low pass filter

[AudioAddicted]

Hey guys, I have just entered in this forum and it seems it's great.
Well first of all I had a question about preamp filtering:
I've got a 200w car amplifier(phonocar) briged and a 12" boschman sub. my problem is that my amp has got no preamp filtering. just full pass.Listening to music in that way is terrible, believe me.
I was wandering if I can construct a low pass preamplifier filter which allows frequencies under 80Hz and goes at 18 or 24 db/octave. It doesn't matter to me if it is a passive or active filter.

[richie00boy]

You need an active filter. Search here and Google for "sallen-key filter" it's pretty easy.

[AudioAddicted]

Yeah it is really simple but I don't understand what kind of operational amplifier should I use. Any identifying code will be a great help.

Here is the link if u need to have a glance

http://www.ecircuitcenter.com/Circui.../opsalkey1.htm

[richie00boy]

It's really not that critical TL071 is a good place to start.

[AudioAddicted]

OK I will try that. I hope it helps.
I just had a look at the google and there is something i don't understand in that Operational Amplifier. (I might admit it, I am not at all experienced in making up electronic circuits)
What do I connect to
OFFSET N1, OFFSET N2, and NC pins. Well I imagine NC is connected to ground but I can't figure it out what to connect to the other 2 pins.
I really need that help.

If u need to chek that out the link below might be usefull
http://focus.ti.com/lit/ds/symlink/tl071.pdf

[richie00boy]

Do NOT connect NC to ground, it's "Not Connected".

Offset 1 and 2 are for nulling out DC offset, leave them unconnected or whatever the manufacturer tells you to do in the data sheet.

[AudioAddicted]

Woow what a terrible mistake i have done. . .
I told u that I was unexperienced in circuit building.

Now I see... NC stands for "not connected".
Well does it matter if I don't connect the offset terminals at all. Will it affect the results I expect?

Did I thanked you for the help?I guess i haven't. well, thanks a lot. I really appreciate your advices. I'm glad I am part of this forum now.

[Dan2]

don't connect the offset terminals to anything - the opamp is running at unity gain so it won't make a noticable difference

[richie00boy]

Just read the datasheet of the op-amp chip you intend to use.

[AudioAddicted]

I just made up the low pass circuit based on TL072 op amp. Well let me say that the result was great. but.... after a few minutes working in music a terrible noise came out of the sub and I immediately disconnected the amp from power source. Then I removed the filter and the amp was completely OK. connected again the filter but the noise was again present. I think i have blown up the op amp of the filter cause i supplied it at +18 -18 volts dual voltage of the amplifier. may this have caused the problem? If so then at what voltage should I supply the TL072 op amp? specifications from the producer say from +/-5 to +/-18 volts. can u help me please?