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#481592 04/28/09 08:14 PM
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I've been talking to my roommates about some car audio stuff and even though I have been doing sound for a while I find myself confused.

1)Say you have a 2 channel amplifier rated 150W at 8 Ohms.
Putting one speaker on each side is fine and it does not change the impedance of the amp. However when you start adding more speakers, it changes the impedance. I was getting series and parallel mixed up in terms of how it changes the impedance.
Can someone clarify this for me?

2)With car audio, most speakers and amps are rated at 2 or 4 ohms. What's the reason for using a lower impedance?

3)Someone was asking me if speakers are AC voltage or DC voltage. I was pretty sure that it's AC...was I correct?

4)When an amp is measured in RMS, is this more or less the safe amount of power it can put out?

As always, thanks in advance!


Jeff Moss

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Jeff check this out it may help you web page


1. Speakers in parallel will lower the load on an amp drawing more power. Speakers in series will combine all the voicecoil impedance and split the power between them.


2. The lower impedance is to draw more power.

Power= Voltage²/Impedance
If you start out with 12 volts then you get the following.

12²=144
144/4 ohm = 36 watts
144/2 ohm = 72 watts


3. Yes audio is AC that has multiple frequencies (20Hz-20,000Hz is normal range) unlike your homes electricity which is at a constant frequency (60Hz in USA)

4. It is best to calculate the usable amp power at about 2/3 of the rated power
Amp with 100 watt RMS I would expect to be able to drive at about 65-70 watts.


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1. Impedance is perceived inversely as one would expect. Taking two 8 ohm speakers and wiring them in parallel will result in a 4 ohm load. People would think that 8 + 8 = 16, right? Wiring them in series instead will result in a 16 ohm load. By mixing and matching series and parallel speaker wiring combinations in an effort to maintain the 2, 4 or 8 ohm expectations of the amp, many speakers can be added. It is important that any speakers that are part of series circuits have the same power ratings though.

2. I've often asked the same question, Jeff. My guess would be to make the systems more tolerant of people not understanding your first question. Several speakers can be wired in parallel and still be supported by an amp that will work with two ohm loads.

3. AC at a tremendous range of frequencies, unlike our fixed 60 Hz AC power.

4. I'm probably going to get shot down in flames on this one since I always screw this up. RMS ratings are like VA ratings on a UPS. An amp with a rating of 1500 watts RMS has a true output of 1200 watts. RMS takes into account the loss associated with power loss. There's also the shaping of the wave form that affects RMS vs: label ratings.

With AC power, we take amps (A) times volts (V) to equal watts (W). Variations of this equation will determine amps, volts, etc. These are in a perfect world where there is no loss in wiring or components in the circuit, which can never happen. A phone system may have a wattage of 500, but the VA (Volts times Amps) load is higher due to losses.

What I'm trying to say as I confuse myself is that the wattage and VA rating are different because of losses that need to be factored (power factor multipliers). The power rating vs: the true RMS ratings follow similar differences.

I'm hoping one of our audiophiles, like Merritt will chime in on this one.


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series adds ie: 2x 8ohm in series is 16ohms.
parallel divides ie: 2x 8ohm in parallel is 4ohm. 4x 8ohm in parallel is 2ohm.

amplifier ratings: don't get me started..

amps are normally rated for ~x watts at 8ohms, ~2x watts at 4ohms, etc.. by rating their amps at a lower impedance, they can give the impression that it is more powerful than it really is.

Think of it this way, the higher the impedance, the harder the amp has to work to get the same output level. The lower the impedance, the more current the amp has to supply.

Amps generally don't like to drive low impedance loads, normally 4 ohms is as low at the pro-audio amps used to go, although you are seeing more amps these days that are rated down to 2ohms. I don't think I've ever seen one that was rated down to 1ohm, except possibly for the Mcintosh amps..

speakers are AC.

There is an art to properly matching speakers and amplifiers..

In the pro audio world, you generally want to oversize the amp by 30-50% or so, but it depends a lot on the application and the speaker. You want to be able to drive your speakers close to their limits, without blowing them up, and you want it to sound good while you do so.

You want to allow for some amplifier headroom, to prevent clipping, and you want to avoid sending the speaker cones flying across the room, melting the voice coils, or blowing up the crossover (if you are using passive crossovers).

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RMS ratings are like VA ratings on a UPS. An amp with a rating of 1500 watts RMS has a true output of 1200 watts. RMS takes into account the loss associated with power loss. There's also the shaping of the wave form that affects RMS vs: label ratings.

Sorry Ed, but nope.

An AC voltage normally is expressed as a peak or peak-to-peak value which would be it's excursion from zero to maximum (peak) or it's excursion from it's maximum negative value to it's maximum positive value (peak-to-peak). A scope is one way to visualize and perform these measurements.

RMS, which stands for Root Mean Squared is a measurement method that indicates the heating value of an AC voltage if it were applied to a resistive load. It came about back in the days when Edison was converting power from DC to AC as a means of comparison. An RMS AC voltage is supposed to provide the same heating power as a DC voltage of the same value.

Most good meters today are "true RMS" and will indicate an AC voltage accurately in RMS, even complex waveforms.

-Hal


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I recently tried to tackle this question for my recent theater production.

I use a very basic 30-watt(max) home-stereo amplifier to drive a 1980's vintage pair of Kenwood speakers, three-elements, the woofer being 10". This sounded fine for years, just for music and sound effects.

When we added stage mics, I quickly realized that these speakers didn't push the mic sound to the back of the audience, without feedback, of course. So I picked up a pair of 8" bookshelf speakers, and hung them about half-way in the room, where our average audience back row would be. Wired in parallel. It works fine, sounds OK, the mic levels still can't go as high as I'd like.

This last show had one actress that couldn't even project (with British accent) to the first row. Even listening in the headphones, I had a hard time hearing her. My stage manager, had a set of DJ-size speakers and a 200 watt amp. My home-stereo amp had been fed by the tape-out on the mixer, so I still had the standard line-out available. I fed the big amp with the line-out. And mounted the Kenwoods in the rear instead of the bookshelf speakers.

STUPID us, we did this on opening night, and failed to try it out with complete silence... as the curtain went up and we were greeted by a very noticeable hum. A hum so bad, that the reviewer moved to the back row (where the hum wasn't noticeable, because the speakers back there were fed from the home-stereo amp) at which point he couldn't hear the actress very well. As I frantically tried everything on the mixer, I finally cut the amp, and the hum went away. It was so bad I had to buy my producer an extra drink after the show to make up for it.

After the show (and drinks) he and I stayed late and fiddled with everything. No matter what, the amp seemed to be the culprit. We tried the church's powered mixer, and then all we heard was a loud static. Then we also tried the church's speakers, and the static was less pronounced. (difference in speaker construction - plastic vs wood/carpet) I even tried breaking the ground-path, which only solved a hum coming from my computer. (which my producer never complained about until now, even though I knew was there all along) Finally, I adapted my red/black pin speaker wire to 1/4" and fed the speakers directly with my little amp. No hum, the static was barely perceptible from 5 feet. Music sounded fine, and the stage mics could be heard very well, without being turned up to feedback. The home-stereo amp was still only turned up to half-dial.

We ran like this for the rest of the performances, and I got compliments from a number of people on the sound.

I was totally surprised by this.. and I'm not sure why. I thought that PA-level speakers wouldn't work with home-stereo-level amps... not enough voltage or something. So now I was running both speakers in parallel, both specified to be 8 ohm loads, so I know that overall, each channel is still representing 4 ohms to the amp, just as before, only the speakers involved are larger.


Rob Cashman
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Without being there it is difficult to say what the problem is. Your description is also confusing. All I get is that you changed to speaker connections to 1/4" TRS plugs and your hum problem went away.

-Hal


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No, ultimately, instead of trying to drive the big speakers with a big amp, I'm driving both big and small speakers with a little amp. I am now just as confused about how to properly match speakers as Jeff was.... It's obvious not to drive 100 watt speakers with a 400 watt amp... but what about a 30 watt amp and 200 watt speakers?

Something that occurs to me, is that the amp only had XLR inputs, which meant that we had to use this guy's DJ mixer (with XLR outputs) to convert the line-level to the XLR. That mixer may be the culprit, if it didn't properly balance the line. (The thought of 1/4" TRS is what made me think of this - the speakers are just Tip and Shield)


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It's obvious not to drive 100 watt speakers with a 400 watt amp... but what about a 30 watt amp and 200 watt speakers

Certainly can, isn't that what the DJ guy was doing? Crank it up and you will blow the speakers though (assuming 400W/ch). As far as the 30W amp, nothing wrong there either- if you have enough power to do what you want.

The speakers had nothing to do with the hum, how you connected the amps did.

-Hal


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