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amplifier output at different ohm loads


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15 replies to this topic

#1 of 16 Daryl_hawk

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Posted February 06 2005 - 08:35 AM

I was hoping someone could shed a little more light for me on the subject of amplifier output at the various loads (8 ohm, 4 ohm 2 ohm).

From what I can gather thus far is that a speaker is rated nominally and actually resistance varies with frequency.....low frequencies having a low resistance value. So if I understand this correctly, then an amp that has a good low end, tight defined bass etc will be able to go down to 4 and even 2 ohms????? Am I on the right track here?

I have heard a lot of talk about the "ideal" having power at 4 ohms twice that at eight and power at 2 ohms twice that at 4. Is this correct. And if an amp has only maybe a 30% increase in 4 ohms over 8 and maybe not even stable at 2 ohms, would this mean that it would not have as nice of a low end and tight defined bass???

thanks.

#2 of 16 amatala

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Posted February 06 2005 - 10:20 AM

In reality it is not as simple as that. Some amplifiers handle 2ohm/4ohm loads better than others. For example it is a bad idea to use 4ohm speakers in combination with Denon receivers because they don't handle 4ohm loads very well.
Here are some lab measurements performed on various receivers powering 2/4/8ohm spakers:

Onkyo TR-NR801:
-Continuous power to 5ch - 8/4ohm: 33/47W
-20ms burst power to 5ch - 8/4/2ohm: 34/49/58W
-Continuous power to 2ch - 8/4ohm: 38/63W

Denon AVR3805:
-Continuous power to 5ch - 8/4ohm: 39W/20W
-20ms burst power to 5ch - 8/4/2ohm: 110W/145W/155W
-Continuous power to 2ch - 8/4ohm: 135/120W

Marantz SR7400:
-Continuous power to 5ch - 8/4ohm: 93/97W
-20ms burst power to 5ch - 8/4/2ohm: 115/125/67W
-Continuous power to 2ch - 8/4ohm: 125/135W

H/K AVR630:
-Continuous power to 5ch (fronts and rears) - 8/4ohm: 84/115W
-Continuous power to 5ch (center) - 8/4ohm: 105/155W
-20ms burst power to 5ch (fronts and rears) - 8/4/2ohm: 92/140/155W
-20ms burst power to 5ch (center) - 8/4/2ohm: 122/185/155W
-Continuous power to 2ch - 8/4ohm: 100/155W
(The center-ch amp is more powerful)

CA Azur 540R:
-Continuous power to 5ch - 8/4ohm: 75/84W
-20ms burst power to 5ch - 8/4/2ohm: 82/92/92W
-Continuous power to 2ch - 8/4ohm: 105/130W

Denon AVR2803:
-continuous power 5ch to 8/4ohm: 30/15W
-20ms power 5ch to 8/4/2ohm: 87/110/130W
-continuous power 2ch to 8/4ohm: 115/89W
-output impedance: 0,09ohm

H/K AVR4550 (=AVR325):
-continuous power 5ch to 8/4ohm: 76/99W
-20ms power 5ch to 8/4/2ohm: 87/110/120W
-continuous power 2ch to 8/4ohm: 94/145W
-output impedance: 0,09ohm

Marantz SR6300:
-continuous power 5ch to 8/4ohm: 73/115W
-20ms power 5ch to 8/4/2ohm: 98/120/35W
-continuous power 2ch to 8/4ohm: 110/165W
-output impedance: 0,17ohm

Pioneer VSX-AX3 (=53TX):
-continuous power 5ch to 8/4ohm: 105/145W
-20ms power 5ch to 8/4/2ohm: 115/175/72W
-continuous power 2ch to 8/4ohm: 125/200W
-output impedance: 0,06ohm

Sony STR-DB2000 (=DA2000ES):
-continuous power 5ch to 8/4ohm: 90/84W
-20ms power 5ch to 8/4/2ohm: 115/125/98W
-continuous power 2ch to 8/4ohm: 160/210W
-output impedance: 0,67ohm

Yamaha RX-V1400RDS:
-continuous power 5ch to 8/4ohm: 100/110W
-20ms power 5ch to 8/4/2ohm: 130/125/54W
-continuous power 2ch to 8/4ohm: 145/200W
-output impedance: 0,07ohm

Enjoy!
Adrian Matala

#3 of 16 Justin_D

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Posted February 06 2005 - 12:35 PM

Some of those look a bit low to me!
Onkyo TR-NR801:
-Continuous power to 5ch - 8/4ohm: 33/47W
-20ms burst power to 5ch - 8/4/2ohm: 34/49/58W
-Continuous power to 2ch - 8/4ohm: 38/63W

Denon AVR3805:
-Continuous power to 5ch - 8/4ohm: 39W/20W
-20ms burst power to 5ch - 8/4/2ohm: 110W/145W/155W
-Continuous power to 2ch - 8/4ohm: 135/120W

WAH!?!?! Are you telling me that a $1000 reciever only really puts out 33-40 watts? This "overrating" wattage is getting BAAAAAD!

:::saves Adrian's list to disk:::

Thanks for the heads up Adrian!

#4 of 16 Shiu

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Posted February 06 2005 - 12:38 PM

No idea where you got your data from, but H.T. mag happened to have tested both the Denon 3805 and the HK 630 (Aug 2004 issue), the 2 channel results are as follow (direct quote from their lab measurements):

"This graph shows that the AVR-3805's left channel, from CD input to speaker output with two channels driving 8-ohm loads, reaches 0.1% distortion at 132.2 watts and 1% distortion at 162.3 watts. Into 4 ohms, the amplifier reaches 0.1% distortion at 218.4 watts and 1% distortion at 243.0 watts".

"This graph shows that the AVR 630's left channel, from CD input to speaker output with two channels driving 8-ohm loads, reaches 0.1% distortion at 84.6 watts and 1% distortion at 100.6 watts. Into 4 ohms, the amplifier reaches 0.1% distortion at 146.9 watts and 1% distortion at 170.8 watts".

Here are the links if anyone are interested:

http://hometheaterma....on/index2.html

http://hometheaterma...4hk/index2.html

They did not do the "20ms burst" though. I can't see why Denon would have too much trouble driving 4 ohm loads. In fact, Denon (in the FAQ section) did say their receivers can drive 4 ohm speakers.

Theorectically, no amplifiers can "double down" from 8 to 4 ohms because no matter how strong the power supply is, there will always be internal resistance/impedance resulting in higher voltage drop when current doubles. Some amplifiers specify say 200/400W into 8/4 ohms. In those cases, the manufacturer probably understates their power output into 8 ohms.

#5 of 16 Shiu

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Posted February 06 2005 - 12:43 PM

It is hard to believe the Onkyo can only put out 38WX2. I hope it was just a typing error.

#6 of 16 Daryl_hawk

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Posted February 06 2005 - 03:06 PM

So can anyone elaborate on why having more power in 4 or 2 ohms is good?

Also, just to note, I have a denon receiver (lower than the 3805) and it does not even specify 4 ohms in the specs or on the back. It only says down to 6 ohms and even at that the wattage is given only at 1kHz, so I would assume it probably stinks from 20 to 20. This is where my questions about this and how it could relate to low end reproduction come from because this receiver definitely has poor, weak and muddy bass.

#7 of 16 John S

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Posted February 07 2005 - 01:45 AM

It really isn't anything to get hung up on... But some do and will.....

Your really just playing with the dynamics of any given amp here....

To the ear performance, isn't really going to be all that different. You also loose half of your dynamic range too.
No biggie, and with a nice enough amp, no real reason not to do it, as long as it can take the heat.

I wouldn't run an entire set of 4 ohm speakers on a Denon myself. One channel like the center would be fine though.

#8 of 16 Chris Fleming

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Posted February 07 2005 - 04:06 AM

John, do you think its okay to run the surround channels with 4 ohm speakers with an "8 ohm" reciever? I've been considering the swans for my HT set-up. All but the surrounds are 8 ohm. If, for instance, I was using the Pio 1014, would I have to worry about a 4 ohm load on the surround channels only?

Sorry for re-hashing old material, but for some reason, I can't seem to wrap my mind around the 4/8 ohm issue!

Thanks!

#9 of 16 Pauli

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Posted February 07 2005 - 09:04 AM

The values posted by amatala are from a finnish Hifi magazine that has been around for quite a while, and are quite reliable.

The reasons why the measurments differ are probably caused by the fact that the US and European models differ a bit (different power supplies, etc.) and that the measurment techniques probably aren't completely identical.

#10 of 16 John S

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Posted February 07 2005 - 09:11 AM

2 channels at 4 ohm may start to push it. I'll bet it would be fine, and just start shutting down. But those would be two ideal channels if your going to do it, as they only see special effect stuff during DD/DTS programming, so I am going to say for movies, it would be fine.

2 fronts would probably not be so good, to much continuous output there.

#11 of 16 amatala

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Posted February 07 2005 - 09:33 AM

The values I've posted do indeed come from a finnish magazine (http://www.hifilehti.fi/). They have been seen quite a lot on different forums.
There are no typos and not many people have doubted their reliability, so I personally keep them as THE reference.
Adrian Matala

#12 of 16 Shiu

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Posted February 07 2005 - 11:26 AM

Quote:
The reasons why the measurments differ are probably caused by the fact that the US and European models differ a bit (different power supplies, etc.) and that the measurment techniques probably aren't completely identical.


That makes sense, thanks.

By the way, I pasted the Denon FAQ section that is relevant to the 4 ohm question below:

" 9) CAN I USE 4 OHM LOUDSPEAKERS WITH MY DENON RECEIVER OR POWER AMPLIFIER?

Yes, you certainly can.

To understand this a bit better, first realize that all amplifiers are designed to deliver a signal into an electrical "load" or resistance presented by the loudspeaker. We measure resistance in units called "ohms" (after the German physicist Georg Simon Ohm, 1787–1854).

Conventional wisdom makes an 8 ohm loudspeaker load the most acceptable because it "protects" the amplifier from delivering too much current. A 4 ohm loudspeaker can encourage a marginally designed amplifier to deliver more current than it comfortably can.

However, you should remember that a loudspeaker’s impedance rating is a nominal or average one: A speaker rated at 8 ohms may actually vary from 5 (sometimes even less) to 20 ohms or higher, depending on the frequency at which you measure the impedance. (Don’t worry about this too much -- good speaker engineers are well aware of these variations and take them into consideration when designing products.)

In general, you’ll find that Denon products are designed to function with a wide variety of loudspeakers and have power supplies and output circuitry more than able to meet the current demands of low impedance loads.

In the rare event that very low impedances tax the amplifier, quick acting circuitry will protect it from damage. If unusual operating conditions trigger this circuitry, the word "PROTECTION" will appear on the unit’s front panel. If this happens, simply turn the unit off, wait a moment or two, and turn the unit back on again. The protection circuitry will automatically reset. If it re-engages, check your system for possible malfunctions. "



Regardless of what Denon says, I would get a separate amplifier to take care of any 4 ohm speakers.

#13 of 16 Shiu

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Posted February 07 2005 - 11:38 AM

Quote:
It only says down to 6 ohms and even at that the wattage is given only at 1kHz, so I would assume it probably stinks from 20 to 20.


You may be pleasantly surprised to find that in general, most mid priced receivers (at least those made by Denon, Yamaha, Pioneer etc.), will give you 20 to 20,000 Hz power output very close to their 1 KHz output. May be 10 to 15% less, not more.

An amplifier that delivers significantly more power output into 4, 2 ohms would indicate that the amplifier is high current capable, and that it has a power supply that can maintain its output voltage under heavy load (current) demand. That, obviously, is a good thing.

#14 of 16 Kevin C Brown

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Posted February 07 2005 - 12:24 PM

More power can give deeper tighter bass because it's all about moving air. Low freqs require more power because it takes more speaker cone movement (area and excusion) to produce lower freqs than higher.

But, most people however cross over their mains to a sub for low freqs. In that case, the power demands on the receiver aren't as great.

I always think about reduced ohm loads as follows: a 4 ohm speaker *pulls* (or *demands*) more power out of the receiver (or amp) than an 8 ohm speaker. If the receiver can't deliver it, poor sound quality, distortion, etc result. Worst case, if you try to drive a speaker with an impedance too low for a particular receiver, you can fry the output stage of the receiver (for that specific channel). That's ulitmately why you need a more powerful receiver for reduced impedance speakers. There is also the speaker's efficiency (or sensitivity) too. 89 dB is about average, 86 is low, 91 or 92 dB is higher. (Some go up over 100 dB. That's a very easy speaker to drive.) So effiency and impedance work hand in hand. There's also your listening habits, how big your room is, etc.
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#15 of 16 Claude M

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Posted February 08 2005 - 12:25 AM

"There is also the speaker's efficiency (or sensitivity) too."

I was reading on hoping this point would be mentioned. Many variables at play here.

#16 of 16 Chris Fleming

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Posted February 08 2005 - 02:45 AM

John, Thanks!





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