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 Forum index » DIY Hardware and Software » Jürgen Haible designs
Polymoog Resonator problems
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morganzw



Joined: Jun 22, 2011
Posts: 1
Location: brooklyn

PostPosted: Mon Aug 08, 2011 6:02 pm    Post subject: Polymoog Resonator problems
Subject description: feedback, clipping
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i'm a new DIYer, and a huge fan of the polymoog, so I decided to try to build the JH resonator clone.

so far i've gotten it working okay, except the gain is exceptionally low for the dry signal. i've put a true bypass in, and compared with the bypass signal, i'd say the dry side is about half of bypass. the resonator is better, but still low. i've fiddled with some of the resistor values - changed r16 to 10k. but still no luck...

also the stranger part for me is whenever i turn the gain entirely down in any of the frequency bands and put signal through i get a horrible nasty clicking / clipping sound that repeats endlessly until i move the gain pot back up a little. sounds like some kind of feedback loop or something but i don't really know enough to figure it out. i know there's a virtual ground, and i'm not sure if that's got something to do with it (i'm not too experienced with virtual ground).

any help would be appreciated!
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davebr



Joined: Jun 09, 2007
Posts: 182
Location: portland, or

PostPosted: Sun Sep 09, 2012 8:44 am    Post subject: Re: Polymoog Resonator problems
Subject description: feedback, clipping
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morganzw wrote:
i'm a new DIYer, and a huge fan of the polymoog, so I decided to try to build the JH resonator clone.

so far i've gotten it working okay, except the gain is exceptionally low for the dry signal. i've put a true bypass in, and compared with the bypass signal, i'd say the dry side is about half of bypass. the resonator is better, but still low. i've fiddled with some of the resistor values - changed r16 to 10k. but still no luck...

also the stranger part for me is whenever i turn the gain entirely down in any of the frequency bands and put signal through i get a horrible nasty clicking / clipping sound that repeats endlessly until i move the gain pot back up a little. sounds like some kind of feedback loop or something but i don't really know enough to figure it out. i know there's a virtual ground, and i'm not sure if that's got something to do with it (i'm not too experienced with virtual ground).

any help would be appreciated!

First off, the gain of the dry is 1/2 so yours is normal. Note the resistor values of R11 and R12. I would not change the gain here as this circuit feeds the resonator section but instead boost the gain after the dry potentiometer by changing R16 to 11K. The dry level being lower is fine for me as I mostly use the resonator.

Are your potentiometers mounted directly to the PCB? If so, verify the pinout is correct for the PCB. I have helped one other who had a potentiometer with a different pinout.

If they are instead connected by wires to the PCB make sure the wiring of the dual potentiometer is correct so that both halves track each other.

This circuit is biased at +5 volts which is supplied by pin 6 of U3. This supply is not very robust and I've worked on units where this is dead. The +5V bias is on some of the controls and if they are not mounted on the PCB it is possible to take out this bias supply with a short from loose controls touching something that is grounded (as in testing the module before mounting the controls to a panel).

When the gain is turned all the way down it connects the input of the resonator cells to the +5 volt bias. You might try measuring pin 6 of U3 to verify it is at +5 volts. Note there are two U3's on the schematics so you want to measure pin 6 of the 741.

Good luck.

Dave
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