Impedance: Measured VS. Nominal

[needaglock]

Hello all--new poster/former lurker with another n00b question regarding impedance. I am currently searching for an amp that will drive my speakers at the upper range of their power handling, listed as 25-150 watts. The speakers in question are a pair of Infinity's dating back from 1996; the manual lists them at 6 ohms nominal impedance, but when measured with a multimeter show an impedance of 7.3 ohms at the terminals.

My question: Should I go by the nominal impedance stated in the manual, or the actual readings I came up with? Infinity had a habit of mis-labeling their impedance back in the mid 90's (Such as my CS-3009's that the manual lists as 8 ohm compatible, but actually measure in as 4 ohms at the terminals.)

Secondly, the amp I am considering is 125 watts per channel into 8 ohms, 200watts per chanel into 4 ohms, and ~170 watts into 6 ohms. This should (if I have done my math correctly,) result in a little under 150 watts per channel into an impedance of 7 ohms. Am I correct in my assumption, or am I a little off base here? Could I safely run my speakers with the for-mentioned amp, or would I be at risk of overpowering them and possibly burning them out? I've listed the actual specs from the product documentation below:

Power Handling, RMS 25 to 150 Watts
Nominal Impedance 6 Ohms
Frequency Response 90Hz - 35kHz +/-2dB
Crossover Frequencies 4000Hz
Sensitivity (2.83V/1m) 89dB
Dimensions 20-3/4" x 8-1/8" x 6-5/16"

Sorry if this has already been covered somewhere else in the forum, but I've tried searching under Impedance and haven't found enough info to adequatley make a decision--thanks again.

 

[MDS]

The 'nominal' impedance is an average over the entire frequency range the speaker can handle. A nominal 6 Ohm speaker may dip way below 4 Ohms in the low frequencies and be in the double digits for the high frequencies.

The only thing that is misleading about nominal impedance is that two speakers that have the same nominal rating my behave very differently with the minimum impedance of one being much lower than the minimum impedance of the other.

If the amp you are considering can honestly handle 4 Ohm speakers it will have no problem driving a nominal 6 Ohm speaker.

 

[markw]

addendum

What you measured was the DC resistance of the speaker system, not any sort of impedance reading.

Impedance is, like MDS said, dependent on the frequenct being reproduced by the speaker, which implies an AC signal, and this can vary widely over the frequency range. "nominal" would be the average impedance over the entire range while a "minimum" impedance (if they ever show this) would be the lowest it gets.

And, you're right. Some older large, inefficient, dual woofer, Infinity speakers could drop to two ohms (or possibly less) when pumping out the bass, as many lesser receivers and amps discovered the hard way.

Perhaps if you said exactly what speakers you're talking about, more info might be possible.

 

[Swerd]

To illustrate what MDS and markw are saying, see the attached frequency response and impedance curve. In this example, the black curve shows how impedance varies with frequency. Read the impedance values on the vertical axis on the right side - the values on the left side go with the blue frequency response curve.

Note how the impedance varies quite a lot. There is no one single value for the speaker. The lowest point for this particular speaker is at about 4.5 ohms at about 7 or 8 kHz. The "nominal" impedance value for this speaker might be the valley at about 200 Hz where the impedance goes as low as 6 or 7 ohms, although I'm not certain. Even the average value for all the points on this curve would be misleading.

Although there is a typical impedance curve pattern that many speakers share, no two speakers have identical impedance curves.

 

[WmAx]

The only thing that is misleading about nominal impedance is that two speakers that have the same nominal rating my behave very differently with the minimum impedance of one being much lower than the minimum impedance of the other.

If the amp you are considering can honestly handle 4 Ohm speakers it will have no problem driving a nominal 6 Ohm speaker.

That is not the only thing that can be misleading. The electrical phase lag/advance of the current vs. the voltage phase of the speaker is of great importance. This variation of electrical phase angle requires more current to drive a load than the impedance alone may indicate. The inductive/capacitive reactivity of the load requires amplifiers to handle much higher VA on the output stages than the impedance and watt specifications alone would suggest, or the amplifiers would fail due to over-loaded output devices. A load with a higher degree of phase angle variation than average would be commonly referred to as highly reactive. Amplifiers can vary dramatically with performance into highly reactive loads, because most amplifiers are designed with an average reactive load in mind. Most speakers are designed to have minimum reactivity. There are exceptions. Especially in high end speakers. This is where some problems can occur. The amplifier may distort earlier than expected and even overheat/shutdown seemingly randomly.

However, despite the high relevance, it is not common for detailed electrical phase response data to be given for loudspeakers. Nor is detailed load impedance vs. phase capacity load ability commonly given for amplifiers. This is a crap shoot(though, since most speakers are designed to be a minimum reactant, the odds are in your favor) unless you know of a 3rd party that has measured a particular speaker and/or amplifier of interest.

-Chris

 

[Swerd]

As usual, we've done thorough job of "educating the noobie", without actually answering any of his questions !

Stick with the nominal impedance ratings in the speaker's manual. Using a handheld meter does not count.

The amp you are considering, 125 watts per channel into 8 ohms, 200 watts per chanel into 4 ohms, should be fine. You can safely run speakers that have 6 ohm impedance and 89 dB sensitivity with that amp. Yes, in theory you might overpower them, but you probably would have to drive them much louder than you could stand to listen. 89 dB is actually quite loud, and that is with 1 watt and the measuring microphone 1 meter away. Just don't let drunks at parties turn up the volume and you'll be OK.

 

[needaglock]

As usual, we've done thorough job of "educating the noobie", without actually answering any of his questions !

Stick with the nominal impedance ratings in the speaker's manual. Using a handheld meter does not count.

The amp you are considering, 125 watts per channel into 8 ohms, 200 watts per chanel into 4 ohms, should be fine. You can safely run speakers that have 6 ohm impedance and 89 dB sensitivity with that amp. Yes, in theory you might overpower them, but you probably would have to drive them much louder than you could stand to listen. 89 dB is actually quite loud, and that is with 1 watt and the measuring microphone 1 meter away. Just don't let drunks at parties turn up the volume and you'll be OK.

Thank you Swerd, that's basically what I wanted to know. I've been upgrading and swapping out components in my system lately, and I've been burned once back in the late 90's with a dead amp thanks to Infinity's labeling.

 

[needaglock]

What you measured was the DC resistance of the speaker system, not any sort of impedance reading.

Impedance is, like MDS said, dependent on the frequenct being reproduced by the speaker, which implies an AC signal, and this can vary widely over the frequency range. "nominal" would be the average impedance over the entire range while a "minimum" impedance (if they ever show this) would be the lowest it gets.

And, you're right. Some older large, inefficient, dual woofer, Infinity speakers could drop to two ohms (or possibly less) when pumping out the bass, as many lesser receivers and amps discovered the hard way.

Perhaps if you said exactly what speakers you're talking about, more info might be possible.

My mains are a pair of Infinity Crescendo Series 3009's bi-amped, each speaker basically running off it's own Adcom-Gfa 5300. My rear surrounds are a pair of CS 3007's, again bi amped but this time running off Adcom Gfa 5200's.

My left and right surrounds, and my dual centers (one on top and one below the TV) are again Crescendo Series, CS-Video to be exact--these are the speakers I was referring to in my original post. I am currently also powering them with GFA-5300's, but I wanted to push them a little more to their max rating, right now the 5300's only dump in about 104 watts each at that impedance.

I've been considering getting an Adcom gfa-7605 instead to power the CS-video's, which would pump out 150 watts at 6 ohms, but that would leave me with an extra unused channel. I toyed with the idea of setting up some tactile transducers with the fifth channel from that amp, but I'm already running a ssw-212 as my sub so I'd be bordering on overkill.

Lastly, I know on audioreview.com the adcom 5300 has mixed reviews, but I haven't experienced any of the negatives some of the reviewers have mentioned--I've been perfectly satisfied with them. My set up isn't audiophile by any means, but it still rocks while being affordable, especially when picking up the odd component here and there off of ebay. Thanks again for all the responces to my question.