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tolaemon 25th September 2008 08:30 PM

OP based low pass audio filter
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Hi, I've spent nearly 1 day trying to make this simple low pass filter work, and I haven't been able. I don't know what I am doing wrong. I want to remove high frecuencies from a square wave which I generate with one of the pins of a microcontroller to make it sound smoother. So at the input of the filter I have (A) and would like to have (B) but i get (C), in other words: I get nothing. I tried to use a passive RC filter but I had undesired atenuations at all frecuencies so I decided to build this active filter. The operational is an LM833 which is suposed to be valid for audio purpouses. In fact the schematic is taken from the LM833 application note (I've added the power supply symbols and the signal pictures ). Can the capacitors be de cause of the problem? they are standar poliester capacitors... Or probably is that the design is only valid for symetric power ( I mena +5 -5 , instead of +5 0 ).

Thanks in advance, and congratulations to all the members of the forum. It's time since I read it's posts, but I havent decided to write until today!

richie00boy 25th September 2008 08:43 PM

The circuit need +5 and -5 with ground/0V to work, not single ended supply.

Minion 25th September 2008 10:22 PM

You could use a single supply if you add a Voltage referance at half supply at the input and use an Output cap.....

But a Dual supply would be better....


tolaemon 25th September 2008 10:34 PM

Man, you are right! I've just tested it with a symetric power supply +5V, 0, -5V and It works. I was mistaken. I know that operationals normally work with symetrical power supplies, but I used some operationals which could be used with not simetrical power supplies ( 0, 5V) and thought this would also.
Now I have a problem because I'll have to decouple the 0 to 5V square input signal to remove it's DC component ( -2.5 to 2.5 ) and I will also need a negative supply on my board to power the opamp.

tolaemon 25th September 2008 11:26 PM

I've read the 358 opamp datasheet with more detail, and says:
"..Wide power supply range:
Single supply: 3V to 32V
or dual supplies: 1.5V to 16V
then, in case of using this IC, would not be necessary to use a dual supply. But I'm using the 833 and on the datasheet of this chip doesn't say anything about single or dual power supply, it only says:
"...Input Voltage Range (Note 3) VIC 15V..."
so I suppose that only works with dual supply.. I don't know. In fact I tested it with single supply and did not work...

pacificblue 26th September 2008 06:49 AM


Originally posted by Minion
You could use a single supply if you add a Voltage referance at half supply at the input and use an Output cap.....

But a Dual supply would be better....


richie00boy 26th September 2008 09:26 AM

I think the 833 will not like just 5V supply, from mrmory minimum is 9V. Any op-amp will run from split or single supply as long as you have the correct voltage and circuit implementation.

I don't think trying to use single supply with coupling cap is good in this application as you are trying to pass square waves.

AndrewT 26th September 2008 11:59 AM

if your input signal is 5Vpk then the supply rails should exceed this by at least a volt and preferably more.
I suggest >=+-8Vdc to pass that 5Vpk signal.

tolaemon 27th September 2008 04:19 PM

Thanks for your help and suggestions! They have been very usefull :)

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