Kevin. W
Screenwriter
- Joined
- Oct 27, 1999
- Messages
- 1,534
Don't most recievers in the 70-90w/channel reach reference level sound? If your speakers are 80watts do we really need a reciever or AMP that produces 120w+?
Kevin
Kevin
But when you dont have a external sub, have full range speakers (with woofers) and then play the lobby shoot-out scene on The Matrix at a interesting volume, well a lot of people were posting "My receiver said 'PROTECT' and turned off!"
Even a 80 wpc receiver will produce good bass for several seconds. But it heats up. After a few minutes, the receiver should shut itself off to prevent damage. (The lobby shoot-out scene is several minutes of intense bass).
A reciever will go into Protect mode when it's over-driven or is over-heating. Over-heating is easy to cure, especially since almost every picture I see of people's home theater's the reciever has only a 2-4" of space above it and is usually covered on 3-4 sides (bottom, sides and sometimes the back) that leaves very little room for ventilation. Most reciever manufactures recommend like 12" of space above the reciever for the heat to dissipate.
Over-driving, well a 70wpc amp isn't going to be able to cut it if you want to listen loud, say you are listening to the Matrix on your 70wpc reciever, hooked up to your 85db@1w1m efficient speakers...but you're listening loud, let's say it's averaging around 85db (plus it makes this easier to explain)....
85db 1watt
88db 2watts
91db 4watts
94db 8watts
97db 16watts
100db 32watts
103db 64watts
106db 128watts
So you can see if you have a large transient you can run into problems. Of course this isn't taking into account that most people sit more than 1 meter away from their speakers, and for a cone driven speaker you lose I belive 6db for every meter you are away from it, so if you sit 3 meters away (around 9ft) your 85db speakers are really only 73db with 1 watt (at 3 meters), so to get back to your original 85db range you're using 16watts of power, at 64watts of power you're only at 91db...to exceed the 70w reciever and jump to 128watts you're only at 94db.
It's very easy to over-drive a small amp like this.
Here's a good small read on transients and amp power in regards to music that came over an email list yesterday.
Art Ludwig looked at three pieces of music that he likes to listen to, and found a peak power to RMS ratio of about 30:1 to 40:1, see;
http://www.silcom.com/~aludwig/EARS....nd_loud_sounds
on his site. Even with a 100dB/1W efficiency, giving you a, say, 90dB eff in the listening position, you'll need 40W to cover the peaks when listening at an RMS level of 90dB (atleast for the three pieces of music he tested with).
A more normal example, I loose about 10dB eff to the distance between the listening position and my speakers, my speakers have a 92dB eff, and I like to sometimes listen at an RMS level of ~95dB. It seems I would need about 20W for the RMS level and up to 800W to cover the peaks. My current
amp only gives me 85W, so in theory I have to keep the RMS SPL in the listening position at a lowly ~85dB or below, due to lack of power.
Bottom line - big enough is .. very big! A whole lot bigger than I thought before reading that page on Art's site.
Hope this helps some more.
Andrew
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[Edited last by AjayM on September 07, 2001 at 12:56 PM]
I'm just a bit perplexed at people listening at -25 and -10 and 0! whats's going on?
reference on a good amp should be no more than -28 (10 o'clock), average listening at -40 (9 o'clock).
Where did you get that idea? I guess first what kind of reciever are you using? My denon goes past 0 and up into a fair amount of positive numbers and the lowest it will go is around 40-42 (hardly average listening level).
And the reference to the clock for the volume adjustment only works on some recievers. But set reference for where-ever you want to.
But achieving reference volume has nothing to do with the little number on your display or where your volume knob is at, to make so many db takes so much power. If you crank the knob down, you end up going into the speaker calibration area and cranking up all those adjustments, or vice versa...the end result is still the same.
Andrew
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reference on a good amp should be no more than -28 (10 o'clock), average listening at -40 (9 o'clock).
This is completely dependant on your PRE-AMP, not your amp, and the display has nothing whatsoever to do with how much SPL a system is putting out.
Just pay no attention to what your pre-amp/reciever volume knob says and use a Sound Pressure Level meter to determine what "volume reading" your system produces reference level at.
Alos speaker wattage ratings are essentially meaningless. The Ohm rating and the sensitivity are the important numbers.
It will take more Wattage (on average) to power low Ohm (low resistance), low sensitivity speakers to the same SPL's as a high Ohm, high sensitivity speakers being powered by a smaller amp.
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http://www.ricperrott.com
Ric Perrott - My DVD's
and the display has nothing whatsoever to do with how much SPL a system is putting out.
...unless, of course, you calibrate reference level (in my case, 75db from Video Essentials) to correspond to the 0db indication on the front panel. With my setup, it was a breeze to do.
I can't imagine that I would ever listen to a movie or music at anything above reference level (the 75db "average" I stated above) in my room. After all, I like having my hearing.
Todd
...unless, of course, you calibrate reference level (in my case, 75db from Video Essentials) to correspond to the 0db indication on the front panel. With my setup, it was a breeze to do.
Sure, IF you're capable of doing that. I couldn't get my system to register 75db at 0db on the dial if I tried, it's much too loud. I can only kick each channel down 10db in the calibration, and I register over 88db at 0db with all channels zeroed.
But anyway, the point is it's virtually impossible to say to someone that reference levl is at "x" setting on your volume knob...there are just WAY too many variables.
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http://www.ricperrott.com
Ric Perrott - My DVD's
the woofer doesn't need more power or the tweeter less, etc
You seem to be saying that it takes nearly the same ammount of power to produce a sound at any frequency.
I'd like to challenge this (if you are up to a friendly debate ) by asking you a few questions:
- Why do external subs come with their own power supplies that range from 200-2,500 watts, but the mid-range and tweeters are not self-powered?
- Those personal "shreeker" alarms produce 120 db of sound from a 9-volt transistor battery. If you took out the little tweeter and wired in a woofer, how much sound would you get? Why or why not?
- When you look at receiver power ratings, why are the more reputable brands careful to report watts-per-channel from "20 - 20,000 hz" if it takes the same power to produce a sound at any frequency?
- Why is a 4 ohm speaker more likely to over-heat a receiver than a 8 ohm speaker? Why?[/list=a]
The amp needs more current to create the same power at a lower impedence (sort of a generalization, but applies to the "average" amp), again nothing to do with the speaker.
This is not true.
The speaker IS the impedence in the circuit.
Your speaker is a coil of wire. And a coil of wire has an impedence.
But if you hooked a ohm meter to a speaker with a 8 ohm impedence, will it read 8 ohms? No. It will read nearly 0.0 ohms. Thats because to DC, a coil of wire is a direct short.
Fact: Impedence is not a DC phenonom, but an AC (like a audio signal).
When you start to make the DC signal an AC signal, the impedence increases. As the frequency goes up, so does the impedence. (At some high frequency, the coil of wire will act like a broken wire and not let any current flow.)
Fact: impedence is FREQUENCY dependent.
Fact: low frequency into a coil is nearly a dead short and draws lots of current. High frequency into a coil has lots of impedence.
Fact: Power = Voltage X current.
Conclusion: low-frequency signals draw more power than higher frequency signals from your amp.
Second argument: Put your hand in front of a tweeter while it is producing sound. Can you feel the air moving? No.
Put your hand in front of the port on a sub while it is producing sound. Can you feel the air moving? Yes.
Since power is the ability to do work and moving air takes work, the driver that moves more air is doing more work.
And more work takes ... MORE POWER! (I feel like Tim Allen )
While the above is true for a pure inductor, the impedance of speakers usually doesn't monotonically increase with frequency. It usually has dips and peaks throughout the audio range.