Speakers, ohms, and wattage

Hey guys,

I'm thinking about designing a circuit with a built in amp. Not sure what class of amp I'd be using yet because I don't really know the difference between them, but based on some brief research I suspect Class D will end up being the easiest to build and the audio quality will be sufficient. This amp will need to be powered by 8 D cell batteries at most, and drive at least 2 large speakers, and maybe a small one. To this end I have been researching what ohm ratings and wattage really mean, and I think I have the basics down, but I've still got some questions.

So here's what I think I know, and what I'm still confused about:

1. The lower the ohm rating, the more current the speaker will draw. A 4ohm speaker will draw twice as much current as an 8 ohm speaker.
2. Most speakers list their peak wattage. The RMS wattage is 1/2 peak.
3. I don't see a voltage rating for any of the speakers I've looked at, but I suspect if I use ohm's law I can calculate that based on the peak or RMS wattage and the speaker's ohm rating.
4. I suspect a speaker like this one with "dual 4 ohm voice coils" will draw as much current as a 2 ohm speaker:
   Robot or human?
5. Assuming said speaker behaves as a 2 ohm speaker, given it's listed rating of 1800 watts at peak, this speaker should be rated for a peak of 60v.
6. That being the case, can I assume a car amp must be able to step 12v up to 60v?

Finally, I'm wondering if anyone has any recommendations for an amplifier chip to use for this application. To answer that, you probably want to know how loud I want to make this. Unfortunately, I can't give you an answer in decibels. But I can say that this will be used at noisy conventions, and that those eight D cell batteries should probably last around an hour.

A D cell, accoridng to wikipedia, has around 12000mAh, so divide that by 1A and that gives 12 hours, but I don't know how accurate that is since the voltage will drop as it discharges. Looking for some graphs with discharge curves now.

I estimate if I were to drive two 8 ohm speakers at 12v then I would need to put 3A through them, but that may be the wrong way of going about my calculations if the amp actually steps up the voltage. I suspect I am incorrect because 4 ohms at 12v is only 36 watts which is well below what even the cheapest Walmart speakers seem to be rated for.

Speaking of which:
http://www.walmart.com/ip/Xpress-6-x-9-3-way-700W-Universal-Premium-Car-Speakers/14335639

Can I assume that that 3 way speaker behaves as three 4 ohm speakers, resulting in a mere 1.3 ohms of impedance, and that the 700W rating is the peak for all three combined? Cause I get 30 volts needed to max those out with 23 amps. Obviously I don't plan to generate that kind of power from eight D cells. Though if it does turn out the batteries are kinda weak it might be possible to run the amp off something like a NiMH pack. I want to keep the option open to run off alkaline though.

Just doing some battery research. Found this LiPoly:

Looks like it could put out 5A at 7.4v for an hour. That's 36 watts for an hour. Assuming an amp chip can step up the voltage, I think that would provide only half what even a cheap speaker like this could handle:
http://www.walmart.com/ip/Boss-60-Watt-5.25-Dual-Cone-Replacement-Speaker/11968793

Of course it also says it's rated for 20C which I think means it can put out 100A for short bursts? Anyway I think that means that it could max out that speaker with 60 watts, on average using 30, and lasting for around an hour.

Makes me wonder if the guys I know who have been hooking up two speakers to a wimpy amp that can only put put 7W have been doing things all wrong and they'd get the same volume out of one speaker... and even then they wouldn't be coming close to maxing it out.

(On a side note, I'm now thinking that a 4 ohm dual cone or dual coil speaker is still a 4 ohm speaker. But I'm still not certain about 2 way and 3 way speakers. I think those probably do behave like 3 4 or 8 ohm speakers.)

Was thinking about the guys using two speakers with the weak amp. Decided to do some calculations based on that.

So this is the amp they're using:
http://www.electronickits.com/kit/complete/ampl/CANUK153.htm

7 watt output. Most of the guys supply it with 12v. I'm going to assume it then outputs 12v.

7W / 12V = 0.58333A which matches what they say the amp draws.

But:
12V / 4 OHMS = 3A which one would assume is what the speaker would try to draw.

So what's going on here? How can a 7 watt amp drive a 4 ohm speaker at 12V without being damaged?

So I've decided after looking at some amps that it's probably not worthwhile to design my own. There are some class T amps from China which have decent sound quality and are really cheap and will do the job, and it's clear that I won't be able to do a better job in a smaller form factor more cheaply.

I'd still like to know how I can tell if I'm going to blow the amp up with the speakers I connect to it though. That Canuck amp in particular I'd lke to know more abut because I have friends who use it, and one plans to connect some speakers he has in serial right now in parralel to it and put 12v into the amp and I don't know if that is going to damage it or if it simply won't deliver the power needed to drive the speakers at full volume.

But I can say that this will be used at noisy conventions,

Theres no AC power???

For that situation, I'd recommend about 100 Watts driving a couple of P.A. type speakers with 8-inch or larger woofers, and with the speakers mounted around head-height.

If you are not too concerned with sound quality, a horn-type speaker ([u]example[/u]) is very efficient and can go very loud with a few watts. But, the sound quality won't be much better than a bullhorn.

Maybe you can rent a battery-powered portable P.A. system to get an idea of what you need before you build something that might not work.

2. Most speakers list their peak wattage. The RMS wattage is 1/2 peak.

You can't always trust power ratings for speakers or amplifiers. "Peak" ratings are usually total B.S.

You usually can trust RMS ratings for amplifiers, especially if it says something like, "100W RMS continuous, 20-20kHz at less than 1% THD."

If a reputable speaker manufacturer says a speaker is rated at 100W RMS, it's generally safe to use it with a 100W RMS amp as long as you don't drive the amp into distortion. But, this is tricky too... Tweeters can't take as much power as woofers. The idea is that the speaker should be able to take 100W of voice or music. You can easily burn-out most tweeters with test-tones, or highly distorted music.

JBL says (for their pro P.A. speakers) that with normal undistorted music, it's safe to use an amp with twice the rating of the speaker. In applications like guitar amplifiers, the opposite is true... You should use a speaker with twice the rating of the amp.

Music has peaks that are higher than the average/RMS levels. Usually, the peaks are around 10 times the average (depending on the particular music). This means that if you have a 100W amplifer running at full power (but undistorted), you are probably only putting-out an average of 10 Watts or so. If you amplifer is 100% efficient (which it is not), you'd only be consuming 10W from the power supply.

3. I don't see a voltage rating for any of the speakers I've looked at, but I suspect if I use ohm's law I can calculate that based on the peak or RMS wattage and the speaker's ohm rating.

Speakers are rated by wattage. If you are designing an amp, you need to know the voltage. If you are buying an amp or speakers, you just need to know the power and impedance. (Unless they are the "constant voltage" 70.7V type used in supermarkets.)

4. I suspect a speaker like this one with "dual 4 ohm voice coils" will draw as much current as a 2 ohm speaker:

You have 3 choices. You can use a single voice coil for 4 Ohms, Wire them in parallel for 2 Ohms, or wire them in series for 8 Ohms. Since power is related to heat, I'd assume the same power rating no matter how it's wired. But 1800 watts... Yeah, right... Do you realize how hot a 1500W toaster gets? Very few speakers can withstand that much heat.

6. That being the case, can I assume a car amp must be able to step 12v up to 60v?

Yes, most high-power car amps use a voltage-boosting power supply. Automobile voltage is usually "considered" 14.4V. A "regular" amplifier (with the output referenced to ground) can "ideally" put-out 14.4V peak-to-peak (ignoring the voltage-loss across the amp). That's ~5V RMS. With a bridged (push-pull) design, you can get double the voltage (28.8V peak-to peak), for 4 times the power.

Makes me wonder if the guys I know who have been hooking up two speakers to a wimpy amp that can only put put 7W have been doing things all wrong and they'd get the same volume out of one speaker... and even then they wouldn't be coming close to maxing it out.

Maybe. You are always limited by power (Watts), but depending on the amp, you may be able to get twice the power at half the total impedance. And of course, for stereo you need two speakers. Two speakers in separate cabinets can cover a larger area. And, two speakers gives you more cone area for potentially more bass.

(On a side note, I'm now thinking that a 4 ohm dual cone or dual coil speaker is still a 4 ohm speaker. But I'm still not certain about 2 way and 3 way speakers. I think those probably do behave like 3 4 or 8 ohm speakers.)

No! The crossover network (if designed properly) sends different frequencies to the different drivers. i.e. the high frequencies don't "see" the woofer at all. Three 8 Ohm divers with a proper crossover results in an 8 Ohm speaker.

Except... Some cheap 2-way speakers use a single capacitor as a crossover. This blocks the bass from the tweeter, and the bass "sees" an 8-ohm load. But, the high frequencies go to both drivers, and you've got 4 Ohms at high frequencies. (These speakers are usually marked as "8 Ohms").

I'd still like to know how I can tell if I'm going to blow the amp up with the speakers I connect to it though.

The important thing is the impedance of the speakers. The amp has a minimum impedance spec. As long as your total load is not less than the rated impedance, you won't blow the amp.

Most home & pro amps are rated at 4 Ohms, and most home/pro speakers are 8 Ohms, so you can connect a pair of 8 Ohm speakers to each channel. Most car amps are also rated at 4 Ohms, and most car speakers are 4 Ohms. But, car amps that go down to 2 ohms are not uncommon, and some are rated down to 1 Ohm.

Thanks for all the information. I think I understand things a bit better now.

DVDdoug:
Theres no AC power???

No. This is intended to be part of a costume. It will be worn on the back. So it has to be small and lightweight. It must also be inexpensive. No more than $35. Loudness is a secondary concern, but it is a concern, so it needs to be as loud as possible within the other parameters. Sound quality is also a secondary concern, but it needs to have reasonable sound quality.

I found this amp after browsing ebay and google looking for the cheapest amps available, then searching for reviews. Most of the $20 amps out there are complete crap, with horrible clipping, and not putting out anywhere near the power they claim. I even saw a video of one catch fire. This amp however looks decent for the money:

And of course, for stereo you need two speakers. Two speakers in separate cabinets can cover a larger area. And, two speakers gives you more cone area for potentially more bass.

More bass is good. Stereo is not necessary for this application though.

Except... Some cheap 2-way speakers use a single capacitor as a crossover. This blocks the bass from the tweeter, and the bass "sees" an 8-ohm load. But, the high frequencies go to both drivers, and you've got 4 Ohms at high frequencies. (These speakers are usually marked as "8 Ohms").

So if i just see one capacitor on the tweeter, it's a cheap 2-way speaker? What should I see on a properly wired 2-way speaker?