Sony Receiver / Yamaha Speakers? help!

I want to purchase new speakers for a 7.1 receiver that I purchased last July. The receiver is a Sony STR-DE995. The power rating (reference) is 100 W/ch @ 8 ohms 20Hz-20kHz,THD 0.09% and 110 W/ch @ 8 ohms 1 kHz, THD 0.7%. I am interested in using (2 ) Yamaha NS-777's or NS-555's for my mains and (4) Ns-333's as the surrounds. I plan to keep my center channel speaker - a Yamaha NS-AC143. The NS-AC143 is rated @ 8 ohms, 10 - 160 watts (89 dB/W/M). The 777's (89dB/2.83v/1m) & 555's (88dB/2.83v/1m) are rated @ 6 ohms, 100 watts nominal power and the 333's are rated @ 6 ohms, 60 watts nominal (87dB/2.83v/1m).

Isn't ohms the amount of resistance? I've seen it said that it's harder to drive speakers with lower ohms, but wouldn't more resistance (higher ohms) be harder to drive?

I've seen different calculations:

Amplifier Output = Amplifier Watts x (Amplifier Rated at Ohms / Speaker Chain Ohms)
Amplifier Output = 100 watts x (8 ohms / 6 ohms)
Amplifier Output = 100 watts x 1.33333
Amplifier Output = 133 watts
Does this mean my receiver will put out 133 watts?

Will this system work OK? Will using speakers rated at 6 ohms hurt my receiver? Will there be any problems using an 8 ohm center speaker, while, my other speakers will be 6 ohm? Please help, explain, or make suggestions.

You're thinking too much :)

Firstly, that receiver will put out maybe 30 good clean watts per channel on a good day. It's impossible for an amp like that to deliver a clean 100 watts over all 5 channels. You can't defy physics. One channel working at one time might deliver 100 watts for .000001 seconds before it bursts into flames, which where they get the 100 watt rating. I'm not dogging your amp, that's the case with most mid-level consumer stuff.

Now with impedance, a 4 ohm speaker is harder to drive than an 8 ohm speaker because there is twice the resistance so the amp has to work twice as hard. If your amp says '8 ohms minimum load' then definitely do NOT go below 8 ohms. 6 ohms is probably okay, but borderline because it will vary anywhere from 3 ohms to 10 ohms during operation.

Also if you are not running the fronts and rears full range, that will help. If they don't have to reproduce bass frequencies then the amp won't have to work so hard.

Your impedance thinking is backwards. Think of impedance as a doorway, and your signal as a group of people. Would it be easier for those people to all run through a 4 foot wide door (4 ohms) or an 8 foot wide door (8 ohms)...or even a 16 foot wide door (16 ohms)?

Thanks for the help. I think I should look for some speakers rated 8ohms to match the receiver and give up on the yamaha's.

Quote:

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Originally Posted by Tomr

Isn't ohms the amount of resistance? I've seen it said that it's harder to drive speakers with lower ohms, but wouldn't more resistance (higher ohms) be harder to drive?

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More resistance is easier to drive since the higher resistance, the less current amp have to pump into speakers. So higher resistance, less current and easier on the amp.

Quote:

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Originally Posted by N. Abstentia

Now with impedance, a 4 ohm speaker is harder to drive than an 8 ohm speaker because there is twice the resistance so the amp has to work twice as hard.

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That might be a typo, but it should read:"Now with impedance, a 4 ohm speaker is harder to drive than an 8 ohm speaker because there is HALF the resistance, so the amp has to work twice as hard and pump out twice the current.

Quote:

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Originally Posted by N. Abstentia

Your impedance thinking is backwards. Think of impedance as a doorway, and your signal as a group of people. Would it be easier for those people to all run through a 4 foot wide door (4 ohms) or an 8 foot wide door (8 ohms)...or even a 16 foot wide door (16 ohms)?

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I know this is a typo :D

Better statement would be to think of current as water in a pipe, and impedance as the size of the pipe. The lower impedance (thicker the pipe), more water (current) will flow. And higher impedance (thinner pipe), less water (current) will flow :)

The amps on the current Sony receivers are especially prone to problems (damage) using speakers that are not 8ohm or are harder to drive.

I decided that I had better get 8ohm speakers. Yamaha said to check with Sony. Sony said to use 8-16 ohm. Now i am looking at Fluance speakers. They seem to have good reviews and they are 8ohm. Also they seem really inexpensive.

You won't get much support for Fluance speakers here. But in general the advice you will receive is very sound. That said, I do have a pair of Fluance surrounds that I am very happy with. Understand I am not the most discriminating listener. For the most part you get what you pay for. Still there are bargains to be had.

cubdog