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Old 27th September 2013, 11:26 AM   #1
AndrewT is offline AndrewT  Scotland
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Default Power consumption of computer speakers

I opened up a pair of Dan computer speakers.
The "ON/OFF" switch only interrupted the secondary feed to the PSU.
The 9-0-9Vac 1.2A mains transformer is permanently connected to mains supply, whether the switch is ON or OFF. There is a thermal fuse inside the transformer.

Is the ON/OFF switch usually not in the mains feed?


Before powering up I used the bulb tester just in case I had done anything wrong in the reassembly.

Surprise !!!!
the speaker consumes more power when OFF, than it does when ON

These are described on the label as "active" speakers.
Nonsense.
There are one pair of TDA2030 chipamps inside one speaker box.
One chip drives the bass/mid directly and the treble via a 4u7F electrolytic.

The other chip drives a cable feeding the other almost empty box. Again the bass mid has no crossover components and the treble has the 4u7F.
These are passive speakers with an inbuilt stereo amplifier to make them "self powered".

Is this a common mis description of "active" computer speakers?
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Old 27th September 2013, 12:14 PM   #2
AJT is offline AJT  Philippines
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what was the power consumption in watts?

power transformer must be very small yes? maybe no more than 10VA...

powered speakers are speakers with the amplifiers built into the speaker cabinets afaik....
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Old 27th September 2013, 12:23 PM   #3
AndrewT is offline AndrewT  Scotland
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Transformer is as stated, 9-0-9Vac 1.2A = 21.6VA @ 230Vac supply.

Power consumption is hopefully low, since I cannot turn it OFF !
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Old 27th September 2013, 12:51 PM   #4
Mooly is offline Mooly  United Kingdom
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Switching the secondaries has been common for many years in a lot of equipment. Reputable manufacturers (and I'm going back a while) used to put labelling of "standby/on rather than off/on but I'm not surprised if thats not the case for a lot of stuff these days. Why do they do it... cost I guess. Its cheaper and means a cheap non mains rated (i.e. a safety component) switch can be used.

Current consumption will be low but these things all add up as you know. Could be a project there, a small PCB thats say nicad powered to detect audio and turn on a mains relay.
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Old 27th September 2013, 01:02 PM   #5
DF96 is offline DF96  England
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Small cheap transformers may be on the verge of core saturation when unloaded, hence draw more current.
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Old 27th September 2013, 03:41 PM   #6
AndrewT is offline AndrewT  Scotland
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That might explain what I am seeing.
Since the transformer is stamped 230Vac and I am on a nominal 240Vac supply then further up the saturation curve is possible.

It seems that drawing a small load to power up the quiescent chip is enough to reduce the saturation AND to reduce the total current consumption.
That is a surprise to me.
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Old 27th September 2013, 10:39 PM   #7
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Alternatively, could it be that the highly inductive unloaded xfmr would be throwing off the "measurement"?
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Old 27th September 2013, 11:06 PM   #8
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Sorry about your bogus transformer. If you can't have half decent true power measurements as through something like a Kill A Watt, there's always the heat test. (Does it actually get warm, or not?) It doesn't sound like the system is really worth upgrading, but switching wall warts have come a Long way and now have very high efficiency, extraordinarily high in standby / no load with the use of chirp mode capable controllers. You could always get two wall wart switchers for your +/- supply, but you might do just as well shopping for a better product.

Last edited by Andrew Eckhardt; 27th September 2013 at 11:08 PM.
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Old 27th September 2013, 11:21 PM   #9
AJT is offline AJT  Philippines
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Quote:
Originally Posted by AndrewT View Post
Transformer is as stated, 9-0-9Vac 1.2A = 21.6VA @ 230Vac supply.

Power consumption is hopefully low, since I cannot turn it OFF !
would be interesting to see the actual consumption using a kill-a-watt device.
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Old 28th September 2013, 08:53 AM   #10
AndrewT is offline AndrewT  Scotland
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I'll set up an actual current measurement when I get back.

I have no intention of upgrading the double insulated box. No changes allowed in any Class11 equipment that cannot be guaranteed as 100% safe.
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