The number of stereo receivers available has shrunk to almost nothing in the last five years or so. Receivers are generally rated to support 8 ohm nominal impedance speakers. Problem is, many speakers drop to 4 ohms or less at some point of their frequency range. This wouldn?t normally be a problem unless the speakers were being played at the upper power limits of the receiver, which could cause the receiver to overheat and go into protection mode.
The problem with running two sets of speakers concurrently is that the impedance seen by the receiver halves. Most speaker switching (A+B) is wired in parallel, so the load seen is 8/2=4 ohms. But if the speakers drop down to 4 ohms at some point in their range, then the load could go as low as 2 ohms. Each time the impedance decreases, the receiver has to provide more current to drive them to the same level. A reasonably trustworthy spec is if the receiver can deliver in the neighbourhood of twice the output power (ignore dynamic power figures) at half the impedance, ie: 100wpc at 8 ohms, 200wpc at 4 ohms.
Another wrinkle with your set-up is that the Klipsches have a sensitivity of 93dB@1 watt/metre, while the Infinitys are only 88dB. If you drive both sets at once, the Klipsches will be considerably louder ? you should have noticed this in your previous set-up as well. If you?re running both sets at once, I?d recommend something in the 70-100wpc range with reasonable current delivery.
You could just replace your old Onkyo with a new one like this: http://www.us.onkyo.com/model.cfm?m=TX-8511&class=Receiver&p=i
or you could try something from Yamaha like this: http://www.yamaha.com/yec/products/receivers/RX777.htm
or you could move to a more up-market receiver like this Rotel offering: http://www.rotel.com/products/specs/rx1052.htm
or you could buy an integrated amplifier and a tuner (gives you more flexibility of amplifier choices) like either of the following two integrateds:
http://www.nadelectronics.com/hifi_amplifiers/C352_framset.htm
http://www.cambridgeaudio.com/summary.php?PID=13&Title=Azur%20640A.%20The%20ultimate%20award%20winning%20integrated%20amplifier
Hope this didn?t muddy the water more. Cheers.
I am looking for a stereo receiver for nice sounding older A and B shelf speakers and aux imputs for computer and tv, but not surround sound. I need something simple where you just plug in the speakers and go - (I'm old school - just care about the sound!). I don't want the remote to be the only control for anything. And I don't understand the Ohms and impedence thing, but I think it might be important. Somebody help! I am running Infinity Alpha 20 (8 ohms - nominal impedence) and Klipsch (KG 2.2 - I have no idea about the impedence - I don't see it anywhere, but they are from about 1990 or earlier and they sounded great with my old Onkyo receiver from the same period). I want to plug those into Speakers A and Speakers B, and as I said, just hit "play". I have a CD player to plug in and a TV, and would also like a couple of additional aux imputs for my computers for when I play music on them.
What should I buy? Can someone explain impedence and Ohms to me?

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