Big 2-way

Dedicated to those large boxes at one end of the room
chris661
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#1 Big 2-way

Post by chris661 »

So, I've been using my PA system a while, which is a pair of 12" two-way tops and a pair of 15" subs and ~3.4kW of amplification, and have decided that its much too bulky - the majority of the time, I have to move the kit using small hatchback cars (think VW Polo).

I opened up the top speakers, and read around about the compression driver used. With its current horn, its good for 1kHz.
The sub drivers are Beyma 15P1200Nd, and will go to about 1.2kHz. They'll also go to 45Hz flat in a smallish ported box, or 50Hz with an absolute wallop.

Anyone see where this is going?

So far, I have one of the subs on its side with the compression driver on top (for the smallest centre-to-centre distance), playing around with the Behringer DCX crossover. Its a critical range, so integration isn't proving easy. There's definitely potential here, though.

Once I'm happy that I can integrate the drivers without having to resort to a dedicated midrange speaker, I'll look into building some cabinets. Probably trapezoidal and made from ply.

If all goes well, these might make it to a future Owston.

Photo of the try-it-and-see rig coming shortly.

Chris
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#2

Post by pre65 »

I look forward to following your progress young Chris. :)
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chris661
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#3

Post by chris661 »

Hopefully this'll whet your appetite, Phil.

Image

Currently I think I have an elliptical filter (that is, a deep notch filter with a shallow crossover, giving a different resultant slope) on the bass driver. It has some break-up problems in the 1.4kHz range, so the breakup is notched down to sensible levels.
Then I looked at the -6dB point and played with the compression driver crossover frequency (4th order Linkwitz-Riley, to ensure no out-of-band signal gets in) so that the CD -6dB point matched.
Then I ran the auto-align function on the DCX, which time-aligned the two drivers. Reversing the phase of one of the drivers shows a nice deep notch with my (very basic) measuring gear. It looks like I'm on to something.

The 15" is in a transmission-line cabinet that leaks quite a lot of midrange, so I've stuffed a pillow up it and have applied some bass boost, as a sort-of Linkwitz Transform.

I'm listening to music through it at the mo, and it sounds alright, but only alright. Good for a PA speaker (many of which sound damn awful), but not just good. I know these drivers can do better. All the parallel walls in the sub enclosure aren't helping.

Chris
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#4

Post by Scottmoose »

All the parallel walls in the sub enclosure aren't helping
If they're not properly damped they won't.
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#5

Post by chris661 »

The current amount of damping in the sub is zero: I was aiming to get the most LF gain I could out of it.
I'll shove a dressing gown in there around the driver and see how that does.


Fired up the Behringer DEQ this time. Gave the tweeter one channel and the woofer the other, put a condenser mic on the RTA input and pressed go.

Wooooooow.

Much, much better. The tweeter has a constant-directivity horn that (apparently) warrants some serious EQ to get the HF back.
Once done, though, it really has transformed the sound.

I need to drag this lot outdoors to do the measurements properly, and then transfer the EQ results into the DCX for safe-keeping: the DEQ has a nice parametric EQ that I keep for getting rid of feedback on the monitors.

Chris
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#6

Post by chris661 »

Next step:
Downloaded REW, sat a cardiod condenser (the datasheet shows flat from 200Hz up into the high kHz) in front of the speaker, and then played around.

The horn needed a touch of 4kHz boost and some other messing, and the woofer's breakup peak was at 1.6kHz, not fully attenuated by the LR4 crossover. A notch on the DCX got that.

Then I inverted one of the drivers and messed with the phase control until I got the deepest notch possible at the crossover point (now 1.3kHz). De-invert, and the crossover transition looks rather smooth on the graphs - a slight raise at the XO point, with a slight droop either side. Still some work to be done, then.

On a cursory listen, though, I'm incredibly impressed with how well this has worked. This is no longer a dirty PA system. This is approaching HiFi.

Chris
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#7

Post by IslandPink »

Good to see someone else getting stuck in !
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#8

Post by chris661 »

Playing around some more today, there was a 6dB rise around 3kHz that put a lot of "edge" on the vocals of Hello by Evanescence. 4kHz is still showing a notch of 10dB, but I'm hesitant to EQ that out, since it'd take a lot of headroom on the compression driver.

For laughs, I dialed in a serious bass-boost, of order 20dB or so, at 20Hz (shelving). Then played this:
Which I found on AVS forums, where the subwoofer maniacs hang out. There's content to 17Hz and below there. Its amazing how much air a long-throw 15" moves. I also noted that, while the woofer was blowing the pillow out of the transmission line, the midrange stayed clean. I have a lot of faith in these drivers.

Once the cabinets are built, I'll make the ports blockable, so that I can have bass down to 20Hz for home use, or really loud 50Hz for PA-mode.

Chris
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#9

Post by chris661 »

Well, had a little playing around and a lot of listening.

With a sensible amount of bass boost dialed in, I can say this is actually astonishing: real 20Hz-20kHz performance, with only one crossover slap-bang in the middle.
Dynamic range is basically unlimited: there's a lot of efficiency and a lot of power here. Even sub-20Hz stuff can be loud enough to put a tactile feel through the floor.

I think perhaps the crossover/horn could use a touch more messing around with, but this is a very satisfying sound, even in the transmission line.

Chris
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#10

Post by chris661 »

More listening.
There's more to be had here. I'm certain of it.

Playing around with REW some more, it turns out that it has an AutoEQ function. It'll even show a list of parametric EQs to dial in with the DCX2496.

First job tomorrow will be to get the horn flat from 1.3kHz up, and then the woofer below.

To make some attempt at evening out room effects, I'll do a few different mic positions with only a LR4 crossover on the drivers, then see what REW suggests. Until now, I'd been messing around with the RTA analyser and altering EQs on the DCX. This'll definitely take the guesswork out.

Chris
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#11

Post by chris661 »

Ordered a 2nd hand ECM8000 of eBay. Online reports suggest they have declined in quality recently, so I'm hoping this one will be an earlier run that's more in-line with what you'd expect.

That said, all the ECM graphs I've seen suggest they're very flat from 200Hz to 3kHz or so, which is plenty of room each side of the crossover region.
The mic will plug into my mixing desk, which then feeds into the soundcard and REW.
I'm going to measure the frequency response of the desk itself to make sure the EQs etc are at zero when they say they are.

Looking forward to getting into proper measurements.
Hopefully I'll be picking up a pair of EV Deltamax cabinets in the near future. They have a few at the Union, all with blown/missing drivers. The cabinet volume's about right, they're made of decent plywood, and covered in carpet with connectors, handles, etc. If the money's right, they'll be much cheaper than I could build them.
The horns are 1" throat entry, which means I'll be able to use my current compression drivers for the time being.


For cabinet bracing, I plan to buy a cheap piezo pickup off eBay - the sort that have sticky pads on them for guitars and the like. I'll attach the pickup to a panel, then I'll sweep the bass driver while looking at the pickup's output on REW, spotting panel resonances. I'm hoping a few well-placed bars of plywood (or whatever) will get everthing nice and rigid without having to do the OTT bracing seen elsewhere.

Standing waves will be a seperate issue, but I think the broom handles deployed in Dad's (brig001) coaxials did a rather good job, so I'll investigate similar if necessary. Front-to-back and top-to-bottom will be the ones to look out for, since the side walls aren't parallel.


I haven't made any changes so far to the set-up at the moment, but I'm going to have another mess-about tonight before it gets too late on.

That's all for now.

Chris

PS - if anyone's interested, I'll post up a few graphs of how its getting on.
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#12

Post by chris661 »

Well, that wasn't productive.

I decided to let REW try its digital hand at Auto-EQ. That is, it takes a look at a frequency response, and suggests settings for the DCX to get a near-flat response.

I put a different mic (first mistake - I sent the nice mics away for a gig) in the measuring position, threw a duvet over and started messing around.

Lifted the duvet up at the end to find the mic had shifted somewhat. No idea when that happened, but it might well explain the 3kHz nulls I was getting fairly consistently.

The REW auto-EQ results showed very quick-and-easy results for the woofer - a pair of parametric filters, one a couple of dB of cut, the other a couple of dB of boost.
The tweeter, however, was all over the shop.


Ahaaaa!!!!

Combined this evening's woofer results with an out-of-band notch at 1.6kHz to get the cone break-up, kept the previous-best tweeter settings (including manufacturer pre-sets) and things have definitely improved - vocals have taken a step forward, but the rest of the music is still there.

Wahey.

Can't wait for the proper measurement mic now - this mic is a vocal mic with a big presence boost. Not much use for frequency response measurements past 2kHz.

Happily, the woofer is showing to be very flat below 700Hz, so that's some work averted.

Chris

PS - can anyone recommend an image sharer? The forum file sizes are rather limiting.
PPS - there's an upper harmony on the chorus of Don't Marry Her by Beautiful South. Hadn't noticed that before.
PPPS - To quote DTB, Phhhhhwwwwwwwooooooaaaaarrrrrrr. This can actually do voices now. Listening to Bob Geldof's Sex, Age and Death album.
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Dave the bass
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#13

Post by Dave the bass »

I use PhotoBucket for image hosting. Its free. After uploading your pics it generates the IMG code so you can insert pics straight into the reply page on BB forums.

Like this...
Image

Ta dah

DTB
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#14

Post by chris661 »

Thanks for the heads-up, Dave. I'll have a play.


Compared the best settings I have so far with the Behringer monitors I have here, that have a known-flat frequency response. Immediately, there's something very obviously not up to scratch with the big 2-way. Funny how our ears adjust.

I think the off-axis response of the 15" towards 1kHz is letting the side down a bit, so I think I'll be making some specific measurements of the off-axis response, figure out what's going on there.
Hopefully 800Hz is possible with these drivers - that's the lower limit to the bigger compression drivers. Some CDs with 4" diaphrams can make it to 500Hz, but get ragged above 7kHz, so a 3-way would be needed. The other option for a 3-way is a 6-8" midrange driver, then a more dainty tweeter, but I really don't want to go there.

Chris

PS - test photo:
Image
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#15

Post by chris661 »

Okay, so here's a couple of graphs. Both of them have a LR4 filter around 1.2kHz. Firstly, the woofer on its own:

Image

Next, with a deep notch filter in place, to make sure the cone break-up isn't agitated. The break-up is even worse without the crossover filter in place.

Image


Chris
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