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 9700 filter mod---zener diodes---HELP 
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Joined: Thu Aug 29, 2013 7:35 pm
Posts: 7
Post 9700 filter mod---zener diodes---HELP
I just got yet another 9700s (I had sold my previous one a few years back).

On my other one I had done a mod to the VCFs that I found on the netz but can't locate now. :(

It used zener diodes to clamp the gain of the filters to control (but not eliminate) the shrieks from too much gain. Supposedly Scott had come up with this mod.

Can anyone help?


Sat Sep 07, 2013 10:10 am
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Joined: Thu Sep 24, 2009 2:14 pm
Posts: 90
Post Re: 9700 filter mod---zener diodes---HELP
No, Scott didn't.
I came up with that zener filter mod.

posted it in the files section at the YahooPaiaGroup.
You have to be a member to access the files section.

http://launch.groups.yahoo.com/group/PAiASynth/

http://launch.groups.yahoo.com/group/PAiASynth/files/

Not the greatest of mods,
but it DID keep the filter from oscillating rail-2-rail. <ouch>

Just remembered! I took it down because other folks were claiming credit.
Also took down the EnvelopeFinishingTrick on the 9700s for the same reason. PM me thru here or thru the YahooPAIAGroup with your email and I can send it to you. The Group has lots of interesting PAIA stuff but is not affiliated with PAIA the company.

What is needed is a feedback limiter, but I have not had any time to persue. This is done on almost all the other MM-VCFs that I've seen.
Sometimes it's done with an opamp stage and some limiting diodes.
Other times it's done with an OTA acting as a limiter/VCA.

tone(man)/toneboy22 aka TonyB

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Sun Sep 08, 2013 12:54 pm
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Joined: Thu Aug 29, 2013 7:35 pm
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Post Re: 9700 filter mod---zener diodes---HELP
Crap, no PM button, FAQ says "contact admin" :( :(

Waiting on membership approval at Yahoo group


Mon Sep 09, 2013 4:16 pm
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Joined: Thu Sep 24, 2009 2:14 pm
Posts: 90
Post Re: 9700 filter mod---zener diodes---HELP
Yep, the PAiAGroupAdmin is usually pretty quick.

I'll still be here.

Do you know about the 9730 trigger mod that I did??

T

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Mon Sep 09, 2013 6:02 pm
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 726
Post Re: 9700 filter mod---zener diodes---HELP
It's best if you can just set the controls and modulation amounts up to or below the onset of this runaway/feedback condition rather than changing the circuit.

With your patch through the 9730 set-up run through all anticipated frequencies of the signal source, and, the setting(s) for the amount of envelope/repeating-envelope (and any external modulation via control input connectors) as the Fc and Q settings are advanced on the first, then second (if cascaded) filter sections.

Think of it as a post calibration, "calibration procedure" and with any more input amplitude or difference in frequency or modulation to vary the filter's initial frequency setting, it will enter the critical condition, but reduced settings and amounts should all be within the limits.

Reducing the amplitude of the input signal makes the settings less critical as there will be more headroom for boost at the center or corner frequencies.

In the P9700S overview,

viewtopic.php?f=22&t=463

, a general, monophonic analog synthesizer patch is described with the two oscillator signals first going through the VCA L and R mixer.

The L+R output is a passive mix and so there is some attenuation and connecting this to the filter input allows for more dynamics through this section.

Even more attenuation of the signal input to the VCF would ensure things didn't go over the edge.

This can be done through an added module or potentiometer, or, by substituting a stereo (trs) connector on the filter input which would work to connect attenuating resistance to ground as a mono (ts) plug is inserted and shunts the ring and sleeve contacts.

A trs plug with no connection to the ring terminal would allow normal, non-attenuated input.

See the schematic in the Quadrafuzz manual for an example of the way a High Level input signal is attenuated via connection to the TRS input connector.

Quadrafuzz manual

viewtopic.php?f=6&t=89

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Scott Lee
scott@paia.com


Tue Sep 10, 2013 11:05 am
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Joined: Thu Aug 29, 2013 7:35 pm
Posts: 7
Post Re: 9700 filter mod---zener diodes---HELP
Scott ,

I know all about keeping the filter input levels down but i did like having the zeners in place as
speaker protection :?


Fri Sep 20, 2013 5:01 pm
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Joined: Thu Sep 24, 2009 2:14 pm
Posts: 90
Post Re: 9700 filter mod---zener diodes---HELP
YahooGroup is kinda slow at approvals lately. I emailed David but have not heard anything back. Maybe Scott can help us with PM here? I don't want to post my eaddress.

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Fri Sep 20, 2013 5:27 pm
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Joined: Thu Aug 29, 2013 7:35 pm
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Post Re: 9700 filter mod---zener diodes---HELP
No rush, I'm off on a vacation for 2 weeks ;)

I'll check in here when I get back


Fri Sep 20, 2013 7:16 pm
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 726
Post Re: 9700 filter mod---zener diodes---HELP
I happened to have a 9730 VCF in for service and may well have found a simple solution to preventing the instability/runaway/oscillation condition that can occur with this module:

http://paia.com/manuals/docs/9730_Res_R.pdf

Note the setting of the scale trim helps keep current peaks with variations in control voltages (Fc, Modulation) in check. I think these are a way it can break into oscillation even if the input signal is attenuated.

I didn't investigate any diode limiting since, it wasn't so clear as to where this might be best applied, other than the input and with it mainly working like a fuzz circuit, it would be making all the input waves like square waves unless it got fairly complex.

This 6790 kit could be put behind an FR-blank for use as a module working to prevent too much signal from getting to following devices. It attenuates, rather than clips, by an amount needed to keep the signal below an adjusted Limit Point.

6790 Dual Limiter
http://www.paia.com/proddetail.asp?prod=6790K

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Scott Lee
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Tue Oct 01, 2013 1:56 pm
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Joined: Thu Aug 29, 2013 7:35 pm
Posts: 7
Post Re: 9700 filter mod---zener diodes---HELP
Thx Scott, I'll definitely give that a try this week when I have a chance.


Sat Oct 05, 2013 10:46 pm
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