amplifier questions

MaxPowers · 06-10-2004, 02:59 PM

Allright i have a rockford fosgate 4ch amp that i was going to hook up to some new speakers that i will buy soon but i had a question for all you audio experts. It says that the amp is 175wx4 per channel for a 2ohm load, 99wx4 per channel for a 4ohm load and it also says 75wx4 for RMS continuous power per channel, both channels driven into a 4ohm load from 20 to 20,000Hz with less than 0.05% total Harmonic distortion. I was thinking of getting some infinity speakers with 100W RMS power in the front speakers and 60W RMS power in the rear speakers. I was just wondering which one would be right for the speakers i want to get? I notice on the info on the speakers that they are 4ohm so does that mean that it would be 99x4 or 75x4? Does any of this make sense cuz i'm an amateur at this kind of stuff and i want to get some speakers that would match the amp well.

eakes · 06-10-2004, 03:25 PM

Quote:

Originally Posted by MaxPowers

Allright i have a rockford fosgate 4ch amp that i was going to hook up to some new speakers that i will buy soon but i had a question for all you audio experts. It says that the amp is 175wx4 per channel for a 2ohm load, 99wx4 per channel for a 4ohm load and it also says 75wx4 for RMS continuous power per channel, both channels driven into a 4ohm load from 20 to 20,000Hz with less than 0.05% total Harmonic distortion. I was thinking of getting some infinity speakers with 100W RMS power in the front speakers and 60W RMS power in the rear speakers. I was just wondering which one would be right for the speakers i want to get? I notice on the info on the speakers that they are 4ohm so does that mean that it would be 99x4 or 75x4? Does any of this make sense cuz i'm an amateur at this kind of stuff and i want to get some speakers that would match the amp well.

The 175 and 99 numbers are peak power into their specified loads (2 or 4 ohms). In your case with 4 ohm speakers the amp could deliver peak power of 99 watts per speaker. The 75 number is stated to be RMS power into 4 ohms with less the 0.05% harmonic distortion. The RMS rating is more 'real world' than peak, RMS is the sustained power the amp can deliver over a period of time longer that a few microseconds. Peak power is an instantaneous measurement that looks good on the spec sheet because it's a larger number that the RMS value for the same amp.

The speaker ratings of 100watts and 60 watts RMS indicate how much power the speakers can handle. If you are going to run your system at high volume for any period of time then you should have speakers that will handle the RMS output of the amp. Using 60 watt speakers connected to a 75 watt amp is not the best situation, eventually the speakers will blow if you crank the volume up.

MaxPowers · 06-10-2004, 06:38 PM

I see what you are saying, but since the rear speakers are a 5.25 size which i can maybe fit 6.5 size, but most speakers i see that size don't have that high of a rating. So possibly for the rear say i get two 5.25 speakers with an RMS rating of 45 watts each could i hook them um just to one channel and have each speaker in the front going through it's own channel? So basically one channel woudln't be used but the power ratings would be close, is that possibly? Thanks for the help.

pciley · 06-10-2004, 07:03 PM

Everything Eakes said is right on. As a general rule, you should have speakers that have a slightly higher RMS rating than the amplifier's RMS rating (how much higher is up to the installer, everyone says something different.. go too high and the speaker will damage itself by clipping). For your 75 watt RMS you are ok to get those 100 watt RMS speakers for the front. Like Eakes said, if you put those 60 watt RMS speakers in the rear, and frequently sustain high volumes, the speaker will most likely blow.

A couple of things you can do... look at HIGHER quality (usually more expensive) speakers for the rear. Since they are a smaller size, a higher quality speaker of better construction will be needed to get closer to that 100 watt RMS.

or... since it is mostly the lower bass frequencies that cause the speaker to excurd farther than it is intended, which causes premature wear ("blowing a speaker"), You can try some 'bass blockers' which are kind of like little crossovers except much cheaper. They block a range of lower frequencies associated with bass notes. You can get them in 150, 600, and 800 hz (maybe more now, haven't installed them in a couple of years). Sounds like you should go with the 800 hz bass blockers for your high powered application (over 50 watts). These get wired inline, one per speaker on the positive wire or directly to the terminal, and will help prolong the life of a cheaper speaker that would otherwise be receiving too much power.

You can find these at Crutchfield.com or at your local Best Buy store.. probably at Circuit City or Good Guys also.

As for your new question: You CAN run two speakers off of one channel. in my opinion unless you are going to do some magician work with positioning the speakers elsewhere in the cab, you are better off running one speaker on each channel. It is what is intended for the truck's door panel. You won't be able to put TWO 5.25 inch speakers in the rear doors unless you do some cutting.
I think that by running two speakers off of one channel you might lose some signal clarity and possibly even have a little distortion. You might even be under powering the speakers which can cause clipping and result in damage that way also. Might not happen, but... just get better speakers.
Splitting the channel really only works well when you have a MONO subwoofer amplifier and more than one subwoofer.
Good luck!
-Paul

pciley · 06-10-2004, 07:26 PM

Sorry I didn't spot this earlier.. Who told you that you have 5.25" speaker for the rear? Maybe I'm missing something?

Should be a 6.75", and 6"x9" for the fronts, right? I see plenty of 6.75" speakers on Crutchfield.com that can handle your RMS.

MaxPowers · 06-10-2004, 07:59 PM

For the non RF unit king cab it looks to be a 5.25 by taking off the cover on the rear door. Maybe a 6.5 could fit there?

sleightofhand · 06-10-2004, 11:36 PM

Speakers blow from to little power much more easily then to high. When choosing amp always get one that atleast matches the recomended minimum source. Many speakers that are rated to handle 100 watts continuous wont actually blow with upward of 500 watts. Clipping is caused by underpowered speakers. Clipping causes rounded waves to be chopped or squared off and prolonged will destroy speakers. The small amount of power your amp puts out(just in comparison to what it would take to blow a speaker with to much power) will not harm those speakers. Check out how efficient they are to figure out how much power you will need to produce the volume you want. You need to check at what voltage your amp is rated if its 14.4 then you probably are not even putting out as much as its rated at , alot of automobile electrical systems put out between 12.5 and 13.8 volts. If you need a second opinion check the web for info, always go high on power. By the way a speaker rated at 90db sensitivity rating is considered efficient.

sleightofhand · 06-10-2004, 11:49 PM

http://www.audiovideo101.com/learn/a...speakers22.asp.
Try the above website to get some general speaker info.

MaxPowers · 06-11-2004, 12:26 AM

So i read some of that and i kinda understand what is going on a little more. They are saying it is better to overpower the speaker rather than underpowering which like you said causes clipping. Here is a link to the amp that i have http://www.cardomain.com/item/ROCG16004

I was basically just thinking of putting some infinity speakers in the front and back but just wanted to make sure i didn't have to get the most expensive speakers out tehre.

stevemis · 06-11-2004, 05:19 AM

Quote:

Originally Posted by MaxPowers

So i read some of that and i kinda understand what is going on a little more. They are saying it is better to overpower the speaker rather than underpowering which like you said causes clipping. Here is a link to the amp that i have http://www.cardomain.com/item/ROCG16004

I was basically just thinking of putting some infinity speakers in the front and back but just wanted to make sure i didn't have to get the most expensive speakers out tehre.

That's good advice - tweeters especially do not handle distortion well. When I was a kid, my father upgraded speakers and he gave me his AR-3a's (ca 1960's). I went through tweeters like a madman driving them with my crappy receiver.

Now I've got a great set of 1980's speakers (AR-9LSi) and still have the original drivers. I can only attribute this to the amplifier, which has gobs and gobs of clean power.

When looking at an amplifier, ignore peak power - it's just a number the marketing people put on to make you think you're getting more than you actually are. You want to pay attention to RMS wattage, distortion and impedance. Specifically, you want the highest RMS wattage and lowest distortion you can find for your expected impedance (2/4/sometimes even 8) ohms.

-Steve