[sdiy] need help with a filter

Nugent, Allison C. (NIH/NIMH) [E] nugenta at mail.nih.gov
Sun Dec 22 01:33:08 CET 2013


I already have flickr:

http://www.flickr.com/photos/75864362@N00/11487359823/

Again, I apologize for a crude hand-drawn schematic.

The power supply is a DC 9V adapter I nicked from one of my husband's other synthesizer modules, so I'm assuming its reasonably optimized for this application.  There are two simple virtual grounds, one for the oscillators and one for the mixer.  The was done more because it was convenient to wire than for any necessary reason.

I only drew one of the oscillators for simplicity.  On the oscillator, the pots don't have one input hanging, it's tied to the wiper, I just left it out for clarity.  And on the level pot in the mixer, I currently have one leg tied to the ground of the power supply, although it now occurs to me that maybe they should be tied to the virtual ground?  I'm not sure if that makes a difference.

I had planned to run the mixer and the filter off the same TL072, but since the mixer is already built, I'm using a separate chip on a breadboard.  I've tried using both an independent virtual ground for the filter, as well as tying into the mixer virtual ground, both of which produced oscillations, and neither of which resulted in the filter actually filtering.

One question - I've been unable to find the equations for the characteristics of this filter.  That would be extremely helpful to know, as I think some of my resistor values are approximate...

Thank you SOOOO much for your help.

Allison
________________________________
From: cheater00 . [cheater00 at gmail.com]
Sent: Saturday, December 21, 2013 6:36 PM
To: Dan Snazelle
Cc: synth-diy; Nugent, Allison C. (NIH/NIMH) [E]
Subject: Re: [sdiy] need help with a filter


I like to use imgur.com<http://imgur.com>, no need to register.

On 22 Dec 2013 00:30, "Dan Snazelle" <subjectivity at hotmail.com<mailto:subjectivity at hotmail.com>> wrote:
just so you know
attached photos do not show up on the list
could you post on flicker or some other website and post the link?

thanks

Sent from my iPad

> On Dec 21, 2013, at 3:34 PM, "Nugent, Allison C. (NIH/NIMH) [E]" <nugenta at mail.nih.gov<mailto:nugenta at mail.nih.gov>> wrote:
>
> One more thing I can add - in addition to the oscillation when there's no input, it also appears to not actually be filtering when I do give it input.
>
> Allison Nugent
> MRI Physicist
> Experimental Therapeutics and Pathophysiology Branch
> NIMH/NIH/DHHS
> Ph 301-451-8863<tel:301-451-8863>
> ________________________________________
> From: Gordon JC Pearce [gordonjcp at gjcp.net<mailto:gordonjcp at gjcp.net>]
> Sent: Saturday, December 21, 2013 2:41 PM
> To: synth-diy at dropmix.xs4all.nl<mailto:synth-diy at dropmix.xs4all.nl>
> Subject: Re: [sdiy] need help with a filter
>
>> On Sat, Dec 21, 2013 at 06:33:45PM +0000, Nugent, Allison C. (NIH/NIMH) [E] wrote:
>> Hello,
>>
>>
>> I don't have a lot of experience building stuff like this, and problems that are obvious to more experienced folks are not terribly obvious to me.   But I really want to get this thing finished...  if you have any ideas, please let me know.  I'm happy to post a schematic, if that would be helpful.
>
> If you could draw out a diagram of exactly what you've built and how you're creating the virtual ground, that would be helpful.
>
> Quite often opamp circuits get right uppity about being run with the inputs and outputs floating, especially if there's some feedback with a bit of phase shift around them.  Like, oh for instance, a filter...
>
> Check your virtual ground is properly decoupled, and try sticking a resistor to that point from both your output and input to ensure they're not floating.
>
> Have you got an oscilloscope?
>
> --
> Gordonjcp MM0YEQ
>
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