Wattage, Ohms and things...

ihardcas

Hi All,

I have a Sony STR DE505 recvr which does me fine for most things. That is hooked up to a pair of Polk RT10's which, after replacing the drivers, also do me fine.

The specs for the receiver say 150W + 150W for 8 ohms and 210W + 210W for 4 ohms. The speakers say they are good for 20-150W. They speakers also say they are 'compatible with 8 ohms'.

What is happening here? Does the amp switch automatically. there are references to 'dynamic power', as opposed to 'surround mode' which only details specs for 8 ohms.

Am I running at 4 ohms or 8 ohms. If 4, could that account for a driver blowing on me a while ago (Thanks to LA Woman)?

And what is the second figure relating to when it says '150W + 150W'?

Cheers for any help
Iain.

George Grand

The ohm rating, is the speaker systems resistance to electrical current flow from your amp. Eight ohms is a GREATER amount of resistance to flow than four. A four ohm pair of speakers places a greater demand on an amplifier(in terms of heat generated inside the amp), than an eight ohm pair.

As far as the power ratings for the amp, they make sense. The front channel amps APPEAR to be robust enough to drive either 4 or 8 ohm (or anything in between) speakers. As is typical with mass-market receivers, the rear channel amps are NOT. Generate enough heat with those, and either the amp, or some speaker drivers (or both) will fail.

The Sony's are not well regarded for their heat tolerating abilities.

Books in the public library address this stuff.

George Grand (of the Jersey Grand's)

blemons

Hello there:

At the risk of being criticized by "real audiophiles" I'll post what I know regarding your posting, from my real world experience.

ANY good quality speaker should be able to handle the maximum CLEAN power output from all but the very largest amplifiers. If the speakers are being destroyed while being listened to in a "regular" size room at non ear-bleeding levels it is almost certain that the amplifier is "clipping". Once an amplifier is driven past it's limits, the audio output typically will appear as a square wave, or worse, a flat DC level. This is most stressful for speakers.

If you view music with an o-scope, you will see that it is comprised of many "hills and valleys" with regards to waveforms. The waveforms typically have ascending and descending slopes to the signals. The ability of the amplifier to accurately reproduce these slopes without distortion is critical to speaker survival. When an amplifier can no longer reproduce these slopes, the output is often referred to as "square waves" (if there are any waves at all). This is also evidenced by audible distortion and blown tweeters if it is not corrected quickly enough.

Your receiver does not "switch" it's output to accomodate 4 or 8 ohm loads. Sony is simply telling the user that the receiver can deliver approximately 7.25 amps of current into a 4 ohm load (per channel). If you lower the impedance (load) on the amp, it will try to deliver more current. This is where most amplifiers typically (not always) outperform their receiver cousins.

(P) Power = (I) Current Squared x (R) Resistance

Therefore, your receiver will output approximately 29 volts across the 4 ohm load resulting in 7.25 amps of current flow and 210 watts of energy dissipated. If you exceed this for an appreciable amount of time, there is a good chance that the amp will shut itself down, or overheat and fail.

Note that in reality, the speakers are rated in impedance, which is different than true resistance. But, for simple calculations like above, they can be interchanged fairly reliably. This assumes that the speaker load you are driving has an AVERAGE impedance of 4 to 8 ohms. Some speakers present extremely low or high impedances to the amplifiers, based on the frequency of the signal being amplified. Hence the importance of the amplifier's frequency response.

Typically, amplifiers are expensive because they can deliver large amounts of current without distorting or overheating. A high-end amp like a McIntosh, Krell, etc. can produce enough output voltage and current to run a upright vacuum cleaner without failing. While this may seem like a silly comparison, it is evidence of the output capability of those devices (110 volts, 6-12 amps of current, 660 - 1320 watts of power!!!:D )

150W + 150W simply means 150 watts per channel, 2 channels driven.

Remember, the total output wattage of the receiver (or amplifier) can not be more than the input wattage that is listed on the UL label on the power supply of the amp. If it only draws 100 watts of power from the wall, it cannot deliver 100 watts RMS to your speakers (There is some loss in the amplifying process).

As a general rule, you can not have too much CLEAN power in a stereo system. More speakers are destroyed by inadequate amplifiers than are by large, "clean" amplifiers.

Sorry about the length of my reply... Hope this helps.

jcaut

I don't mean to be overly picky, but I wanted to clarify one point, at least as I see it.

ANY good quality speaker should be able to handle the maximum CLEAN power output from all but the very largest amplifiers.

I understand the point you're making, and that statement may be true, speaking from a "thermal power-handling" point of view. Besides burning out the voice coil, there are several other ways you might damage a speaker. (I'm sure you already know this, I just wanted to clarify) For instance, if Iain, above were to feed his R10's a healthy dose of power at, say, 30Hz, for more than a few seconds, there's a good chance the speakers could be damaged. This might happen at considerably less than 150 watts. There should be a clearly audible indication that the speakers were being stressed before serious damage would occur.

I completely agree with your point that most people damage their speakers by overdriving a receiver or amplifier, to the point of clipping.

gidrah

I recently blew a woofer in a 250 watt@6 ohm speaker, while doing a listening test with a 30 watt@8 ohm speaker. The 30 didn't even noticably distort. The 250 was bottoming out all over. The 250s were even rated to go lower (Hz)then the 30s. I was testing mainly the bass at the time (<100+5db/>100-15db). This was in a left/right comparison and sounded very similar in sensitivity
I was doing this on a decent 110w/ch (@8 ohms)Sony receiver that never went past 1:00 (7:00 being zero).
I have since replaced the Sony, with a receiver higer current amp.
My question is, what happened. I know that it coulda' clipped, but so soon?

George Grand

I have been told that on a conventional rotary type volume knob, the 2 or 3 o'clock position is nearing or at full rated power. Just for giggles, what kind of speaker made it, and which one didn't? Were you using test tones, or music for the test? Could the 6 ohm rating of the speaker have something to do with it? Was there a large "spike" in the program material you were using?

George Grand (of the Jersey Grand's)

gidrah

I didn't want to name any names, but it was a Monitor 10b that blew. The speaker that survived is an older Acoustiflex model 17. I can't find out anything about these except for the sticker on back. Right now I'm listening to some SunRa and they sound great.
I was listening to music at the time of destruction. Prodigy - Smack my **** up.
P.S. If anybody knows about Acoustiflex, please let me know. I picked them up used almost 20 years ago. I'm wanting to open them up, but will wait until I repair my Polk as they have some sort of sealant around the cones and horn.

George Grand

Hey I guess you're right. That 10B is nothing to giggle about. Sun Ra huh? Nice to meet another dinosaur.

I have a pretty good library of Stereo Review and recently deceased Audio magazines, going back to the 60's in some cases. When I get some time I'll see what I can research on Acoustiflex. Maybe at least come up with an old address or something.

George Grand (of the Jersey Grand's)

blemons

Hi all:

Sorry for the confusion I seem to have created. I should have been more specific about my "ANY good quality speaker should be able to handle the maximum CLEAN power output from all but the very largest amplifiers" statement in my earlier post.

I was referring to music, not sine waves, when I suggested that decent speakers can handle enormous amounts of power, as long as it is clean.

The majority of music I listen to does not have sustained, low frequency tones. If there is a large bass strike (a tympani drum, for example) it last for a short time period, relatively speaking.

It is true that you can overdrive even good quality speakers if you have a large enough amplifier.

It is also true that trying to use an incorrect driver for a particular application (ie: 6" woofer at 500 watts to play Rap music in an auditorium) will certainly damage the driver.

Sorry. I figured people understood more about proper application than appears to be true here.

By the way, a Sony receiver is a poor test bed for power handling capabilities of speakers. (I know, I have one)

My Carver amp makes it look silly when driving speakers...

rskarvan

Kinda off topic, but, I recently figured out the following.

Power = current^2 x Resistance.
Current = sqr-root (power/resistance).

100 Watts, 8 Ohms
Current = 3.53 Amps

200 Watts, 8 Ohms
Current = 5.0 Amps

400 Watts, 8 Ohms
Current = 7.07 Amps.

THEREFORE, to double the current to your speakers, you need to quadruple the power. Speakers see current - not power.

Its a marketing trick to get consumers to think that a 200W amp is twice as powerful as a 100W amp...when, in-fact, it delivers only 40% more current.

If honesty were in audio marketing, which it isn't, we would be discussing amps (current) rather than watts. Anyway, this answers the question I originally had regarding why (exactly) when you switch a 2-channel amp to mono operation the power quadruples. NO. In reality, the current doubles (as expected). Scwooo. I feel better now. I hope somebody (besides me) learned something today.

George Grand

Yo Ron,
Not being an electrical engineer, I did learn something. Great post. I WAS going to learn if there were monsters under my bed during daylight today, but nobody else is home...and I'm too afraid. I'm gonna stick with what I learned from you.

George Grand (of the Jersey Grand's)

blemons

Nice work, rskarvan !!

Also somewhat irrelevant, but interesting enough to note:

Human ears do not relate the increase of electrical power (watts) identically as to the apparent loudness of the sound.

In other words, a 200 watt amp does not sound twice as loud as a 100 watt amp.

I don't know where I read it, but I once read that it requires approximately 10 times the power to register twice the loudness with most humans.

Also, if you play the identical music, on identical speakers (blind however, so the listener does not know the source) over 95% of listeners will choose a system that is played even slightly louder than the other. For whatever reason, most people believe that the louder system (within reason, of course) sounds better.

This is where the "trained ear" scenario begins to come into play. I'm fortunate enough to not be able to hear the difference between my Carver amp and one costing 10 or 20 times as much. I could not justify it without a really apparent difference.

George Grand

YES! The loudness (db) scale is exactly like the Richter Scale for rating earthquakes. A "2" on the Richter scale, is an earthquake TEN TIMES as powerful as a "1" on the scale.

Also, doubling the power output into the same speaker and cable, only yields a slight increase in percieved loudness. An increase that is almost imperceptible to humans. I also remember reading, that an actual 3db INCREASE in loudness, is perceived by HUMANS as a doubling of loudness.

Blemons and me are alike. My Carver's are just fine for this almost 50 yr old pair of ears.

George Grand (of the Jersey Grand's)