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samsam
Joined: Mar 01, 2017 Posts: 22 Location: France
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Posted: Sun Jun 18, 2017 5:57 am Post subject:
TL071 problem. ultra simple VCF |
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Hi everyone,
I have been trying to build the Ultra simple VCF for the last two days and things are not working properly.
Here is the schematic : http://electro-music.com/forum/phpbb-files/ultrasimplevcf_132.png
I have a power supply that deliver 9 volt and 4,5 volt. Thats what i read when the vcf circuit is not hooked to the power supply. Once hooked to the TL071 of the VCF, pin 4 shows 7 volt instead of 4.5 volt, and the circuit is not working.
Is that normal ? I have changed IC 's but they all behave the same. 7 volt on pin 4.
Did anybody tried that schematic ? or experienced the same problem ?
Thank you. Last edited by samsam on Mon Jun 19, 2017 7:34 am; edited 1 time in total |
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samsam
Joined: Mar 01, 2017 Posts: 22 Location: France
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Posted: Sun Jun 18, 2017 6:09 am Post subject:
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Ok just found a solution, the circuit seems to work when i plug pin 4 to ground.
Problem is that now my op amp picks up some signal of my oscillator from the ground of my power rail.
Any idea ? |
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PHOBoS
Joined: Jan 14, 2010 Posts: 5591 Location: Moon Base
Audio files: 705
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Posted: Mon Jun 19, 2017 11:38 am Post subject:
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This filter is designed for a single supply so in this case pin 4 should indeed be connected to GND.
Pin 3 is connected to a fake GND created with resistors R4 & R5, so if your 4.5V supply is more
stable you could use that. As for picking up the oscillator signals it can have varies causes, even
having two signal/power wires parallel to eachother could cause it. Using bypass capacitors connected
at strategic places can help out sometimes.
Do you still get the oscillator bleeding through with the input connected to GND or 4.5V ? _________________ "My perf, it's full of holes!"
http://phobos.000space.com/
SoundCloud BandCamp MixCloud Stickney Synthyards Captain Collider Twitch YouTube |
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samsam
Joined: Mar 01, 2017 Posts: 22 Location: France
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Posted: Mon Jun 19, 2017 2:09 pm Post subject:
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Hi PHObos,
Thanks for your help. When the op-amp is connected to ground the filter is working but i still get this bleeding from the oscillator.
How do you know wich value or where the bypass capacitor should be placed in the circuit ? |
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elektrouwe
Joined: May 27, 2012 Posts: 143 Location: Germany
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Posted: Tue Jun 20, 2017 6:54 am Post subject:
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samsam wrote: | but i still get this bleeding from the oscillator.
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you mean oscillator @ CV ? That's a "ultrasimplicity" feature not a bug |
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samsam
Joined: Mar 01, 2017 Posts: 22 Location: France
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Posted: Tue Jun 20, 2017 8:05 am Post subject:
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Hi elektrouwe,
What do you mean ? I am not using the cv input at the moment. |
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samsam
Joined: Mar 01, 2017 Posts: 22 Location: France
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Posted: Tue Jun 20, 2017 8:34 am Post subject:
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OK a 22uf capacitor between pin 3 and ground solved the bleeding problem.
Haha, this is really one step at the time.
Now i have to figure why the cutoff pot behave strangely. there is like a bump (sudden change in tone) in the middle of the course. |
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elektrouwe
Joined: May 27, 2012 Posts: 143 Location: Germany
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Posted: Tue Jun 20, 2017 8:47 am Post subject:
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I thought you have a modulation oscillator connected to CV. This will always bleed through, because this simple design has no intrinsic CV feedthrough cancelation (as eg. ladder filters have) |
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