VTF 2 Setup

[Ecruz]

i've got a VTF 2 with Magnepan MMG's in a stereo set-up. the sub is connected via the speaker level inputs run right off of the amp. the MMG's are suppose to go to about 53hz. i set up the sub using a sound level meter and i can get the response to be pretty smooth throughout the range, but no matter what i set the crossover to i'm getting a dip of about 8db at 50hz and a hump of about 8db at 30hz. i've moved the sub to several different locations and this is the smoothest response i can get, any suggestions?

[DavidD]

Assuming it is a positioning issue, try this. Put the sub at your listening position, and then measure the response at various positions around the room. The place with the smoothest response should be the location of your VTF 2.

[Lwang]

Did you play around with the phase setting? A dip near the crossover freq usually means there is some cancellation at that freq. Try flipping the switch and see if there is any increase in sound in that freq. If not, it is usually a room issue.

[Ecruz]

i'll try switching the phase and if that doesn't work i'll try the other suggestion.

thanks,
E-

[DavidD]

Good decision. I think the phase suggestion is more likely to be the problem. I wish I'd thought of it.

[Ecruz]

once i looked, i realized that i had the phase switched to 180. i switched it to 0 and 50hz dropped even more. i guess i'm going to have to try moving it around the room.

[DavidD]

Regrding positioning, I suggest you consult Dr. Hsu for suggestions. Send a diagram of your room, including dimensions. I wonder if there is something in the Magnepan radiation patterns which make positioning more difficult. Dr. Hsu should have some insights there.

[Lwang]

You should not rely on the marking of the crossover dial to set your xo freq. Those VCA type crossovers are not very accurate.

You should measure the sub's upper end freq resp (w/o the MMG) and determine at what point it ocmplements the MMG's rolloff. And the adjust the level so that the -3dB point of the MMG roughly intersects the -6dB point of the subwoofer. Then play both sub and MMG and play with the phase switch to see which one has the least dropoff.


If you are going to do the walkaround in the room, you should turn off the amp to your MMG so that you only hear the sound of the sub.

[Ecruz]

i did send Dr Hsu a diagram of the room when i first ordered the sub and it's placed in the spot he recommended.

Lwang-
all of my measurements have been with both the VTF 2 and the MMG's. i'll try your suggestion of measureing them seperately and see if i can get a smoother response.

once again, thanks and i'll let you know how it works.

[Ecruz]

ok...measured seperately.
the MMG's were 75db @ 80hz, 75db @ 63hz and 62db @ 50hz. so they clearly drop off between 63 and 50.

the smoothest i could get the VTF 2 was 62db @ 63hz, 67db @ 50hz, 77db @ 40hz, 77db @ 30hz and 73db @ 25hz (if i set it to 75db @ 40hz, then 50hz dropped to 63db).

measured together phase set to 0.
75db @ 80hz, 73db @ 63hz, 65 db @ 50 hz, 77 db @ 40 hz, 77db @ 30hz and 73db @ 25hz

measured together phase set to 180.
75db @ 80hz, 76db @ 63hz, 68db @ 50hz, 77db @ 40hz, 77db @ 30hz and 73db @ 25hz.

so there is still a dip around the crossover. but now i'm thinking it must be the room, since i couldn't get it smoothed out even when i was just running the VTF 2.

any other thoughts?

just for reference, i'm using a Radio Shack Digital Sound Meter mounted on a tripod at the listening position.

thanks for all of the help,
E-

[DavidD]

With the exception of the crossover, this looks pretty good. I think this must be the room, somehow. One questions however... are you using an external crossover, or a receiver with a crossover? If so, what is it set to?

[DavidD]

Never mind - you already said you were just using speaker level inputs into the sub. One more thing to try (just for fun), run the sub only, and measure the response with the subwoofer crossover switched out. Does the response change?

[Lwang]

You are probably have the sub's crossover freq set too low. By plotting the numbers into a graph, the sub and your MMG crosses each other at 54hz, roughly at their -10dB point. If you want to have a smooth transition, it would probably be best if you have them cross one of the 3 ways:

both at their -3dB point
MMG at its -3dB point, VTF-2 at its -6dB point
both at their -6dB point

I would assume option one or option 2 would be the right way, but given that the MMG drops off at close to 24dB/oct, it looks like a 4th order dropoff, but at the same time, the summation of the sub and MMG does not arise to 6dB increase in volume (more like 3dB), leading me to assume that it is not a in-phase summation.

Here is chart in non freq log scale. Note, how the freq the sub & MMG crosses over (red dot, @ -10dB) results in summed output (green dot, @ -7dB, for a 3dB increase over the individual levels)

[Ecruz]

DavidD-
i will try measuring only the sub with the crossover switched out.

Lwang-
that's all a bit over my head. but i did understand "you probably have the sub's crossover freq set too low", so i will try setting it higher and re-measure.

i really appreciate all of the help. i'll keep you guys posted.

[Lwang]

The -3dB point for the MMG is at roughly 60hz, while the -3dB point for the sub is around 42hz (the 4th order xo is usually defined by its -6dB point (defined here as at 46hz), but since you don't have a L-R 4th order HP, it is pointless to match their -6dB point).

So turn the xo freq roughly 20hz up (I am using relative numbers instead of absolute, since the indicators on the knob are usually not very accurate) and see if they overlap at roughly 60hz, at their -3dB point.

[Ecruz]

first i tried setting the crossover higher. even turning it all the way to the max, didn't increase the 50hz level. it did boost 63hz and 80hz. 50hz dropped even lower when i set the phase to 0.

then i turned the maggies off and switched the crossover out. the same thing happened, 50hz didn't increase, but everything above it did.

much to my wife's dismay, i think i'm going to start moving the sub around the room to see if i can get a smoother response.

again, i really appreciate all of the help.

E-

[Lwang]

Probably the best thing would be to inquire with Dr. Hsu regarding the layout you sent him and the room null you are encountering. Be sure that your seating position is on the diagram since that will also affect bass eveness.

You could keep the sub in the current location and measure 1 ft in front and behind the current listening position and see if there is a change in the room null. When measuring at your current seating position, be sure the mic is at your head position instead of being in front of you.

[Chris A H]

Lots of threads in the Home Theater forum speaker section talk about how to achieve flat room response. Most of them eventually recommend getting a Behringer Feedback Destroyer (BFD) to remove stubborn dips.

See one such thread here. (http://www.hometheaterforum.com/htforum/showthread.php?s=&threadid=170688&highlight=bfd)

[Lwang]

I think it is repeated in all forums that you can't correct dips in the room response. You are only pumping more power into a accoustic null. To correct that 10dB dip, you would need 10x the power, turn your VTF-2 into a 15 watt sub in the 50hz region.

[Ddavidson]

Eq 's are only good for cutting and even then you have to be careful of the phase problems you can introduce by incorrect use. That is why doing corrections while the signal is still in the digital domain is prefered. The B&K surround Ref 50 pre-amp and receiver has a good inbuilt eq that can help. The BFD also works but both can not solve really bad positioning or really bad room modes. Run the test tone and see if you can pick up a little of the null by moving your "listening seat" a little.

Even try a slightly higher and lower mic position to see if you can pick up a bit of the null. I put a riser under my main seat so I could pick up a little of the slight null around 75Hz. Of course nulls are quite hard without dimension change. Where peaks can even be tuned out with a specially tuned bass traps if they are too strong. When I made my bass trap I used the excellent Master Handbook of Acoustics by F. Alton Everest. It has lots of great info on room problems and solutions and is available at Amazon.

Ddavidson

[Sasha_G]

This piece of info might help: the markings on the old crossover dials were not as accurate as the new ones on the STF series. So, consider the markings as only approximate.

At work, we realize that EQ is not a panacea--it won't solve everything. You still should find a place with relatively even response for the subwoofer. The EQ should only be used to remove peaks, so if you have to decide on a position pick one with peaks, not dips. The so called "subwoofer crawling" method works great--put the sub in the listening position and move about the room with a meter or your ears.

If you start using extreem EQ, it can cause more problems than it solves. The EQ works at the sweet spot, but with extreem EQ moving your head a foot could result in worse sound than no EQ at all.

[Ecruz]

ok, i finally had some time to mess with this again.

i moved the sub to the opposite corner on the same(front) wall and didn't change any settings. here are the results:

75db @ 80hz, 77db @ 63hz, 73db @ 50hz(the best so far), 77db @ 40hz, 83db @ 30hz and 76db @ 25hz (note that 76db @ 25hz is what the meter showed but then you have to add 4db per the manual that came with the VTF 2). moving the sub up to about a foot in and out of the corner didn't change the 30hz or 25hz at all.

so now the crossover point is smooth but there is an increase @ 30hz and 25hz.

i also took measurements one foot in front/behind of the listening position. in front was pretty much the same. as i moved farther behind, everything got lower from 50hz on down.

where do i go from here?