Understanding impedance/watts of my powered mixer and monitors.

[KipH]

Well, I'm finally starting to learn about impedance, wattage, and all that good stuff. I read what I could find on this forum as well as some stuff I randomly found via Google.

 

When piecing together my simple vocal PA for our band's rehersal space, I knew nothing of amps in relation to speakers. In my ignorance, I ended up purchasing a cheap Behringer PMX2000 powered mixer and 2 Wharfedale VS-12M wedge monitors.

 

I finally actually researched these things.

 

PMX2000 output: Bridged 500W/8Ohm or Stereo 250W/4Ohm

 

VS-12Ms: Continuous: 125W, Music: 250W, Peak: 500W. 8Ohms Nominal Imp.

 

So, this means that each speaker can take a maximum of 500W, correct? Does this mean that each speaker is most efficient at 500W/8Ohm?

 

I have had 1 250W-4Ohm output into each of the 2 monitors. So, that means that each monitor is receiving 4 ohms instead of the normal 8 ohms they are supposed to. If I use the bridge and chain the monitors, I will be getting the exact same result, right? The wattage and ohms will split between the two monitors at 250W/4ohms.

 

What effect does having less than the nominal 8 ohms have on the monitors? Also, what are the difference between the input ratings for the speaker (cont., music, peak)?

 

Ideally, should I have a 1000W/16ohm amp powering these 2 monitors?

 

Thanks.

[tony k]

[Craig Vecchione]

 

Originally posted by KipH Well, I'm finally starting to learn about impedance, wattage, and all that good stuff. I read what I could find on this forum as well as some stuff I randomly found via Google. When piecing together my simple vocal PA for our band's rehersal space, I knew nothing of amps in relation to speakers. In my ignorance, I ended up purchasing a cheap Behringer PMX2000 powered mixer and 2 Wharfedale VS-12M wedge monitors. I finally actually researched these things. PMX2000 output: Bridged 500W/8Ohm or Stereo 250W/4Ohm VS-12Ms: Continuous: 125W, Music: 250W, Peak: 500W. 8Ohms Nominal Imp. So, this means that each speaker can take a maximum of 500W, correct? Does this mean that each speaker is most efficient at 500W/8Ohm? I have had 1 250W-4Ohm output into each of the 2 monitors. So, that means that each monitor is receiving 4 ohms instead of the normal 8 ohms they are supposed to. If I use the bridge and chain the monitors, I will be getting the exact same result, right? The wattage and ohms will split between the two monitors at 250W/4ohms. What effect does having less than the nominal 8 ohms have on the monitors? Also, what are the difference between the input ratings for the speaker (cont., music, peak)? Ideally, should I have a 1000W/16ohm amp powering these 2 monitors? Thanks.

 

 

First, to understand the terms a little, understand that amplifiers produce a certain amount of power, expressed in watts, when presented with a speaker load having a certain impedance, expressed in ohms. Speakers don't "receive ohms" from an amp, they present a load, or resistance (more accurately impedance).

 

You need to know what power the amp produces when presented with various impedance loads. Then you need to know what the speaker impedances are. With this info, you can determine what's going to happen when you plug things in.

 

So in order to figure out your system, you have an amp that can provide 250 watts into a 4-ohm load, or 125w into an 8-ohm load.

 

Since your speakers are 4-ohms, the amp will produce 125w maximum to one speaker on each channel. If you daisy chain (run in parallel) the two speakers, and then plug the combined speakers into one channel of the amp, here's what happens;...the two 8ohm cabinets in parallel, will present a total impedance of 4-ohms (two similar loads in series would present double the impedance, or 16ohms). The amp is presented with a 4-ohm load and will provide 250w to this circuit. Obviously, each speaker shares this power, and "sees" 125w each. However, you've freed up the other amp, which can be used to power other speakers, if needed.

 

The usual rule of thumb for powering speakers is to provide 1.5 times the Continuous (also called RMS) power rating of the speaker at its impedance rating. So ideally in this discussion you'd want an amp capable of providing 200w into an 8-ohm load for each speaker, or 400w into 4-ohms if you daisy-chain the two cabs.

[tlbonehead]

Originally posted by KipH Well, I'm finally starting to learn about impedance, wattage, and all that good stuff. I read what I could find on this forum as well as some stuff I randomly found via Google. When piecing together my simple vocal PA for our band's rehersal space, I knew nothing of amps in relation to speakers. In my ignorance, I ended up purchasing a cheap Behringer PMX2000 powered mixer and 2 Wharfedale VS-12M wedge monitors. I finally actually researched these things. PMX2000 output: Bridged 500W/8Ohm or Stereo 250W/4Ohm VS-12Ms: Continuous: 125W, Music: 250W, Peak: 500W. 8Ohms Nominal Imp. So, this means that each speaker can take a maximum of 500W, correct? Does this mean that each speaker is most efficient at 500W/8Ohm? I have had 1 250W-4Ohm output into each of the 2 monitors. So, that means that each monitor is receiving 4 ohms instead of the normal 8 ohms they are supposed to. If I use the bridge and chain the monitors, I will be getting the exact same result, right? The wattage and ohms will split between the two monitors at 250W/4ohms. What effect does having less than the nominal 8 ohms have on the monitors? Also, what are the difference between the input ratings for the speaker (cont., music, peak)? Ideally, should I have a 1000W/16ohm amp powering these 2 monitors? Thanks.

No offense,but you really aren't grasping it yet. Impedence is amount of resistance,measured in ohms. Your speakers provide the resistance(and a small amount from the speaker wire,etc)which in this case is 8 ohms. That is a constant. Your speakers don't "receive" a certain impedence,they provide it. Therefore,to know how much power your amp is putting out,check the ratings @8 ohms/channel. It will be somewhat less than the 4 ohm rating,depending on the amp's design. Probably in the 150-175 watt range. In most cases,you want to be somewhere in between the RMS rating and the music/program rating. It looks like you are ok. If you ran two of the cabs on one channel of the amp(in parallel,or daisy-chaining them)your amp would see roughly 4 ohms of resistance and would deliver the 250 watts @4 ohms to the speakers.(about 125 to each) And you never want to run an amp at a lower impedence than its minimum rating. Does this help?

[Craig Vecchione]

*obviously tlbonehead was typing his explanation at the same time as I was mine* :D

[KipH]

If I plug one speaker into the bridge connection on the amp (mixer) I will get 500W into that one speaker, and it's impedance is 8 ohms. The bridge output can handle a 8 ohm load, so the wattage is unaffected.

 

If I plug a speaker into each of the 2 powered outputs (the non-bridge connections), each speaker will get 250W at 8 ohm, but the amp can only handle 4 ohms on each of the 2 outputs. So, it will only be able to provide 125W @ 4 ohms instead of 250W @ 8ohms.

 

Now, say I had 4 ohm impedance speakers as opposed to 8 ohm, and used both powered outputs, each of the 4 ohm speakers would receive 250W.

 

Is this right?

[tlbonehead]

Originally posted by KipH If I plug one speaker into the bridge connection on the amp (mixer) I will get 500W into that one speaker, and it's impedance is 8 ohms. The bridge output can handle a 8 ohm load, so the wattage is unaffected. If I plug a speaker into each of the 2 powered outputs (the non-bridge connections), each speaker will get 250W at 8 ohm, but the amp can only handle 4 ohms on each of the 2 outputs. So, it will only be able to provide 125W @ 4 ohms instead of 250W @ 8ohms. Now, say I had 4 ohm impedance speakers as opposed to 8 ohm, and used both powered outputs, each of the 4 ohm speakers would receive 250W. Is this right?

You are corect except on the part about plugging an 8 ohm speaker into each of 2 powered outputs. As I stated before,your amp will put out less power at 8 ohms than at 4 ohms. So the amp will be putting out less than 250 watts to each speaker when they are 8 ohm speakers. Check your manual or the full specs,it should tell you how much power each channel puts out @ 8 ohms,as well as 4 ohms.

[KipH]

Originally posted by tlbonehead As I stated before,your amp will put out less power at 8 ohms than at 4 ohms. So the amp will be putting out less than 250 watts to each speaker when they are 8 ohm speakers.

 

Yeah. That's what I said, or at least what I meant.

 

Since each powered output can handle a 4 ohm load, when an 8 ohm speaker is attached, it can only provide that speaker with 125W instead of 250W. If the speaker was 4 ohm, it would get 250W.

 

My reading continues...

[tlbonehead]

Originally posted by KipH Yeah. That's what I said, or at least what I meant. Since each powered output can handle a 4 ohm load, when an 8 ohm speaker is attached, it can only provide that speaker with 125W instead of 250W. If the speaker was 4 ohm, it would get 250W. My reading continues...

But it isn't automatically half. Every design is a little different. Check your manual and it should tell you how much power/channel it puts out at 8 ohms.

[KipH]

I understand. I was just talking theoretically, assuming it was 8.0, etc.

[tlbonehead]

Originally posted by KipH I understand. I was just talking theoretically, assuming it was 8.0, etc.

Ok,I did your work for you. Behringer rates it at 180 watts/channel at 8 ohms. Don't forget,those are Behringer watts.

[KipH]

hahaha.. Thanks

[Craig Vecchione]

Originally posted by KipH hahaha.. Thanks

 

He laughs now....

[Tommy Tune]

I love this place.

[RickJ]

 

tlbonehead wrote: Impedence is amount of resistance,measured in ohms.

 

Nit-picking: Resistance is unit of measurement in a direct-current (DC) circuit. Impedance is a unit of measurement in an alternating current (AC) circuit.

 

Although both DC and AC circuits use Ohms as their unit of measure, they are referenced to two entirely different sets of circumstances.

 

Sound signals are always AC.

 

In a DC circuit, an ohm of resistance is an ohm.

 

In an AC circuit, an ohm of impedance is dependent upon the resistance, capacitance, frequency and amplitude.

 

IOW, a pure DC resistive circuit doesn't change. An AC circuit is constantly changing. That's why you'll usually see the word "nominal" when you see the word "impedance." It's been averaged for us spec-readers.