A few questions about AVR / Speaker impedence

[aris3256]

I'm having a hard time understanding how impedance works, and am not finding any good sources that answer the questions I have, so I'm hoping that one of you can provide some clarity.

My current AVR is a moderately old Denon AVR1611, with 75w/ch @8ohms. On the back panel, it indicates that the speaker impedance for the fronts, center, and surrounds should be 6 - 16ohms. If I understand this correctly, the 8ohm rating just allows them to specify the watts/channel, and the 6ohm rating indicates that it can run 4 or 8ohm speakers (not sure about this).

The reason why this matters is that I'm trying to buy a new set of speakers, and most of the ones I am gravitating toward (e.g., Airmotiv B1+, Elac DBR62, and Polk R200) are 4 ohm speakers. The Polks are en route, and indicate 4 / 6 / 8 ohm impedance, which I don't understand.

Here are the pieces of advice I've found that I'm having trouble squaring:

• I've seen in several reviews of these speakers that, because they're 4 ohm impedance, typical low/mid AVR's will have trouble powering them. I run into this advice a lot.
• Other people say
  • it should be fine if only the fronts and maybe center are 4 ohm, you'll just need to turn the volume up (but not too much--I tend to keep the volume moderate anyhow).
  • it helps if you have a higher crossover with powered subs, which will take some of the demand off of the fronts (I have two 10" Monoliths, so this wouldn't be a problem).
  • if low/mid AVR's couldn't handle these 4 ohm speakers, both the AVR makers and speaker makers would be shooting themselves in the foot, so of course they'll be able to drive those speakers.

For those of you who understand this better than I do, what is the real deal on all of this? Am I going to need a new AVR to power 4 ohm speakers? Can I use the AVR I have now, carefully, for the time being, with an eye to eventually upgrading? Will I be just fine with what I have?

 

[alex-z]

The 6-16 Ohm rating usually means the nominal impedance. Your receiver can likely drive a speaker that dips to 4 Ohms for brief periods of time.

The Emotiva B1+ would be a safe example, the minimum impedance is 4.6 Ohms as measured by amir. As a general rule, impedance ratings are meaningless unless a third party measurements confirm them. Some companies have models which are "8 Ohm" but dip to 2 Ohm, those you would want to avoid running on an AVR rated for 6 Ohm loads.

The higher crossover does help because the speakers are handling less bass, which is where most of the power is required.

An AV receiver has a power supply built for driving 5, 7 or even more channels. This gives you some leeway to run low impedance speakers on 2-3 channels. Hence why you don't see people screaming about how their Klipsch R-41M that dips to 2 Ohms blew up their 4 Ohm receiver.

 

[Doodski]

I'm having a hard time understanding how impedance works, and am not finding any good sources that answer the questions I have, so I'm hoping that one of you can provide some clarity.

The impedance is another term for resistance but impedance is a resistance that changes with the frequency of the energy going into the speaker. Here is a graph of a speaker tested here at ASR and you can see the resistance is on the left side in Ohms and the frequency of the energy going into the speaker is on the bottom and on the right side is the degrees of phase shift. For now don't worry about blue colored phase shift until you can read the red magnitude impedance graph. You can see that @ ~205Hz the resistance of the speaker is about 3.3 Ohms. As the frequency of energy going into the speaker changes so does the resistance of the speaker. Because the resistance changes it is called impedance to differentiate that it is a changing resistance. If you look at the top highest peak of the red impedance graph information you will see it goes as high as ~23ish Ohms resistance for the frequency of energy at ~1.7kHz. Then follow the graph red lines and look at the various resistance at various frequency and then you will know how to read a impedance graph.

 

[Doodski]

I've seen in several reviews of these speakers that, because they're 4 ohm impedance, typical low/mid AVR's will have trouble powering them. I run into this advice a lot.

The amplifier will run hotter with lower impedance speakers. Sometimes a amp can manage the 4 ohm speakers and sometimes they get hot and bake the PCB and overheat components and cause a failure. So one needs to be careful when running low impedance speakers on a amp that is rated for 6-16 ohms or even a 8-16 ohms rating. I've repaired lots of amps with cooked overheated stuff that caused a failure so it does happen. Whether this Denon can squeak by and manage a 4 ohm speaker is not known and running a speaker that dips down low in the low impedance on a graph is a bad idea for this Denon. If the speaker has a impedance graph that show there is no deep dip in the impedance then it should be OK although with these surround sound receivers they don't have the best cooling and will run hot with the lower impedance speaker(s).

 

[Doodski]

if low/mid AVR's couldn't handle these 4 ohm speakers, both the AVR makers and speaker makers would be shooting themselves in the foot, so of course they'll be able to drive those speakers.

There is no politics in proper ratings/specs. They are there to inform you of what the amp can manage. If the amp says no 4 ohm speakers then it is maybe wise to avoid that because they might dip down to 2 ohms or something really low and cause additional heat in the amp as explained previously.

Like @alex-z stated some 4 ohm and 6 ohm speakers don't dip very low and sometimes maybe can be considered for the task. Take it on a one by one basis and not a general rule that you can apply to all speakers.

 

[Doodski]

For those of you who understand this better than I do, what is the real deal on all of this? Am I going to need a new AVR to power 4 ohm speakers? Can I use the AVR I have now, carefully, for the time being, with an eye to eventually upgrading? Will I be just fine with what I have?

Nobody really knows 100% the answer to the issue of the amp blowing due to high heat and increased current flow but operating the amp at lower power output will lower the heat output and some peeps place cooling fans on the top of their surround sound receivers because they run hot.

 

[aris3256]

Thank you both for your help with this. As an update, the Polk R200's showed up in the mail today, and I was looking through the manual and found the numbers it lists for impedance (though remembering what you said about the reliability of numbers prior to independent testing). Here's what it says:

Amplifier output compatibility 8ohm / 6ohm / 4ohm
Minimum impedance 3.8ohm

Do I understand this right as their claim that the speakers should work with a 6ohm or an 8ohm AVR (I'm still not sure which mine is) and that--by their measurements, at least--the impedance should never dip below 3.8ohms (as per the graph you posted above)?

 

[Chrispy]

Does it state where the minimum impedance occurs? Phase angle to go along with it? Not likely all that important. Many amps are okay with 4 ohm loads to an extent, but if you want to push that to high spl levels consistently a more capable amp is likely in order. Having an avr with pre-outs for such flexibility can be a good thing, altho you'll need to go up the Denon model numbers a ways to get a model with a full set of pre-outs (and many other features will come along with that as well).

 

[Doodski]

@aris3256
I Googled extensively searching for a impedance/phase graph on the Polk R200's and found none. I did find a little more details about the speaker.

• Amplifier output compatibility: 8Ω / 6Ω / 4Ω
• Minimum impedance: 3.8Ω
• Sensitivity (2.83V/1m): 86 dB SPL

So according to Polk they are OK for a 8Ω rated amp. As Polk has rated them as safe for 8Ω rated amps I would say they are OK. They are not the most efficient/sensitive speaker out there and so will require a bit more power to get to the sound pressure levels required. If you find the amp feels hot then get a small fan and lay it on the top of the amp over the heatsinks where the heat originated from. The Denon surround sound receivers can run hot even with 8Ω speakers so don't be surprised if you think a small fan is in order. There are 5 volt and 12 volt fairly quiet 120mm fans and even more silent 200mm fans available if you want to make a fan power supply from a extra cel tel charger or have a12 volt power supply laying about.

 

[Head_Unit]

Nobody really knows 100% the answer to the issue of the amp blowing due to high heat and increased current flow but operating the amp at lower power output will lower the heat output and some peeps place cooling fans on the top of their surround sound receivers because they run hot.

Yeah! This is highly recommended, because to riff on @Doodski the AVRs can run somewhat hot even at LOW volume (there's a lot of stuff packed into those chassis, and most AVRs are powering every channel even when no speakers are connected to some, still generating waste heat). We really like this https://www.acinfinity.com/receiver-amp-cooling-fans/

As for the impedance and the AVR, it's simple:
- If you're not cranking to the max, impedance does not matter, let me repeat it does not matter.
- If you crank to 11 all the time, sh!t will break unless you TURN IT DOWN when things sound bad.
- Speaker impedance is too complex to really be "4 ohms" or "8 ohms" or whatever and seems to have become something lied about almost as much as sensitivity/efficiency
- AVRs can play most speakers pretty dang loud. If you really need louder then you need a powerful external amp, like an ATI 525NC we're using now. For a mere $3400 it sounds...pretty much the same as the Denon AVR-X3600H it replaced. Runs cool as a cucumber! (Hybrid Class D output with old school power supply)