Author Topic: Low pass filter help please...  (Read 7946 times)

Offline Barry (NJ)

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Low pass filter help please...
« on: August 27, 2012, 07:57:12 AM »
I'd like to create a low-pass filter, or crossover, to feed a sub. I'd like the pass frequency to be in the neighborhood of 30-35 Hz. I don't need to recover the high-end, just the low.

Would someone here be able to recommend a design with component values(?)

Thanks, Barry
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Offline Barry (NJ)

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Re: Low pass filter help please...
« Reply #1 on: August 27, 2012, 08:04:01 AM »
P.S. this is for a speaker level feed, not line level.
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Offline _Scotty_

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Re: Low pass filter help please...
« Reply #2 on: August 27, 2012, 09:40:51 AM »
Barry, let's take this as an example. In my speakers I have a three-way crossover with the high frequency xover set at 3.5khz and the mid to woofer at 150Hz, the penalty I pay for doing this passively is that the 12ga. coil in line with the woofers has about .3ohms DCR. This DC resistance is unavoidable and that coil is a 12ga to minimize its negative impact on the damping factor that the amplifier can apply to the woofers.
 If you have a 4ohm sub-woofer driver and you crossover at 35Hz and 12dB/oct. you will need a coil value of 25.73mH and a capacitor value of 803 uF. If you have an 8ohm woofer
you will need a 51.46 mH coil and a 400uF capacitor.
 The biggest steel laminate cored inductor I can find on the Madisound website is 20mH coil with .7ohms DCR, cost $39. A 25mH coil, if you can find one will probably have around a 1ohm DCR. What this means is that you will have muddy bass from your sub with this much resistance between it and the driving amplifier.
 A couple of Solen caps with a value of 200uF at 400volts will set you back $52.50 each. At these kind prices a non-polarized electrolytic cap is better alternative.
 You can go with a passive design but you have to be aware of the costs and sonic penalties associated with this approach. As you can see it is not very practical and that 1ohm resistance, 2ohms if the sub-woofer is an 8ohm impedance, between the amp and the sub-woofer driver is not your friend.
Scotty

Offline richidoo

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Re: Low pass filter help please...
« Reply #3 on: August 27, 2012, 10:37:09 AM »
The driver's impedance peak at resonance will screw up the response if it falls within the passive lowpass filter band, so you might need a series notch to flatten the impedance.

Scotty is right, if it is at all possible to run the bass driver actively, you will be much happier. Then you can use a little active filter like Marchand XM-1.

A couple active subwoofers might be the best value solution. Add even more subs to smooth in room bass response. :D

DaveC

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Re: Low pass filter help please...
« Reply #4 on: August 27, 2012, 12:00:27 PM »
I agree an electronic line level crossover is the best solution. Also, there is the crossover slope, and IMO a 24dB/Octave slope integrates much better than 12dB in most cases. Anyway, I'd much rather avoid passive xo's anyway with my 6 wpc SET amp.

Also, there should be a selection of subwoofer amps with the xo built in...




Offline Barry (NJ)

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Re: Low pass filter help please...
« Reply #5 on: August 27, 2012, 12:03:14 PM »
Thanks guys! Your input is greatly appreciated. I was thinking of running the second VC in my Gallo Reference 3.5 speaker's woofer off of the same signal as the first, but crossing it over lower to avoid doubling all but the lower bass out-put. You've now helped to explain why Anthony doesn't do this from the factory, and instead sells a "Bass Amp" for this purpose.

Best, Barry :thumb:
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Offline Barry (NJ)

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Re: Low pass filter help please...
« Reply #6 on: August 27, 2012, 12:32:27 PM »
P.S. I may slap one of these on each one just to see what happens...

http://www.parts-express.com/pe/showdetl.cfm?partnumber=266-442

Though in my room I'm already pretty flat to about 35-40Hz
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Offline richidoo

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Re: Low pass filter help please...
« Reply #7 on: August 27, 2012, 02:09:56 PM »
That's a clever idea Barry! But the only problem is the filter would not only roll off the volume but it also twists the phase of the signal, which would make the amps fight over where the driver cone should be at any given moment.

It would be better to apply EQ to one amp controlling both VCs. You can do that with a line level filter, diy would be easy and cheap.  It is a sealed woofer, iirc, so a Linkwitz transform is what you want, and low impedance, powerful amp like class D. ;)  That's probably what the Gallo amp is, an amp with the transform EQ already applied, specific to the Gallo woofer.

I once heard Ref 3.1 demoed with and without the Gallo bass amp, it makes a pretty big improvement, iirc, and as I'm sure you already know! Gosh that was a long time ago...

Offline _Scotty_

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Re: Low pass filter help please...
« Reply #8 on: August 27, 2012, 02:26:33 PM »
It sounds like you have a second unused amplifier that you wish to press into service. A Behringer Ultra Drive Pro DCX2496 might be useful thing to have.
Scotty

Offline Barry (NJ)

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Re: Low pass filter help please...
« Reply #9 on: August 27, 2012, 02:33:04 PM »
What about a Capacitive/Resistive filter as opposed to the Inductive type?

http://www.allaboutcircuits.com/vol_2/chpt_8/2.html#02117.png

Please understand I'm a n00b to this stuff ;)

Actually Scotty it's the exact opposite. I want to use a single amp for this. The speaker as designed has two sets of binding posts. The primary set powers the full range of the speaker. The second set is wired directly to a second voice-coil on the woofer with no filtering. It is designed to be used with a second amplifier with filetering to adjust the signal to enhance lower bass.
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Offline _Scotty_

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Re: Low pass filter help please...
« Reply #10 on: August 27, 2012, 03:14:56 PM »

 You might be able to get there from here.
I have to amend my first statement. If you parallel the voice-coils in the woofer over part of its range you could, on paper, gain 3dB of increased output by halving the impedance the amplifier sees over the frequency range where the voice coils overlap. The halved impedance will draw twice the the current the single voice-coil does, however the amplifier may not appreciate this and problems may ensue. Just a guess, but your choice of inductor will have a significant DCR,perhaps more than .5ohm which will reduce the amount of current that can flow to the second voice-coil. This would still be an interesting experiment, 3db might be a worthwhile increase.
Scotty
« Last Edit: August 27, 2012, 04:47:05 PM by _Scotty_ »

Offline richidoo

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Re: Low pass filter help please...
« Reply #11 on: August 27, 2012, 04:43:05 PM »
Barry, do you mean that you want to use a single amp to power just the woofer (with another for the M/T,) or do you mean a single amp to passively power the whole speaker?


A line level filter will use resistors and caps, no coils (although it could use coils, but not necessary.) Like all analog filters, Line level filters are sensitive to their load impedance, so it helps to use a current stage amplifier to make sure the filter parts don't rob highs or lows due to insufficient drive. They are called active when they are powered by an opamp or some active amp stage. But pure passive line level filters are possible in some cases. These would just insert into the line before the woofer amp.  If you want any kind of boosting of low freqs then a passive filter is not possible.

A Linkwitz Transform is a type of filter that boosts bass in sealed boxes. It must be active, that is use a buffer like an opamp to power the filter.

Any kind of analog filter will cause phase error, so they can't be applied to the same cone with two amps, no matter whether passive or active. The phase difference will cause the amps to overheat, protect, or break the VC former from the tug of war.

A linear phase digital crossover could concievably accomplish what you are imagining,  because that type of filter does not add phase error to the filtered signal, just amplitude attenuation and that's all. But it has a half second latency so the whole spekaer must be active crossover, and they are pretty expensive because it requires huge CPU power to calculate the filter values for each sample individually.

OK now you got me rambling... DANGIT