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Old 8th June 2011, 07:57 AM   #1
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Default Passive low-pass passive for line input

Hello, I'd want to ask you for a question: I have to use an old amplifier to pilot a passive DIY subwoofer, so I have to put a low-pass filter across its line input. Planning to use the power output signal of the master amplifier using the old amplifier as a booster, my question is: being necessary to use a resistive partitor to attenuate the high level signal, can I add to it a passive low-pass filter using simply inductance and capacitors, 4th order, 60 Hz, calculated for a load of about 100 ohms? I'd want to avoid a classic electronic filter with op-amps and necessary power supply, to maintain the signal clean and as it is and to build the simplest possible circuit.
I think: in the high-level signal there is enough energy to avoid active filtering!
Thank you!
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Old 8th June 2011, 09:09 AM   #2
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Ciao Andrea ,l'ho fatto anch'io ..ehm
I did it once ,but I gave up in having the LP filter . I just made a node with two 500 Ohm 1 watt resistors , then put two 1000 uF 50 VL in anti-phase ,a 4K7 pot (unnecessary ). Putting RC nets in that point brings to serious phase alterations ( you can try...) , I guess a tailored RCL would do the job .
You can then rely on a pass-band design for the cabinet ,thus eliminating the need of the LP filter ...
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Old 8th June 2011, 09:35 AM   #3
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I think that you will quickly find that the inductors required will be too large to be practical.

Using the filter calculator here Crossover Design Chart and Inductance vs. Frequency Calculator(Low-pass) a 4th order filter will require a 500mH and 250mH coil for 60 hz and a 100ohm load.

I designed a low pass filter using an FDNR, but have not yet built it (so still don't know if it works). The idea with it is that it works like a classic passive LC filter with the opamp in the shunt leg, so none of the signal passes (directly) through an opamp. It still requires buffers on either side, but these can be discrete. It's in my blog http://www.diyaudio.com/forums/blogs...crossover.html as you can see from the date, I've made slow progress you would need two of these to get 4th order...

In reality for a frequency as low as 60 hz a plain old sallen key filter would probably be the best bet.

Tony.
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Old 8th June 2011, 10:26 AM   #4
ampmade is offline ampmade  Belgium
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Hello,
I'm agree with wintermute. If you want to make a 4th order filter, you need two 2nd order cell to do the job.
Do you have any particular reason to avoid active filters?
Sallen-key cell is very easy to design, you could make a little PCB with a proto-board and connect it to your power supply.
You could also (like mentionned by wintermute) make a simple second order low pass for your cabinet. Choose a lower cutting frequency to avoid the use of 4th order.

Fabrice.
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Old 8th June 2011, 11:27 AM   #5
AndrewT is offline AndrewT  Scotland
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investigate the MFB active filter. Compare it to unity gain Sallen and Key and to the equal component value Sallen and Key (it has variable gain).
Each type have advantages and I believe that the MFB is unbeatable for a multiple pole bass low pass filter.
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Old 8th June 2011, 04:14 PM   #6
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Thank you for your answers. Admitting to use however the classic LC circuitry as indicated and calculated by Wintermute but on a 20 ohms load (to use inductors of 100 mH, more easily foundable..), being necessary a resistive partitor to attenuate the signal, can I put a 150 ohms resistor in serie before the filter and the 20 ohms resistor in parallel after the filter? In other words, can I place the filter between the node of the resistive partitor of 150/20 ohms?
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