Cv out to filter problem. Help please. Pics added.

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Marcuz
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Cv out to filter problem. Help please. Pics added.

Post by Marcuz » Tue Jan 05, 2021 1:57 pm

Hello I have just built my second module after I managed to get the first a VCO working well. I have now built the LMNC simple low pass filter but got greedy and added the switch which also makes it high pass aswell. I tried to test it by hooking it up to my behringer td3 cv out which worked with the VCO I built. I took the cv out to the vcf cv in and it was audible and the led on the filter pulsed with the pattern playing but the cutoff and resonance and also the switch did nothing. The cv input level also appeared to work as the volume got louder as I turned it. Any ideas before I rip it down and change it to low pass only? Thanks.
Last edited by Marcuz on Wed Jan 13, 2021 9:21 pm, edited 2 times in total.

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Bartimaeus
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Re: Cv out to filter problem.

Post by Bartimaeus » Tue Jan 05, 2021 5:25 pm

do you have a schematic and photos? it's hard to say why your switch isn't working when we don't even know how you've wired it up

pjbulls
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Re: Cv out to filter problem.

Post by pjbulls » Wed Jan 06, 2021 4:24 am

I remember building this way back when and the high-pass didn't work for me either. It might be that we both made a mistake, or that there's a flaw in the original design :hihi:

More recently I built Kassutronics' MS-20 style filter, which is almost the same circuit & works great.

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KSS
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Re: Cv out to filter problem.

Post by KSS » Wed Jan 06, 2021 6:18 am

Without seeing the actual circuit involved I can only answer in a general way. When an LPF is turned into an HPF, the levels-mix is really important to get exactly right or its doesn't really work. Ideally you have a pot to adjust for the required signal cancelling.

Resonance in this type of LPF into HPF is hard to achieve also.

Marcuz
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Re: Cv out to filter problem.

Post by Marcuz » Wed Jan 06, 2021 3:38 pm

Thanks people. I thought about it and think I'm going to walk before I can run so will get rid of the switch.

Marcuz
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Re: Cv out to filter problem.

Post by Marcuz » Wed Jan 06, 2021 10:03 pm

Hi I have now got rid of the hp part of the filter. I have ran my vco to the vcf and when inputting a signal to the filters cv input the leds change when I change the cv input level and sound comes through. The sou d is slightly affected by the cutoff but resonance doesnt do anything. If I use the other input I dont get any sound could this be because I dont have a ground wire attached thus open circuit?? This is well frustrating. I have attached pics. Thanks for everyones help.
Attachments
20210107_025000.jpg
20210107_024939.jpg

Marcuz
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Re: Cv out to filter problem. Help please. Pics added.

Post by Marcuz » Wed Jan 13, 2021 9:26 pm

I have now earthed the input Jack but only the cv input appears to be working the leds get brighter as I turn the cutoff and sound is slightly affected. The resonance does not appear to be doing anything.

Marcuz
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Re: Cv out to filter problem. Help please. Pics added.

Post by Marcuz » Thu Jan 14, 2021 2:36 am

Is it possible the blue leds I have used are causing too much voltage drop? I know yellows and reds cause less.

pjbulls
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Re: Cv out to filter problem. Help please. Pics added.

Post by pjbulls » Thu Jan 14, 2021 2:45 am

No, I doubt that's the problem. The original design had three regular diodes instead of a LED, and blue and green LEDs have pretty similar forward voltage. If the voltage drop is too high you will have too much resonance if anything: they serve as limiters for the signal that goes back into the filter.

It is more likely that the gain in the resonance circuit is too low, did you use the correct values for the resistors? Also, do I understand correctly that, when you say 'the LEDs get brighter', this is the resonance ones? That would indicate there's at least some signal going through the resonance path.

Marcuz
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Re: Cv out to filter problem. Help please. Pics added.

Post by Marcuz » Thu Jan 14, 2021 2:57 am

They get brighter while moving the cutoff mate and there is slight modulation to the sound mid range of the cutoff pot. The resonance is doing nothing unfortunately. However the leds do glow faintly when the resonance pot is twisted.

Marcuz
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Re: Cv out to filter problem. Help please. Pics added.

Post by Marcuz » Thu Jan 14, 2021 2:58 am

Also nothing is happening on the standard input I have to use cv input to get it to do anything?

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