Speaker impedance question

I have a concern about impedance myself..
I like to run channels A & B simultaneously on my Outlaw 2160 stereo receiver.
The 2 pairs of 8 ohm speakers I currently run don't match very well in sensitivity. I'm considering upgrading my towers and in the process getting lower sensitivity speakers for a better match with my Outlaw bookshelf speakers. Some speakers I have been particularly interested in are the KEF Q series 550/750/950. In KEF's specs they show Nominal Impedance 8Ω (min.3.5Ω)..
The minimum 3.5Ω in parenthesis has me concerned that these speakers are likely not gonna fit the bill. What's up with this, does the impedance vary, and how? Or should this not be a concern at all. I just don't wanna blow up my receiver.

If the (min.3.5Ω) is a deal breaker, my next consideration is Polk, Is this variance in impedance a common thing? Can manufacturers be trusted to tell you about it?

 

Impedance does vary because it is related to ac (Alternating current). It 'alternates' - its not a steady value that remains constant, as is the case with DC (direct current). For example, the lights in your house actually turn on and off 60 times very second, since house electricity is provided and distributed by your local power company at 60 Hz (cycles per second), unlike batteries that are put into a flashlight (DC). Capacitors and Inductors (components used in the crossover circuits of loudspeakers) behave differently when used in ac circuits, compared to DC. For those who havent gone to school for electronics, engineering, etc, you can browse/google search at your leisure for more online info/theory on the subject which can then help to better understand how it applies to audio/hi fi systems. You can think of ac as a repeating sine wave- it has a peak max and min value at a particular frequency. But even though it constantly changes, it does so in repetitive manner, thus, if you were to take a basic multimeter and stick the probes into a wall outlet, you can still obtain a reading, which is actually an average (DC) reading. Thus, if you measure 120 volts from an outlet (or have that displayed on a power strip /conditioner)- its really measuring a sine wave with a peak of 170 volts (which is 1.414 more than 120, or vice versa, 120 is .707 of 170, aka "rms" root mean square.)

Anyhow... most speakers have an impedance value that varies over the audible 20-20K frequency range, its not a steady 8 ohm or 4 ohm as given by the manufacturer. 4 ohms being lower, causes an amplifier to "work" a bit harder, but usually its ok, unless you really crank up the volume in which case, the amp's protection circuit will kick-in and shut it down. The Outlaw you mentioned has two sets of speaker terminals, so I think it should be able to handle 4 ohms, whether its two pairs of 8 ohm speakers, or one pair of 4 ohm speakers. I noticed it does have a separate pre and power amp section via the jumpers in the back which is always a nice feature. If you really needed extra power and wanted to keep the Outlaw, you could use it as a preamp and connect to another power amp.

I have a pair of vintage Infinity RSIIA's in one of my room setups- even though its rated 4 ohms, they seem to do better with extra power, so in my case, I bi-amp them with a Sansui G-901 switched to power amp mode, a Sansui B-2101 power amp, that are both fed by a Sony TA-E77ESD (has two preamp outputs to drive each amp). Before the B-2101, I had used a Sansui AU-717 integrated (switched to power amp mode). The 717 is a great unit, but at 85 Watts, it did go into protect mode.. a good test track: Bach's "Fugue in G Minor" - nothing like the power of a church pipe organ to shake the house Crank that piece up a bit and can then see which amps are up to the task or not.

 

Thank you for that! Awesome response.
I'm already running two pairs together most of the time. I was just wondering if it would be safe to run the KEFs-- Nominal Impedance 8Ω (min.3.5Ω)--that minimum had me concerned.

 

Alone?
No!

 

I was only able to take a quick peek at the speakers online and their specs.., amp power suggested 15-130, and so forth.
I get the feeling that they really arent too hard on amps. The 3.5 min is probably just at a limited # of frequencies
for short duration, so to speak. And 3.5 is not much below 4. Its when you approach 2 ohms that its really not good for an amp
(unless you have a vintage Sansui BA-5000 as I do

To feel more reassured, simply contact KEF and let them know of your Outlaw amp. I'm sure if you send a note to their website,
they would be happy to answer your concern and your interest in their products. They can probably send the frequency response
chart for whichever model you're curious about. I'm fairly certain the impedance will not be an issue, if it was at 3.5 throughout the
entire audible freq-spectrum, they would have given it a 4 ohm rating, not 8.

 

Thanks, and great suggestion, I'll see what they say.