Monitor Sub Crossover Point

[lernith]

Hello!
I am building a stereo monitor power amp with an active crossover for a single sub. The monitors are Tannoy PBM 6.5 IIs ((±3dB) 55Hz - 20kHz (-10 dB @ 40Hz)). I have not yet selected a sub.
I'm mixing in a Mac with a MBox. The room is about 18' x 12' x 8'. TMI?
Most of the information I've found on crossover use seems like hifi foolery, please set me straight when something silly creeps in. One practice in particular is worrying me: setting things up, then sweeping the crossover point around to find the best point for the room.
I was planning on a 24dB/Octave Linkwitz-Riley crossover because it is phase coherent and has no peaks or dips.  The aggressive curve is supposed to lower the workload for the monitors, but it is not a sweepable design.  I thought at worst I could have a handful of crossover freqs built into several banks of filters, to be selected and bypassed as needed.  Each point would need five op amps and 20 caps/resistors, so it'd be do-able, but I'd feel pretty silly investing time, money, space, and power for a complicated crossover when 95% of rooms are happy at a 80Hz crossover point.
The thought turning over and over in my mind: 100Hz is supposed to be the approximate point of directionality, and I'd want to give the Tannoy's 55Hz rating some berth. So could steps between 65Hz and 100Hz really be that big of a deal?
Usually one can trust GroupDIY to cut to the heart of things so here I am.  Thanks!

 

[erikb1971]

Especially after your last words, I am a bit scared to chime in first... but even as a beginner I recognize a lot of words in your post looking like words I am working on at the moment, so here I go.

I am working an a 2.1 monitor set myself. For the mid / high speakers I decided to go for a pair of Audience A3 3" full rangers. I got a lot of tips on the sub-woofer and crossover points at a thread I started here: http://www.diyaudio.com/forums/subwoofers/198117-what-subwoofer-driver-go-audience-a3.html

About the crossover point: I am afraid to say it, but I bought a cheap adjustable crossover (for a different reason) from the company that shall not be named but starts with a B and ends on ehringer. I will use it to experiment with the crossover point to get some ideas. Then I will build something decent.

Also about the crossover: in another thread, especially John Roberts had a lot of interesting remarks on crossover filters. Ha no clue what he was talking about half of the time, but maybe it is valuable to you: http://www.groupdiy.com/index.php?topic=45282.0

And of course I am very curious about your sub amp... what ... how and so on.. I have not been able to find a decent diy sub amp, so please enlighten me!

Good luck and keep us posted!

Erik

 

[abbey road d enfer]

Hello!
I am building a stereo monitor power amp with an active crossover for a single sub. The monitors are Tannoy PBM 6.5 IIs ((±3dB) 55Hz - 20kHz (-10 dB @ 40Hz)). I have not yet selected a sub.
I'm mixing in a Mac with a MBox. The room is about 18' x 12' x 8'. TMI?
Most of the information I've found on crossover use seems like hifi foolery, please set me straight when something silly creeps in. One practice in particular is worrying me: setting things up, then sweeping the crossover point around to find the best point for the room.
I was planning on a 24dB/Octave Linkwitz-Riley crossover because it is phase coherent and has no peaks or dips.  The aggressive curve is supposed to lower the workload for the monitors, but it is not a sweepable design.  I thought at worst I could have a handful of crossover freqs built into several banks of filters, to be selected and bypassed as needed.  Each point would need five op amps and 20 caps/resistors, so it'd be do-able, but I'd feel pretty silly investing time, money, space, and power for a complicated crossover when 95% of rooms are happy at a 80Hz crossover point.
The thought turning over and over in my mind: 100Hz is supposed to be the approximate point of directionality, and I'd want to give the Tannoy's 55Hz rating some berth. So could steps between 65Hz and 100Hz really be that big of a deal?
Usually one can trust GroupDIY to cut to the heart of things so here I am.  Thanks!

It is not so much the frequency that is critical than the placement of the subwoofer and the consequential alignment necessity.
The subwoofer should be placed in a location that doesn't trigger too much resonances; this is done by sending LF program to the sub and listening or measuring. Obviously, the sub should be placed as close as possible to an identical distance to the listening position as the main speakers.
When this is done, the polarity of the sub vs. mains must be checked. Switching the sub in and ou of phase clearly indicates which position is correct; the wrong position sounds clearly wrong. Then is time for phase-alignment; you can use purely mechanical means, i.e. moving the sub to improve the response, which needs some measurement capability (and you will need to check if the sub is still in a position that doesn't make the room too boomy), or you can use an electronic circuit that will allow you to electronically "move" the sub.
This circuit is an all-pass filter.
With the actual values, this gives 90° phase rotation at ca. 100Hz.
You need to switch it in and out and decide which is the best position; it can be done auditively.
Now, if you want to be punctilious, you can build the APF with a 22-50k potentiometer intead of the 10k R. Then you'll need some measurement tool.
Since you're building your xover, adding the APF and a phase-reverse switch is trivial and you won't regret it.

 

[LeeYoo]

Hi,
I'm affraid, you won't get a very straight answer here.
Some things to consider.
Ported speakers already have a 3rd order roll-off, just below ? the port tuning frequency.
Forget LR 4th order at these frequencies. You will make a 6.5th order filter.
Speaker phase is all over the place at these frequencies.
Leave the main speakers as-is, or if cone excursion is a problem, just add a passive first order filter at the input of the main amp. (R/C).
Sub needs a low-pass filter, maybe 3rd order, IMHO near the main speaker's port frequency, and as said by "abby road d efender", add an adjustable all-pass.
All-pass to "tune" the phase of the sub to the main, so they work together instead of fighting each other, partially out of phase.
Just a 0/180 degrees switch is crude.
Then comes the art of placement and level ballancing.
Have fun.
Leo..

 

[abbey road d enfer]

Hi,
I'm affraid, you won't get a very straight answer here.
Some things to consider.
Ported speakers already have a 3rd order roll-off, just below ? the port tuning frequency.

  More like a 4th order. So has the subwoofer, probably, if it is also ported. So the phase shift of the two units will be more or less complementary, because the tuning frequencies are not very far apart, probably less than a half-octave.

Forget LR 4th order at these frequencies. You will make a 6.5th order filter.

It would be silly to crossover electronically at the box tuning frequency, because of this, but also because it wouldn't relieve the main speakers of the bass energy, which is one of the benefits of bi-amping. But crossing-over at 100Hz, this won't be a problem anymore. 

Speaker phase is all over the place at these frequencies.

When evaluating phase problems, you have to have a full-range view on it. If you compare on the same graph the phase response of the sub and the mains, if they are both of the same design, i.e. both bass-reflex, you will see that they are parallel, with probably less than 45° difference between them in the 80-125Hz area.

 

[lernith]

Thank you everyone for the posts! Good links and information to chase down and kill with a rock.

Stalking: Sweetwater recommends plugging the port of the Tannoys if they are used with a high-pass filter as this "can improve the mid-bass response". http://www.sweetwater.com/NearField/

This seems like one of those "it depends" issues.
Still, I have to build sometime, so for the prototype I'll have a single bypass-able crossover at 100Hz and an adjustable all pass filter to play with phase.

This will let me observe ported monitors and subs with their built in HPF and LPF, and compare with sealed, electronically cross'd monitors and subs.

Please post more comments and experiences, and I'll soon post PCBs, pics and outcomes.

Thanks again.

 

[abbey road d enfer]

Thank you everyone for the posts! Good links and information to chase down and kill with a rock.

Stalking: Sweetwater recommends plugging the port of the Tannoys if they are used with a high-pass filter as this "can improve the mid-bass response". http://www.sweetwater.com/NearField/

You can experiment with that. Don't expect spectacular differences (+/-1 dB at 100Hz).