Subwoofer crossover questions..... (1 Viewer)

[Jason Wilcox]

I'm a little confused with subwoofer crossovers.

1. A low pass filter sends everything below a certain frequency to the driver and a high pass sends everything above a certain frequency to a driver right?

2. When using the line level inputs on a sub the crossover on a subwoofer acts as a low pass crossover ( according to my definition ) right?

3. ( here's where I'm really confused ) I had always thought that when you use the speaker level inputs on a subwoofer the crossover on the sub would act as a low pass for the subwoofer and a high pass for the other speakers depending on what you set the frequency as. But, I've seen a lot of subs with their high pass frequencies set in stone while the low pass can be changed. ...help?

 

[Dustin B]

Nope it appears you've got it. Most sub plate amps have a fixed highpass around 120hz and a variable low pass. However, there are plate amps available to DIYers where both are adjustable and some higher end subs have the same type of feature.

 

[Jason Wilcox]

Oh ok Thanks.

Why 120hz though? I know a sub should be below 100hz to avoid directional sound. So if I did set the sub to 100hz there would be a dip in db between 100 and 120 right?

 

[Dustin B]

I don't understand why they do that either. Doesn't make any sense to me for exactly what you just said.

Anybody else know?

 

[Jason Wilcox]

hey how bout this........since the sub and mains would be producing the frequencies between 100hz and 120hz....the combined volume in db of the sub and mains in that 20 hz region would be closer to that of the rest of the frequency range....2 speakers creating the same frequency is equal to some boost in db....i'm not sure what the number is....6db i think....of course this all depends on how steep the crossover is...

 

[Jason Wilcox]

wait what am i saying...that involves the sub producing sound above 100hz....thus being directional.........

 

[ling_w]

Those fixed freq high pass filters that are on some plate amp subs are useless since it has no knowledge of the input impedence of the next stage that it is feeding to. A passive high pass would have to include that information as part of its complete circuit. 99% of the time, they will probably use a cheap electrolytic or ceramic capacitor.

If you are hooking a sub to a 2ch system, skip the sub's high pass crossover and run the mains full range. If you don't want that, get a active high-pass filter (not a cheap passive one) and let that set the crossover freq accurately.

I think Hsu's Bass optimizer could be custom configured to become a high pass filter, with 2 custom frequencies selectable by a toggle switch.

 

[Jason Wilcox]

ok.....so to clarify....when i see a sub with a single adjustable crossover knob.....this knob only acts as a low pass filter for the subwoofer....it doesn't double as a high pass filter for the speakers right?

 

[Jason Wilcox]

anyone?

 

[Dustin B]

Correct. But there still is a high pass filter for the left and right speakers. It's just fixed, set rather high, and crap.

 

[Chris PC]

No it does not double as a high pass. That sucks. If your subwoofer has an input that does not go through the crossover, then you could try that and get an outboard crossover. Mirage used to make a nice crossover that was adjustable from 50-100 hz for both the high pass for your speakers and the low pass for your subwoofer but you needed to insert the crossover between a pre-amp and amp, or in the main-out main-in loop of a receiver that has those connections.

 

[Jason Wilcox]

so the sub takes the frequencies below the crossover....but also passes the same ones on to the main speakers?

sorry if this seems like i keep asking the same question...i'm just surprised by all this

 

[Chris PC]

The subwoofers adjustable crossover, the actual knob on the subwoofer that you turn yourself to adjust, that is used as a low-pass, to pass only freq below the selected point on to the subwoofer. If you are using speaker wire, then the subwoofer probably has a highpass crossover in the circuit for the main speakers. You connect the speaker wire to the subwoofer, which strips away the low bass and sends it to the sub, and then the signal which still contains the full range, it is filtered before being sent out to the main speakers, and this xover is usually between 80 and 120 hz. The best being 80 hz.

You are surprised because all of this is counter intuitive, as is the crappy fixed crossovers that are in most receivers, which we all complain about. When using crossovers for a subwoofer, there should be an adjustable control for both the subwoofers low pass and the main front speakers high pass. Thing is, we are now connecting pre-amp level signals to the subwoofer, so the crossover for the main speakers has migrated to the receiver, but now its been relagated to crappy frequencies to accomodate people with tiny front speakers. So receivers end up with crossovers like 90 and 100 hz (Yamaha, Technics, Marantz)when a fixed 80hz (in all THX receivers, better but not the best) would at least be reasonable. The main front speakers high pass doesn't even need to be an infinitely variable control. A simple choice of 50,60,70,80 or 100 and 120 hz would cover everybody (Many Sony receivers have quite good crossovers, as do the Outlaw 1050 receiver and 950 preamp-processor).

Welcome to bass management. It means you do your best and compromise if you don't have the crossovers you want and then eventually get a receiver with the proper crossover, OR you can use a pre-amp and amp, and insert your own crossover between them (or in the main-out/main-in loop of a receiver that has such a feature).

What receiver do you have? If you connect your subwoofer to your receiver, you can still use the crossover on the subwoofer for the subs low pass, but you may as well try the SMALL setting on your receiver, this is what acts as a high-pass crossover for the front main speakers, taking the bass out of their signal.

So what receiver do you have?

 

[Dustin B]

Jason, I suggest you give this a read. Should help your understanding of crossovers a lot.
http://www.hometheaterforum.com/htfo...602#post523602
The slope of the crossover is a bigger problem than the point at which the crossover occurs. I'd personally like a 4th order in both directions at 80hz. But THX receivers have a 4th order for the sub, and a 2nd order for the speakers (THX relies on the speakers haveing a 2nd order roll off at 80hz so when combined you get a 4th order). But most receivers use a 2nd order crossover (including mine) in both directions which isn't steep enough.

 

[Jason Wilcox]

i've got a marantz 2235B stereo receiver.

what i don't really understand is....if the sub takes the frequencies below say...100hz (with the slope of course)....how could it send the mains the frequencies below 100hz when the sub has already taken them?

oh yeah....thanks for the replies!

 

[ling_w]

what i don't really understand is....if the sub takes the frequencies below say...100hz (with the slope of course)....how could it send the mains the frequencies below 100hz when the sub has already taken them?

This isn't like owning a car, where if you take it from the garage, it isn't there anymore. The sub could take a copy of the freq below 100hz. Electricity could be duplicated, summed, subtracted in various ways that doesn't apply to the objects in the physical world.

 

[Jason Wilcox]

oh ok...thanks

think i've got it all now

 

[Chris PC]

Exactly. When I said the subwoofer low pass "strips away the bass" and I refered to the high pass filter acting on the signal that still had the full range signal including the bass, I should have said, the bass copied the signal before filtering it. Bascially, when you connect your speaker wires to the subwoofer, the signal is split equally, one full range signal goes to the low pass crossover within the subwoofer, before going to the sub amp, while the other full range signal goes to the high pass filter which removes the bass before sending it to the output which you connect your front main speakers to.
The problem is one of convenience. Really, what they should do is simply make it like an auto and manual camera. If they don't want to make it too complicated for general purpose non-audiophile owners, they could have the receiver with a default setup of 80 hz with whatever the most universally useful crossover slopes would be sensible. But for those who want to tinker, they could have a manual setting where we choose a crossover for the sub AND a crossover for the front mains, from say 50,60,70, 80 hz or 100,120 hz, and then also choose the slope and we can fine tune it. I think people should get a product and learn how it works. People are not unable to learn. I think the reason we don't have highly adjustable and flexible crossovers is because our rooms have lots of variability, and because good flexible crossovers cost more.
This is all very important because receiver, speaker and subwoofer companies should realize that we as consumers know that a full range speaker works well because its built together, with the crossovers working in a predictable way. Once you try to take the low bass out and you have to play with crossovers between speakers and subwoofers from different companies, and not to mention that the subwoofer is not always in the same place in each persons room, these companies should start making better crossovers like those in the Sony and Outlaw receivers and pre-pro's. Marantz, Yamaha, Denon, Technics and Panasonic and NAD should listen to the demand for having full proper crossover adjustments available on their receivers. How can they think that a single preset crossover frequency with preset slopes will work for an infinite number of different subwoofers and different speakers in different rooms? It doesn't. Thank-you for reading installment # 17 of my rant about crappy receiver crossovers

 

[Jason Wilcox]

wow thanks again....i'll just hook up the sub with normal interconnects out of the receivers pre outs and set the sub's crossover to around 95hz as the speakers start to roll off fast below 100hz.

koss m/80+ speakers
yamaha yst-sw105 sub

 

[Chris PC]

Enjoy your subwoofer