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Old 10th June 2009, 03:07 PM   #1
AndrewT is offline AndrewT  Scotland
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Default B&W power claims

1x 50W (subwoofers)
5x 25W (center/ surround)
Power input voltage 100V 120V ~ 50/60Hz
220V 240V ~ 50/60Hz
Rated power consumption 32W

the above extract from B&W's Panorama website shows that this previously reputable British Speaker company has descended to the depths.

175W of total power from a PSU that is rated at 32W.
Are we supposed to believe that?
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Old 10th June 2009, 06:35 PM   #2
Eva is offline Eva  Spain
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You are probably comparing different things.

I have seen many many old-school integrated class AB amps from reputable brands that were not even able to play full volume music on 4 ohms stereo without overheating after some time (parties).

If it has good class D and SMPS in it, it will draw less than 32W average while playing music at full volume, and it will do it producing very little heat.

My latest class D amplifier does 10KW peak on 2 ohms of low-pass filtered music (bass) all night long while drawing around 600W average from mains and without even requiring a fan...

It's your standard what is wrong. Music efficiency of class AB is around 30% at full volume while class D efficiency is around 90%. In other words, class AB requires a 3 times bigger power supply for normal operation.

Also, even if a PSU is rated for 32W long term, short term power ratings are usually higher.

Do you really expect a 500VA toroid to provide 500VA long term with a rectifier as a load without overheating? These ratings are for resistive load, for rectifier it has to be divided by 2.5 or so...
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Old 10th June 2009, 08:16 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
if you look at the Panorama web site I think you'll agree they are transformer powered.

Normally these conventional PSUs are rated at the maximum Current/power draw, often the start up current/power being the stated figure.

Look at the back of any conventionally powered ClassAB amplifier and see what I am referring to.
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Old 10th June 2009, 08:51 PM   #4
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That rating doesn't say very much if we don't know what the amplifier is doing when that is drawn. It is very common to use for example 1/8 average power pink noise just at clipping when measuring power consumption. It is probably something similar in this case.

Quote:
Originally posted by Eva
[...]
My latest class D amplifier does 10KW peak on 2 ohms of low-pass filtered music (bass) all night long while drawing around 600W average from mains and without even requiring a fan...
[...]
That is very impressive, especially no fan required. Does it have an isolated power supply or do you "cheat" and run it unisolated from the mains?
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Old 10th June 2009, 09:07 PM   #5
Eva is offline Eva  Spain
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I took a look at the specifications on manufacturer web site. It says SMPS. It also says 32W consumption at rated power, which seems like a correct figure when playing music.

The meaning is not the same as the value found on the back of those integrated class AB amps, which often refers to absolute maximum power consumption (probably for 30 seconds before tripping the thermal fuse of the transformer, of which I had to repair too many).

Note that a 32W SMPS is physically tiny, they probably use something capable of 100W to 200W short term, which is still easy to fit. Long term power rating of a SMPS is usually determined by heatsinking of semiconductors and cooling of the magnetics.

They also use DSP. With DSP you can easily measure heatsink temperature and reduce volume to keep it at a safe value. You can even compute the RMS power involved in the signal going to each speaker and cut gain if required to keep voice coils safe too. It's one of the features I'm working in, but in analog flavour.

Then again, voice coils tend to give up faster than amplifiers when overloaded. They have less chances of cooling.


To megajocke:
Sorry I can't give more details until it's released (soon). But it's quite obvious that some unconventional things are being done anyway It was just an example of the performance that you can achieve by doing "unconventional" things (and not just big toroids and a dozen or two of pairs of output devices like 30 years ago).
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