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 Forum index » DIY Hardware and Software » Developers' Corner
TL071 problem. ultra simple VCF
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Sun Jun 18, 2017 5:57 am    Post subject: TL071 problem. ultra simple VCF Reply with quote  Mark this post and the followings unread

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

PostPosted: Sun Jun 18, 2017 6:09 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 3272
Location: Moon Base
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PostPosted: Mon Jun 19, 2017 11:38 am    Post subject: Reply with quote  Mark this post and the followings unread

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 ?

_________________
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http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Mon Jun 19, 2017 2:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 102
Location: Germany

PostPosted: Tue Jun 20, 2017 6:54 am    Post subject: Reply with quote  Mark this post and the followings unread

samsam wrote:
but i still get this bleeding from the oscillator.

you mean oscillator @ CV ? That's a "ultrasimplicity" feature not a bug Wink
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samsam



Joined: Mar 01, 2017
Posts: 22
Location: France

PostPosted: Tue Jun 20, 2017 8:05 am    Post subject: Reply with quote  Mark this post and the followings unread

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

PostPosted: Tue Jun 20, 2017 8:34 am    Post subject: Reply with quote  Mark this post and the followings unread

OK a 22uf capacitor between pin 3 and ground solved the bleeding problem. Cool

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: 102
Location: Germany

PostPosted: Tue Jun 20, 2017 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

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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