First Audio Build (Need Math Help!)

[MidnightMonkey]

Ok smarties. Here's what I've got:

Pioneer DEH-80PRS
Polk MM6501 Components

I was stoked that my head unit came in the mail yesterday until I was reading that it's highly recommended that you don't hook it up to speakers with an impedance <4 ohms. I was going to just use the stock speakers until I picked out an amp, but it seems that that's out of the question as I don't want to fry the amp circuitry in the HU.

SO. I have these Polk speakers I got last Christmas (bite me, I'm slow to finish stuff). Handling for each mid/tweeter is 125w @ 2.7 ohms. The box says "Compatible with 4 ohm Outputs", which leads me to believe that means when used in conjunction with the supplied crossover (i.e. total impedance is the sum of each component in series). I'll probably use the thing unless someone can recommend against it from experience.

That makes it kind of difficult for me to pick out an amplifier. If:

power(w) = voltage^2/impedence

Then should I calculate an amplifier's output for 2.7 ohms or 5.4 ohms?

I'd like to be within about 110-112.5 per channel, I think. Isn't too little juice just as bad as too much? Not looking to use any other speakers with whatever amp I get either. And I don't want to spend a freakish amount of money. Sorry if this is super noobish or if I'm totally wrong in my understanding of how this works. Any help would be appreciated! (do I hand out bananas or potatoes for long posts?)

[Chimera]

The components impedance will be 2.7 as seen by an amp. you want to select a 2ohm stable amp.

The HU will recommend 4ohm+, but should be fine with 2ohm from the OEM, just don't push it. at least not until you have the rest of your equipment in.

[TOOSTUBBORN2FAIL]

Also keep in mind, the only things that can damage a speaker is mechanical damage and thermal damage. Mechanical damage is either caused by mishandling a speaker or sending it too much power at too low of a frequency causing it to bottom out. Thermal damage is caused by sending it too much average power over time. Clipping will not cause damage unless you are sending it too much average power over time.

[MidnightMonkey]

Originally Posted by Chimera

The components impedance will be 2.7 as seen by an amp. you want to select a 2ohm stable amp.

So, even when the components are wired as such:



The impedance will still be 2.7 ohms? I thought that the impedance would be the sum of both, since they're drawing from the same channel.

Or, would it be better SQ wise to run each speaker off it's own channel from the amp?

[TOOSTUBBORN2FAIL]

It would give far more control to run them actively off their own channels, and ditch the passive crossover completely. This is possible with your head unit, and better, since you can accurately control the crossover frequency, slope, level, and time alignment.

[Chimera]

it may be possible.

The picture you have shows 2 sets of components, each on their own channel. with that configuration each channel would have 2.7ohm load.

IF you ran them off one channel and wired them in parallel then the impedance would be 1.35. I would recommend against this and you'd be hard pressed to find a reason to do so.

IF you ran them in series then then impedance would be 5.4. I would also recommend against this.

the impedance for a load is measured per channel, not per amp. so if a given amp has 4 channels, you can run 4 loads on it. IF the amp is bridgeable (many are) then you can convert 4 channels to 2 channels usually with a higher load tolerance (ie 2ohm stable usually will become 4)

For those speakers if you don't care about the back you want to find a 2 channel 2ohm stable amp that puts out somewhere around 45-125W.

If you are planning on adding more components you can change up some of this.


FOR EXAMPLE
my current build has a 3 way crossover (audiopipe crx-303)
hanging off it I have an infinity 6.5, 3.5, and tweeter, and its input is a 45W ch from an alpine ktp-445u
even though I have 3 (5 really) speakers hanging off 1 channel on the amp, the total impendence for the load is still 3 (the speakers all have 3 ohm impedance on their own)

I additionally have a 4" coax run off the other 2 45w channels in the rear.

[TOOSTUBBORN2FAIL]

Don't forget however, that the crossover affects the impedance as well. I'll try to look up the speaker later, but I'd bet the 2.7 rating is with the crossover in place and both speakers on one channel. But I'll try to double check tonight. It's important to remember that the crossover affects it though.

For instance, I once had a set of infinity kappa perfect 6.1's. Infinity rated them at 4 ohms. But they also gave an impedance chart. While for the majority of the frequency range, they were around 4 ohms, at the crossover frequency they dipped to 1.5 ohms. Very few amps tolerate that. However, I could remove the crossover and run them active, with no issues at all, because the crossover was causing the dip in impedance.




I would still recommend using a 4 channel amp, and running them active. It's just better than any passive crossover, period.

[stugray]

Two things regarding speakers:
1 - Impedance is a little more complicated than just adding "Ohms" together.
The "Ohms" mentioned in the impedance spec of a speaker is different than the "Ohms" of a simple resistor.
They don't add in series and parallel like a resistor network, so be careful.
They are frequency dependent and have a complex component (remember 'i' the square root of -1 from math class?)


AND


2 - I have heard TOO MANY times "My speakers are rated at 40 watts and my stereo is only rated at 30 watts so it is impossible to blow the speakers" - WRONG!
As mentioned above, a speaker can be blown (ruined) by overheating which can be caused by impedance mismatch or driving the speaker with too much distortion.
So you could blow a speaker playing slipknot but NOT blow the same speaker at the same volume playing classical Beethoven.

[TOOSTUBBORN2FAIL]

Distortion by itself can not kill a speaker, that is a myth that needs to die an ugly death.

You mentioned that a speaker that survives beethoven may be killed by slipknot. If that happens, it is because of user error in the lack of volume control. Slipknot is recorded louder, with much higher levels on average than beethoven. That means if you keep ot at the same volume with both types of music, the speakers will see more heat due to the increased average power over time with slipknot.

It is not due to distortion, but too much heat building up because of too much power for the coil to thermally handle.

You could run a 10 watt square wave to a speaker that can handle 50 watts thermally, and it would not fail.

[TOOSTUBBORN2FAIL]

Also, it after reading the manual, the speakers are rated at 2.7 ohms as connected to the amp as a component set with the crossover, so you would be fine running it off almost all 2 channel amps, although I still highly recommend running them active off a 4 channel amp.

[stugray]

Originally Posted by TOOSTUBBORN2FAIL

Distortion by itself can not kill a speaker, that is a myth that needs to die an ugly death.

You mentioned that a speaker that survives beethoven may be killed by slipknot. If that happens, it is because of user error in the lack of volume control. Slipknot is recorded louder, with much higher levels on average than beethoven. That means if you keep ot at the same volume with both types of music, the speakers will see more heat due to the increased average power over time with slipknot.

It is not due to distortion, but too much heat building up because of too much power for the coil to thermally handle.

You could run a 10 watt square wave to a speaker that can handle 50 watts thermally, and it would not fail.

Attempting to drive a speaker with a mismatched impedance can over heat the speaker drive coil, the amplifier, or both.

Another thing that can overheat a speaker is by driving it in a way that it cannot respond.
If a speaker is given a drive current that it cannot respond to (say too high of a frequency on a very large {heavy} cone) the result is not movement of the cone, it generates heat in the coil.

So if you have a set of speakers that cannot respond to a large range of frequencies yet you try to drive it simultaneously at a large set of frequencies, the speaker coils will get hot.
Another way to say "large set of frequencies" is a "spread spectrum".

Classical music is typically composed of a small number of pure frequencies so will look like a forest of trees on a spectrum analyzer.
This type of signal is typically easy for a speaker system to respond to.

The example of Slipknot is a perfect example of a wide-band spread spectrum "NOISE".
This type of signal will look like more level "sea" or noise floor on a spectrum analyzer.
Speaker systems typically cannot respond to NOISE as well as they can respond to TONES.

Trying to drive speakers with noise will invariably heat the coils more than driving them at pure tones (within the response of the speaker coil/cone system).

Therefore I will repeat:

"So you could blow a speaker playing slipknot but NOT blow the same speaker at the same volume playing classical Beethoven."

Driving at high levels of distortion is the same as adding noise.

[Xbdition]

How about some practical direction? If you want to be absolutely sure your amp will match your speakers, buy a DMM (doesn't have to be expensive), measure what the actual (vs stated) impedance is after the crossover (please spare me geeking out over impedance vs frequency response), and make sure your amp can handle it.

Going active has its benefits in the right circumstances, but if the crossover supplied with the set you have was well matched and engineered to the speakers, you may end up taking the long way home for no reason.

Keep it simple, try out what you have and if you're happy, job done.

[stugray]

Originally Posted by Xbdition

How about some practical direction? If you want to be absolutely sure your amp will match your speakers, buy a DMM (doesn't have to be expensive), measure what the actual (vs stated) impedance is after the crossover (please spare me geeking out over impedance vs frequency response), and make sure your amp can handle it.

Keep it simple, try out what you have and if you're happy, job done.

That's what I am saying: Take a "4.0 Ohm" speaker and measure the resistance across the coil, it will not be 4 ohms.

Impedance is not the same as resistance.
You cant just measure it with a DMM and add it up like they are resistors.

[Xbdition]

Does all the discussion about whether distortion kills speakers or not really matter? The pointless distinction that people make is that its not distortion "alone" that kills speakers. So what? Would you ever recommend someone drive their amps with a distorted signal even if they only listen at low volumes? The added harmonics may not sound bad at low volumes, but it would still be a stupid suggestion to run a distorted signal.

Take for example a speaker rated for 50watts thermally. Why even discuss whether that speaker could thermally handle 10 watts of a square wave? That 50 watt thermal rating is not for a clipped signal and a speakers thermal rating goes out the window when you are sending a square wave. So much hot air. See what I did there?