car stereo amp for p.a.

[mikeyhasnobones]

hey all!
so i have a quick (possibly dumb?) question! my drummer and i are trying to build a p.a. on the cheap (i.e. free...we're deadbeats) while cleaning out the basement we came across a 50 watt car stereo power amp. building the cabinets is no problem but we were definitely curious to see if we could use the amp to power them by hooking up a little mixer to it. is this gonna work at all?? can we just rig up a 12 volt power supply for it and go ahead? or is there more to it? thanks in advance for your help, i'm a total noob at this
mikey

[mikeyhasnobones]

also, i wanted to say that this is a GREAT addition to the HR community! excellent work!

[suprstar]

Sure, it'd work, but it wouldn't be very loud... 50w isn't enough power for a PA, your actual singing would prolly be louder than the mix coming from the PA! If you had a bigger amp, ie a few hundred watts, you'd need a pretty big power supply, one where it'd be more cost efficient to just go buy a regular poweramp.

[ido1957]

I read somewhere that car stereos are pretty crappy because they are played in a loud noisy vehicle where super quality doesn't matter.... So if a good regular PA amp was Class A, they would be Class D....

[stevieb]

No reason it would not work- but the advise above, that power will be disapointing, is prob. true. Still, what do you have to lose? Go for it- PA's are "sound reinforcement," meaning that if you sing just into the room, your voice will be of a certain volume; if you put any "PA" between you and the room, and sing thru it, your voice will be somewhat lounder. Maybe not enough to make a difference, but, again, try it.

If you are using, say, 200-watt power capacity speakers, I would not try to drive them with a 50-watt amp, esp. a car stereo amp. Clipping-produced distortion will destroy your speakers in one practice session.

And for what it's worth, in the world of audio amplifiers, terms like "Class A" do not necessarly refer to the quality of the amp, it's build or output, but rather to the way the amp is built to produce the power it does.

[suprstar]

Technically that's true. Maybe not the quality of the amp itself, but class A or D generally affects sound quality, because:

Class D's simply try to push every last ounce of juice it gets to the speakers with more/less total disregard for sound quality. A 500w class D amp would draw about 45-50 amps, delivering 80-90% of the power it takes in to the speakers. Noisier, definately, but it doesnt matter much cuz what ido said.

Class A's show a little restraint and are designed to get a clean signal to the speakers. They only run ~25-30% efficiency, so those same 50 amps in a class A amp would only deliver a total of about 150w to the speakers.

[Rust E-Z]

That car amps 50 watts is probably @ 4 ohms, maybe even 2 ohms, your PA speakers are almost certainly 8 ohms so for your car amp you would be @ 25watts or less.

[suprstar]

Originally Posted by Rust E-Z

That car amps 50 watts is probably @ 4 ohms, maybe even 2 ohms, your PA speakers are almost certainly 8 ohms so for your car amp you would be @ 25watts or less.

Yep, very true.. Then you could easily double, triple, or even quadruple your output by parallel'ing 2, 3, or 4 of em together to drop the load to 4, 2.6666, or 2 ohms.. If you dont know how low your amp goes, I wouldnt put anything lower than 4 ohms on it. Unless you don't care about the amp, then I'd wire up a 1 ohm cab just to watch the amp melt..

[dementedchord]

dont even try it... it's not worth the hassle or the chance of blowing shit up... and there's a whole lot of the info you're being givin is just plain wrong...

the class of an amp has to do with how it acheives it's gain... things like being on all the time (passing current at max all the time even when signal is not present)come into play... effeciency does inter into it some what but only to the degree that a particular way an amp approach might tend to be more effecient... so in other words the class is not defined by the effeiciency...
and if you think for instance that a class d amp sounds bad just beacuse it's class d your full of shit... there are lots of great sounding class d amps... many of them in the high-end powered monitors that most of us cant afford... and there's a real good chance that the pa you heard at the local shed last weekend was being powered by class d amps... QSC makes a few great ones...

[SEDstar]

actually, a lot of whats said about the various classes of amplifier operation, is "3/4 correct" on a few small points....

if anyone really cares, do a WIKI on "Class A amp" and a article comes up explaining the differences.



for teh budget minded... "Pyramid" audio power amps provide a pretty big margin of "watts per dollar"

tip: if you think you need 100 watts? get a 300 watt one and let it "loaf" along... you'll run cooler, and wont beat on the power transistors

[Farview]

Originally Posted by ido1957

I read somewhere that car stereos are pretty crappy because they are played in a loud noisy vehicle where super quality doesn't matter.... So if a good regular PA amp was Class A, they would be Class D....

Class A is a type of circuit design. I have never seen a PA power amp that was class A. Most are class A/B design.

Class A means that one device (bipolar transistors, MOSFETs or tube) handles the entire signal.

Class B is when two devices are used together. One device handles the positive slope of the wave and the other does the negative slope.

Class D uses pulse width modulation. These are mostly used for subwoofers because they are very efficient.

The class system in amplifier design doesn't work the same way as it does with beef. It just tells you what type of designe the amplifier is, it's not a value judgement.

[Farview]

As far as the car amplifier goes, don't bother. Most car audio amplifiers are rated in such a way that it inflates the wattage number.

ie. 50watts @ 2ohms with 3%THD

This means that you get 12.5 watts at 8 ohms and you are still at 3% distortion. If you back off to get below 1%THD, you are probably down to around 5 watts.

3% distortion might be just enough to blow your tweeters too.

Don't use a battery charger as a power supply, it will hum like you wouldn't believe.

[suprstar]

Originally Posted by SEDstar

for teh budget minded... "Pyramid" audio power amps provide a pretty big margin of "watts per dollar"

tip: if you think you need 100 watts? get a 300 watt one and let it "loaf" along... you'll run cooler, and wont beat on the power transistors

Pyramid is the biggest joke in car audio, makes Behringer look like Soundcraft.. Hell, it makes RADIOSHACK look like A&H! All garbage.. A '2000w' amp, but it only takes an 8 guage power cable?? A 30 amp fuse?? Puhleeeeze.. Pyramids put out a quarter of their rated power if you're lucky. I swear they keep feeding it higher and higher voltage til it poofs, then they say "Well at 54 volts it peaked 2000w, so its a 2000w amp!" Thats PEAK power too, not rms... Yeah, 2000w when it gets hit by lightning.... A lot of cheapo amps rate way high, it's all marketing bullshit. Pyramid is about the worst offender..

Then there's other GOOD amps - kicker, mtx, JL, alpine, that actually push HIGHER than rated, in RMS. I have a '560w' kicker amp that actually pushes 730 RMS. They are basically honest, and make sure you get at least what the box says. A 200w Alpine amp will blow the doors off anything Pyramid makes. Reputable companies make amps that last forever too, my Kicker IX704 must be 12 years old now, and I beat the ever loving shit out of it on a daily basis. (on the freeway, i respect my neighbors and community)

Then there's a whole opposite end - cheater amps, like the Orion HCCA series.. They have a '70w' amp, but what they dont say is 'at 4 ohms'. The HCCA's were stable to like 1/2 an ohm, so you could actually suck about 500 RMS out of that "70w" amp. They made em to keep your car in a lower wattage class at competitions like iasca db drags, etc.. A '200w' car that does 152 db - yeah right! I beleive iasca re-wrote the competition rules specifically because of this.

[dementedchord]

Originally Posted by suprstar

Yeah, 2000w when it gets hit by lightning....

hey where's my writers credit???

some buddies and i made that up over twenty years ago while working in retail...

we called it the ISL wattage... if struck by lightening...

[mikeyhasnobones]

wow, thanks for the help everyone! it never ceases to amaze me the wealth of knowledge on this board! well, we decided to bite the bullet and just bought an amp since the general consensus here was to forget about it, but i feel i know that much more about the subject. well done!
mikey

[stevieb]

Well, darn! I was looking forward to hearing how it worked out.