Relationship of Ohm's and Watt's [Archive] - Audio & Video Forums

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astravitz
02-03-2005, 12:38 PM
I'm a bit confused by all of the specifications presented by vendors sometimes. For example, Arcam provides the following specifications:

8 Ohms, five channels 70W
8 Ohms, two channels 90W
8 Ohms, single channel 100W

However, they allow you to set the amplifier to either 4 ohms or 8 ohms depending on the speakers. When you set it to less ohms do you get more or less wattage? What are you supposed to set it to if your speakers say Nominal 6 Ohms, Minimum 4 Ohms. What is the difference between Nominal and minimum? If a speaker is rated from 10-150 Watts (Program 100 / Dynamic Peak 200), what size receiver do you need each channel to put out for a good match? Is 70 Watts enough, or is 100 Watts Better?

risabet
02-03-2005, 01:31 PM
I'm a bit confused by all of the specifications presented by vendors sometimes. For example, Arcam provides the following specifications:

8 Ohms, five channels 70W
8 Ohms, two channels 90W
8 Ohms, single channel 100W

However, they allow you to set the amplifier to either 4 ohms or 8 ohms depending on the speakers. When you set it to less ohms do you get more or less wattage? What are you supposed to set it to if your speakers say Nominal 6 Ohms, Minimum 4 Ohms. What is the difference between Nominal and minimum? If a speaker is rated from 10-150 Watts (Program 100 / Dynamic Peak 200), what size receiver do you need each channel to put out for a good match? Is 70 Watts enough, or is 100 Watts Better?

RE: the Arcam, when all the channels are being driven simultaneously, the amps is capable of putting out 70 watts per channel. When two channels are driven 90 watts per channel. One channel driven 100 watts per channel. In this case, either the power supply is sagging, running out of steam, or the unit is designed to limit the output as the # of channels driven increases.

The lower the impedance (ohms), the greater the current draw on the output devices and the greater the power in watts.

For nominal 6 ohm speakers either the 4 or 8 ohm position should suffice. Nominal is the average impedance whereas minimum is just that, the lowest impedance the speaker reaches.

You can't IMO, have too much power. The difference between 70 and 100 watts is on the order of 1db, an insignificant difference at best.

Quagmire
02-03-2005, 02:01 PM
Ah... the numbers game. First of all, you need to understand a few things about speaker impedances. Impedance, simply stated, is a resistance that is dependant on frequency. As the frequency changes, so the resistive value changes. Speakers are given a nominal or average value, but that doesn't mean a constant value, it will change based upon the frequency of the signal being applied to it. As the resistive value drops, the relative power value will increase. However, amp and receiver manufacturers often measure their products by attaching them to a 8 ohm resister (to simulate a nominal 8 ohm speaker load) and then take that measurement at a single frequency; typically 1 khz. This does not really simulate a true speaker load very well, but like I said in the beginning, it's a numbers game.

What is really more important than a High Watts rating is the ability of an amp to deliver high amounts of current when needed. You see, when a speaker's impedance drops down to say, 2 ohms, if offers very little resistance to electrical current flow. That's fine if the amp is able to provide the needed current, but if it can't, it can quickly over tax the amps power supply. Without protective devices, such as the ohms selector switch you spoke of, the amp would probably self destruct. That's really all the ohms selector switch is. It doesn't make an amp more capable of driving low impedance loads. In fact, it does just the opposite. I limits the amount of power the amp will deliver so as not to overdrive the the power supply.

If you want to make any sense of the numbers, first look for reputable manufacturers who design "High Current" products. Then look for measurements into lower impedance loads, ALL channels driven, through a range of frequencies (such as 20 hz to 20khz). It doesn't take a huge Watts rating to make a very capable and good quality amp. It is very possible for a good quality, high current, 30 watt amp to outperform a poorly made 100 watt amp. As far as speaker ratings go, the numbers represent a whole operating range for the speaker. Unless it is a very inefficient, low impedance speaker, most any amp will be able to power it through its normal operating range. When you start driving multiple low impedance speakers with many of todays surround sound amps, you can get into some trouble. But for stereo operation (2 channels only), most will be alright.

Q

paul_pci
02-03-2005, 09:52 PM
We should write a fabulous story whereby Dr. Watson and Holmes try to unravel the mystery of watts and ohms.


Well, it was funny when I thought of it.

kexodusc
02-04-2005, 05:26 AM
Good one paul_pci...Dr. Watts, and Sherlock Ohms...lol!!!

Astravitz: Not much else I can add here, some good advice already. Don't worry about the Arcam's ability to deliver power...it is a very beefy unit, most receivers see the power output per channel fall by 1/2 or more when going from 2 channels driven to 5. 70 watts per channel will play pretty freakin' loud.

astravitz
02-04-2005, 08:06 AM
I must thank everyone on this board for thier expert input. You take techno-jargin and help translate it into english.

So, while we're on the topic. Could anyone explain I guess the other half of the puzzel which is more on the Speaker side. I've sort of figured out that speaker SENSITIVITY, which I beleive some people call EFFIECENCY is measured by dB. It seems that the more efficient the less power required to drive the speakers. What I don't understand is how can the exact same woofer and tweeter result in various mesurements of Sensitivity. This does not make sense to me being that the only differene is the number of woofers or tweeters or the size of the case. What exactly does Sensitivity measurement mean?

Perhaps a little understanding of what is actually measured and meant by the Sensitivity rating that all manufactures put on their speakers. It seems that measurements in the 80's dB are not as good as speakers in the 90's? Are speakers with lower sensitivity better for music and higher better for HT?

PSB Produces all identically matching speakers (5 1/4"woofers & 1" tweeters), yet has the measurements all over the scale (plus they include two measurements "Anechoic Chamber" & "typical listening room." Why should I care about these numbers?

http://www.psbspeakers.com/s/ImageSeries.html

PSB Image C40 Center - Anechoic Chamber 90dB, Typical Listening Room 92dB
PSB Image B15 Bookshelf - Anechoic Chamber 89dB, Typical Listening Room 91dB
PSB Image T45 Tower - Anechoic Chamber 90dB, Typical Listening Room 93dB
PSB Image S50 Surround - Anechoic Chamber 91dB, Typical Listening Room 93dB

If all the above speakers have the exact same materials, how can all the measurements be so different?

kexodusc
02-04-2005, 08:24 AM
First, the overall speaker sensitivity is expressed in dB, efficiency is a % ratio of power out to power in...but yeah, they refer to the same thing sorta.

The difference in number of drivers (tweeters, woofers etc) and the size of the cabinet is exactly why the numbers are different.
Sensitivity is not merely a function of the drivers, but also of the crossover and even the cabinet of the speaker. How much loudness or volume does the speaker produce at a given level of power input. Consider a speaker with 2 drivers, one a woofer with a sensitivity of 90 dB, the other a tweeter with a sensitivity of 87 dB...clearly, to have all the frequencies sound the same you either have to boost the signal to the tweeter, or diminish the output of the woofer somehow so that they are both at the same level...

Low sensitivity is not necessarily better OR worse, but will require more power. That's all.

As you add drivers, you typically (but not always) increase the sensitivity. It's sort of counter intuitive, but think of it as the juice running through yet another driver, so even more volume is emitted.

I assume you are aware that sensitivity is generally given at a distance of one meter with 1 watt of power. You can probably deduce that 90 dB is pretty frickin' loud inside your room. Did you know that it takes double the power (2 watts) to that speaker to push the volume up to 93 dB, a barely noticeable difference? You can see that to get to 100 dB, you need about 10 times the power...hence the advantages of an efficient speaker.

Food for thought.

As for the typical room, vs anechoic chamber...every room adds a certain amount of gain (and diminish) to certain frequencies, anechoic chambers basically remove that variability by reducing echoes, nodes, etc...

The number itself is only so useful, but tells you more about how a speaker will behave when compared to another number. If your speakers are rated at 87 dB sensitivity, and at your listening position (ie: greater than 1 meter) you need 50 watts to create sound of 87 dB, you will need an amp capable of delivering 100 watts of power to get up to 90 dB...a speaker rated at 90 dB might save you from having to upgrade your amp. (over simplified).

Typically one chooses speakers before the amp, but not always.

Jace
02-06-2005, 10:09 AM
as was mentioned before a bit. % efficiency is a ratio of power out divided by power in. sensitivity in dB's is measured as "10 log (power out/power in). it is just another way of indicating the ratio of efficiency. it is also a logarithmic scale which is why it takes twice the power to push the volume up 3dB and ten times the power to push it up 10dB.

-Jace