Quentin

Too Much or Too Little Power?

My center is rated 6 ohm with max wattage of 400, and fronts are rated at 8 ohm with max wattage of 200. My surrounds are 8 ohm with max wattage of 150. I'll be using the AVM 20 as my processor.

I'm looking at 7 channel amps now, but I'm not sure if I should get a 200 watt amp (sherbourn or b&k) or 105/125 watt (PVA 7). None of the top amps are rated at 6 ohms...they're all 8/4/2.

I'm wondering which way to go? Would the 200 watt amps overpower my surrounds? Would a 105 amp be too little for my center channel?

Mark Dickerson

Quentin:

If the amps you are looking at are rated 8/4/2 ohms, you are looking at a very high quality amp that can drive any speaker out there. It will have no trouble driving a 6 ohm. Heck, 6 ohms is a very easy load that even the mass market receivers can handle. It is when they drop below 6 ohms that they begin to choke.

Pick the amp that sounds best to you. Use your pre/pro to balance the sound levels and you won't have a problem with either amp or any of the speakers.

Enjoy!

MikeTz

Quentin:

Sounds like you are looking at some quality amplifiers. Mark is right about your speaker impedance (dynamic speakers with 6-8 ohms impedance should be easy for these amps to handle). So now the questions are how big is your room, how loud do you listen, and do you intend to play a 5.1 music format (SACD, DVD-A)? For large rooms a more powerful amp may be better. Same goes if you like to listen at loud volume. Remember that in most movie tracks the side/rear speakers do not require much power to reproduce ambiance and surround effects.

If you are serious about 5.1 music and you intend to have 5 large speakers then the larger amps may be better.

If you listen in a modest size room, mostly to movies, at reasonable volume with efficient speakers, then an amp like the Anthem PVA-7 will make you happy.

MT

Parker Clack

Quentin:

The reason that I recommend amps with more watts per channel is the headroom that they give you from the input signal from the preamp. Having a 200 watt amp doesn't mean you are going to be putting 200 watts out all the time. It does give you the ability to play really soft and loud passages in the music or dialogue without strain or distortion.

Since you obviously care about the quality of the sound that your system has I would go with a good quality 200 watt per channel amp. Another power amp to look into is the Outlaw 770 that is 200 watts per channel x 7. At $1,799 is a real bargain.

Parker

Bob McElfresh

Short answer: go for the 200 wpc or larger amp.

A few things to keep in mind:

- the same speaker changes it's impedence with the frequency. A "8 ohm" speaker actually varies from about 3-30 ohms as it creates sounds from 20 - 20,000 hz.

- Most of the time, your speakers pull 5-10 watts. There are wild swings in volume/power for action movies, but the average power draw is still about 5-10 watts.

- Your rear speakers (the ones you are worried about over-powering) are only producing sound less than half the time.


Ok, amp power ratings are COMPLEX. There are several variables that need to be identical or you cannot really compare 2 amps.

Note: this is more of an issue for receivers which sometimes hide these details amidst all the other features, but it's a good idea to review.

When looking at power ratings, make sure these variables are similar:

• Non-inductive load - Remember that speakers change their impedence? It's more consistant to measure power with a 2/4/6/8 ohm RESISTIVE load.
• Average Power - Make sure the power spec mentions "Average" or "RMS" power and not "Peak-to-Peak". Peak-power looks better, but it really does not represent the steady-state power ability.
• All Channels Driven - Some manufactures report power in stereo/2-channel mode because the numbers are larger. But you run with 5 or 7 speakers so each speaker now gets less power. Better companies report both.
• THD (Total Harmonic Distortion) - In truth, a 80 wpc amp can provide .. 100 watts per channel. But at a cost of increased distortion. Check some of the cheap receivers on eBay and you will see "120 watts of power", then see "7% THD". The distortion numbers should be around 0.05-0.07%.
• What's the frequency Kenneth? - Better amps state that the power is measured from 20-20,000 hz. Cheap amps say something like "measured at 1 Khz". You want an amp that is rated through a range of frequencies.
• Weight - if EVERYTHING else is identical between 2 amps/receiver, buy the unit that weighs more. The transformer in a amp is the heaviest (and often the most expensive) part. But a heavier transformer will run cooler/handle power needs better than a lighter one.

Hope this helps.