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Old 28th April 2003, 08:52 AM   #1
Kanga is offline Kanga  Australia
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Default Turning a 2 way into a 3 way with biamping

I have recently almost finished my project to build a bass speaker to add-on to my existing 2 way mini monitors, the Acoustic Energy AE1. The AE1s have some wonderful qualities, but going down low is not one of them. They were about 3dB down at 63 Hz, falling very rapidly below that.
Having been inspired by Rod Elliotís article on biamping, I decided to turn my 2 way into 3 ways with an extra amp to power the bass speaker and an active XO. Last summer holidays I built an AKSA 100W amp for the bass speakers, and this has been sitting around not doing much until now, waiting for me to build the speakers.

The drivers I used were Scan Speak 25W8565-01 units, which were one of the few I found that could give reasonably low F3 in a sealed enclosure. They are mounted in a 25mm wall MDF box with an internal volume of around 50 litres, giving a Qt without stuffing of 0.8. The internal dimensions of the enclosure are roughly in the golden ratio to avoid standing waves occuring at the same frequencies, not that it is that important in a bass cabinet, but just in case I even want to cross higher. There is a shelf brace fitted at the level of the bottom of the driver, and an extra MDF brace connecting the side walls below this.

Mounting of the driver was unexpectedly difficult. I bought M5 cap screws to suit driver holes, but unfortunately you canít get M5 tee-nuts. Trying to glue the M5 nuts securely on the inside of the box was an exercise in futility, so in the end I used wood screws. I have some black painted ones for the final mounting, to match the drivers.

Aesthetics are provided by Australian silky Oak veneer, which has millions of lovely eyes that look back at you. I gave the veneer two coats of shellac to seal it, then painted the veneer and the box with slightly thinned down PVA glue and let them dry. Then I ironed on each piece. Edge trimming was down with a router with a bearing guided bit. This wasnít the best choice, as I had some tearing out when cutting across the grain. I think using a sharp trimming knife or veneer saw would have been better. Overall this technique worked pretty well, although I tried to iron on one of the top pieces to the wrong cabinet that had not been painted with glue, and of course it didnít stick properly. Final finishing was down with a grain filler coat, and then a Cabots gel polyurethane. I didnít think much of the polyurethane Ė brush marks were lift behind, and the resultant finish is not very hard and easily scratched. You can see in the photo where the front got scratched when I used the circle cutting router template. Iíll have to sand this and put down another coat.

I liked the look of the Dynaudio audience cabinets and so the black plinths are inspired by them. There is a 19mm layer of MDF, then 3mm felt impregnated with car noise deadner, and then 25mm MDF at the bottom. The plan was that vibrations get damped by the felt/bitumen on their way to the floor. Did it work Ė who knows (have to get that accelerometerÖ). There are spikes that attach to the bottom too, but these arenít on yet.
Access to the inside of the cabinet is possible by removing the plinths, which screw up into the enclosure walls. This means I can mess around with stuffing etc in the area below the shelf brace.

For internal stuffing I plan to fill the box with normal fibreglass insulation, with Dacron just behind the drivers.

Modifying my Cyrus 3 amp to allow pre-power connections was a bit beyond me, so I got a professional place to do this. The active XO is a 12 dB/Oct built from an ESP (Rod Elliot) board with the matching ESP power supply. XO frequency is 300 Hz Ė not sure whether this is the best place for it, but its not that easy to change, with 8 resistors or caps needing to be changed to change the frequency.

So how does it sound?:
The extra bass extension has been a little bit disappointing, given the new boxes are about 6 times bigger than the AE1s. Although near field the F3 seems to be 40 Hz, at the listening position its more like about 50 Hz at the moment. Maybe this can be improved by changing the speaker and listener positioning. Iím also contemplating building a Linkwitz transform circuit to see what difference a lower response will make.
Having said that, the bass is much more authoritative, and is a big improvement. The Reference Recording of the Rite of Spring is a great test, with a bass drum that drowns out the orchestra sounding absolutely fantastic. The improvement from the extra bass is most noticeable in orchestral pieces, compared to jazz and similar. I donít listen to a lot of pop music with heavy bass, so I canít comment there.
The midrange also sounds clearer, with a more spacious sound. Maybe I lost a bit of accuracy in the imaging, but Iím not sure on that one.
I donít want to comment too much on the sound at this point, as I was without the amp for around a week, and have changed the speaker positioning, and the stand for the AE1s, so comparisons are a little difficult. I will be setting things up in the next week or so to allow me to swap easily from the AE1s full range to the biamped setup, so much better comparisons will be possible then.

Mick
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Old 28th April 2003, 08:55 AM   #2
Kanga is offline Kanga  Australia
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...and here's the back view, showing the speakon connectors that I used - very neat.
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Old 28th April 2003, 09:18 AM   #3
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Default Sealed vs ported.

Hi Mick,
I notice that your subs are below your AE-1. Ducted enclosures
( bass reflex) tend to produce deeper bass away from the walls than sealed boxes ( same driver). But your same sealed sub would get a huge bass lift if located along a wall . The location along a wall or in a corner or thereabouts will provide a bass lift starting at some low frequency depending on your room dimensions. You can very easily get a 6db lift about 50 Hz. Then if your sub is properly matched ( the Q and F3 ) the bass can go down very low with a sealed box. Reflex boxes designed to be reasonably flat will be OK away from the walls ( and tuned properly) but could then tend to boom if you place them near the walls.
What I am saying is that if you want to use the sub as shown , you should try out the reflex enclosure also. It might give you the deeper bass you want.
Let me see if I can check the response of your sub in the same enclosure with a duct.
Cheers.
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Old 28th April 2003, 11:06 AM   #4
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Default Sealed box is right.

I checked the SS driver. It is suited for sealed box use.
The ideal box size for a Qtc of 0.707 is about 54 lit. So you are about right without stuffing. I modelled the box with a 1.5 db rise at 50 Hz when kept closer to a side wall. You can see that the sub will go much lower without the use of any bass boost.
Did you use the 3way crossover ? To use the 3 way facility you would need to connect the drivers in the AE1 directly !
In any case it is not difficult to modify the 3 way for a different crossover frequency. In fact to make it a two way Linkwitz Riley 12 dB per octave unit it is pretty easy.
Your 300Hz crossover is too high. You should crossover maybe at about 90Hz. What are your current component values? You can keep the capacitors the same and change the resistors only. At least 4 resistors are just removed !
Have a look at my attached document. The AE1 frequency response is also shown. It is flat to about 65 Hz and drops off very rapidly. Subs are generally not expected to work very much above 100Hz. The idea is also to try and keep the vocal range out of it . 300 Hz is just not acceptable for a sub. You 'could' end up with a lot of problems.
Cheers.
The zipped file mentions some opamps. The OPA826 is wrong. It is AD826 !
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Old 28th April 2003, 04:51 PM   #5
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Kanga,

mail me if you want some M5 t-nuts..

Rob
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Old 28th April 2003, 10:25 PM   #6
Kanga is offline Kanga  Australia
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Thanks for your comments and suggestions Ashok

With all the work that went into the current boxes I can assure you I'm not about to replace them with much larger reflex boxes!! I went sealed because of simplicity, and also because of better group delay/impulse response. Building a linkwitz transform will be quite easy and cheap, and adjustable too.

The AE1 keeps the passive XO it was born with. I thought about 3 way biamping, but I've decided that next step is building new mid/high speakers, possibly biamping these, and then using the AE1s for something else.

My design is not so much having stereo subs, but creating a 3 way loudspeaker, so I don't think that a 300 Hz XO frequency is unusual in this case. My reason for selecting this frequency is:
*Balance the amp load better between the bass and mid/high amps. Before biamping the 50W cyrus was a little small for the very inefficient AE1s.
*Get the XO frequency well away from the natural roll off of the AE1s, and thereby avoid phase problems.
*By letting the bass speakers handle frequencies up to 300 Hz, the AE1s have less work to do and can have cleaner midrange - this seems to have worked.

I'd like to keep the AE1s on the bass boxes, as they are good stands! I've been meaning to buy the AE1s good stands for a while now, and so this was another benefit in building the bass boxes.

Re the active XO - I used OPA 2134 op amps.

Rob - nice to know that M5 tee nuts exist - just not in Australia obviously. Thanks for the info, but I think that I'll stay with the screws - they seem to be OK.

Mick
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Old 28th April 2003, 11:08 PM   #7
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Default scan-speak's

The bass augumentation you tried is just what I've been working on. (not done)
the only comment I have on X-over is 1: you only need to unsolder resistors, not caps. They'll come out easily.
2: a crossover around 200 gives you a chance to adjust the floor notch; two different path lengths because of two different drivers.

Also, since your long winter is coming-This thought.
I have heard that particular SS in approximately 2.3 ft# with a 10" passive radiator (stereo subs)& the sound was awesome, tight, all kinds of good adjectives. We listened to Japanese ensemble drummers, and I was there! I don't expect to ever hear better bass! This driver is highly recommended.
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Old 29th April 2003, 01:12 AM   #8
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You should try stuffing the ports on the AE-1s.

And i agree with the others that your XO is probably too high. I'd bring the XO down an octave -- 150 Hz.

dave
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Old 29th April 2003, 02:02 AM   #9
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Quote:
Originally posted by planet10
You should try stuffing the ports on the AE-1s.

And i agree with the others that your XO is probably too high. I'd bring the XO down an octave -- 150 Hz.

dave
I agree with Dave on both counts.

Whenever my main speakers were down for update or service, I would use a pair of Polk 4 mini-monitors in a bi-amp mode as you have discribed. I would stuff the port of my little 2-way mini-monitors because I found I had a hole in the bass response if I didn't. I can only assume that port output was out of phase with the subwoofer output and cancelled.

I would never xover to a sub much above 100Hz and usually 90Hz or below with a steep lo-pass filter.

By blocking your port and crossing over in the 70 to 80Hz range, you may even be able to get by without a hi-pass on the mini-monitor.

Rodd Yamashita
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Old 29th April 2003, 02:27 AM   #10
Kanga is offline Kanga  Australia
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Dave - I have tried briefly stuffing the ports with socks, but the inside of them is fairly rough, so the socks didn't want to go in. I'll try to get some better material and have another go. Squash balls may almost be the right size...

Comments on XO frequency appreciated - I might try 200 Hz, so that I'm still well away from the natural AE1 rolloff, and try for a bit of a peak to address the floor bounce dip. I suspect that the peak will be too broad to counter the sharp floor bounce dip.

Rod - I don't think that the 200 Hz dip was due to port output, as I could make the problem go away by turning the bass cabinets upside down, so that the driver was near the floor. If I were to close the ports and use natural rolloff of the AE1s, what are the phase reponses of the L-R 12 dB/Oct low pass active and a sealed cabinet, and will they match properly?

Mick
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