Sub woofer

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Hi guys, I'm planning to build a DIY subwoofer but I'm new to these audio stuffs.
I have a 20w speaker and a 25w amp to drive the speaker. The amplifier is driving the speaker with all audio frequency signals but I need only low frequency bass signals to be amplified. So I made a low pass filter using 741 Operational amplifier and connected between output of amp and the sub. But the result was worse. Instead of filtering the low frequency signals the LPF attenuated the signals.So my question is, was my choice of putting a low pass filter is correct? If yes, where is the correct position to insert the LPF? Before amp or after amp?
And my last doubt is, whether 20w sub is powerful enough in a 4*4 metre room.
Please give me some suggestions.
 
Operational amplifiers work at signal levels, the output of a power amplifer is high power and completely unsuitable for active filtering - passive cross-over filters are used after a power amp, using large inductors and capacitors.

To low-pass filter with an opamp you need to do this before the power amp at line-level.

The 741 is not appropriate for audio, its noisy and poor performance. NE5532's are the to-go easy to source opamp for general audio use, there are others too but for price/performance ratio few compete.

How low in frequency do you want to go with the sub? What driver and enclosure are you using/contemplating? Ported (bass reflex) or not?
 
Hello,


You can't put an op amp or DSP based low pass between the amp and the speaker. Op amps are for doing very precise things to small, weak signals. Amps deliver big, high-power signals to speakers to drive the magnets. Assuming the amp didn't break the opamp filter (a possibility if it is only 25W) then go ahead and put it at the input of the amp.


Brian
 
Today I tested my speaker with a low pass filter connected before the amp, low frequency signals got filtered out and amplified pretty well. But I had observed an other problem..... The bass was not at all audible even though the cone paper of the driver was vibrating pretty well.
I tested this by just keeping the driver on the table without any enclosure. What will be the problem?
 
Sorry it's a 5.25 inch driver
This like pulling teeth...


Which exact driver are you using? Do you have all the specifications for it?

You'll need the Thiele-Small parameters if you want to design a subwoofer, and there are various web apps out there that can do the work for you, but without the data for the driver you'll be guessing wildly, and drivers differ a lot.
 
I think what you’ll soon find out is that you cannot design (nor can anyone recommend) a box without the TS parameters. I’ve never measured my own but perhaps someone here will instruct you.

Or, just build a modest sealed box and see how it sounds. Make the box as big as you can tolerate. It will make more bass than sitting on a table, not as much as a properly tuned ported enclosure, but something.
 

PRR

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For the vast majority of domestic speakers: multiply advertised diameter by about 1.1. Cube that. Build a box that size. Preferably different on all three dimensions. Half fill it with wool blanket, socks, or even fiberglass. This will almost always give a Q of 0.8 to 1.6, and pretty near optimum output.

So 5.25", times 1.1, is 5.775. Cubed, is 193 cubic inches.

7" times 6" times 4.5" is 189 cubic inches.

This is NOT a precision prescription. A 4 liter box is fine. 7 inches of 6-inch drain pipe is fine.
 
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