Modular CP3 "Click Filter"

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cloudhop
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Modular CP3 "Click Filter"

Post by cloudhop » Thu Aug 27, 2009 3:29 pm

Although I've made several CP3 mixer modules in my DIY modular I am still unsure as to the purpose of the "Click Filter", was it designed for some specific task?

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Kevin Lightner
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Post by Kevin Lightner » Thu Aug 27, 2009 11:48 pm

I don't know the actual reason (or at least I don't think I do), but the early Moogs didn't have integrated circuits.
The CP3's were discrete and could develop offset voltages with temperature deviations. So could the VCAs, which changed design many times.
So in my opinion (I stress: opinion), it was a simple filter (it IS just a capacitor) to remove offset clicks that were more prevalent in the earlier systems.
If anybody knows the real answer (or a better theory), please chime in.

The later CP3A's had opamps and better offset characteristics and didn't need a click filter.
Once you trimmed them for no offset, they stayed put.

Btw, the CP3A's can run on higher voltages. If run on +/-15volts, they can actually amplify sounds or DC signals and be used to get the higher voltages necessary to gate a sequencer from a VCO or other source.
They can also boost the noise source or other mod sources for deeper modulation effects than when run on normal voltages.
One small trick to make a Moog do a bit more. :)

cloudhop
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clicks

Post by cloudhop » Fri Aug 28, 2009 6:41 am

Thanks again for your knowledge Kevin, thats the most info Ive had about the Click filter in 15 years. Theres nothing mentioned anywhere about it but the old design team must have thought it important enough to put it in.
Interesting about + - 15v supply for the CP3, I never thought about it, I will probably set up the 4th CP3 for this supply as I have + - 15v supply within the cabs as well as the +12 / -6 / -10

kind regards

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Kevin Lightner
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Post by Kevin Lightner » Fri Aug 28, 2009 8:21 am

All good, but what I mentioned was in regard to the CP3A, not the CP3.
While the CP3 might handle higher voltages, I've never tried it and don't know how well it will cooperate.
The transistors can likely handle more voltage too, but it's a different design.

The 3A uses opamps and they have no problem with the higher voltages.
You'll still have to trim them to have no offset and be at a virtual ground because the previous supply voltages weren't equal to begin with.

But these mixers are DC coupled and intended for CVs as well as audio, so I imagine Moog welcomed redesigning them to be more accurate and with less offset. They didn't appear to design in any gain since that would change familiarity and previously notated patches on their systems and possibly overdrive in some audio applications.

Btw, the fact that some old mixer panels also had switches with lamps that got warm was enough to cause headaches.
If used for audio, a thump or click could develop.
If used for DC, a tuning error or other undesirable offset could occur.

If you have a DC coupled scope, check the offset of your CP3(s).
You can also use two VCOs, one as a reference, one hooked up to the CP3 and tuned to unison with the other.
Simply touching or breathing on the CP3 transistors can send them into a noticeable offset.

grayedout
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Re: Modular CP3 "Click Filter"

Post by grayedout » Mon Jun 28, 2021 5:16 am

Hi All , I just bought an old Modular , which has been made up of all kinds of bits including 2 Mini Moog OSC banks , I am going to make 1 or 2 new top cabinets and start to add to the system using Moon and Other 3rd party modules can anyone help with the Cp3 I have , I do have much info and I am not sure what original in them . I have 4

I can see there is nothing connected to the Click Filter which is how I found this thread via a google search


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analogmonster
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Re: Modular CP3 "Click Filter"

Post by analogmonster » Mon Jun 28, 2021 5:58 am

Isn't this the back side of a CP3A frontpanel?

And where is the pcb?

The circuit of a CP3A does not provide a click filter. But you can add one by providing a switch and a 1µF capacitor between input sum and ground. See service manual. But at least my CP3A clone works fine without.

grayedout
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Re: Modular CP3 "Click Filter"

Post by grayedout » Mon Jun 28, 2021 8:44 am

I don't have the PCBs ? these seem to work and a line level mixer without them what's it the PCBs did ?

I see you have made some do you sell them ?

Andy

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analogmonster
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Re: Modular CP3 "Click Filter"

Post by analogmonster » Mon Jun 28, 2021 9:47 am

The switches of the module I can see on your picture are not connected to anything. Seems that they have never been connected to anything :shock: 4 input channels seem to be passively mixed and provided at the outputs. This seems to work anyway.

A lets say complete CP3A does an active mixing with opamps incl. amplifying!!! Two outputs are phase inverted, as engraved on the front panel. You need a pcb for this, as you can see with my clone. Has Moog ever sold passive mixers? I don't think so, as the swiches are provided, but not connected. So what is this here?

As we are in personal contact via mail, I will give further answers there.

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