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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Help with BP filter
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acidblue



Joined: Jun 26, 2009
Posts: 192
Location: The Darkside

PostPosted: Fri Apr 11, 2014 4:25 pm    Post subject: Help with BP filter Reply with quote  Mark this post and the followings unread

I have a BP filter schematic , ( I forgot where I found it),
But Im not getting any sound if the first pot is connected to ground like the
schem says, I get a some sound, although low volume if I disconnect it from ground.
Using single supply LM358 like the schem says, with no virtual ground.

Also the schem is missing values for R1 thru R3 I tried using 1K resistors cause thats what I have on hand, but I dont what effect that has if any.

It's such a simple schem, but it's providing endless frustration.
Any advice would be appreciated.


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gdavis



Joined: Feb 27, 2013
Posts: 159
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PostPosted: Fri Apr 11, 2014 9:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Google search turned up this blog which discusses the circuit and suggests values for the resistors:

http://blog.skincontact.com/2010/08/

"Suggested values are R1 & R3 1k-10k, R2 & R4 100k or more, and C3 maybe another 1uF. Trial and Error."

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acidblue



Joined: Jun 26, 2009
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Location: The Darkside

PostPosted: Sat Apr 12, 2014 6:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

That link is dead.
Getting "Server not found" error.

But I'll try different R values when I get a chance and see if that makes any difference.
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gdavis



Joined: Feb 27, 2013
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Location: San Diego

PostPosted: Sun Apr 13, 2014 10:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Wow, I quoted the relevant info in case someone came across this thread a year from now or something and the link was dead, but didn't expect it to die after one day Shocked

There wasn't much else that would be helpful as I recall. The impression I got is that it's just a simple but crude circuit for people to experiment with, not really something that's reliable and stable enough to just build and expect to work perfectly, even if built "correctly".

Anyway, good luck.

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acidblue



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PostPosted: Tue Apr 15, 2014 2:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've tried everything, 1k , 10k , 100k, No k, just straight from pin 3 to the pot
and I just can't get this circuit to work. Sad



It's a shame cause it's a simple shcem.
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ElektroBoy



Joined: Apr 16, 2014
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Location: Sweden

PostPosted: Wed Apr 16, 2014 12:31 am    Post subject: Reply with quote  Mark this post and the followings unread

acidblue wrote:
That link is dead.
Getting "Server not found" error.

But I'll try different R values when I get a chance and see if that makes any difference.


When ever a link is gone, try the archive. Smile

https://web.archive.org/web/20120419231009/http://blog.skincontact.com/2010/08/
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wackelpeter



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PostPosted: Wed Apr 16, 2014 8:56 am    Post subject: Reply with quote  Mark this post and the followings unread

stumbled this week across this pdf file... seems to be very useful, especially if anyone wants to stuff some op amps in his Lunetta which mostly run with single supply...

http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf
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acidblue



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PostPosted: Wed Apr 16, 2014 12:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your replies.
But I've tried every thing and still cant get this to work.
When I disconnect the first pot I get an amplified signal from
my square waves osc's. but nothing when connected.

I've tried single supply and VR supply, I think the LM358 will work
with a straight single supply, but I tried it with a VR as well, still nothing.
Crying or Very sad
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Sat May 10, 2014 6:42 am    Post subject: Reply with quote  Mark this post and the followings unread

ok I decided to give this one a try. The first thing I noticed is that the LP filter section can't really work well because it's directly connected to the low
impedance output of the opamp. So instead of taking the output signal from pin 1 use the connection between R3/VR2 and use the other half of the
LM358 as another voltage follower connected to that. So you end up with: signal -> passive HP -> buffer -> passive LP -> buffer.

I did a test with my modular lunetta as a signal source but becauseI have diodes on every output I needed an extra pulldown resistor. Since the output
signal is also very high I used a voltage divider (2x 10K) and then used another opamp as a buffer before going to the filter input. Component values
I used are:
HP: C1 = 100nF, R1 = 10K, VR1 = 100K
LP: C2 = 100nF, R3 = 10K, VR2 = 10K
(voltage supply 12V)

and that seems to work, but I did not test the CV inputs,. (yet)

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acidblue



Joined: Jun 26, 2009
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PostPosted: Sat May 10, 2014 8:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Something like this?
Thanks for your input BTW.
I'll have to make room on my bread board and test this.


BandPass Filter_VoltageFollower.png
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PHOBoS



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PostPosted: Sun May 11, 2014 4:31 am    Post subject: Reply with quote  Mark this post and the followings unread

yep, like that Very Happy.
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acidblue



Joined: Jun 26, 2009
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PostPosted: Sun May 11, 2014 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok I got it to work, mostly Question

The Low Pass side works fine, however the High Pass is proving to be difficult.

I'll explain, if I wire it exactly like the schematic, with the resistor/pot combo after the cap, I get nothing.
If I put the resistor/pot combo before the cap, I get sound, but adjusting the pot does little if anything, this is on the HP side of the circuit.

I tried different values for the resistor, (1k, 100k), same result.

Any ideas??
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PHOBoS



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PostPosted: Sun May 11, 2014 11:46 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm you could try if it works when you put an extra opamp buffer before the filter aswell (either another LM358 or you could use a quad opamp like LM324).
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Last edited by PHOBoS on Sun May 11, 2014 3:47 pm; edited 1 time in total
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acidblue



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PostPosted: Sun May 11, 2014 12:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Adding buffer at the input does help, but the variation on the HP still doesn't
seem that much.

Any way thanks for the help PHOBos thumright

Here is the updated schematic, caution the CV inputs are untested.


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