hello all! I have started this blog so that others may benefit from my discoveries as I advance my knowledge of building portable, battery powered, rechargeable audio devices. I will arrange this blog by subject as best I can. this way, you can start with little to no knowledge like I did and move on up into the more technical aspects as you read along.
for my first entry I will break it down to the first four things you should consider when designing your portable audio device… Power, amplifier, speakers, and enclosures.
I. what will supply power to my boom box?
1. There are many different types of batteries available out there. dry cell, lithium, nickel, acid etc… if you’re already familiar with different types of batteries you might already have a one in mind. what I have used so far, and what many people use, are sealed lead acid batteries. these are often referred to as SLA’s. also available in this category are glass Matt and gel batteries. these batteries provide a 12 volt power source and have a variety of amp hour ratings available. please make sure that you choose a SEALED lead acid battery. the are 12 volt automobile batteries available with amazing amp hour ratings, but if you tilt those it is possible for acid to leak out into your boom boxes. trust me, you don’t want that.
please take some time to become more familiar with batteries here
A. things to consider when picking out a battery.
a. voltage. there are a lot of amplifiers out there that are built for car audio, which a lot of people use in their nomadic devices. these amps are designed to work with a 12 volt DC power source. so you can pretty much buy a 12 volt battery and hook it up to a car amp without any guesswork. if you decide to build your own amplifier (yeah, try it at least once.) you’ll notice that they have a minimum and maximum supply voltage. if you find an amplifier that can operate at 7 to 32volts then you’re going to want to shoot for as close to the maximum as possible. you can hook two 12 volt batteries in series to get 24 volts and more watts out of your amplifier. I will make a post soon on wiring batteries in series and parallel to increase either amp hours, or voltage.
b. amp hours. there is a mathematical formula that will help you determine how long a battery will last with the amp/speaker setup you have…aaaand I will find that later. I’m sure it deserves it’s own post. but for now let’s just say that the more amp hours (AH) it’s rated for the longer that battery will last.
c. weight/size. because you’re building a nomadic stereo you need to be able to move it around. in my experience so far, the more amp hours a battery has the heavier it gets. if it’s too heavy, you won’t have a lot of fun lugging it around. you also need your battery to fit inside your nomadic stereo. higher amp hour batteries are not only heavy, but tend to be larger as well.
d. cost. generally, the more amp hours the battery has the more expensive it will be. the more exotic the chemicals are inside the battery, the more expensive it will be. if you are trying to sell what you make you probably want a cheaper battery. buying a $100 dollar battery will add more than $100 dollars to your selling price, if it’s too expensive no one will buy it. if you’re building it for yourself, and if you don’t mind paying a lot for a battery, try out the gel cells.
II. what will I use to amplify my audio signal?
1. I’m going to assume that you are designing your nomadic stereo to be used with a MP3 player of some sort. it would be wonderful if you could just plug a 10 inch speaker into the headphone jack and hear music, but that’s just not the way it works. your mp3 player puts out a low voltage signal and you’re going to need to amplify that signal with an audio amplifier. there are several classes of amplifiers, each with their own strengths and weaknesses. class a, b, ab, c, d, t, and I think there is a class H as well… here’s the thing about choosing an amplifier for your portable audio device… you’re going to want a nice combination of efficiency, and sound quality. so far, in my search I have found class D and T amplifiers fill those requirements quite nicely. class AB amplifiers are supposed to sound really nice but they are less efficient than class D or T. there are a few
things to consider when choosing an amplifier.
learn more about amplifiers here
A. wattage. don’t go over board on the watts. if you want your portable audio device to last longer than 30 minutes on a single charge then you probably don’t want to run a 1000 watt amplifier off of a 7 amp hour 12 volt battery. there is a mathematical formula that will help you figure out how many watts you will get for how long from whatever battery you choose, and I will post that later, but for now my advice to you is to be conservative. there is no greater disappointment after all your planing and hard work than to have your device die 30 minutes after you pull it out. my understanding of wattage is that you need 10x the wattage to get twice as loud. on the bright side you don’t need 600 or 1000 watts if you pick the right speakers and design the right box (more on that in the speakers section)… keep in mind that the more watts your amplifier puts out the more power its sucks up, which will require more battery and more weight in your boom box.
a. RMS and max watt ratings. usually your amplifier will have two ratings. the RMS and the MAX. the RMS rating is what your amp can push all day without any damage to your amp, the MAX is the peak it can handle. most amps are advertised at their MAX rating, so watch out for that. some of them are able to boast ridiculous MAX power because one time the amplifier got hit by lightning and produced 1.21 gigawatts for a nanosecond and didn’t die. amps like this one do not produce the advertised power under normal conditions. read the product reviews. there is usually someone who has bench tested one of these amps and can tell you the type of wattage you should really expect from that amp. sometimes the wattage rating will have a “x2″ or “x4″ after it. this means you can get that wattage from each channel the amp has. if the amp says it provides 200watts, and does not have “xwhatever”, then look at the specs in the product description. if it’s a four channel amp then you’re probably going to get around 25 watts from each channel.
B. Ohms. look for this symbol Ω. ohm is the unit of measurement for resistance in a circuit. you might have heard of ohms law by now. well, usually RMS ratings are also paired with different Ohms ratings. generally, the smaller the number in the ohms rating the less resistance the circuit has.
a. what does this mean for you? in my experience I’ve found half the ohms means more wattage. for example. if you have found an amp that is rated for 25watts RMS at 8 ohms. then Ideally, at 4 ohms you get 50watts. it’s not usually twice as much, but usually what you can count on is the watts rating increases as resistance decreases.
b. how do you change the ohms of an amp? you don’t. that rating lets you know that the amp can handle a certain load. usually amps are 8,4, and 2 ohm stable, which means you can run your amp with those loads and not have it blow up. yes, there are 1 ohm stable amps, but they are expensive.
c. how do I change the ohms then? you can increase or decrease the load on your amp by choosing speakers rated at different ohms and how they are wired. I will devote an entire post to this later, but for now let’s just say your speakers are the load on your amp. the speakers you choose and whether you have them wired in series, parallel, or series parallel will determine the load on your amp.
C. channels. amplifiers can typically have 1 to 6 channels. how do you know many channels you need? well, you can hook one speaker to one channel if you want to keep it simple, you can also hook up three speakers to one channel, or four speakers to two channels… what you want to do is wire the speakers in a way to present the nominal load on your amp, which is usually 4 or 2 ohms. lets say you have three speakers you want to use. a left full range, right full range, and a woofer. I would hook the full range speakers into separate channels, and bridge the woofer on the other two.
a. what do you mean “bridge”? first, let me say not all amplifiers can be bridged, find out if the one you’re looking at can or can not be bridged. second, it’s not as complicated as it sounds and it has a handy effect. by connecting a speaker to the positive output of one channel and the negative output of another channel you bridge those channels into one and you get the wattage from both channels in one speaker. for example, two 25 watt channels bridged is like one 50watt channel.
D. build or buy? if you want your nomadic audio device to be simple to set up and build moral then buy a car audio amplifier. That’s what I did for my first build. I used a sound storm labs 2×100 watt full range amplifier. no, I don’t recommend it, it sounded good, and it was cheap ($40), but it lasted about two months. however, if this is the route you choose to take it’s a very simple set up; screw your amp onto your box, connect battery, connect speakers. done. a decent car audio amp will cost about $300 or more brand new, and you’re going to want a decent amp because it will sound better and last longer. try Craigslist for used and/or stolen amplifiers at a discounted price, or amazon for new stuff at a fair price with a larger selection and less back alley transactions.
a. why would you want to build your own amp if it’s that easy? well, most of those car audio amps are designed to be used with in dash head units where you can control the bass, mid, treble, and volume. without a head unit you don’t have much control over these things. it’s also hard to find car audio amps with lower wattage ratings, like a 25 watt amp. if you build a boom box with two 35watt full range speakers you aren’t going to need a 200 watts RMS amplifier and most dudes with two tens in the trunk of their caprice would laugh at your 200 watt amp, so there’s not much of a market for them… also, usually these amps can be set for either full range or low pass amplification but not both. this can be a setback with your speakers, but you can get around it with a little extra work. learn more in the speakers section. Finally, the car audio amps are big in size. if you need a small amp I suggest buying an amp board, or amp kit.
b. if you are going to try to build your own amplifier you’ll need to develop your electronics skills. that’s what I’m doing right now. why am I doing this? because I can tailor an amplifier to my specific needs. I can find schematics for free online, order the parts for an incredibly low price, add tone control, volume adjustment, assign low pass, bandpass, and highpass to specific channels. basically, you can make the specific amplifier you need for your build. I think you can see the benefits of that. also, it’s waaay cheaper, and it can sound better than most anything you would buy pre assembled.
E. efficiency. you want your amp to be efficient. high efficiency means more of your power is turned into sound and less of it is turned into heat. that means means more play time for you. look for amps that people say run “cool”. heat is the product of inefficiency. so if you find a $40 amp but several of the product reviews say it gets pretty hot, find a different amp.
III. I don’t know a whole lot about speakers, can you tell me about them?
at this point I’m still fairly new to this but I will tell you what I know.
IV. what should I put all this stuff into?
this is the fun part. there are two ways to go here.
A. you can build a box that is sized, tuned, and built for the speakers you’ve chosen, which will be your best sounding option. an example would be the boominator by saturnus.
here you can see several variations of the Boominator. the black pads on the top of the orange one are solar panels used to recharge the batteries. the boominators are usually built from scratch. the speaker enclosure, or box, is built specifically to maximize the sound output of the speakers chosen, and to house the battery and amplifier. boominators are typically built using component speakers placed magnet to magnet inside the box. four 10″s and four tweeters for impressive loudness, bass response, and a 360 degree sound stage. you can find lots of info on how to build a boominator at the boominator thread on http://www.diyaudio.com. I plan to build one of these, I will document the build, and let you know how it sounds. the one with the wood grain uses two small full range speakers and it’s more like an ipod dock.
B. the other route you can take is using a template. what I’m referring to as a template is an already fabricated box or enclosure like a wooden box, cooler, pipe, or any other hollow thing you can think of. with these you simply have to cut holes and instal the speakers, battery, and amp. the downside is they are not usually made of the best materials for sound, and they are not built specifically sized and tuned for the best sound from your speakers. you can make them work, but the boominator will sound better, usually. sometimes, you can get lucky and find a template that is close to what you would have to design.
here is a cooler radio like the one I built. these are built for durability and their protection against the elements. cooler radio builders tend to use water resistant materials and marine certified speakers. helpful hint; at the moment, pretty much any speaker Polk audio makes is marine certified. the reason they use marine certified materials is because most of these river radios are designed to be put in a tube so they can float down a river and provide tunes for a group of beer drinking, sun soaking, floaters. some of these guys actually design them so they play music AND hold beer. I can personally say the one I built has taken several trips on the back of my kayak, gotten splashed, dunked, rained on, stored on the porch all summer, and still sounds better than anything I ever bought at a store. I will have a whole post or two devoted to this type of build but for now you can go to http://www.motorboatyourself.com and look through the forums for all the help you will ever need.
as you can see, the cooler radio builds can get pretty outrageous, but the guys at motorboat.com will know more about 12 volt battery power consumption than the guys at DIYaudio forums. the strength of this type of build is its durability and a lot of these coolers have wheels, so weight is not as much of an issue which means you can use batteries rated at 72 amp hours, huge speakers, and party for three days in a row. these are also great for tailgating at football games.
2. old suitcases.
these are examples of the boomcase, made famous by mr. simo at theboomcase.com. these look really cool. if you place the speakers right, and combine the right speakers with the right suitcase, what you end up with is a retro looking piece of functional art. these are typically designed using home audio component drivers. car audio coaxial speakers can be used but take away from the aesthetics. also, a lot of these are mono, instead of stereo. some of these speakers are repurposed from vintage hi end three way floor models, making the suitcase build an upcycled product. if you look at them you can see three speakers working together.
Do you see the similarities?
mr. simo is either buying new drivers and designing a three way speaker in a suitcase, or taking the drivers out of old three way speakers and putting them in suitcases with an amplifier and a power source. if you use old speakers you can just use the drivers and the cross over that’s inside the enclosure. that way you don’t need to design and make a crossover yourself. the strength of this build is the aesthetic. they look rad. audiophiles hate the idea of the suitcase radios and claim that they will sound like a turtle fart and make you want to punch a baby, so don’t go to hifi audio forums looking for help to build something like this. I haven’t heard one in person yet, but I am building my own and I will let you know how it turns out.
3. PVC pipe.
these are built for durability like the cooler radios. but they can also look kind of funky, and they are really fun to play with. pvc pipes are almost like Legos and they conveniently come in the same sizes as speakers. so you can mess with a variety of elbows and junctions and create a pipe stereo version of Poseidon’s trident. PVC stereos require the minimum amount of tools. which if you don’t have any tools already makes them the cheapest option. all you need to cut these is a $4 hacksaw, and a screwdriver or power drill to screw in the speakers. the strength of this build is the ease and speed and durability. these are typically built with coaxial full range speakers and car audio amplifiers or head units. some of them don’t require a space for the battery because people hook them up to the battery of their four wheeler or golf cart which is a huge advantage. if you design these sealed they will sound great. I don’t recommend designing these ported because the audio will have a strong “pipe sound”. if you do port, try to use polyfill or socks to get rid of the “pipe sound”.
4. miscellaneous items.
use your imagination. it’s not always the best option, but you really can make a boom box out of just about anything that is hollow and rigid. all you need is power, amplification, and speakers, and a box.
yeah, that last picture is meant to be there. that’s right, books. that one is not portable but that pictures is of an amplifier and speakers hidden in books.
I hope all this helps you get your footing.
please feel free to email me with your ideas, tips, tricks, links to your own build blog, links to someone else’s build blog, schematics, and helpful advice. send me pictures of you and your friends throwing a rave at the top of a mountain.
help me turn this blog into a “how to build a boombox for dummies” kind of place.