low pass filter

AudioAddicted · 2006-10-12 1:04 pm

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 · 2006-10-12 1:09 pm

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

AudioAddicted · 2006-10-12 2:46 pm

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/Circuits/opsalkey1/opsalkey1.htm

richie00boy · 2006-10-12 3:27 pm

It's really not that critical

TL071 is a good place to start.

AudioAddicted · 2006-10-13 7:19 am

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 · 2006-10-13 9:12 am

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 · 2006-10-13 9:49 am

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 · 2006-10-17 12:38 pm

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

richie00boy · 2006-10-17 12:53 pm

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

AudioAddicted · 2006-10-20 10:19 am

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?

richie00boy · 2006-10-20 10:30 am

If the 18 volts was unregulated it might have fried the chip. 12 or even 9 volts is fine for an active filter to run off, so you could drop a regulator in or just a few series diodes with each rail to drop it down a bit if unregulated.

AudioAddicted · 2006-10-20 10:44 am

In fact it isn't regulated. U mentioned some diodes to drop it a little. How can I make this up, I don't understand how to lower the voltage with some diodes. can u explain it to me please?

richie00boy · 2006-10-20 11:37 am

Well each diode will drop about 0.65 volts... put two in series and you drop about 1.3 volts...

AudioAddicted · 2006-10-20 12:42 pm

I see now. It is pretty easy. I will certainly try that and tell about it tomorrow. the only problem is that I need plenty of diodes to drop the voltage from 18v to 12v or so. is that a problem?

Dan2 · 2006-10-30 7:02 am

you could just put a regulator in - this will give you a constant voltage out. if you want 9 volts out, just use LM7809 and LM7909 instead of LM7812 and LM7912. the caps in the diagram shouldn't be necissary- but you do need heatsinks on the regulators.

AudioAddicted · 2006-10-30 2:57 pm

Thanx Dan2. It looks good. I will try to find those regulators.
First I think to try it with two 9V batteries to form a dual power supply. If it is OK then I will go to by the regulator chips. Thanx again.

jgrimsley · 2006-11-01 9:46 pm

Or you could use small switching supply that produces +/- 12V. Check out the ones made by TDK. I'm relatively sure model number CC3-1212DF-E will work for you. It produces +/-12V and
+/-125mA from a 9V to 18V input power supply. You will probably will need something like this when you put the filter in your car, since your car battery only produces a positive voltage.

Best of luck with your project.

AudioAddicted · 2006-11-02 7:44 am

Well thanx but I am thinking to put that filter inside the Amplifier where I have got a dual voltage power supply of +/-18 volts.
The reason is that this will make no mess around my vehicle.
Anyway thanks. I think someday I will need sth like that TDK you told me.

low pass filter

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