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  #1  
Old 08-02-2010, 02:42 AM
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larrylover larrylover is offline
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Default OMG OHMS. what the hecka is impedance?

here is the deal. I've got an amp with a stereo and bridge mode. it also has a 1.4v or 26db sensitivity switch. SO how do i power two 8 ohm speakers with this amp? found thisw but i am still confused. http://www.crownaudio.com/pdf/legacy/125645.pdf

i know its not a drum question, but it directly affects my drumming experience.
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  #2  
Old 08-02-2010, 04:22 AM
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Default Re: OMG OHMS. what the hecka is impedance?

Leave the amp in stereo mode and you can power two speakers as normal from the two outputs.
You only want to be in Bridge-Mono mode when you're only powering one speaker (usually a sub). You said you have two speakers, so like all amps, leave it in stereo mode, and connect speakers as normal. The manual looks like the normal mode for the amp is 26dB. You switch it over to 1.4V mode when you're running only one output.

Sorry, forgot about your original question. Impedance is a measure of resistance. And in this case, this is determining how many watts will be output from the amp. Speakers are the "resistance" being put on the circuit. In this case, mostly all speakers are 8 ohms, and in this case, your amp is rated for a certain amount of wattage at a certain amount of ohms. So, your amp is probably putting out the correct wattage because it's matched with the correct ohm-age of speakers. If you want your power to come up quite a bit, do this: daisy-chain your two speakers together and plug into either the left or right side of your stereo output. When you daisy-chain the speakers, the ohms get cut in half, so now each speaker is only resisting 4 ohms each, but you have two of them in line, so the measured output should be the same.

If you could "re-ohm" your speakers to 4 ohms each (I had a pair of big Yamahas that had a knob to select ohm-age), then the amp will put out more wattage (usually almost double, that's that rating you see in the ads). The trade-off is that the less resistance, you get more power, but your signal sounds grittier the less ohm-age you have. Does that help?
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Old 08-02-2010, 06:36 PM
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Default Re: OMG OHMS. what the hecka is impedance?

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Originally Posted by Bo Eder View Post
Leave the amp in stereo mode and you can power two speakers as normal from the two outputs.
You only want to be in Bridge-Mono mode when you're only powering one speaker (usually a sub). You said you have two speakers, so like all amps, leave it in stereo mode, and connect speakers as normal. The manual looks like the normal mode for the amp is 26dB. You switch it over to 1.4V mode when you're running only one output.

Sorry, forgot about your original question. Impedance is a measure of resistance. And in this case, this is determining how many watts will be output from the amp. Speakers are the "resistance" being put on the circuit. In this case, mostly all speakers are 8 ohms, and in this case, your amp is rated for a certain amount of wattage at a certain amount of ohms. So, your amp is probably putting out the correct wattage because it's matched with the correct ohm-age of speakers. If you want your power to come up quite a bit, do this: daisy-chain your two speakers together and plug into either the left or right side of your stereo output. When you daisy-chain the speakers, the ohms get cut in half, so now each speaker is only resisting 4 ohms each, but you have two of them in line, so the measured output should be the same.

If you could "re-ohm" your speakers to 4 ohms each (I had a pair of big Yamahas that had a knob to select ohm-age), then the amp will put out more wattage (usually almost double, that's that rating you see in the ads). The trade-off is that the less resistance, you get more power, but your signal sounds grittier the less ohm-age you have. Does that help?
Yes that helps a lot. thanks!! but if I daisy chain, will I be in any danger of heating something and damaging my stuff?
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Old 08-02-2010, 08:10 PM
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Default Re: OMG OHMS. what the hecka is impedance?

Larry. The basic thing is resistances sum, or divide. In series they sum, in parallel they divide.

Maybe a bit counter intuitively, daisy chaining is hooking the speakers in parallel. You aren't changing the connections, just running multiples together. The pluses are all hooked together, and the minuses are all hooked together. It doesn't matter if you string the connections from cabinet to cabinet, or hook them to multiple jacks on the back of the amp (which some PA and guitar amp heads have).

Series would be where the signal flows though each speaker to the next, using the speaker as part of the circuit. i.e. going from the + terminal of the speaker, through the speaker, out the - side and into the + side of another speaker, out it's - side and back to the amp. The problem with series connections is that if a speaker blows, it opens the connection and you have nothing out of anything, and in the case of a tube guitar amp which typically doesn't have the built in fault protection of a modern SS amp, leaving the output open while playing into it can damage the amp. For this reason you typically only find series connections as part of a combination of series and parallel. Most common example would be a four speaker guitar cabinet.

Now you may have already wrapped your head around the math of the 4 speaker cabinet. Four 8 ohm speakers if run in series would add up to 32 ohms, far too high. Run all in parallel it ends up at 2 ohms, far too low. So if you hook two speakers each in series, each pair would be 16 ohms (two 8 ohm speakers summing), then if you hook the two pairs together in parallel, the two 16 ohm pairs divide down to 8. Now you're back to your original 8 ohm load.

Impedance is basically the reactive resistance of something. A light bulb is mostly pure resistance. It doesn't do anything except resist electrical current flow and use that resistance to create enough heat to generate light. But if you could imagine the instant of turning it on, the cold wire has less resistance than it does when it's hot, so for that brief moment, the resistance rises while the electricity is going though it.

In a speaker, there are bunches of things going on. Mainly, you're using the electricity to yank a mechanical cone back and forth. Basic Newtonian laws say that something put in motion wants to keep going that way. Then you are trying to reverse the electricity and yank it back. So for a moment it acts like a generator and then it sucks extra electricity to reverse it’s direction. Then once it gets going, it doesn’t need as much power to keep going, until you try to reverse it again.

All this means that the amount of resistance, or load, the speaker shows to the amplifier’s output, varies all the time. It isn’t pure resistance. And it varies with frequency depending on things like the strength of the magnet, the weight of the cone, porting on the cabinet which resists the cone’s moving at some frequencies and provides no resistance at all at other frequencies, and many other factors. Typically the average impedance is what is written on the speaker and what people go by, unless you are a speaker designer and trying to get all the factors to play well together.

Back to your question of running your amp in bridge mode. The basic thing to remember is that bridge mode halves the load on the amp. So if you run an 8 ohm load on a bridged amp, you are actually running the amp at 4 ohms. This is because in bridge mode, each channel of the amp is acting on one half (either the positive or negative swing) of the signal.

So, in your case, if you want to hook two 8 ohm speakers to your amp in bridge mode you have to first calculate the speaker load. 8 ohms divided by 2 speakers leaves 4 ohms total load. Then you halve that for bridge mode, leaving a 2 ohm load on the amp. Looking at the attached data sheet for the amp, it is rated for a 2 ohm load. It also lists a power output into a 4 ohm load in bridged operation, meaning to finally directly answer the original question, it is okay to hook two 8 ohm speakers to it in bridge mode. In fact, that gives the highest power output.

Now, if you daisy chain more than two speakers, you have to recalculate the load. E.g. if you have three 8 ohm cabinets, 8 divided by 3 gives 2.6 ohms. Since the amp is only rated down to 4 ohms in bridged mode you can’t daisy chain 3 cabinets to it. But each channel running normally is rated down to 2 ohms, so you could run 3 cabinets off of one channel.

There is some debate in the pro sound community on using bridged operation however. One side wants more power. And there is engineering math to justify running around twice the amplifier power into a speaker’s rating to optimize performance (e.g. 1200W amp into a 600W rated speaker). So when you have 2000W speaker cabinets, you kind of need to run amps in bridged mode to get the 4000W that would optimize their performance. But running flat out in bridged mode (like at a large concert, not what will happen at your corner bar with a band, DJ’s are another kettle of fish altogether) means that the amp is passing all the electrical current it can. Meaning it will run harder and hotter. More distortion and less reliability, which means something at the varsity level.

Now I hope you’re thoroughly confused.
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  #5  
Old 08-02-2010, 08:32 PM
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soham2rescue soham2rescue is offline
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Default Re: OMG OHMS. what the hecka is impedance?

being a electrical-electronics student.. technically speaking..

a A.C curcuit mainly consists of three components: resistor, inductor and capacitor...

magnitude of resistor is resistance...R
of inductor is inductance is.... L
of capacitor is capacitor is... C

the opposition of resistor is called resistance.. R
of inductor is called inductive reactance.. XL
of capacitor is called capacitive reactance Xc

here R=R
XL= WL
Xc = 1/Wc ...........{ here W (omega) is the angular speed of the AC current.. where W = 2 x pi x f ... (here f is frequency of AC current) }

the total opposition provided to the AC current in this circuit due to these three components is called impedience [Z]

given by formula
........_______________ ........................____________
Z = V [ R + ( XL - Xc ) ] .....................{ V indicates square root }


i wrote this all by myself.. m proud.! :D
thanks dude.. u made me a effortless revision of the last chapter i studied!

i hope it helped!
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  #6  
Old 08-02-2010, 08:39 PM
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Default Re: OMG OHMS. what the hecka is impedance?

Quote:
Originally Posted by larrylover View Post
Yes that helps a lot. thanks!! but if I daisy chain, will I be in any danger of heating something and damaging my stuff?
Sorry, I don't have as complete answers as the two posters below, so I hope I'm not confusing you anymore.

But to attempt an answer, I think you would be in danger of damaginging stuff if you daisy chain too much. As I said, as you daisy chain, you're splitting ohmage. If you have an 8 ohm load that breaks down to 4 ohms when you daisy chain two speakers, sooner than later you'll have no ohmage, which means it's pure power going to each speaker, and your sound will definitely sound wrong. I haven't tried this though, I probably should so I can say I know what will happen when you do. I would say don't daisy chain more than two.

BUt if you only have two speakers you're good, yes?
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Old 08-02-2010, 11:16 PM
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Default Re: OMG OHMS. what the hecka is impedance?

In simple terms, think of it like a bunch of knights trying to get though a castle gate. If the gate is too small (too much resistance) not many get out and you have no power. But if the gate is too big, you have the whole brigade charging though at once and there's nobody to back them up and keep the flow going.

By convention, most speakers are in the 4-16 ohm range. The standard middle ground being 8 ohms. So the amps are designed to work against that resistance range.

So, you take the impedance rating of the speakers and divide by the number of speakers in parallel to get the load or resistance the amp will be pushing against. You'll never have no resistance at all as Bo opined, but at some point, the floodgates will be open so to speak, and the amp will be spewing it's little guts out with electrons charging ahead. And the reserves will run dry while the gate is overheated by the activity.

A lot of times, if you're not running real loud or bass heavy stuff, you can get away with less load than the amp would like, because it has the capacity to handle that much current flow. But when things get cranking, it will overheat and go into self imposed thermal shutdown. Picture, but no sound. Unlikely that a Crown CE will actually burn up the first time you do this. But if you keep at it and keep running it into protection mode, cooling it off and then overheating it again, you may run into problems.
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  #8  
Old 08-02-2010, 11:32 PM
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Default Re: OMG OHMS. what the hecka is impedance?

Larrylover - since I do have a somewhat rudimentary understanding of this subject, albeit not nearly as comprehensive as Aeolian or Bo's (or especially Sohan2rescue's), if you'd be willing to send that wonderful little Crown amp to me, I volunteer to put it through it's paces, figure out exactly what it can do in a variety of configurations, write up a full report and send it back to you with said documentation so you can fully optimize it's capabilities.

I only offer this out the goodness of my heart and my desire to help you out. It's got nothing to do with a personal desire to use an amp with such a distinguished lineage - honest. ;)
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  #9  
Old 08-02-2010, 11:40 PM
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Default Re: OMG OHMS. what the hecka is impedance?

As a matter of fact, many pro series amps are speced to be run into loads as low as 2 ohms. Many of our feeders are either number 12 or 10 gauge stranded conductor cable to insure very low resistance from the amp the the arrays.

Dennis
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Old 08-04-2010, 04:46 AM
Numberonefan Numberonefan is offline
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Default Re: OMG OHMS. what the hecka is impedance?

Quote:
Originally Posted by audiotech View Post
As a matter of fact, many pro series amps are speced to be run into loads as low as 2 ohms. Many of our feeders are either number 12 or 10 gauge stranded conductor cable to insure very low resistance from the amp the the arrays.

Dennis
Yea but its terrifying! :-)

I have a QSC1850HD designed to run at 2 ohms of both sides all night long…. Yea right! They can say it in the literature but even with a respected company like QSC when it’s my own equipment I worry… I ran 8 monitors (1mix) of that amp one night for a choir. It lived! LOL
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  #11  
Old 08-04-2010, 10:37 PM
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Default Re: OMG OHMS. what the hecka is impedance?

@Aeolian Wow. ok I think I've got a handle on this now. thanks for the in depth review for the n00b :D

@soham2rescue that class sounds like a punch in the face. I think I am taking it next year lol.


@Bo Eder yup. just 2 speakers. dont have to worry about any 8 monitor mixes like Numberonefan... lol


Thanks guys. When I get home I'm gonna make some noise. hopefully not the sound of blowing speakers. Gonna hook 'er up to some old yamahas and take that as a small PA to school. Otherwise its hooked up to 2 kustom PAs
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