Hello

I want to build a portable stereo but im having trouble working out the math on the expected battery life. I plan on building 2 portable stereos the first one will be small and the second will be large.

My plan so far is using a 6X9 speaker with a small amp like this one:
http://www.ebay.com.au/itm/Mini-Hi-...ctronics_Audio_Amplifiers&hash=item3cb7cc9be1

which has an input power of 12v 6A and i was planning on using say a 7AH recharge battery but how much battery life would that give me? i am hoping to get around 5-6 hours of battery life.

Also would i benefit from using 2 6X9 speakers rather than one? Any help is appreciated.

 

Hello,

With ONE amplifier that will draw 6 Ampere at full power, you will run about 1 hour with the 7 Ah battery.
When used at low output, the time will be longer.

Bertus

 

Bringing in human factors, I expect you'll be using 12 to 24 watts per channel when it's turned up pretty loud, so its more like 2 to 4 amps. Besides that, I've never seen a 6X9 speaker that could survive 180 watts (peak) (as advertised). 6 amps times 12 volts is 72 watts, and that's for both channels. If you like lots of distortion, you could push this amp to 36 watts per channel, but you're going to want it down around 12 watts per channel if you like dynamic range and low distortion. That would get you 2 to 3&1/2 hours out of a 7 amp-hour battery.

 

Okay thanks for your help i thought that's how it worked just wanted to make sure.

I have found other amps that draw 2 Ampere like this one http://www.ebay.com.au/itm/Mini-MP3...ctronics_Audio_Amplifiers&hash=item461f9d205f

Do you think an amp like this would be worthwhile for a portable stereo system? I plan on taking it camping and to the beach. Also would i be better off with 1 or 2 speakers? i understand it depends on the speakers but would having one speaker be louder than having 2 speakers?

 

No. six by nines don't move enough air to have good bass, so 2 would work better. Besides, you can pour twice as much power into two speakers.

 

How loud are you going to play this thing?

12V @ 6A input power is the maximum draw.

As #12 said you are not likely going to draw 6A. Figure out how much power you need and use that as an estimate based on 80% efficiency.

If you need 10W then estimate a current draw of 1A @ 12V.
A 12V/7Ah battery will last for about 5 hours.

That is about what I get from my battery powered guitar amp.

 

Increasing the number of speakers does not make the sound louder or does it draw more current.

As I said, go with a simple estimate on how much power you need, i.e. 12V x ?A.

The reason for adding extra speakers is if one speaker cannot handle the power without distorting.

I presume you want a wild party on the beach.

Try one speaker of sufficient power, one for each of the two stereo channels and try it and see if it is loud enough to drive the neighbours crazy. Then measure the current draw from the battery with an ammeter.

While you're doing that, check to see how hot the amp gets.

 

Increasing the number of speakers does not make the sound louder or does it draw more current.

MrChips, You're a smart guy, but sometimes I don't understand you. Could you elaborate?

My primary consideration was for the bass response, as experience tells me that a single 6 X 9 speaker in a car is pretty wimpy. Having 2 speakers of equal wattage, the increased sound pressure level will be just barely perceivable to a person, but I was after frequency range. We all know that a speaker on each channel of the amplifier will have power running to both speakers and each of them will use current from the power supply, so what was on your mind?

 

Yes, you're correct.

You want to add more speaker area for better bass response. Unfortunately, one large speaker cone starts to break up at higher frequencies. In such a case it is better to use a number of speakers with smaller diameter cones.

For the op, the usual three-way speaker system with tweeter, mid-range and woofer would work fine.

We're not talking 1000W rock concert on the beach, are we?

 

We're not talking 1000W rock concert on the beach, are we?

No. It's a stereo amp that can do 36 w/channel (minus conversion losses) before the fuse blows.
It's really tiny, so it must be a switching amplifier, and that puts it in the 80% to 90% efficiency range (I think).
It's labeled, "180 watts peak" but we all know that's some kind of pretend number hiding behind some weasel words in some advertising office.
Still, it will produce good sound if you have good speakers.

 

If you want to use minimum battery power then you should use a Class D type audio switching amplifier. They have much higher efficiency than standard Class AB type linear amps.

Don't know what type of amps either of those your referenced are.

 

thanks for both your input. these were my goals for the project
1- make it portable so no more than 10kg
2- battery life of 5-6 hours

I do want it to be loud but i understand it will be no party stereo system. I want to achieve a decent volume level without to much distortion but considering the amps i am planning on using im unsure of what volume i can achieve without distortion. For the battery i will use either a 7AH, 9AH or 12AH. I think its better to spend more time and money on some good speakers like MrChips suggested say three way or better, it depends on how my budget goes i don't plan on spending too much on this first project.

I plan on making a bigger version later where i will use a much bigger battery and have it on wheels. I was going to try and have a small sub but it seems very unlikely as it will draw much power. From the research i have done its best to use more efficient speakers and amps, its common to use PA speakers as they have a much higher efficiency than car stereo equipment. i did some research on a good car amp that could run a decent sub and 2 speakers and it would draw 33 Ampere at full power.

Whats are your thoughts, ideas and comments?

 

Let's get some design specs down.
So you want to have a party on the beach.
How many friends are you planning on entertaining?
How many enemies you plan on creating?
Is this a public beach?

If I am on a beach, the worst thing I would want is to have to listen to someone else's music. I've had this happen to me before.

Camping would be even worse. I would want to put a bullet through your sound system.

 

The speakers are very important. I've seen them as low as 87 db for the first watt, but I bought the speakers that put out 103 db for the first watt. That's like the difference between having a 10 watt amplifier and having a 320 watt amplifier.

Put your money in the speakers.

 

I don't want a party, usually around 10 people we usually go camping where we are away from others. Normally we use a car for a radio and have it up very loud but i know this wont be as loud as that but it doesn't need to be because it can be positioned closer to us. we having a swimming hole we go to in summer which is in the middle of no where and no one else is rarely there. but its a 10 min walk so we cant take car in there.

Yeah i agree #12 i was planning at the start to have one powerful 6X9 rated at around 50-60rms but im unsure now. One speaker would mean a lighter and smaller product but what would the sound quality be like compared to having two of them? Im not even sure if i can bridge this amp or would i just be running it off one channel?

I have been researching amps and this one is looking promising: http://www.ebay.com.au/itm/281319217312?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 

Just for reference material, a 6 x 9 speaker has about the same air pumping surface as an 8 inch round speaker. Having a, "powerful" rating like 100 watts, means nothing about loudness, as I just demonstrated by comparing a speaker that is rated at 87 db/watt @ 1 meter to a speaker that is rated at 103 db @ 1 watt and 1 meter distance. If you buy the worst speaker I have ever seen, you will need 320 watts of amplifier to get as much sound as a good speaker at 10 watts. You don't have an energy budget in the hundreds of watts, so you are shopping for a sensitive speaker, not a, "powerful" speaker. ps, if the speaker does not tell what its sensitivity rating is, don't buy it.

What's the sound like with one speaker? Get in a car with the stereo on, driving a pair of 6 x 9 speakers, and put a seat cushion over one of the speakers.

 

What sensitivity would you recommend for this project remembering that I'm on a budget? Most of the speakers I am looking at are around 90-95

 

consider impedance matching

 

#12, I reread what I wrote. Of course, you and I both know what we're referring to.
If you set the amp to the same level output, adding more speakers wouldn't change the acoustic power output or draw more current.

Of course, if you change the speaker impedance you will get a different result.

Let's make an assumption that the speakers are standard 8Ω impedance.
The reason they go to 4Ω and 2Ω speakers is so that they can deliver more power at the same voltage output.

What is more important here, as #12 has said, is the performance and efficiency of the speaker. For what the OP is asking, that is, longest running time with reasonably good quality sound and high power output and low weight, here are your choices:

1) a class D amplifier,
2) a bigger battery vs weight consideration,
3) high efficiency speaker with a power rating suitably matched to the amp.

With a 12V 7Ah-12Ah battery lasting for 5-6 hours you want to pull no more than 1-2A average. Remember that the average current draw is going to be much less that the max current.

Hence your average power draw is about 12-20W, i.e. 6-10W per channel.

For good efficiency, match the speaker to this by choosing about 4 times this power, i.e. 25-50W speakers. Avoid going higher.

Keep it simple and you will get good sound with high efficiency.

Remember, you're simply adding to your camping/beach pleasure, making sound waves not water waves.

 

And another thing, if you haven't considered this already,
the acoustics in the wide open outdoors is very different from that of your living room.
With no walls to reflect the sound, open space is essentially a "dead" room. Hence you will need much more power and this goes against the quest for longer playing time.

I would compromise the hi-fidelity for efficiency. Instead of a 2-way or 3-way system I would choose a single 50W 10" speaker in a compact basic bass reflex design, i.e. a closed box with a tuned vented front port.