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Old 18th February 2013, 02:31 PM   #1
child1 is offline child1  Denmark
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Default Subwoofer low output

Hello there!

I have just finished a build of an active subwoofer with a TDA7294 chip amp inside. I have connected it to a soundbar I have build also. Inside the soundbar is a 2. order active filter for the sub. Now, my problem is that the output of the subwoofer is very low compared to the soundbar. I would like it to be twice as loud i think. What can I do. Do i really need to add a small amplifier circuit on the input of the subwoofer to make the input signal amplitude larger?

Any tips or ideas is appreciated

/Mads
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Old 18th February 2013, 03:08 PM   #2
Mooly is offline Mooly  United Kingdom
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You could probably just alter the feedback factor of the power amp. Halve the value of R2 (approx) and double the value of C2
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Old 18th February 2013, 03:27 PM   #3
DUG is offline DUG  Canada
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The RC combination of 22uF and 680R has a 3dB point of 10.6Hz.

For a sub I would have used a 220uF - 680R for a 3Db point of 1 Hz.

Or is that just me?

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Old 18th February 2013, 05:51 PM   #4
child1 is offline child1  Denmark
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@mooley - would it be okay with 47uF and 330ohm? Sounds like an easy fix... any downsides?

@DUG - My sub is not going further down than about 36 Hz (its a small system), so i dont think that is a problem, but i could be wrong?
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Old 18th February 2013, 05:58 PM   #5
Mooly is offline Mooly  United Kingdom
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There shouldn't be any downsides for your application.

Try and locate those actual components in your amp and we'll take it from there. The circuit I posted is the standard application note. Yours will be similar but may have slightly different values. So see what is fitted. Its just a case of lowering the value of that resistor (whatever it is) and raising the value of the cap in a similar ratio.
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Old 18th February 2013, 10:27 PM   #6
gootee is offline gootee  United States
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Quote:
Originally Posted by child1 View Post
@mooley - would it be okay with 47uF and 330ohm? Sounds like an easy fix... any downsides?

@DUG - My sub is not going further down than about 36 Hz (its a small system), so i dont think that is a problem, but i could be wrong?
Typical recommendation is to set the -3 dB point at no more than one-tenth to one-hundredth of the frequency that you don't want to have any effect on. So, 3.6 Hz or lower. Not sure how much difference it might make.
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Old 20th February 2013, 12:17 PM   #7
child1 is offline child1  Denmark
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Well - tried to change the value of C2 and R2 to 44F (two 22F in parallel) and 330R, but it has not caused any improvements. (The original values where 22F and 680R). Bummer!
Any other suggestions?
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Old 20th February 2013, 02:18 PM   #8
Mooly is offline Mooly  United Kingdom
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If you've identified the components correctly then it must be doing what it should... it can't not do. It is absolutely "cast in stone" that those components set the voltage gain of the amp. In the above circuit the voltage gain is 22K/680+1 which is 32. Your new values are 22K/330 + 1 which is 66.

So what do we do...

We prove it (which will make sure the correct parts have been identified). Two minutes of a job to test. A scope or DVM can be used (DVM on AC voltage).

1. Get yourself a test tone burned onto CDR (there are two here in post number #2 if you haven't any. Choose the 120Hz track),

A Test. How much Voltage (power) do your speakers need?

2. Connect the DVM across the speaker and set the meter to AC volts. Use a low range for best resolution.

3. Leaving your amp exactly as it is now play the tone and adjust the volume for some convenient level on the DVM. It can be anything that is accurately resolved and displayed on the meter, for example 100 millivolts or 1.394 volts, anything.

4. Write down the result and leave the volume settings exactly as they are.

5. Now, either refit the 680 ohm back in place of the new 330 ohm OR, tack the 680 ohm across the 330 ohm currently fitted. Whichever is easiest. Leave the new cap as it is.

So you now have either 680 ohm or a new value of 222 ohm (the two in parallel) in circuit.

6. Repeat the test and write down the new level.

Whatcha' got ?
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Old 20th February 2013, 02:35 PM   #9
child1 is offline child1  Denmark
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I bet you are right - perhaps I just cant hear the difference with the input signal I have tested with. I'll try some other signals tonight, and post what happens.

Anywho - I am absolutely certain of the values as I have made the amplifier from scratch... so I'll give it another go and see what happens.

Thanks!
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Old 20th February 2013, 02:39 PM   #10
Mooly is offline Mooly  United Kingdom
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OK

If you need even more gain you can go lower on the resistor (within reason). Perhaps as low 82 ohm (and a bigger cap).
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