy.blogspot.com/2020/03/piece-of-paia-2720-bandpass-vcf-lives.html. I will revise the board again to add the buffers then post some sounds.

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cslammy



Joined: Apr 27, 2018
Posts: 202
Location: USA
Audio files: 1

PostPosted: Mon Apr 06, 2020 10:02 am    Post subject: OK it's built--issue with audio "Bleed" Reply with quote  Mark this post and the followings unread

Gang:

I have built the filter using custom PCBs. I used "tall trimmers" and "skiff" construction, and it's all working.

For this v2 I am using a 2nd board to set CV bias, which IMO really helps control the filter's sweep.

Problem: on both versions of this I am having the same issue, what I call "audio bleed". Some of the initial unfiltered signal, very audible, is turning up at output.

I have enclosed a sound clip. You can hear the "ring" of the bleed pretty clearly after the filter has closed.

Anyone know how to stop this? I have checked my work a few times, can't find a mistake. My analog design skills aren't good enough to know where this bleed is coming from, but some of the unfiltered signal is obviously making it from input to output.

I can't remember if the initial paia filter had this issue, but perhaps it did.

(You can circumvent this by gating the filter output, using a 2nd filter in series, LP works or just adjusting VCA envelope to close before the filter. But overall I'd like to solve this somehow.)

BTW: I changed R9 to 27K; input audio resistance to 3M for one input and 1.5M for the other. R12 was change to 68K. I am using +/- 15V. the size of LED doesn't for me make that much of a difference. Besides that, it's an exact copy of GASBOSS's linear current design, with an additional couple of op amps downstream from CV in to set the CV bias relative to ground.

One way or the other, I am going to work on this more and then will post more details on blog incoluding a lot more sound clips.

Thanks in advance for any help....


NoICs=done.png
 Description:
Here is the unit, yes, no ICs, these were removed for testing a few things. The build uses 3x TL072
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NoICs=done.png



paia2720-audio-bleed.mp3
 Description:
What can I do to make this ringing "bleed" sound go away?

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gasboss775



Joined: Jan 02, 2016
Posts: 217
Location: Scotland

PostPosted: Thu Apr 16, 2020 4:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

@cslammy:

Nice to see such a professional looking implementation of this circuit, probably better than the PiAI original!

It's been awhile since I worked on this but I seem to recall that it wasn't possible to completely shut off the signal with this filter as the cut off range couldn't go that low. You could experiment with different capacitors in the branch of the circuit connected to the controlling LED but you would probably sacrifice some of the higher end of the cut off span.

Gordon.
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cslammy



Joined: Apr 27, 2018
Posts: 202
Location: USA
Audio files: 1

PostPosted: Fri Apr 17, 2020 8:34 am    Post subject: Reply with quote  Mark this post and the followings unread

gasboss775 wrote:
@cslammy:


It's been awhile since I worked on this but I seem to recall that it wasn't possible to completely shut off the signal with this filter as the cut off range couldn't go that low.


CL: For what it's worth I have compared the modified 2720 here to other synth bandpass filters including some from the current big-bucks, load-up-your- credit-card-the-bank-will-love-you modular mfgrs.

Bandpass filters, even the really fancy ones, seem to not always close all the way, this one appears to be no exception.

I need to finish a front panel for this and make a few other tweaks, but your cap values etc. seem to be spot on....Cap and resistor wise I can't improve on the sound from the values you have now. Nice work!

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cslammy



Joined: Apr 27, 2018
Posts: 202
Location: USA
Audio files: 1

PostPosted: Tue May 05, 2020 11:39 am    Post subject: Reply with quote  Mark this post and the followings unread

MapacheRaper wrote:
Great!! That´s exactly the kind of project for your blog. Waiting to read the post Smile



I have gasboss's design working. Wanted to A-B it with the orig PAIA before I post.

You can see the orig schem at the top of this thread.

Question: to make the orig PAIA filter work w/ 15V (Paia uses 18V) my guess is that R2 and R5 needs to decrease, to say 120K (R2) and 4.7K (R5). This would set the bias for Q1 and Q2 appropraitely for the lower VCC of +15V.

Agreed? Any guesses?

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cslammy



Joined: Apr 27, 2018
Posts: 202
Location: USA
Audio files: 1

PostPosted: Mon Jun 15, 2020 6:29 am    Post subject: Filters done Reply with quote  Mark this post and the followings unread

OK--done w/ this I think, I've finished my build of Gasboss's 2720 mods, also I went back to orig paia schematics and built one of those that works on +/-15V.

blog post about gasboss' design and orig PAIA filter repop: https://audiodiwhy.blogspot.com/2020/06/paia-2720-3b-filters.html

Website: https://www.charlielamm.com/gear/vcf/forums.html#2720

Sound clip: https://soundcloud.com/cslamm/2720bandpass-final


2720stock 1024x1388.png
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