How much power is enough... which amp would you buy? - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-25-08, 01:17 PM

Andrew

Join Date: Jul 2007
Location: College Station, TX
Posts: 1,131
Re: How much power is enough... which amp would you buy?

I am not saying all amps sound the same in all situations. The amp must be sufficiently able to power the speakers in question at all frequencies with inaudible levels of distortion in a linear manner at required SPLS. Not every amp is able to do this with every set of speakers due to varying impedance curves and such. So before determining if an amp is actually transparent one would have to see measurements of the amp and correlate them to the amps performance with a specific loudspeaker.

Before publishing your article I strongly recommend you read the articles I mentioned here: http://www.hometheatershack.com/foru...50-post26.html
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Old 01-25-08, 02:04 PM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: How much power is enough... which amp would you buy?

For sure! I've read those both in the past but I'll take another look at them. Thanks for the input!

Edit: I read those in the past but now the articles seem to have disappeared from the internet. Any idea where I can find them? Checked the local library as well... Thanks

Last edited by thxgoon; 01-25-08 at 02:23 PM.
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Old 01-25-08, 04:38 PM
Elite Shackster

Jacen

Join Date: Apr 2006
Location: Bay Area, California
Posts: 1,582
Re: How much power is enough... which amp would you buy?

My quick answer -- watts are over-rated, so I'd stick with the 125watt system.

• Watts are needed, but not as much as we might think. In normal operation, you're only using ~10watts at any given time.
• All things being equal, a higher wattage amp will have a higher noise floor.
• Watts to dB's is a logarithmic equation -- increasing the wattage from 125watts to 400watts will only give you an additional 5dB's at max output.
• It takes an additional 10dB to perceive sound to be twice as loud. So you'd need an amp that was 1250watts to sound twice as loud.
• Doubling the drivers will boost your SPL by 3dB -- if they're close enough to be "coupled".
• You loose 6dB for every doubling of distance
• Room gain will get some of that back. I'm pretty sure you get 3dB, at least in the bass region.
• It'd be very hard to do a proper a/b test -- the respective levels would have to be matched precisely and the switching would have to be REALLY quick.
• Without a blind test, perception is going to rule the roost rather than whatever the "real" answer may be.

JCD
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Old 01-25-08, 06:17 PM
stephenb1
Inactive

Posts: n/a
Re: How much power is enough... which amp would you buy?

Hi i use Kef q55.2 's tri-amped with a parasound 1205 for lower bass and a Nad 216Thx for mids, plus an Arcam alpha 8p for the tweeters.
A Yamaha Z9 for the rear outputs q60 's 8" bi-amped, and for 1 front centre and rear centre and then the parasound running 2 further front centres, plus some other kefs and mission di-pole speakers powered with arcam power amps, through arcam dvd and cd. (its a small 10.2 system).

Its around 3000w rms as all speakers are 4 ohm also i have 17 speakers in total including 3 subs 3 q95.2 's for the front and 1 rear, a dcx-2496, 2 cx3400 cross-overs, a bfd, a peq2200 and some more q55s off ebay on there way, plus to much more to mention.

I can assure it sounds anything but nasal even at very high volumes and the increased output after upgrading most of my amps has gone from loud to very loud and is not the "placebo effect" as im sure my closet neighbours would confirm.
Its very loud and i have n't found the stress level yet at 11-12 o'clock not sure how far it will go ? but quite alot more I'm sure.

It was worth every penny upgrading to more powerfull amps.
Old 01-25-08, 06:42 PM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: How much power is enough... which amp would you buy?

Quote:
DS-21 wrote: View Post
Indeed. Which is why I'm always puzzled why the people who most often bleat about finding actually nonexistent differences in commodity parts such as amps, digital sources, wires, etc.
I'd agree with digital sources being a bit of a mixed bag these days as they all have great noise floor and resolution. There might be other factors involved like quality bass management that would make one digital source preferable over another, and then there's always the ipod...

For amps and cables, it's a different subjective story and as mentioned earlier can get quite religious. A speaker system (amp, cables, speakers) act in concert as an rlc system. Changing an amp or cables may indeed have a difference on the overall sound of the system. These differences are small compared to better speakers or proper setup but they do exist for some systems. The problem IMO, is that these differences cannot be attributed to any particular spec given to us by manufacturers because there are too many variables and opinions are too subjective.

I think a blind test would be perfectly achievable and if differences were to be found I think the observations would be consistent. In my experience, when there is a difference between to amps on a system, the difference is noticeable enough to be memorable.

Anyone up for an htshack convention somewhere to try this? Vegas anyone
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Old 01-26-08, 03:01 PM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: How much power is enough... which amp would you buy?

Quote:
DS-21 wrote: View Post
Religious is exactly the right word, because there really is nothing to go on here except for blind faith. Personally, I'd rather have faith in something important than in something stupid like saying that ugly black boxes sound different from each other.
I have to completely disagree. How do you explain the difference Sonnie heard between his Denon receiver and his Anthem gear with his ML's? All plecebo???

In your line of logic, a receiver taken from a \$60 htib, driving whatever speakers you want, will sound identical to the best electronics offered in the world (under normal operating conditions) in a blind test? If digital devices, amps and cables have no affect on sq, what in the signal chain other than speakers, does?
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Old 01-26-08, 03:33 PM
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Josuah

Join Date: Apr 2006
Location: San Jose, CA
Posts: 636
Re: How much power is enough... which amp would you buy?

Quote:
thxgoon wrote: View Post
The Voltage or Amperage into a given load is independent of the amplifier. Cheapo brand A will have the exact same voltage and amperage as expensive brand B if driving the same speaker at the same level (assuming the level is within the design parameters of the amp). It's a function of the speaker characteristics that place the power demands on the amp.
Well the original question said all things being equal except the transformer, watts per channel, and number of channels. Under those rules, the transformer of cheapo brand A may not be able to drive a 2 ohm load whereas expensive brand B may. The load demanded by the speaker will be the same, but the amp may not be able to satisfy it. Thinking about it, simple VA rating isn't enough there either.

(I don't think I'm saying things correctly here. But transformers are rated for both voltage and current, or at least they will be subject to those things once you've got them paired up with all the other circuit components involved in regulating the power.)

That's why I also said the 8 ohm rating isn't useful enough when pairing it with 4 ohm speakers. Maybe cheapo brand A shuts down (or blows up) if attached to 4 ohm speakers.

Quote:
thxgoon wrote: View Post
Josuah, how do you figure 300 watts is the minimum you need? At 105 db and 90 db sensitivity you only need 32 watts per channel to reach this level. I have all 4 ohm speakers with 87db sensitivity using a 125watt amp and I've driven the system well beyond reference with no trouble.
90dB/1W/1m with 300W gives you approx. +25dB, so 115dB/300W/1m. I sit ~10 feet away so I lose about 10dB. Places me at ~105dB. It's only 32W if you're sitting 1m away.

Sitting 10 feet away from your speakers, if 125W is your amp's peak output, you can only hit about 98dB.
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Old 01-26-08, 04:26 PM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: How much power is enough... which amp would you buy?

Quote:
Josuah wrote: View Post
Well the original question said all things being equal except the transformer, watts per channel, and number of channels. Under those rules, the transformer of cheapo brand A may not be able to drive a 2 ohm load whereas expensive brand B may. The load demanded by the speaker will be the same, but the amp may not be able to satisfy it. Thinking about it, simple VA rating isn't enough there either.
Right. Manufactureres of power supplies rate them in VA, which gives you an idea of the power capabilities of the transformer. Volts time Amps = Power. ie 1200VA transformer=1200 watt transformer for all practical purposes. In order for us to know what voltage and current capabilites it has independently, the manufacturer would have to give us those individual specs, which they normally don't. Also, the VA specs do not take into account efficiency in the amp design. A 1200 watt transformer does not necessarily mean the amp can deliver 1200 watts continuously.. so another difficult spec to interpret.

Quote:
Josuah wrote: View Post
90dB/1W/1m with 300W gives you approx. +25dB, so 115dB/300W/1m. I sit ~10 feet away so I lose about 10dB. Places me at ~105dB. It's only 32W if you're sitting 1m away.

Sitting 10 feet away from your speakers, if 125W is your amp's peak output, you can only hit about 98dB.
Correct, using a single speaker in a perfect environment. In practical usage though this is a very grey area. Practially, I've never seen more than a db or two when measuring between 3 feet and 10 feet from my speakers with standard noise. Also, in my case, the system is calibrated for reference leves at the seat and I've never had clipping issues at reference levels and higher. This brings us to another discussion about dynamic power capabilities of an amp which for home theater is one of the more relavent specs.

Edit: I think your math might be a bit fuzzy.. 10ft=approx 3 meters which means you 'double' that 1m distance about 1.5 times, for about 4.5db, not 10.

Last edited by thxgoon; 01-26-08 at 04:32 PM.
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Old 01-27-08, 02:59 AM
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Josuah

Join Date: Apr 2006
Location: San Jose, CA
Posts: 636
Re: How much power is enough... which amp would you buy?

I think line sources are -3dB per doubling of distance, but otherwise it's -6dB. ???

I also know that while my amp is rated at 300W/ch at 4 ohms, if I push it close to there then distortion kicks in. Arguably not audible distortion.

Regardless, I don't think anyone can provide Sonnie with an answer that's correct.
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Old 01-27-08, 12:51 PM
Elite Shackster

thxgoon

Join Date: Feb 2007
Posts: 1,506
Re: How much power is enough... which amp would you buy?

Quote:
DS-21 wrote: View Post
That's nice. I like the way you bring evidence to support your position. But when you're runnin' on faith, I guess evidence doesn't so much matter, now does it?
Common man, comments like this don't belong at the shack... Please read the forum rules/regs. We're all here to learn and discuss...

Throughout this thread I've posted my experiences and some mathmatical discussion pertaining to the question at hand. I've addressed my position relative to both subjective/objective camps. It isn't 'faith', I've heard it, I've experienced it. I've seen and heard others experience it. I have educational background that says differences could occur. Sonnie's ML's are indeed an extreme case, but many other speakers are also different from an ideal test load where most amp measurements are taken.

As to the original post, you represent the strictly objective camp, where you would probably agree with me that anything over 150 watts or so with 90db speakers is overkill, but would disagree by saying he should find the cheapest one he can that can deliver 1.2 ohms without burning up. Others on this thread represent the subjective camp where more money and more power always = better sound.

I fall in the middle. Where I DO believe there are differences in sound, but I DON'T think they can be directly attributed to any specifications that a manufacturer hands out in a literature brochure. Simple math can tell you what power requirements you need. I'm going to agree to disagree with you until evidence is provided that contradicts anything I've said.

Quote:
Josuah wrote:
I think line sources are -3dB per doubling of distance, but otherwise it's -6dB. ???
That's a good point and I can't remember. Blast... heading downstairs to take measurements...
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