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Old 21st May 2011, 10:55 PM   #1
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Default passive bass boost?

If i put a cap in series with the speaker i can have bass roll off. How do i boost bass? I just want like a 3db boost from 150hz on, but i dont want a slope. I guess you could say i just want to boost the gain on my magic subwoofer.
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Old 21st May 2011, 11:08 PM   #2
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Passively, you can't boost, you can only attenuate. But boosting bass and attenuating midrange / treble is pretty much the same thing.

So if you want to attenuate 3db at about 150Hz and higher, you could use a coil and resistor in parallel with each other, attached to the driver's red / positive terminal. The value of the coil determines the frequency, the resistor determines the amount of attenuation.

This also happens to be the same as what's called a baffle step compensation (BSC) circuit. You might be able to use a BSC calculator to determine the values you need, for example:

Difraction Loss / Baffle Step Compensation (BSC) Circuit Calculator
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Old 21st May 2011, 11:39 PM   #3
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What 'magic' acoustic load your sub uses ?
If it is sealed ,adding a cap before the speaker may help in lowering the first peak,together with very little stuffing inside ,so a high Qtc ,otherwise damped by heavy stuffing ,might make the sound punchy but not well extended .
So ,yes , a cap ( me too , I was thinking of it as a natural High pass filter ...)
may help in sealed boxes to extend low freq reproduction .
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Old 22nd May 2011, 01:07 PM   #4
borispm is offline borispm  Hong Kong
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Actually I run into the same question, but my case is a bit different. I want to see if I can "boost" the bass for about 2 octaves, but I can't figure it out... Can someone give me some advice?
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Old 22nd May 2011, 01:40 PM   #5
borispm is offline borispm  Hong Kong
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I should have made things clearer. I want the frequency above 400Hz to be 12dB softer than that of below 100Hz and the roll off slope is still 6dB/Oct. I just can't figure it out...

Thanks.
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Old 22nd May 2011, 10:38 PM   #6
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look up knotch, band pass, and pass band filters
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Old 22nd May 2011, 10:44 PM   #7
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Quote:
Originally Posted by rjbond3rd View Post
Passively, you can't boost, you can only attenuate. But boosting bass and attenuating midrange / treble is pretty much the same thing.

So if you want to attenuate 3db at about 150Hz and higher, you could use a coil and resistor in parallel with each other, attached to the driver's red / positive terminal. The value of the coil determines the frequency, the resistor determines the amount of attenuation.

This also happens to be the same as what's called a baffle step compensation (BSC) circuit. You might be able to use a BSC calculator to determine the values you need, for example:

Difraction Loss / Baffle Step Compensation (BSC) Circuit Calculator
ouch a 8mh coil costs $30 each...
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Old 22nd May 2011, 10:53 PM   #8
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Hmm this might be cheaper and more efficient, put the baffle step at the line input, looks like it turns the source into a 5k though.


Baffle Step Compensation
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Old 29th April 2012, 11:57 PM   #9
diyLeo is offline diyLeo  Europe
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How about just putting a capacitor in parallel with the speaker? Would this not short out anything but the low frequencies? Or could this somehow be dangerous to the amp?

I'm thinking of trying this with headphones for my iPod to have the effect of a bass boost. The volume is plenty loud enough (especially when using and in-line remote into the dock connector, which is much louder) but the bass to mid and treble ratio is too low. I have no idea what capacitor value to try using for 70 Ohm headphones though, or whether to try a variable capacitor.

I'm also thinking of trying a resistor, probably a variable resistor, in series with the capacitor to see if that can affect the shape of the frequency response curve, perhaps making it a less sudden low-pass, but I'll only have my ears to measure this. Or perhaps even multiple parallel capacitors with resistors. Can't remember the equations off the top of my head.
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Old 30th April 2012, 05:22 AM   #10
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re:'could this somehow be dangerous to the amp?' - yes, it'll short it out, $$$$

re:' variable resistor, in series with the capacitor' - this will change the turnover frequency; if you want to change the amount of low pass, the pot will have to be in parallel. This sort of thing is best done at line level, it's less likely to result in smoke
Baffle Step Compensation
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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