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Class D Switching Power Amplifiers and Power D/A conversion

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Old 7th June 2008, 01:59 PM   #111
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Yeah, I'd also go for front facing speakers until you get an extra set (next year perhaps ).

Oh, and we must set up a meeting with all those making their own boomboxes. That could be fun. Maybe even have some sort of competition based on originality, design, loudness, quality of sound, battery consumption and bling value.

I'd volenteer as a judge.
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Old 8th June 2008, 07:52 AM   #112
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Sure! That's a good idea. And if such a contest will never surface, you're always welcome in my camp for a beer or ten :-)

I'm currently considering using my [feriepenge] (which I didn't know I'd get) for two extra speakers and tweeters... I'm so bad at keeping money for hard times
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Old 11th June 2008, 02:17 AM   #113
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Sorry for hijacking, but this thread seemed to be the best match at the moment. Please move this post if I'm mistaken.

I also want to build a small but powerful boombox, smaller than the Boominator though. I'm aming for max 10-15 kg (22-33 lbs) with amp and batteries, and with a smaller volume, preferably with no side longer than 60 cm (2 ft), say 60x30x30 cm (24" x 12" x 12") or smaller.

I'll be playing mainly dance music like techno, outdoors. Fwiw, I have very little experience with PA and speakers, only built a pair of rear loaded horns by following plans.

I've been googling like crazy for any previous work but haven't found anything that really suits me.

Some questions:

1) Is there any real point in going stereo?

2) For efficency, shouldn't I make a folded horn/RLH, or even one of them tapped horns? Or just stick with a simple reflex box?

3) If I try for a folded horn, are there any small horn designs to look at (I haven't found any)? If not, can you simply down-scale a bigger horn?

4) Advice I've found says "don't aim for below 100hz" (this Boominator thread), other say "10 W will do it". Does this sound ok for dance music as well? I've found music where lyrics are significant are easier to amplify into some agreeable form. Techno is usually instrumental.

5) One "big" (I was thinking maybe 6.5") woofer or many small? And whats the deal with back-to-back magnet mounting? Why does that make it more efficient?
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Old 11th June 2008, 06:09 AM   #114
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Quote:
I also want to build a small but powerful boombox, smaller than the Boominator though. I'm aming for max 10-15 kg (22-33 lbs) with amp and batteries, and with a smaller volume, preferably with no side longer than 60 cm (2 ft), say 60x30x30 cm (24" x 12" x 12") or smaller.
If you manage to find a very slim (5cm wide) 12V battery your size approximations are OK. Then the setup could be like this (seen from the top and using 10" speakers).

Code:
|   amp    battery| 5cm
|speakerA speakerB| 5cm
|speakerA speakerB| 5cm
   25cm     25cm
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Old 11th June 2008, 10:29 AM   #115
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@Janoch

The magnet to magnet construction focuses the magnet fields in the driver and gives a slight increase in sensitivity of roughly ½dB per driver, resulting in a 1dB sensitivity increase per speaker pair ... that may not sound like a lot but 1dB is 25% more output for the same power input.

But that's actually just one of the benefits. When used outdoors the back mounted speakers will augment the front mounted speakers output with an natural acoustic filter. This works in the Boominator because I have carefully choosen the drivers and cabinet tuning to function as a acoustic filter in themselves that is the reverse of the first mentioned effect. Effectively giving an almost flat frequency response down to 90 Hz in free field conditions.

The point here is that the bipolar construction, the drivers chosen and the cabinet tuning is all chosen carefully to shrink the cabinet as much as possible and still maintain a linear output as deep as possible.

The third major reason is that the drivers themselves are a major part of the cabinet construction. I use 12mm plywood which in selected places internally is routed down to 8mm thickness to cut down weight, and it would be impossible to use if it weren't for the drivers actually holding the cabinet together, and not the cabinet holding the drivers.

You see I use a center brace and then mount the driver loosely with screws on top of a foam gasket on each front plate, then after having tested the connections and that everything works, I apply a high vicious slow setting two-component glue to the sides of the magnets and around the drivers at the front. Then use vices to force the front plates the last 4 mm in pressing the foam gaskets flat and forcing the glue to set. Then I screw the whole thing together and wait 24 hours for the glue to set. Now each driver is glued to the center brace and the front, as well as pressed together internally.

This is an extremely solid construction that is derived from formula one cars where the chassis and suspension is mounted on the motor and not the other way around and extremely light weight construction which is impossible without the magnet to magnet bipolar construction technique.

I should also note that the 4 10" drivers themselves make up 14 kg, the 2 SLA batteries another 5 kg, so the entire cabinet with solar cell, amplifier and everything else weighs just under 7 kg. You could cut some wieght by using ni-mh or li-ion batteries instead though but it won't make a huge difference except in price.

Now for your other questions;

1) Not really but why not, is there any point in going mono?

2 and 3) Drop the horn idea! The cabinet size and the resulting horn mouth size would dictate that the horn only amplified sound over 400-500 Hz.

4) 100 Hz is nonesense in free field conditions. You'd need multiple 15" drivers in large cabinets to get any output lower than that. And yes, depending on sensitivity of the drivers, yes, 10W is more than enough.

As a note, I play a lot of techno music, and have several friends that make that kind of music and you'll quickly find that it has absolutely no deep bass, almost all the bass output in techno is between 100 and 200 Hz. And the deep bass there is only distortion effects from being to heavily compressed.

Hope this was some help.
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Old 11th June 2008, 01:17 PM   #116
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Thanks, it's great help! I might aim for a scaled-down Boominator after all.

1) I just thought one less channel = one less amp, at least it should be cheaper. Since the dimensions are so small, and it's outdoors, will anybody be able to hear the stereo effect? Won't it just cause some phase cancellations? How do you point the speakers left and right?

4) You said you tuned the Boominator for 82 hz. How low does it go?

Some additional q's:

6) One last q about bipolar: you invert the signals right? So that when the "front" speaker cone goes towards you, the rear one also goes towards you? And this means one pair of magnet-to-magnet speakers is in two separate reflex enclosures?

7) What do you guys think about using 4 full-range woofers, and dropping the piezos? I'm not aiming for the same level of professionalism in sound as the Boominator, low weight/small size is more important to me.

8) I also thought about making folding "barn doors" (on hinges) for the woofers, to extend bass. Could this work at all?
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Old 11th June 2008, 01:51 PM   #117
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1) You'd have to find another mono amp than my amp6basic since that's a 2 two-channel amp (stereo), and that actually might be more expensive to find.

4) The cabinet is tuned to 0 dB at 82 Hz which results in a free field -3 dB at 91 Hz, so roughly 90 Hz after which it rolls off 36 dB/octave due to both acoustic and electronic filtering.

6) No. That would be dipolar where the output from the back cancels out the output from the front, doing this would result in practically no bass output at all. And the each driver pair is "one side" left or right, do not under any circumstances connect left and right outputs to two drivers mounted in the same enclosure volume as it could have some nasty and wierd side effects when the bass output isn't perfect mono.

7) 4 full ranges would work fine. Note that full-ranges have less high frequency dispersion, and as a result you have a less wide area in which the sound is balanced. The Boominator sounds more or less balanced even up to 60 degrees off-axis, with full-ranges you should not expect more than 20-30 degrees off-axis balanced sound.

8) Could work. I have not tested it since it would require loose sides and hinges which would make the construction too heavy and complicated for my use.
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Old 11th June 2008, 01:54 PM   #118
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Hi Janoch23. I can answer a couple of these from my point-of-view.

1. Most of the class D amps come as stereo pairs and can't be bridged, so you're paying for two channels anyway. And if power is the concern, two channels = twice the wattage.

4. I'm using two four-inch full-range drivers, the css fr125S. Even though these have a decent top end, I found that the addition of a pair of small Audax tweeters (from the guy on eBay who's selling them cheap, and with a 3.3uf cap and a 6r resistor in series) add a considerable amount of missing top-end sparkle. I aimed them upwards, as it seems like the boombox is usually positioned way below ear level when it's being used.

--Buckapound
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Old 11th June 2008, 02:26 PM   #119
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Yeah, buckapound, that's the same reason I use those wide dispersion tweeter horns, so I don't have to angle them upwards and/or to the sides. Most off the tweeter output that is aimed down by the horns is absorbed in the grass or sand that would be the normal placement anyways, and therefore don't contribute to the sound with reflections.
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Old 11th June 2008, 10:47 PM   #120
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Those HP-10W's seem to be way ahead of the alternatives for the price (~30 euros ea). I've been searching high and low for something smaller with similiar sensitivity at a decent price (say <€150 for 4), but to no avail.

Saturnus, you said you buy your speakers from Sweden. I can't find anybody except Ljudia.se and Hifikit.se, and the HP10W's stand alone here.

Most 6,5"s seem to be around 85 db, 8"s at 88 db or so.

There's no point in using 4 x low efficency 6,5" as far as I can see. Perhaps I should go for 1 x HP10W instead (95 db at 2.8 kg/€30)? Or perhaps 2 x MGR F-2008W , which would only be 94 db at 3.8 kg/€39, but theres two of them for the 41hz AMP6.

(Btw, to calculate summed sensitivity, you just add 3 db for each additional element right? Sorry if this is FAQ, searched but no find)

Any other driver suggestions appreciated!
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