More Power into Less Ohm - What Does This mean? - Page 2 - AVS Forum
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post #31 of 56 Old 09-11-2013, 08:10 AM
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Originally Posted by FMW View Post

They don't sense impedance. They sense heat. It is heat that can damage an amp, not low impedance. Low impedances simply cause the speaker to draw more current and that can heat up the amp given enough level and time.

AVRs are designed with multiple protection circuits, these include:
A. Thermal breaker within the power transformer
B. Temperature sensor on the amplifiers' heat sink
C. Level/current sensing in driver stage
D. Sensor for low impedance/short circuit

D is connected directly across the output amplifier terminals, and monitors impedance levels/short circuits. A short circuit is essentially 0 Ohms and promptly shuts down the amplifier while showing PROTECT on the front display or in Denons' it flashes the RED LED. An amplifier will self-destruct trying to drive a short circuit...


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post #32 of 56 Old 09-11-2013, 09:50 AM
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Quote:
Originally Posted by M Code View Post

AVRs are designed with multiple protection circuits, these include:
A. Thermal breaker within the power transformer
B. Temperature sensor on the amplifiers' heat sink
C. Level/current sensing in driver stage
D. Sensor for low impedance/short circuit

D is connected directly across the output amplifier terminals, and monitors impedance levels/short circuits. A short circuit is essentially 0 Ohms and promptly shuts down the amplifier while showing PROTECT on the front display or in Denons' it flashes the RED LED. An amplifier will self-destruct trying to drive a short circuit...


Just my $0.05... 👍😉

I've seen electronics place a capacitor in parallel to an IC so that if there's a rapid voltage change, the capacitor will effectively short and draw the current. Frying itself in the process. Cheaper and easier to replace a capacitor than an IC.
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post #33 of 56 Old 09-11-2013, 10:27 AM
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Originally Posted by KidHorn View Post

I've seen electronics place a capacitor in parallel to an IC so that if there's a rapid voltage change, the capacitor will effectively short and draw the current. Frying itself in the process. Cheaper and easier to replace a capacitor than an IC.

Hmm..
Not familiar with that solution....
Most of the mentioned protection circuits in my post are done by a silicon IC which resets itself (except A.), using a solution that requires replacing a capacitor means significant labot costs for doing its replacement. Since the average minimum service bench rate is about $75, the parts costs are a minor part of the rework costs such as an inexpensive capacitor....

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post #34 of 56 Old 09-11-2013, 10:39 AM
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Originally Posted by M Code View Post

Hmm..
Not familiar with that solution....
Most of the mentioned protection circuits in my post are done by a silicon IC which resets itself (except A.), using a solution that requires replacing a capacitor means significant labot costs for doing its replacement. Since the average minimum service bench rate is about $75, the parts costs are a minor part of the rework costs such as an inexpensive capacitor....

Just my $0.05.... 👍😉

The capacitor thing is a failover in case the other circuit breakers don't work. It's not intended to be the primary protection. It can also be used to stabilize the voltage for normal operation of an IC.
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post #35 of 56 Old 09-11-2013, 11:01 AM
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Originally Posted by grigorianvlad View Post

I am probably buying an Emotiva power amp (XPA-200) . It isnt exactly a powerhouse (my receiver by itself is powerful enough, I just want my speakers to open up with a separate unit).

The idea that an external amplifier will cause speakers to "open up" is an audiophile myth.
Quote:
The Emotiva descriptions says:
"Equally at home powering a superb stereo system, or the front channels of an audiophile surround sound system, the XPA-200 delivers 150 watts per channel into 8 ohms, or 240 watts per channel into 4 ohms of powerful, clean, audiophile grade audio power."
My amp is a Pioneer Elite VSX-53, speakers are Polk LSi-9s (4ohm, power hungry).

http://www.polkaudio.com/products/lsi9

I don't know where you got the idea that Lsi-9s are power hungry, They are fairly typical 88 dB/watt 4 ohm speakers. To be truly power hungry you would need speakers that had sensitivity on the order of 80 dB/w or impedance on the order of 2 ohms.
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So, the Emotiva will be providing 240 watts of power into the Polks because they are 4ohms? Or 150W? Why the difference?

In fact the Emotiva won't be providing any more power to your speakers than your AVR unless you play them seriously loud or sit a long distance from your speakers (say 20 feet) etc.

Remember, we are talking the maximum possible power that an amplifier can deliver. Power amplifiers are voltage sources, and are primarily voltage limited.

Using the power calculator at http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm I find that it takes 24.49 volts to develop 150 watts into a 4 ohm resistor. If I apply the same voltage to an 8 ohm resistor we develop only 74.99 watts. The differnce in power comes about because only half the current flows through a resistor with twice the resistance. Power is the product of amps and volts so half the current (amps) means half the power (watts).
Quote:
If I had an 8ohm speaker set, would they be less loud because of 150W power?

All other things being equal, yes.

The difference would be 3 dB, which is not all that much. It takes 10 times the power to create the impression of "twice as loud".
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post #36 of 56 Old 09-11-2013, 11:04 AM
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I have Martin Logan Spires 4 Ohm speakers but dip into .5 Ohms @ 20k and 90db effeciency.

Are you guys saying that if I have a 100 watt amp only rated into 4 Ohms (NAD T787) that I can run it at 50 watts into 2 Ohms and 25 watts into 1 Ohm and 12 watts into .5 Ohms?

Ofcourse I wouldn't blast the volume all the way up anyways but 6 watts would still net me 96 db spikes without clipping. Is this somewhat correct?

And I do hear of AVRs shutting down with Martin Logans and other brands as well which is why I had to buy a top of the line model. Martin Logan says you can drive them with 8 Ohm receivers just fine as long as you don't over drive them. I guess it just depends on how much watts you require and not the Ohms of the speaker?
Thanks

k
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post #37 of 56 Old 09-11-2013, 11:07 AM
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Originally Posted by KidHorn View Post

You do realize that it's very common for all speakers to show a resistance load at times below 4 ohms regardless of the nominal impedance rating of the speaker. it what you state is true then those receivers would routinely shut down for no apparent reason.

It depends on where in the audio band the impedance drop below 4 ohms occurs, and the phase angle degree. A severe impedance drop and severe phase angle in the bass/midbass region where there is significant musical content, versus the upper frequencies where there is less musical content, will present a more difficult load to the driving amp.

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post #38 of 56 Old 09-11-2013, 11:18 AM
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Originally Posted by grigorianvlad View Post

I am probably buying an Emotiva power amp (XPA-200) . It isnt exactly a powerhouse (my receiver by itself is powerful enough, I just want my speakers to open up with a separate unit).
The Emotiva descriptions says:
"Equally at home powering a superb stereo system, or the front channels of an audiophile surround sound system, the XPA-200 delivers 150 watts per channel into 8 ohms, or 240 watts per channel into 4 ohms of powerful, clean, audiophile grade audio power."
My amp is a Pioneer Elite VSX-53, speakers are Polk LSi-9s (4ohm, power hungry). So, the Emotiva will be providing 240 watts of power into the Polks because they are 4ohms? Or 150W? Why the difference? If I had an 8ohm speaker set, would they be less loud because of 150W power? Or this has nothing to do with how loud the signal will be?
I am sure this is a basic question, but I never got a straight answer to it from anyone before.
Thanks a lot.

One thing I don't see mentioned is your choice of upgrade amp. There is only a 40W difference between your AVR and the Emotiva XPA-200. That is too little notice any difference in loudness. It takes a doubling of power (or 3dB increase) to obtain a perceived increase in loudness level. You will be wasting money on anything less than 200W per channel.

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post #39 of 56 Old 09-11-2013, 11:27 AM
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Originally Posted by M Code View Post

Many entry-level AVRs have an impedance sensing circuit if the load impedance is 4 Ohms or less it prevents the AVR from outputting any signal.

There are amps of all power ratings and genres that look at the load and shut down if the load fails some sort of a test, but the test is almost always based on whether the load is ridiculously low or not. I've tested a lot of AVRs in actual use and on the bench and I've never seen one that summarily shut down unless the load was rediculously low. Certainly not 4 ohms because many 8 ohm rated speakers have DC resistance that is below 4 ohms. I even had a Sherwood cheapie that would shut down instantly when shorted, but it worked fine into 4 ohm resistive loads and 4 ohm speakers.

The most common current limiting circuit has been around since the 1960s and is composed of two cheap transistors and a few resistors. It can be connected as a simple current limiter or a more sophisticated SOA limiter. It is almost always configured as a SOA limiter because that protects the output transistors, but is minimally intrusive. Cheap, effective, and minimally intrusive. Self-resetting. Well known.

There are very sophisticated protective circuits that constantly measure the resistance of the speaker voice coil and back off the amplifier's gain if the voice coil's resistance goes up too far, meaning that it is getting very hot. That one is patented!

At a minimum a power amp should have some kind of SOA limiter and a thermal limiter.
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post #40 of 56 Old 09-11-2013, 11:33 AM
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Originally Posted by Glen B View Post

Quote:
Originally Posted by grigorianvlad View Post

I am probably buying an Emotiva power amp (XPA-200) . It isnt exactly a powerhouse (my receiver by itself is powerful enough, I just want my speakers to open up with a separate unit).
The Emotiva descriptions says:
"Equally at home powering a superb stereo system, or the front channels of an audiophile surround sound system, the XPA-200 delivers 150 watts per channel into 8 ohms, or 240 watts per channel into 4 ohms of powerful, clean, audiophile grade audio power."
My amp is a Pioneer Elite VSX-53, speakers are Polk LSi-9s (4ohm, power hungry). So, the Emotiva will be providing 240 watts of power into the Polks because they are 4ohms? Or 150W? Why the difference? If I had an 8ohm speaker set, would they be less loud because of 150W power? Or this has nothing to do with how loud the signal will be?
I am sure this is a basic question, but I never got a straight answer to it from anyone before.
Thanks a lot.

One thing I don't see mentioned is your choice of upgrade amp. There is only a 40W difference between your AVR and the Emotiva XPA-200. That is too little notice any difference in loudness. It takes a doubling of power (or 3dB increase) to obtain a perceived increase in loudness level. You will be wasting money on anything less than 200W per channel.

+1.

It takes 10 times the power to create the subjective impression of "twice as loud". At that point you should have also seriously upgraded your speakers to handle all of that power. Which makes the point that the usual route to greater loudness is to have more efficient speakers, until you can't practically play the efficiency card any more - around 100 dB/W. Going from typcal 90 dB/W speakers to 100 dB/W speakers will give you "twice as loud".

Most people have no actual idea about how much power they are using when they play their AVR, As long as your AVR isn't clipping and producing lots of distortion, having more powerful amps won't give any advantage. A pair of 90 dB/W speakers, sitting about 12 feet away, and 100 wpc music power will get to reference levels - 105 dB peaks, with a nominal safety margin.. It is generally more practical to extend the dynamic range of your system with a good subwoofer than more powerful amps.
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post #41 of 56 Old 09-11-2013, 12:34 PM
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Originally Posted by snyderkv View Post

I have Martin Logan Spires 4 Ohm speakers but dip into .5 Ohms @ 20k and 90db effeciency.

Are you guys saying that if I have a 100 watt amp only rated into 4 Ohms (NAD T787) that I can run it at 50 watts into 2 Ohms and 25 watts into 1 Ohm and 12 watts into .5 Ohms?

No reason to worry about .5 ohm at 20khz. There isn't any recorded content there anyway.
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post #42 of 56 Old 09-11-2013, 12:52 PM
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Originally Posted by Glen B View Post

It depends on where in the audio band the impedance drop below 4 ohms occurs, and the phase angle degree. A severe impedance drop and severe phase angle in the bass/midbass region where there is significant musical content, versus the upper frequencies where there is less musical content, will present a more difficult load to the driving amp.

If what's being discussed is some sort of circuitry that detects the impedance and shuts down, it doesn't really matter the particulars of the waveform being generated. All that would matter is a simple measurement of the voltage and current at that moment and if V/I < 4, shut down = true. I don't believe any such circuitry exists in AVRs for the primary reason that impedance can't be directly measured and must be inferred from other measurements. More likely the unit would shut down based on the current alone exceeding a certain level. After all driving a 1 ohm load isn't a problem if the voltage is 1, but increase the voltage to 100 and you probably have a problem.
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post #43 of 56 Old 09-11-2013, 01:18 PM
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Originally Posted by snyderkv View Post

I have Martin Logan Spires 4 Ohm speakers but dip into .5 Ohms @ 20k and 90db effeciency.

Are you guys saying that if I have a 100 watt amp only rated into 4 Ohms (NAD T787) that I can run it at 50 watts into 2 Ohms and 25 watts into 1 Ohm and 12 watts into .5 Ohms?

The numbers you have provided seem to assume that the amp is current limited and that the current limit is right at 5 amps which is the same as 100 watts into a 4 ohm resistive load. Most real world amps can put out more current than this under these conditions.

Hard to tell for sure, but the numbers you have come up with are probably the worst case. The amp might put out far more power into 2 ohms than your model suggests.

For example here is a real world amplfiier:

http://www.stereophile.com/content/symphonic-line-kraft-400-monoblock-power-amplifier-measurements



"Fig.7 Symphonic Line Kraft 400, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms"

I read 233 watts @ 8 ohms, 375 watts @ 4 ohms, and about 600 watts at 2 ohms. Using your model, we would expect to see more like 375 watts at 8 ohms, 188 watts at 4 ohms, and 94 watts at 2 ohms. Big difference!
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post #44 of 56 Old 09-11-2013, 05:42 PM
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Originally Posted by FMW View Post

No reason to worry about .5 ohm at 20khz. There isn't any recorded content there anyway.
You mean abundance of recorded content but in Classical, Electronic music there are selections and some movies ( WOTW, U-571,Tron) of late there is content of course if he has his speakers crossed over to a sub at 80Hz then no worry.
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post #45 of 56 Old 09-12-2013, 05:14 AM
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Originally Posted by oztech View Post

You mean abundance of recorded content but in Classical, Electronic music there are selections and some movies ( WOTW, U-571,Tron) of late there is content of course if he has his speakers crossed over to a sub at 80Hz then no worry.

Thanks, nah it's not crossed over but I don't think the Ohms just drops right at 20k. I'm sure it drops gradually as you go up in the frequency range and at some point dipping below 4 Ohms long before 20khz. That is why I was concerned although I don't loud movies or music so it shouldn't be an issue..
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post #46 of 56 Old 09-12-2013, 06:31 AM
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Originally Posted by FMW View Post

No reason to worry about .5 ohm at 20khz. There isn't any recorded content there anyway.
I stand corrected found the impedance charts and thought it was at 20Hz but it was at 20KHz and you are correct most things are filtered right at 20KHz .
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post #47 of 56 Old 09-12-2013, 07:11 AM
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Originally Posted by Zen Traveler View Post

I...After comparing everything I could came to the conclusion it was the ability of the power supply to drive 4 Ohm speakers, but don't discount that Audyssey also may have something to do with it. With this in mind I noticed that the 7 channel AVR-3805 weighs 37 lbs 8 oz and the 9 channel AVR-4311ci weighs 38 lbs 2 oz and is rated to drive speakers that are 4 Ohm....

What part of the AVR allows one to drive speakers which are 4 Ohm rated compared to ones that advise against it? I noticed in the example above they both share similar WPC and weight (even with 2 additional channels on the latter) but one states to be careful with under 6 Ohm loads whereas the other says it safe to drive 4 Ohm speakers on all channels?
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post #48 of 56 Old 09-12-2013, 11:10 AM
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Originally Posted by Zen Traveler View Post

What part of the AVR allows one to drive speakers which are 4 Ohm rated compared to ones that advise against it? I noticed in the example above they both share similar WPC and weight (even with 2 additional channels on the latter) but one states to be careful with under 6 Ohm loads whereas the other says it safe to drive 4 Ohm speakers on all channels?

Primary factor is the AVR's power supply....
Older AVRs built 2007 typically have higher quality, more costly parts including power transformer and power capacitors. Since the power transformer is the single most costly component within an AVR, this the 1st place costs were reduced by going to cheaper, lower performance part. Additionally since the majority of AVR are used with subwoofer/satellite speaker systems, the heavy lifting for current is usually done by an external powered subwoofer. So the AVR's amplifiers only had to drive the satellites that had limited bandwidth compared to full-range loudspeakers. Some AVR brands as Harman/Kardon actually used dual power supplies, 1 for the left, right, center and 1 for the left/right surrounds and left/right back surrounds.

My recommendation is to try and use a mid-range AVR (SRP >$699) and avoid the lower priced entry-level models(SRP <$599). Also stay with the established brands, and avoid the factory refurbs, the refurbs may appear attractive but were returned for some reason plus their warranty is shorter and return freight can be costly..


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post #49 of 56 Old 09-12-2013, 11:20 AM
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Originally Posted by Zen Traveler View Post

Quote:
Originally Posted by Zen Traveler View Post

I...After comparing everything I could came to the conclusion it was the ability of the power supply to drive 4 Ohm speakers, but don't discount that Audyssey also may have something to do with it. With this in mind I noticed that the 7 channel AVR-3805 weighs 37 lbs 8 oz and the 9 channel AVR-4311ci weighs 38 lbs 2 oz and is rated to drive speakers that are 4 Ohm....

What part of the AVR allows one to drive speakers which are 4 Ohm rated compared to ones that advise against it?

Advice against driving speakers that are 4 ohm loads is usually based on a misunderstanding of how AVRs are rated to meet government standards.

Just because an AVR is rated based on at the lowest, say 6 ohm loads does not mean that it will have real world problems driving 4 and perhaps even 2 ohm loads.

The 6 ohm rating is based on bench testing with resistive loads and pure sine wave test signals. I seriously doubt that any AVR you or I buy will receive much use of that nature.

The high crest factor of music allows an AVR to drive speakers that go down to well below 4 ohms to achieve reference levels without audible distortion.
Quote:
I noticed in the example above they both share similar WPC and weight (even with 2 additional channels on the latter) but one states to be careful with under 6 Ohm loads whereas the other says it safe to drive 4 Ohm speakers on all channels?

Or, the 7.1 channel AVR 1913 that weighs about half as much - 19 pounds.

But to answer the question, if you want an amp to really grind power out into low impedance loads with sine waves you need an amp with a high current power supply (this usually means lotsa weight unless the power supply is switchmode), big heat sinks (more weight and bulk) or forced air cooling or both, and an output stage with high current transistors. Don't have all 3, and the power transistors will blow, the amp will overheat, and the transformer will fry or the power it supplies will collaps under load. Because of advances in power transistor technology, the cost of high current output transistors has continued to drop. Because of the ability of modern parts to take more heat, heat sinks and power transformers can now be smaller.

An invisible technical advance such as better insulation for transformer wire or output transistors that handle more power and run hotter can vastly shrink and lighten power amps for no apparent reason.
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post #50 of 56 Old 09-13-2013, 04:12 PM
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Originally Posted by arnyk View Post

...Just because an AVR is rated based on at the lowest, say 6 ohm loads does not mean that it will have real world problems driving 4 and perhaps even 2 ohm loads.

The 6 ohm rating is based on bench testing with resistive loads and pure sine wave test signals. I seriously doubt that any AVR you or I buy will receive much use of that nature.

The high crest factor of music allows an AVR to drive speakers that go down to well below 4 ohms to achieve reference levels without audible distortion.
Or, the 7.1 channel AVR 1913 that weighs about half as much - 19 pounds.

....An invisible technical advance such as better insulation for transformer wire or output transistors that handle more power and run hotter can vastly shrink and lighten power amps for no apparent reason.

Arny, thanks for answering my question once again and I listen to alot of Multichannel Music...I have posted links to the Frequency Response Curve of the Klipsch RF-7IIs and trying to figure out why the upper-end AVRs drove them (mine are actually the original RF-7s) in a multispeaker configuration better at louder volume (Reference Level or slightly below) than the (older) mid level ones I tried before...Your last line here is something new I can add to the mix which is:

A) The ability to drive lower impedance loads in a multispeaker configuration
B) With or without better EQing in the AVR (Audyssey)
C) An invisible technical advance such as better insulation for transformer wire or output transistors that handle more power and run hotter can vastly shrink and lighten power amps for no apparent reason.

There are those that think they need an external amp to get full benefit from these speakers but I think it's not more WPC, but one or several of the items above.
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post #51 of 56 Old 09-14-2013, 07:02 AM
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Originally Posted by Zen Traveler View Post


Arny, thanks for answering my question once again and I listen to alot of Multichannel Music...I have posted links to the Frequency Response Curve of the Klipsch RF-7IIs and trying to figure out why the upper-end AVRs drove them (mine are actually the original RF-7s) in a multispeaker configuration better at louder volume (Reference Level or slightly below) than the (older) mid level ones I tried before...

I suspect you determined your opinion in this matter by means of uninstrumented sighted evaluations, and by now you should know what I think about that. Please correct me if I'm wrong.
Quote:
Your last line here is something new I can add to the mix which is:

A) The ability to drive lower impedance loads in a multispeaker configuration
B) With or without better EQing in the AVR (Audyssey)
C) An invisible technical advance such as better insulation for transformer wire or output transistors that handle more power and run hotter can vastly shrink and lighten power amps for no apparent reason.

There are those that think they need an external amp to get full benefit from these speakers but I think it's not more WPC, but one or several of the items above.

I think that modern AVRs benefit from items A-C.
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post #52 of 56 Old 09-14-2013, 08:44 AM
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Originally Posted by arnyk View Post

I suspect you determined your opinion in this matter by means of uninstrumented sighted evaluations, and by now you should know what I think about that. Please correct me if I'm wrong.
I think that modern AVRs benefit from items A-C.

I used a Denon AVR-3805 for a year between the Denon AVR-4806 (and the ARV-4802R prior) and the AVR-4311ci that I have now. Every night I would listen to mostly Multichannel music with my SPL meter in hand and would get slightly frustrated that it became bright at the levels I was accustomed and ended up lowering the volume. For that year I tried and experimented with different settings...I admit it wasn't super-scientific and it did sound good about 6 dB lower than before and now, as per the Rat shack meter.

{EDIT: Even after a few beers when I was rockin' and turning the volume up it became bright and ended up lowering it again--Again, not necessarily scientific but consistent. wink.gif }
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post #53 of 56 Old 09-14-2013, 08:53 AM
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Originally Posted by arnyk View Post

I suspect you determined your opinion in this matter by means of uninstrumented sighted evaluations, and by now you should know what I think about that. Please correct me if I'm wrong.
I think that modern AVRs benefit from items A-C.

I used a Denon AVR-3805 for a year between the Denon AVR-4806 (and the ARV-4802R prior) and the AVR-4311ci that I have now. Every night I would listen to mostly Multichannel music with my SPL meter in hand and would get slightly frustrated that it became bright at the levels I was accustomed and ended up lowering the volume. For that year I tried and experimented with different settings...I admit it wasn't super-scientific and it did sound good about 6 dB lower than before and now, as per the Rat shack meter.

{EDIT: Even after a few beers when I was rockin' and turning the volume up it became bright and ended up lowering it again--Again, not necessarily scientific but consistent. wink.gif }

Well, lightly instrumented sighted evaluations. If you follow my posts, I'm not a big fan of SPL meters, especially while playing something that jumps around like music. I don't know what to attribute your perceptions to, so I really have no further comments.
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post #54 of 56 Old 09-14-2013, 09:00 AM
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Well, lightly instrumented sighted evaluations. If you follow my posts, I'm not a big fan of SPL meters, especially while playing something that jumps around like music. I don't know what to attribute your perceptions to, so I really have no further comments.

His trebble on his avr was too high wink.gif mystery solved
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post #55 of 56 Old 09-14-2013, 09:01 AM
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Well, lightly instrumented sighted evaluations. If you follow my posts, I'm not a big fan of SPL meters, especially while playing something that jumps around like music. I don't know what to attribute your perceptions to, so I really have no further comments.

I appreciate the comments that you have made. Honestly, it sounds good now and I'm just trying to help others do their homework...You have given me several things to think about and no one seems to think that external amps helps as much as others have posted who don't do any level matching experiments. Thanks.
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post #56 of 56 Old 09-14-2013, 09:04 AM
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His trebble on his avr was too high wink.gif mystery solved

Could be but at lower volume it sounded great.
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