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Charan Raju H 30th March 2020 03:13 PM

Sub woofer
 
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.

Mark Tillotson 30th March 2020 04:05 PM

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?

BJosephs 30th March 2020 04:08 PM

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

Charan Raju H 30th March 2020 04:12 PM

Oh ok.....
I would like to amplify the signals with a frequency up to 150Hz.
I'm using a 25w+25w TDA7492P Bluetooth amp as a driver and I'm planning to use a simple wooden enclosure.

Charan Raju H 30th March 2020 04:37 PM

Ok. Thank you

Mark Tillotson 30th March 2020 05:03 PM

Quote:

Originally Posted by Charan Raju H (https://www.diyaudio.com/forums/analog-line-level/351932-sub-woofer-post6140529.html#post6140529)
Oh ok.....
I would like to amplify the signals with a frequency up to 150Hz.
I'm using a 25w+25w TDA7492P Bluetooth amp as a driver and I'm planning to use a simple wooden enclosure.


A driver is a loudspeaker unit, without the box. What driver are you thinking of.


What size of enclosure? Ported or not ported?

Charan Raju H 30th March 2020 05:11 PM

I have a 20w speaker which I got from my friend's old creative home theater system.And I've not yet decided the dimensions of enclosure.

Charan Raju H 30th March 2020 08:23 PM

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?

PRR 31st March 2020 04:59 AM

> 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?


You NEED a box.

Charan Raju H 31st March 2020 06:18 AM

Ok. Which will be the best enclosure if I want to get louder bass. Ported or not ported?
Also can you please give me a best dimensions for the enclosure. The driver I'm using is of 4 inch, 20w RMS


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