AV receiver vs HiFi amp rated power

[splasher]

Hi all.I have a Denon AV receiver rated at 5 x 80w into 8 ohms and a Marantz HiFi amp rated at 2 x 70w into 8 ohms. The HiFi amp runs all day long at high volumes (around 3/4 on the dial) whereas the AV receiver falls over on over-heat at a similar 3/4 on the volume. I have nothing on top of the AV receiver yet I have another hifi box sat on top of the amp so I could understand it if it was the other way around.I have tried to explore the issue on a forum previously but couldn't get an answer I was happy with - the argument seemed to go that the volume scale on the AV receiver wasn't linked directly to the power it produced and so the amp may be trying harder than it was capable of depending on the room calibration. This sounded like nonsense to me - I can't see why the pre-amp would be designed to push more into the power amp than it could cope with. I suspect that thermal design of AV receivers is based on soundtracks where the level of sound varies and so the components can be good for short periods of high power but the thermal design doesn't have to cope with long periods of high power, like when listening to music. I should have probably already said it tends to trip on music, not film. Can anyone shed any light on this - my theory would also seem to be backed up by the fact that hifi amps seem to cost about double AV reveivers in the watts/channel stakes.

 

[Paul.]

the argument seemed to go that the volume scale on the AV receiver wasn't linked directly to the power it produced and so the amp may be trying harder than it was capable of depending on the room calibration. This sounded like nonsense to me - I can't see why the pre-amp would be designed to push more into the power amp than it could cope with.

AV receiver microphone auto setups are designed to hit around 75db at the listening position when the amplifiers digital volume readout is showing 0db (assuming your amps volume is reversed, i.e it starts at -50db ish and increases to 0db). If the speakers are situated in a larger room, the amplifier will have to do more work to generate that 75db at the listening position than it would in a smaller room, so the output power is increased depandant on room size,.

THX rate amps as either Ultra or Select for this very reason, the larger Ultra amps can maintain the volume required in larger rooms (greater than 12 foot viewing distance) than Select amps can (10-12 foot).

Not saying this is your problem, just explaining what was said on the other forum

Some AV receiver manufacturers aren't always honest with the way they rate power, so in reality your amp probably just lacks the power claimed.

 

[Overdose]

Either the AV amp has a fault or it is tripping to protect itself.

In any case, the use that you are giving it is simply too much for it, so you need to either fix it, lower the usage or replace it.

 

[KidKomet]

Some AV receiver manufacturers aren't always honest with the way they rate power, so in reality your amp probably just lacks the power claimed.

This.

 

[Frank Harvey]

Many AV receiver manufacturers quote output power into one channel driven, not five or seven which can be quite a different story. Hi-fi amplifiers are always quoted as two channels driven.

 

[splasher]

AV receiver microphone auto setups are designed to hit around 75db at the listening position when the amplifiers digital volume readout is showing 0db (assuming your amps volume is reversed, i.e it starts at -50db ish and increases to 0db). If the speakers are situated in a larger room, the amplifier will have to do more work to generate that 75db at the listening position than it would in a smaller room, so the output power is increased depandant on room size,.

Paul, thanks for the explanation. I do understand that the AV amp could be put into a situation where to achieve 75dB, it needs to use a lot of power. But my contention, which I couldn't seem to explain last time very well, was that the amp cannot work any harder than it can work and irrespective of what that is, the thermal management of the same amp should cope. It's like having an 80w amp with the cooling arrangements of a 40w amp - it can put itself into situations it can't cope with.

Not sure I've explained it any better.

 

[jcbrum]

I assume that the respective amplifiers are driving different loudspeakers.

The nature of the load can cause amplifiers to run hot or cool, and require varying power levels for the same sound pressure from differing loudspeakers.

JC

 

[splasher]

Yes, the AV amp drives MA bronze 5.1 and the HiFi amp drives Tannoy floorstanders. All speakers are 8 ohm nominal. From memory, I think the front BX2s are rated up to 100w whereas the Tannoys I seem to remember are good for about 170w.

 

[davedotco]

First of all forget about the setting on the volume control, it is arbitary and largely dependent on the level of the incoming signal, even if THX spec says that certain things should happen at certain settings, that will be for a specified input level and your partnering equipment wii probably not be to that standard.

Secondly the available power output of both amplifiers will be determined by the power supply, it will have a maximum rating measured in VA (volts x amps) and is fixed. To all intents and purposes they can be considered as watts in this example, and the total can not be exceeded irrespective of the mumber of channels in use.

If a A/V amp is quoted as (say) 150 watts per channel, this could in effect mean that driving all channels simultaneously could give you just 30 watts per channel. This is an extreme example and I doubt most reputable brands would rate their product in this way but it gives you an idea of what could be going on, so be aware.

 

[splasher]

I'm an engineer so I get P=IV, and obviously the total signals out can only be the power of the transformer minus the losses, like heat - no problem with any of that. Also, an amplifier, by its very nature increases the output in proportion to the input, again no problem with that.

Not sure about forgetting the setting on the volume control though - surely this sets the gain of the amp from 0 to maximum. And the incoming signal will differ but in both cases (av amp and hifi amp) the incoming signal is line level from a CD player. But finally, the issue I just cannot get my head around, is that someone can design a box that can take a line level signal, apply a given amount of gain to that signal, in doing so create a known amount of heat and yet not make sufficient provision for that heat to be dissipated in the design such that it builds up and trips the thermal protection.

 

[davedotco]

I'm an engineer so I get P=IV, and obviously the total signals out can only be the power of the transformer minus the losses, like heat - no problem with any of that. Also, an amplifier, by its very nature increases the output in proportion to the input, again no problem with that.

Not sure about forgetting the setting on the volume control though - surely this sets the gain of the amp from 0 to maximum. And the incoming signal will differ but in both cases (av amp and hifi amp) the incoming signal is line level from a CD player. But finally, the issue I just cannot get my head around, is that someone can design a box that can take a line level signal, apply a given amount of gain to that signal, in doing so create a known amount of heat and yet not make sufficient provision for that heat to be dissipated in the design such that it builds up and trips the thermal protection.

OK.

Re setting of the volume control.

The power amplifier of any amplifier whether a power amp, integrated or an A/V system is always effectively operating at full power, ie the gain is fixed. Typically a power amplifier will have an input sensitivity, ie the input level that will, given the gain of the amplifier give full output, usually this is in the range 500mv to 1 volt. (Pro amps are usually lower sensitivity, idealy +4dbm, about 1.775 volts)

The volume control, on the preamp, separate or integrated, will turn down the level of the input signal, typically 2 volt from a CD player to the level that, subject to the gain of the power amp, gives 'normal' listening levels. Older style source components such as tuners and cassette decks would typically have outputs arounf 500-700mv and therefore require to be turned down less than the CD player, hence higher volume settings.

Furthermore, for marketing reasons, a lot of manufacturers deliberately choose to make their volume controls act very quickly, ie get loud early, say around the 10 o'clock point. This is to impress the uninformed who do not understand that such amplifiers are then in hard clipping much after 12 o'clock.

Re thermal tripping.

This is rarely a problem with any kind of quality amplifier, some less worthy designs will stretch their spec and cut corners, so that driven hard on low dynamic range material, over heating can become an issue. This is simply a matter of cost, providing adequate power supplies, well rated output stages and proper heat sinking is expensive and it is unrealistic to expect all of that in budget equipment, whatever the specs appear to say.

 

[Thompsonuxb]

First of all forget about the setting on the volume control, it is arbitary and largely dependent on the level of the incoming signal, even if THX spec says that certain things should happen at certain settings, that will be for a specified input level and your partnering equipment wii probably not be to that standard.

Secondly the available power output of both amplifiers will be determined by the power supply, it will have a maximum rating measured in VA (volts x amps) and is fixed. To all intents and purposes they can be considered as watts in this example, and the total can not be exceeded irrespective of the mumber of channels in use.

If a A/V amp is quoted as (say) 150 watts per channel, this could in effect mean that driving all channels simultaneously could give you just 30 watts per channel. This is an extreme example and I doubt most reputable brands would rate their product in this way but it gives you an idea of what could be going on, so be aware.

is that right?

usually they give a total output i.e 600watts for a 100wpc reciever (5.1) into a given load (ohms)

 

[Thompsonuxb]

I think your issue is trying to drive your 5.1 system to the levels of your stereo.

Have you tried connecting your rx'er to your stereo speakers driving it in stereo to see if it cuts out or over heats or maybe putting your BX2 on the stereo amp using A and B speaker pairs to drive them to the same levels - you're using the rx'er incorrectly.....lol.

 

[davedotco]

First of all forget about the setting on the volume control, it is arbitary and largely dependent on the level of the incoming signal, even if THX spec says that certain things should happen at certain settings, that will be for a specified input level and your partnering equipment wii probably not be to that standard.

Secondly the available power output of both amplifiers will be determined by the power supply, it will have a maximum rating measured in VA (volts x amps) and is fixed. To all intents and purposes they can be considered as watts in this example, and the total can not be exceeded irrespective of the mumber of channels in use.

If a A/V amp is quoted as (say) 150 watts per channel, this could in effect mean that driving all channels simultaneously could give you just 30 watts per channel. This is an extreme example and I doubt most reputable brands would rate their product in this way but it gives you an idea of what could be going on, so be aware.

is that right?

usually they give a total output i.e 600watts for a 100wpc reciever (5.1) into a given load (ohms)

The maximum power consunption of the unit is often a good guide.

Take the highly regarded Onkyo TX 818 for example. It is rated at 180 watts per channel, into 6 ohm (though that is not important in this case). The maximum power consumption for this unit is just 720 watts, allowing for inefficiencies in the power supply and the fact that the pre-amp and other sections also require some power, the power amplifiers are likely to have to share around 400-500 watts.

This is a 7.1 channel unit, so roughly 65-70 watts per channel with all channels driven and remember, this is probably into 4 or 6 ohms, so a little less into 8 ohms. Now clearly in any kind of a proper surround sound setup the power requirements for the surround channels are not likely to be huge so in this sense the amp is pretty well specified, ie the power output to the front main channels is probably as specified and the overall performance will probably be fine.

Now I accept that this is a pretty 'quick and dirty' analysis but it does give you an idea of what is going on, imagine what might happen with a lower spec, cheaper unit where the manufacturer has been, shall we say, a touch 'optimistic 'in his ratings.