I want to add a low pass filter to a subwoofer I'm working on, and probably will get one of these from Parts Express:
https://www.parts-express.com/100-Hz-Low-Pass-8-Ohm-Crossover-266-446
It comes in an 8 ohm version and a 4 ohm version.
The sub will be a DML that will use either one or two 4 ohm exciters, so could end up being either 4 ohm or 8 ohm.
What would happen if I used the 8 ohm version of the filter, but used only a single 4 ohm exciter? Or if I used the 4 ohm version of the filter, but used it with two exciters (in series, for 8 ohms). Would either mismatch either not function quite right (how?), or result in damage to the amp, exciter, or filter?
Thanks,
Eric
https://www.parts-express.com/100-Hz-Low-Pass-8-Ohm-Crossover-266-446
It comes in an 8 ohm version and a 4 ohm version.
The sub will be a DML that will use either one or two 4 ohm exciters, so could end up being either 4 ohm or 8 ohm.
What would happen if I used the 8 ohm version of the filter, but used only a single 4 ohm exciter? Or if I used the 4 ohm version of the filter, but used it with two exciters (in series, for 8 ohms). Would either mismatch either not function quite right (how?), or result in damage to the amp, exciter, or filter?
Thanks,
Eric
Changing the ohm load seen by the filter will change the frequency of roll-off of the woofer.
Best match the ohm specifications. However, I don't predict any damage your amp etc.
Best match the ohm specifications. However, I don't predict any damage your amp etc.
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passive filter for such a low crossover is usually not ideal - i suppose you could get a cheap class D amp with active crossover for not much more money.
if you connect 4 ohms to an 8 ohm crossover the resulting slope will change and does not result in a "clean" filter anymore. the series inductor will provide attenunation at a lower frequency and the parallel cap will only "shunt" higher frequencies.
this results in a higher impedance than using a 4 ohm crossover - so you will probably not damage the amp.
the other option: connecting a 8 ohm speaker to a 4 ohm x-over may lower the impedance slightly from 8 ohms, so probably no problem either.
anyhow, at low frequencies the impedance of any driver is all over the place but not the nominal one, so i suppose any filter would not behave ideally anyways.
if you connect 4 ohms to an 8 ohm crossover the resulting slope will change and does not result in a "clean" filter anymore. the series inductor will provide attenunation at a lower frequency and the parallel cap will only "shunt" higher frequencies.
this results in a higher impedance than using a 4 ohm crossover - so you will probably not damage the amp.
the other option: connecting a 8 ohm speaker to a 4 ohm x-over may lower the impedance slightly from 8 ohms, so probably no problem either.
anyhow, at low frequencies the impedance of any driver is all over the place but not the nominal one, so i suppose any filter would not behave ideally anyways.
A passive filter is OK (that's not what OP asked). Finding the right response is important with any filter.
If you use this with a different impedance the frequency should be the same, the slope should be the same but the level at the knee can change. Since you are uncertain about the result to begin with, this might actually be a better thing, or not, but maybe you'll want to make adjustments.
If you use this with a different impedance the frequency should be the same, the slope should be the same but the level at the knee can change. Since you are uncertain about the result to begin with, this might actually be a better thing, or not, but maybe you'll want to make adjustments.
It really depends on the efficiency of the subwoofers compared to the satellite speakers. But considering the impedance is indeed a big deal as well. If you like to listen as loud as I do, you would be well served by bi amping and using an active crossover. This will allow you to easily match the subwoofer level and Fc response to your satellites. It will also give you more headroom and lower distortion. I always cross my subs over below 80Hz at better than 18 dB/Octave. But that is my personal opinion.
Thanks for all the responses. For less than 20 bucks each I guess I should just buy both the 4 ohm and 8 ohm versions and see which works out best.
Eric
Eric
If you did that you'd have two different values for each component which means 16 different combinations available for finding the better frequency and filter Q factor.. plus the ability to try third or fourth order.
Allen,
It's two possibilities for driver impedance and two filters. Four combos, not 16. Right? What am I missing?
Eric
It's two possibilities for driver impedance and two filters. Four combos, not 16. Right? What am I missing?
Eric
you are right, of course - that's not what the OP asked.
(good) passive filters for low frequencies tend to be expensive.
also, passive filters will be heavily affected by the changing impedance of a bass driver near resonance frequency (which may not be the case with a low resonance driver).
that's why I suggested to consider going active.
Since you mention these I don't agree that the quality of a passive filter is dependent on price. It is the configuration and values that matter.. I suspect "heavily" affected is an exaggeration of this natural effect, would you like discuss the level to which a filter is in control despite its load?
I did a simulation for a 100 Hz LR lowpass filter designed for an 8 ohm load, to see what would actually happen to the speaker signal and if (and how much) it actually is "heavily" 🙂 affected by the speaker impedance
1) with 8 ohm ideal load
2) with 4 ohm ideal load
3) with a loudspeaker impedance model (8 Ohm, Fs = 25 Hz, no enclosure/infinite baffle for simplicity)
attached the results and the circuit.
edit: I could do a impedance graph later, and a version with 4-ohm filter.
Attachments
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I think the regular solution is to develop the right filter. If someone did use a processor for that very reason, it's possible they're not interested in that level of accuracy. After all, the groundwork of measurement and calculation remains the same for either type of filter, so if you're going to that level anyway....
AllenB, I agree. but developing the right filter would probably mean to linearize the impedance near crossover frequency.
this can be quite a challenge around the bass resonance frequency.
that is what I wanted to point out with my simulation.
and I find it somewhat misleading to offer a passive 100 Hz generic low pass filter.
to veleric: i don't think any of the possible variants will damage amp, filter or speaker/exciter!
this can be quite a challenge around the bass resonance frequency.
that is what I wanted to point out with my simulation.
and I find it somewhat misleading to offer a passive 100 Hz generic low pass filter.
to veleric: i don't think any of the possible variants will damage amp, filter or speaker/exciter!
Let's not forget the insertion loss of passive filters. Nor how a low Fc crossover will affect the damping factor due to the series impedance. Perhaps I am just a perfectionist, but I gave up on using passive crossovers for subwoofers back in the 70's. While they will "work", they are a huge compromise in overall performance.
Allen, you are correct. But to do so likely requires more test equipment and knowledge than many novices possess. And to us that have made that investment, we have only discovered that some battles are not worth fighting. LOL
Doing any any crossover properly requires more test equipment and knowledge than many novices possess. 🙂
There is no insertion loss with this kind of filter. Also it seems you read more into damping factor than I do.
Allen, I am not aware of ANY passive filter that does not incur insertion loss. ALL inductors have DCR, with air being worse than ferrite. Resistance = Loss