Backpack Sound

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First off, thanks for all the response!

What about using 2 hypex UcD180ST they are cheap, have good specs, ready to run auto of the box. they need +-50V.
one li-lion cell will give you 3.7v so you will need about 26 cells to build the battery. You will skip the part of making higher voltage from 12V.
This setup will be very efficient about 92% (datasheet hypex)!
those amps are a little out of my price range, especially if I have to use two of them! (unless I'm looking at the wrong amp here...)

also, i'll just restate my price range right now:
For ALL electronics and speakers, I'd like to limit the price to $250 or less...

On the discussion about the
Mark Audio CHR-70 's, if i need 8 of them, thats still going to be a pretty hefty price. Wouldn't it be just as good to get 2 good quality 8" or 6" woofer/mid range speakers, and two tweeter horns?

here's my idea for that: (yes, I added the LEDs for some neat detailing)
twowoofersandtweetersandLEDs.jpg


for the woofers I'm looking at these two from eminence:
Eminence Alpha 6A speaker. The Eminence Alpha 6A is a 6" 8 ohm speaker. Alpha-6A is a 100 watt RMS 6" speaker.
Eminence Beta 8A speaker. The Eminence Beta 8A is a 8" 8 ohm speaker for all bass applications including bass guitar cabinets. Beta-8A is a 225 watt RMS 8" woofer.
I like the 6 inch, because it will give me more space at the bottom, to put the battery and amp.

and these tweeters seem pretty good:
Pyle Pro PSN1165 4.35" Piezo Horn Tweeter Pair | Parts-Express.com
but how good will they sound? opinions?

I don't know if that is the best way to do it, but it seems the most cost effective to me currently... this all with the lepai tripath, which is, in my mind, the best overall option.

Lastly, for the batery, I'd kinda figured on using gel-cells that i could pick up for free or cheap at a local recycling place of some sort. opinions on that, keeping in mind my price range?

thanks!
 
I just started using Lithium polymer (LiPo) battery packs to power a DAC, but these offer a lot of advantages. Since they're used for R-C helicopters and other vehicles, they're all over eBay and pretty reasonable in cost. Search for 11.1v, as this is actually the bottom end of the voltage they'll supply. For mot of their discharge cycle, they'll actually be around 12 volts. Chargers are small and cheap, too, and you can get monitors that will either show the cell voltages and/or beep when you hit a certain discharge voltage, helpful for the life of the cells. A 5200 mah pack should give you several hours at a pretty good volume with a Tripath 2020 amp. Probably best to build it so packs can easily be swapped out for obvious reasons.

LiFePo cells are cool and do have the advantage of being long-lasting, but they're not widely available in 12v configurations, so they're much more expensive for now.

Lead/gel cells are heavy of course, but also need to be properly charged, and will poop out in a couple of years. Scavenged cells are likely to be problematic, seems to me. But IMO, the key to making this kind of project work is making it somewhat modular and easy to change. Start with gel cells if you want, but don't lock yourself into them. Oh, and also, make the electronics compartment really easy to open. You want to make it easy to make changes and also to show off your work.

--Buckapound
 
hmmm... good ideas! yes, I like those LiPo packs, and they're well within my price range I suppose... there's several 2x 2800mah deals on ebay for 9.99 (the shipping kills though. 29.99)

anyways, would those two packs or similar ones be the way to go? or even more sets of smaller cells, for easy switch-out?
 
Nothing wrong with using two packs, but swapping, charging and keeping track of the discharge level of more than that might get to be a pain. FYI, you'll have to solder on connectors for the power leads yourself. If you use the concentric type of connector that almost all "wall wart" type power supplies use, you can use one of those power supplies when you're close to an outlet. Tip: The power leads are heavy gauge, so you'll need to solder a short length of smaller gauge wire between the connector plug and the battery leads.

Poke around eBay, there are better deals, like 2 for $41.25, shipping included. You might also check your local hobby/RC shop. Base prices will be more expensive, but you won't have that expensive shipping.

--Buckapound
 
You know, I don't have any real issues about what battery you use, given the appropriate care and charging routine.

However, and I can't stress this enough, if you make a mistake with a LiPo battery, are careless in handling, testing or storage, or use a poor or failed charge monitoring and recharge regimen, or even make a slip with a VOM probe, you are in for some serious excitement, perhaps not of the "wow that was cool" kind and more along the "tragedy" kind.

This battery was (deliberately) overcharged:

http://video.google.com/videoplay?docid=3690260570423705609#

Regardless of the cause, you can expect all LiPo fires to be similar to that example.

The RC model guys have a forum and this is a simple listing of the various forum posts there, regarding LiPo fires:

http://www.rcgroups.com/forums/showpost.php?p=1936756&postcount=4

Mostly I just wanted to give a heads-up since this project has a very strict budget.

You will have to be very sure and take extreme care that you don't buy potentially defective or unreliable Lithium Polymer batteries (or Lithium Ion ... they are similar, use the same charge routines, but LiPo are lighter and uncased) or unreliable charge management circuitry in an attempt to save money.

Personally, since this is designed to be used on your person and in public, I would consider Lead-Acid myself. CTEK makes excellent chargers for these batteries. And remember: Wear eye protection when working with or charging batteries.
 
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I would consider nimh AA cells, they give the best bang for the buck. They can be found already welded together in some shops and a charger doesn't have to cost too much.

If you are on a very tight budget and don't mind the weight then indeed SLA's are very good. If you ever consider solar power they are quite the ultimate actually...

Another down side of any lipo or liion battery I find the very limited cycle life. In relation to the cost they are not that good a deal at all!

Then the speaker drivers, I wouldn't recommend a pile of small drivers for this application, it's just too costly and likely to introduce a whole bunch of problems to get a good sound out of the system. Did you care to look at the link I posted earlier on? That is a combination of speakers with super high efficiency, reasonably light weight and it can sound very good.

It is often overlooked with backpack systems that the proximity of a large surface is very helpful to produce low frequencies. If this backpack is worn most of the times instead of put down on the floor it will have a real hard time to produce bass. This effect will happen equally with any speaker, so it is very important to start off with something really capable. PA speakers are designed for such situations and therefore I would look in that corner.

Actually what you are asking for, this whole system for a budget under $250 is rather extreme. The physical aspects of such a system all speak against feasibility, it's all contradictory. Actually you need some really high tech speaker drivers in there, the design itself has to be top notch. (you're super lucky if design number one pulls it off straight away) I don't want to discourage you, actually I guarantee you that you will probably look back upon it as a super valuable learning experience and it's often more important to just finish something instead of keep dreaming about it for years without doing anything. From what I have personally experienced it will take a couple of designs before the next one isn't 200% better....
 
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Man, those LiPo cells blow up real good. I've got to start putting them in a metal pan to recharge. SCARY!

Personally, I've found NIMH cells a little fussy, especially as they tend to drain out in storage due to low internal resistance and seem to need to be recharged just before use.

This is the LiFePo I'm using in my boombox:

LiFePO4 Belt Bag Power Battery: 12.8V 6.6Ah (84Wh, 14A rate) + Fast Smart Charger

I get a lot of playing hours out of it, but the thing can be charged as a whole, rather than balancing each cell, like the LiPos or swapping out AAs with NiMH cells. With the LiFePo, I can have it inside the boombox and flip a switch to run off batteries or run off AC while the battery is charging. Supposed to be very good service life, but the tradeoff is a bit more expense, but that thing is 6.6AH, and the prince includes a charger, case and a bunch of cables that you may or may not need, so by the time you're comparable with LiPos, it might not be that different.

My experience with SLA cells is that with daily use and intelligent charging, I get about two years before they crap out. I think I paid about $60 for a good SLA charger with "desulphation." (VDC BatteryMinder 12V Battery Charger/Conditioner/Maintainer - VDVDC12117). 5 AH SLA was slightly larger in volume and exactly double the weight of the 6.8 AH LiFePo above, as a data point for you.

--Buckapound
 
... yeah, let me repeat, there can be NO explosions! I'm hoping to use this in a school setting, and while I doubt there'd be a problem (I'm pretty cautious with anything pontentially hazardous or explosive. okay, REALLY catious) with it exploding, but if the school catches wind of there being an explosive possibility... well, you get the picture.

so I guess i'm back to gell-cells... Which isn't all that bad of an option i suppose.

As to the speakers, i was thinking, for this application anyways, a bigger speaker would be better, correct? since it will have a better SPL rating. so what if as opposed to multiple 6 or 8" drivers, i go with one 10 or maybe even 12" driver? yeah, its huge.... but what effect will that have? will it produce the same amount of sound as a few smaller ones? (from what i understand, it should... but i'm still trying to wrap my head around all the complex stuff on here!)

and yes, that is quickly becoming apparent to me. a high-quality system for $250 is going to be very difficult to put together! I came into this thinking i could just slap something together... and now, i realize just HOW MUCH planning has to go into all of this stuff! i can't imagine trying to figure all of this out on my own.

and if no-one else has a better idea, i think i'll go with the Lepai Tripath.

thanks!
 
Personally, I've found NIMH cells a little fussy, especially as they tend to drain out in storage due to low internal resistance and seem to need to be recharged just before use.

Not if you buy good ones, like Eneloop....

Lead batteries really shouldn't crap out in two years, usually this is caused by fast charging them. They also live longest when they are kept well within a certain voltage range and very frequently charged at a slow rate. Unlike nicads or nimh, which can whitstand much deeper discharges and don't like to be charged for extended periods.

One large speaker, in the link to that thread I posted you see that guy used a 10" driver. What's louder can be fairly accurately calculated, but the main advantage of one single driver is no comb filtering effects. Even though these are often not a problem at the lower frequencies, in the midrange these can give rather 'shouting' tendencies. This means you would need relatively expensive and complex filters to get rid of it. A two way system is much easier to filter and such a filter is much more common...

Then a Lepai. It's not too bad as long as it works, but if something goes wrong it's a pain in the behind. The board quickly lets go of the solder pads and traces because of quite poor board quality and the overall quality of the parts isn't top notch either. I would rather take a DIYparadise or 41hz amplifier, the latter also delivers a couple of models as ready made and tested board nowadays.
 
LiFePO cells don't pose the fire/explosion risk of other lithium types--that's one of their advantages.

The Arjenhelder boards have a good reputation also.

You do have to match the size of the speaker to the size of the box to some degree. If the box is too small, a larger speaker is not necessarily going to produce more bass than a smaller one. Look at existing speaker designs of the size you're going to be building. For the half a cubic foot or so you'll have available, what driver sizes are common? Maybe look at small MTM designs that use two woofer/wide-range drivers. That way you can have an amp channel on each. This should give you a clue and help narrow down the choices that now seem so overwhelming.

--Buckapound
 
Pick the speaker for the SIZE of box you want to tote around. A larger or more efficient speaker is not always going to give you more low end if you stuff it into a tiny box.
Reinventing the "BoomBox" is no small feat and even with a unlimited budget you can't break the laws of physics.

For "Kicking" bass I have found this driver to deliver the goods with low powered T-Amps. Throw it in a chunk of sonotube and hide it in a backpack and people will be looking for the boomcar!

Also try to cut the really low bass below 75hz from your mp3's and don't try to tune the ports below that as it will only waste power and drain the battery.
 
Dual voice coils, yeah! Now you can use the whole amplifier with just one speaker (plus tweeters).

You can change the value of the input caps on the t-amp to form a high-pass filter to cut out the lower bass as Mr. Doom suggests. Can't remember the exact value--maybe somewhere about 0.5 to 1uh? Easy enough to experiment with that aspect until it sounds right.

--Buckapound
 
You can change the value of the input caps on the t-amp to form a high-pass filter to cut out the lower bass as Mr. Doom suggests. Can't remember the exact value--maybe somewhere about 0.5 to 1uh? Easy enough to experiment with that aspect until it sounds right.

It takes some mighty big caps to measured in anything but pico or nano henry's :p

You mean uF, or microfarads, naturally but the actually value is impossible to say without knowing the input impance which varies from one T-amp to the next.
 
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