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Two different sets of speakers on amp/receiver.
September 24, 2006 8:33 PM   Subscribe

Will two different pairs of speakers work and play well with my new Onkyo receiver?

Hot on the heels of this question... I have a new Onkyo TX-8222 receiver. I've got two pairs of speakers; Boston Acoustics CR-57's in the living room (channel A), and Bose 161's downstairs (channel B). Reading the specs for the receiver, it says that when connecting two sets of speakers, their impedence should be between 8 and 16 ohms. The BA's are rated 8 ohms (OK), but the Bose are rated "4-8" ohms (that's what the spec sheet says). What does that mean? I guess the real question is; why does Bose list a range, and not a single number? Every other speaker I've seen lists a single number.

Will I be safe? I am 99% sure I will, but the left channel on the old receiver (TX-26) blew just two weeks ago. I had it running with the BA's and Bose 161's, and everything sounded fine (both channels on at same time, even running loud). I just assumed that it was an old age thing (the receiver's, not mine, dag-nabbit!).

Specs on the new Onkyo here.

P. S. I did send an email to customer service via the Bose web site. No reply yet.
P. P. S. I found a guy through a friend who buys old stereo components. I'll be calling him tomorrow. If you're on Long Island, and have stereo equipment gathering dust, let me know.
posted by ObscureReferenceMan to Technology (4 answers total)
 
They're talking about impedance, not resistance. Resistance is a number; impedance is a curve.

Impedance refers to the plot of resistance as a function of frequency. "Resistance" is, in fact, a special case: it's the impedance at zero hertz.

For pretty much anything except a piece of wire or a resistor, the impedance curve isn't constant, and thus a single number can't really describe it.

However, many speakers have a relatively flat curve over the relevant frequency range, and usually in that case they pick a single number which is most representative of the curve and that's the number they publish. In the case of your second set of speakers, "4-8 ohms" means the curve isn't vary flat, and tells you that it's a little low on average.

One reason that matters is that all other things being equal, if the impedance is lower then the speakers will draw more current. That may mean that the amplifier will top out and start clipping -- and it can cause the final stage FETs to smoke, which is probably what happened to your previous receiver.

The amplifier will have an output impedance rating and the speakers will have an input impedance rating. The system is safe as long as the amplifier's impedance is equal to or lower than the speakers. If the combined speaker impedance is lower than the amplifier, you're risking destroying the amplifier.

If you run your speakers in series then the impedance adds, and the system is safe. For your speakers it means that the impedance would be between 12 and 16 ohms. There would be no danger of burning out your amplifier, but it might not be as loud as you might like.

If you run them in parallel, the formula is 1/((1/imp1) + (1/imp2)) which results in a lower impedance. In your case the impedance would vary between 2.6 and 4 ohms, and that's way too low for your amplifier to handle safely.
posted by Steven C. Den Beste at 9:03 PM on September 24, 2006


A bit more: if your amplifier has two connections for speakers (i.e. an "A" and a "B") then if both are running at once the amplifier will be wiring them in parallel, thus danger Master Robinson.

However, if the speakers have an impedance of 16 ohms, then two speakers in parallel have a combined impedance of 8 ohms, which is safe for an amplifier with an impedance of 8 ohms. That's what the Onkyo spec was telling you.
posted by Steven C. Den Beste at 9:07 PM on September 24, 2006


Wiring speakers in series doubles the impedance, wiring them in parallel halves it. The amp docs are saying you want to keep the impedance between 8 and 16 ohms, while the Bose speakers are good down to 4 ohms. Too little impedance in the speakers can burn out your amplifier, while too much will quiet everything down. Likely each Bose speaker is 8ohms, like most home speakers (regular automobile speakers are 4ohm), so you'd want to do your math and make sure each speaker connection is actually 8-16 ohms.

If the Onkyo has four speaker outputs (my 15yr old one does), then you can wire each speaker to a separate output and use the speaker selector to play both pairs at the same time.
posted by rhizome at 9:58 PM on September 24, 2006


Here's the deal. I checked the docs for your receiver and speakers. Your receiver can drive either 4 ohm or 8 ohm speakers. Both of your speaker sets are 8 ohms.

The Bose spec doesn't explicitly say that it is 8 ohms. It says that they can be driven by either an 8-ohm or 4-ohm amplifier. This implies that they are really 8-ohm speakers since an 8-ohm speaker can be driven by a 4-ohm amplifier but a 4-ohm speaker can't be driven by a 8-ohm amplifier. That is where the 4-8 ohm confusion came from. They are talking about the range of the amplifier, not the speaker.

So when you hook up both 8-ohm sets and turn on both A and B channels at the same time, you are driving 4 ohms. That is at the limit of your receiver, but should be within spec. Also, your receiver docs imply that they have protection circuits if you connect improper speakers so that is extra insurance. You may hear them cut out if you drive them too hard, which is your clue to turn down the volume.

So you should be okay. You receiver is designed to drive two sets of 8-ohm speakers. Still, I wouldn't turn up to full volume when both A and B channels are on. But then I value what hearing I still have left.
posted by JackFlash at 10:02 PM on September 24, 2006


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