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CGS dual wasp filter - build question
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author CGS dual wasp filter - build question
carynrich
so i ordered the bridechamber full kit and it looks like there are two mods.

i get the cv invert switch but i cant figure out the wiring to the spread cv input jack and pot. cynthia describes it as a divergence control to spread the two filter frequencies apart from center.

heres what i think it looks like so far:


any advice would be appreciated.

thanks, richard.
numbertalk
I actually just went through this. The thing I caught is that Bridechamber were having you put >1 input into a non-summing node on the CV input op-amp. Apparently, though, it works ok. I'll copy and paste both what Scott at Bridechamber initially told me to do and then a quote from Ken regarding having multiple inputs into the non-inverting input of the op-amp:

Jack to left lug of pot
Middle lug via 200k resistors (haven't tested this yet, but should be a good value) to two places:
CVSN of Filter B
Junction of Junction of 100k and 2k2 resistors that go to pin 2 of Filter A CV inputs op-amp
Right lug to ground

So, the effect should be, putting 5V in, with pot turned all the way up, Filter A should go down around 2V5-worth of frequency, and Filter B should go up an equal amount.

It was at this point I caught an error in Ken's schematic and realized Scott was suggesting I feed an extra input into pin 3, not pin 2, so this is what Ken had to say, regarding that with this mod:

You can feed a second signal into it via a
second resistor. The two -CV will AVERAGE with each other, not sum. In other
words, stuff will happen, and it will probably be good enough. The -CV won't
be quite as sensitive as on the other unit, though it probably won't be that
noticable. It will be in keeping with the general "roughness" of the module
though, and that is part of its charm.

Make sense?
carynrich
would the junction of filter A's opamp pin 3 be the CVSN input? then the wiring on the pot would be (from the front of the pot) CCW (left lug?) - tip from jack, SW (middle lug) - two wires to both CVSN inputs on PCB with a 200K resistor each, CW (right lug?) - ground?

i kinda thought i might need to use the CVSN inputs but i read this on ken's website for the VCO: A fine-tune control can be added by wiring a second pot as per the tune control, but wiring the wiper to the CSVN via a high value resistor such as 4M7, or even 10M.

I've never played with resistors that high, plust i don't even have those lying around.

thanks numbertalk! i'll give it a try, richard
numbertalk
carynrich wrote:
would the junction of filter A's opamp pin 3 be the CVSN input? then the wiring on the pot would be (from the front of the pot) CCW (left lug?) - tip from jack, SW (middle lug) - two wires to both CVSN inputs on PCB with a 200K resistor each, CW (right lug?) - ground?


Not quite - if you look at the schematic on Ken's site (which is now corrected), CVSN goes to pin 2 of the CV op-amp. You want to connect here for filter B. For filter A you want to connect to pin 3 of this TL072, so you'll have to hotwire a connection on top of the board somewhere to the junction of the 100K CV- input and the 2K2 pull-down resistor. Just soldering the wire to one of those resistors will do, so long as you wire it to the correct side - the lead that is part of the junction with the other resistor.

Also I'm not sure but you might have your CW & CCW lug connections backwards. Looking at the back of the pot with the lugs at the bottom ground would go to the right lug and the wire from the CV Spread input jack would go to the left lug. So that then when looking at the front of the pot, turning the pot clockwise you increase the signal from the spread input. Make sense?

i kinda thought i might need to use the CVSN inputs but i read this on ken's website for the VCO: A fine-tune control can be added by wiring a second pot as per the tune control, but wiring the wiper to the CSVN via a high value resistor such as 4M7, or even 10M.

I've never played with resistors that high, plust i don't even have those lying around.

thanks numbertalk! i'll give it a try, richard[/quote]
carynrich
i finally got it now.

the first problem was i was counting the pin outs on the op amp wrong (alternating sides instead of down the left side first then the right). basically i thought pin 2 was pin 3. thanks for the detailed explanation connecting between the 100k and 2k2 resistors. its at either of these two junctions on the PCB:


regarding the pots. i was referring to an alpha pot with the lugs on top. if you are referring to a spectrol/bourns pot with the lugs on the bottom, i think we're talking the same thing. can't afford the spectrol pots at $7 each!

thanks again for all the info! richard.
numbertalk
Also if you want to properly sum/mix the voltages into the CV- input of filter A you could feed them first into one of Ken's small DC Mixer boards, feeding the output of that board into the CV- pad of filter A:

http://www.cgs.synth.net/modules/cgs04_mix.html

The board is small enough to probably easily fit somewhere behind your panel. I try to keep a handful of these available for situations like this. Sounds like it's not a big deal to do it the way we discussed which will give you the average, but I'm pretty anal, so I think I'm going to change mine to go this route.
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