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 Forum index » DIY Hardware and Software
Bridged T filter question.
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lorencz



Joined: Oct 04, 2015
Posts: 11
Location: Detroit
Audio files: 1

PostPosted: Wed Nov 18, 2015 10:53 pm    Post subject: Bridged T filter question. Reply with quote  Mark this post and the followings unread

I was working on adapting a filter design from Ray Wilson's Synth DIY Experimenter board to run on a single 9V battery supply, split into a positive and negative rail with a virtual ground. 4.5V each. I changed some values, simplified the CV input/manual cutoff stage and simulated it in LTSpice. Everything went great, and it had a nice response. Then I breadboarded it and...nothing. The filter wasn't loaded down because I had the output running into a LM386 amp before my test speaker. Anyway, I was playing around with it, trying to figure out what I did wrong, and I removed the resonance pot and put a jumper from the 10n/150k resister junction to the inverting input of the TL072, and suddenly, it worked. And it sounded kinda good, but of course, no resonance control. Anyway, I went back to Spice to run some more simulations of the modified design, and it didn't work in Spice.

So I know there's a few things going on here, for instance LTSpice kinda sucks. But here's the thing, when I made the filter the way it should be made, it didn't work, but when I wired it up in a configuration that shouldn't work, it worked just fine. This really irritates me and I wish I knew what was going on here.

Anyway, the first picture, filter1, is the circuit I set out to build.

The second picture, filter2, is the modified, working version of the circuit.

The last picture is my breadboard, of course. You can't really tell, but the lower ceramic cap and 150k are in the same column as the upper cap and resister. It just doesn't look like it because the legs are bent. The upper two were supposed to go to the output via a 10k pot.

I included the schematic for my power supply for clarity. Please note this simulation won't run in LTSpice because it doesn't like the power supply configuration.

Edit: Oops! Let's pretend the -V net isn't mislabled on my schematic. Also, the output decoupling cap is backwards, but that shouldn't really matter because we're dealing with AC, right?


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Last edited by lorencz on Wed Nov 18, 2015 11:26 pm; edited 2 times in total
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lorencz



Joined: Oct 04, 2015
Posts: 11
Location: Detroit
Audio files: 1

PostPosted: Wed Nov 18, 2015 10:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, here's the link to Ray's original design:
http://www.musicfromouterspace.com/forums.html?CATPARTNO=WSG001&PROJARG=WSG2010%2Fwsg_page9.html&MAINTAB=SYNTHDIY&SONGID=NONE&VPW=1082&VPH=500

Also, the 7.5k/2.5k voltage divider on the top of my schematic is the trimpot on my board, to scale the CV.
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lorencz



Joined: Oct 04, 2015
Posts: 11
Location: Detroit
Audio files: 1

PostPosted: Wed Nov 18, 2015 11:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK, good news, I made a stupid mistake. I connected the cap and resistor to the output instead of the inverting input. Whoops. Anyway, it sounds great now, and the first schematic, filter1 is correct.

Good sounding filter, low parts count, single 9V battery.
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lorencz



Joined: Oct 04, 2015
Posts: 11
Location: Detroit
Audio files: 1

PostPosted: Thu Nov 19, 2015 12:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, now that everything is working, I put together a better schematic, and took some screencaps of some spice simulations.

The green trace is a sweep of the CV from the cutoff pot, from 0 to 4.5v. The blue trace is a sine wave running through the filter. All three images show the response with the resonance at 50%. The CV scaling pot should be at 75% or so.

The first picture is the response for 261Hz, or approximately middle C, the second is 1.5kHz, and the third is 17kHz.

Lastly, BOM:
C5 1 .1u
C3,C4 2 10n
C6 1 10u
R5,R6 2 150k
C1,C2 2 220u
T1 1 2N3904
R4 1 330k
R3 1 470k
R1,R2 2 47k
B 1 9V
R7 1 R7
R8 1 R8
R9 1 R9
U1 1 TL072


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L´Andratté



Joined: Sep 23, 2012
Posts: 99
Location: Hamburg, Germany
Audio files: 5

PostPosted: Thu Nov 19, 2015 11:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

How´s the sound? Smile
Cheers
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