Op Amp filters for newbs

[bluzeyonecat]

I started this thread in response to accidentally derailing another one.
We started talking about low pass filters ect. If you want to see where we left off, check out the "astable circuit" thread. Some really useful information there including schems. and formulas.
So this space is dedicated to those of us that are new to op amps and their functions. So ask away! Im sure I'll be back in a few.

[anotherjim]

So, following where the previous thread was headed, it looks like a synth type filter was desired.
With only one op-amp to play with, I'd suggest Escobedo's Quick & Dirty Filter.

It needs a harmonically rich input signal (Square/Pulse/Saw) and it must not be too strong - a volume or trim pot needed on the input.
Certainly the most Synth like simple filter I've ever toyed with, although I've yet to build it into anything.

[GibsonGM]

Just to make sure we have the basics, this is a really great vid that explains how opamps work, how you set the gain, and some of their other properties.  Might help as something to refer to if these questions come up.   Links to many other awesome topics, too!

https://www.youtube.com/watch?v=7FYHt5XviKc

[garcho]

http://www.geofex.com/article_folders/eqs/paramet.htm

[garcho]

http://www.schmitzbits.de/ms20.html

http://www.diystompboxes.com/smfforum/index.php?topic=85331.0;prev_next=next

[bluzeyonecat]

Actually subscribed to eevblog a while ago. He's awesome to learn from! Check out his lasted vid that showed up in my menu. Wow!
https://www.youtube.com/watch?v=x8nbHYOc8ns&feature=youtube_gdata_player
Quick and dirty schem above looks like fun! I'll give it a go. One question, (forgive me Im a greenhorn ) but which resistors would be the pots?
On a positive note, I built my first functioning low pass filter! It worked great! Swapped out different sized caps and could hear the difference in their characteristics. One thing was odd though. I fed it with a 5v oscillator with 5V going into the input of the 741 and the 741 turned the signal into roughly .50 mv. So I sent that to one of my amplifiers and all was good. But is that signal supposed to get squished like that? Thanks for all your help. Feel like Im one step closer to soldering iron.

[GibsonGM]

In the Q&D posted by Anotherjim, the 1M trim and 100k freq, and 10K resonance are pots, just wired with their wipers set up in a way maybe you haven't seen before?

>> I'd expect that if you fed your 741, set up to filter, 5V in and you got .5mv (your sure it was that low??) on the output, that would mean you set it up so that the frequency of signal you fed it was well outside its pass band.   So it was rejecting it, as any filter should by definition.   The test for that is to apply a signal above or below the cutoff (depending on if it's a LPF or HPF) and see how it works when the freq. is 'included' in the pass....

What're you using for an oscillator?  And how did you wire up the filter?  The piece of info missing is "I fed it 5V AT XXX FREQUENCY...my filter's cutoff is set up at XX".