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Old 13th February 2014, 08:24 AM   #1
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lorienblack's Avatar
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Default How do you create a low shelf with passives?

I'm building some studio monitors, twin FR drivers. Mark Alpair 7.3's

I applied a low shelf filter 3db gain rolling off from 100hz to 200hz (active filtering Nano Digi dsp) . It transformed the sound, imaging, soundstage, absolutely fantastic. The speakers will be active, but I want to avoid DSP and plate amps if I can, I don't want to digitize and reconvert signal to analouge using relitively cheap components if I can avoid it.

Is it fairly straightforward to engineer a low shelf filter?

And ideas or pointers would be most welcome.
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Old 14th February 2014, 03:00 PM   #2
DF96 is offline DF96  England
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Is it fairly straightforward to engineer a low shelf filter?
At line level, yes. Two resistors and a capacitor will do it. The two resistors form a potential divider for LF, and the capacitor bypasses the upper resistor for HF.
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Old 14th February 2014, 03:16 PM   #3
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Hi, yep you can do that, like DF96 says. You may need an active buffer at the output though.

I did about same with my Kleos crossover. See the shelving filter just before the opamp, on the low pass section. I applied an adjustable boost (12dB max) below about 120Hz, to correct for dipole roll off of my speaker. P4, R16 (optional), and C13 determine the corner frequency Fc which is 100hz. P4 adjusts the boost. The overall gain of my crossover is 6 (15.5 dB).

For a low pass filter - Fc= 1/(2*pi*(P4+R16)*C13)

This is a low frequency shelf though, not a low pass filter. It starts it's roll off at the corner, and levels off at a point determined by the voltage divider created by P4.

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Last edited by Johnloudb; 14th February 2014 at 03:19 PM.
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