[sdiy] need help with a filter

Nugent, Allison C. (NIH/NIMH) [E] nugenta at mail.nih.gov
Sun Dec 22 03:27:04 CET 2013


Still oscillated with no input (so input is effectively tied to virtual ground) with the battery...  so I added a cap from battery negative to the virtual ground, and that seemed to take care of the oscillation.  There's still ripple in the output, but it's now on the order of 25mV.

Then, I tried putting in input from the output of the mixer.  I wasn't sure what to tie the ground of my oscilloscope probe to.  If I tied it to the virtual ground on the filter, then when I look at the output of the mixer, it looks garbled - I'm guessing because it's riding on top of amplified ripple from the power supply from that board.  If I tie it to the power supply on the main board, the output of the filter doesn't make sense.  So I think I need to add the capacitor to the main supply board as well before I can make sense of this.

Allison

From: cheater00 . [cheater00 at gmail.com]
Sent: Saturday, December 21, 2013 9:12 PM
To: Nugent, Allison C. (NIH/NIMH) [E]
Cc: Paul Schreiber; synth-diy
Subject: Re: [sdiy] need help with a filter


Does it work with the bit?

On 22 Dec 2013 03:05, "Nugent, Allison C. (NIH/NIMH) [E]" <nugenta at mail.nih.gov<mailto:nugenta at mail.nih.gov>> wrote:
The filter comes from here:

http://www.musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&VPW=1313&VPH=504

The only difference is that I've taken out the voltage controlled bit that receives input from an LFO and/or ARG.

The same filter appears in another MFOS toy, here:

http://www.musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&VPW=1313&VPH=504

Slightly different values and wiring.

I'm working on it now with a battery as a supply...

Allison


Allison Nugent
MRI Physicist
Experimental Therapeutics and Pathophysiology Branch
NIMH/NIH/DHHS
Ph 301-451-8863<tel:301-451-8863>
________________________________________
From: Paul Schreiber [synth1 at airmail.net<mailto:synth1 at airmail.net>]
Sent: Saturday, December 21, 2013 8:55 PM
To: Nugent, Allison C. (NIH/NIMH) [E]
Subject: RE: [sdiy] need help with a filter

a) you can't use that 4.7M feedback resistor, it is WAY too high of a value.
The op amp input bias current will be multiplied by 4.7 million and chances
are, the op amp will just drive up to a supply rail. In general, you want op
amp feedback resistors <390K or so. So you would make resistor 10x smaller
and the 2 caps 10x smaller.

b) that is a very odd filter, is it trying to be a twin-T bandpass or
something? Because a twin-T has a very specific operating point: you just
can't "replace" resistors with pots and expect it to work (typical bad
hacking in the guitar pedal world).

Paul Schreiber
Synthesis Technology


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl<mailto:synth-diy-bounces at dropmix.xs4all.nl>
[mailto:synth-diy-bounces at dropmix.xs4all.nl<mailto:synth-diy-bounces at dropmix.xs4all.nl>] On Behalf Of Nugent, Allison C.
(NIH/NIMH) [E]
Sent: Saturday, December 21, 2013 5:33 PM
To: cheater00 .; Dan Snazelle
Cc: synth-diy
Subject: Re: [sdiy] need help with a filter

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<mailto: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><http://imgur.com>, no need to register.

On 22 Dec 2013 00:30, "Dan Snazelle"
<subjectivity at hotmail.com<mailto:subjectivity at hotmail.com><mailto: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><mailto: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><tel:301-451-8863<tel:301-451-8863>>
> ________________________________________
> From: Gordon JC Pearce [gordonjcp at gjcp.net<mailto:gordonjcp at gjcp.net><mailto: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><mailto: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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