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Class D Switching Power Amplifiers and Power D/A conversion

Help on improving my festival stereo? :)
Help on improving my festival stereo? :)
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Old 13th August 2008, 02:10 PM   #1
tfan is offline tfan  Denmark
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Question Help on improving my festival stereo? :)

Hello, I'm new at this forum

I was a bit confused on where to post this, so I ended up posting it here

I started building my own festival stereo back in 2007 for the Roskilde Festival. The first version was a big(!) pushcart, with a car radio, only one car battery and two external speakers. It actually played OK, as the speakers were in prebuild speaker boxes. But.. I like to be able to carry my stereo around with me and start parties at random places at the festival, so I soon realized that it was way to heavy to pull around, especially when the wheels got all muddy and dirt got into them and so on.

At this years festival, I build it in a rectangular box, with 2xtweeters, 4xmidtone and 2x10" speakers in the sites. I bought a shitty 200w amplifier, had three car batteries and a mobile coffemachine(!).
To tell the truth, it sounded like **** So the reason why I'm writing here, is that I'm tired of not knowing what I'm doing.

I've read about the boominator and was really amazed about how much sound you can get out of a small stereo, compared to what mine could offer.

I've decided that the next one is going to be in some kind of a bicycle trailer, as I want to put in all batteries in the trailer at the same time. I'm also thinking about integrating my ipod and the carradio from the past years. I've attached a model of what I'm thinking about building.

So my question is, what kind of amplifier and speakers should I go after, and what would be a good way to build the trailer for optimal sound? I've read something about that I should forgot about having dedicated subs and instead focus on midtone and full range, but I really don't know much about this
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Old 13th August 2008, 05:31 PM   #2
Buckapound is offline Buckapound  United States
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Hi tfan,

Too many choices, right? The Boominator does look like an awesone design, and you would do well to use that as a starting point.

For sure, you want a class D amplifier, because they offer small size, great sound, high efficienct and don't require much cooling. Look at the kits on 41hz.com. Your first big choice will be between the 10-25 wpc amps that run on 12v, and so can be run directly off of 12 batteries or higher wattage modules which require either multiple batteries or a step up switching power supply.

Spealers vary a lot in their efficiency. High efficiency drivers in a proper enclosues can get a lot of sound of a few watts. You can also bi-amp or tri-amp, using one amplifier channel to drive each speaker (woofer, tweeter, etc), although this will require a line-level crossover.

As has been suggested before in connection with boomboxes, plastic-coned drivers with a dustcap as opposed to a phase plug (which has a gap that can let dust into the motor) are the most weatherproof.

As for the speakers cabinet(s), I would start looking at existing designs based on your criteria, These will give you the best chance of a design that sounds really good. It is possibly to modify the dimensions on most designs to suit your trailer design as long as the volumes remain the same. Be sure to make the cabinet very strong and rigid. One thing I like to do is build a prototype out of cheap fiberboard or even foam-core, to make sure the dimensions work out. One other thing I would consider if it were me. Think about making either one or both speakers detachable in case you want to spread them out for better stereo and/or cover a wider area. As for a separate sub, on a thing the size you're proposing, you should be able to use drivers large enough to get pretty decent, if not ear-bleeding bass.

Look at a battery charging circuit that has "desulfinating" circuitry, as this will greatly extend the life of your lead-acid batteries (assuming that's what you're using). Solar power, to aid charging, is not cheap, but is way cool.

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Old 15th August 2008, 03:43 AM   #3
NED 209 is offline NED 209  Ireland
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hey there Tfan, im a total novice also. ! if you type boobox into this forum youll find a lot of informative threads.

when Buckapound said -

It is possibly to modify the dimensions on most designs to suit your trailer design as long as the volumes remain the same.

by volume he was referring to the amount of air inside the cabinet. woofers need a certain amount of air inside the chamber to perform optimally. this volume of air varies from speaker to speaker, and is worked out by ... doing math.... with the TS values and certain other values of the particular speaker.

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Old 21st August 2008, 01:51 AM   #4
moskeeto is offline moskeeto  Denmark
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Hey, I'm one of the guys building the system with tfan.

Buckapound, I really liked your idea about building the system in foam core first. It will save us a lot of problems

I've looked some more at the boominator and it's so well designed, that our project probably will end up as a mini-boominator So currently we are looking at the AMP6-Basic amp from 41khz.
And 2 x P.audio HP10W drivers because of their 96 dB sensitivity and low price.

Will we get higher efficienty if we increase the volume of our speaker cabinets?
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Old 24th January 2009, 07:31 AM   #5
tfan is offline tfan  Denmark
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Okay, it's been a while since I started the thread..

First of all, I'm new in all of this, trying to get to understand what I'm doing, so there might be stupid questions.

I've been doing some calculations, trying to find the ideal measurements for two vent cases with a P. audio HP10W driver in each (data sheet). Those cases are also going to have two piezo tweeters, probably playing in range 4kHz - 30kHz with a resistence of 60-80ohms. I'm almost done with the measurements, but I wouldn't start buliding anything as long as I was in doubt about tuning frequency.

I also have two U.S blaster usb 2620 (data sheet) laying around, but not sure whether I should use them or not.

My amplifier can play with 4x100w or 2x200w (bridged) both at 4ohm. I can choose frequencies and bass boost for two channels at a time.

In that connection, I have some questions that I can't find answered elsewhere.

1) Can anyone tell me short about tuning frequency? I know that thiele-small parametres is a expression for the performance of my speaker in a specified low frequency, and also that the values provided in the data sheet are only averages. But I'm not sure about tuning. I know you "tune" your case for a frequency, but I don't know what it means and what it'll do for the sound in practice. And does this only apply to the case or do you need to do some filtering with the driver also?

2) What frequency would good for me? It is a stereo for a festival, so it's designed to play outside. I want solid bass, not too deep, but I want it to be notified and be loud.
Saturnus said he tune his for 82Hz in the boominator thread, would that be good for me aswell?

3) Do I need to do anything, like filtering or would the sound be great, just connecting everything directly to the amplifier?

- Thanks, Thomas
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Old 24th January 2009, 11:21 AM   #6
HK26147 is offline HK26147  United States
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I have done many parade sound systems; but these were on vehicles and ran from the vehicle's 12V system.
How much volume and weight are you willing and able to move?
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Old 24th January 2009, 06:20 PM   #7
tfan is offline tfan  Denmark
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It can weight quite a bit. I'm putting everything in a cart, so weight won't be that much of a problem.

I'm also running on 12V and car batteries, as I have some from earlier version of the stereo.
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Old 25th January 2009, 12:57 PM   #8
HK26147 is offline HK26147  United States
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You must get as much SPL out for a watt as possible.
I wire for 4 ohms - I don't wire for 2 ohm as the current load is excessive.
Fans of very low powered SET amps use horn loading because of their high SPL output.
Horns of course are not the smallest LF cabs.
Hauling around large sources of power for designs that produce much lower SPL per watt ( consuming limited battery power ) is even less desirable in your scenario, because you are pushing around the weight.
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Old 26th January 2009, 11:55 AM   #9
Saturnus is offline Saturnus  Denmark
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Box tuning is partly understanding how different tuning works and in which enviroments the speaker is used because the enviroment ("room") plays a much larger role in how a speaker sounds than the box tuning or driver themselves does.

In a normal room a box Q of between 0.5 and 1 would be preferred depending on the size of the room. For free field use a box Q of between 1 and 2 is preferred, and gives the same theoretical result as a room box tuning of half the value. Unfortunately no ordinary rooms are perfectly hermetrically sealed and true free field would require an infinite infinite room.

So choosing the box Q basically comes down to a compromise of several factors. For the Boominator I use a Q of 1.207 which gives a 3db boost at twice the Fb. This is what I have found sounds the best for outdoor enviroments provided the speaker is placed directly on the ground. The bass response with this tuning and placement is comparable to a box Q of 0.707 in a standard sized living room.

Lifting it off the ground on a stand or cart will require an even higher box Q depending on height off the ground and tuning frequency.

Keeping the box Q relative low is desirable since it can then be used indoors as well, especially for parties, although for everyday use it will probably have too "boomy" bass, and second of all, the low the Qb, the lower the Fs. Meaning that the speaker will go lower in frequency with a lower Q.

That means you have to recalculate both the box Q (Qb) and the tuning frequency (Fb) which in turn result in a completely different box volume (Vb) if you're not going to use the placement directly on the ground, although on a bicycle cart with it's 10-15 cm off ground elevation it's hardly a problem.
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Old 27th January 2009, 01:42 AM   #10
tfan is offline tfan  Denmark
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Just received the drivers today, must say I'm very impressed. (thanks alot for the recommendation!)

I'm putting it on a push cart, not a bike trailer. So I have plenty of room for a bigger case. (surface on 61cm x 122cm)

Well, I'm not very experienced in this area. So I'm affraid I'm maybe measuring something that dosen't sound properly. I really don't have any clue about what my Qb value should be, and I couldn't find anything about how to calculate it, seems like people usually use it like a constant.

My calculations says that the case should be ~159L with a tuning on 82Hz, seemed a bit unrealistic. So I must admit that I'm a bit confused now..
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