Power output of USB speakers and volume levels

Thread Starter

1dimwit

Joined Jun 2, 2012
8
As I understand things you can only draw 2.5W from a USB 2.0 socket - (5v * 5 units of 100mA = 2.5W)

Yet I see USB powered speakers advertised rated far in excess of this.

How is this possible ?

Will a USB powered speaker system rated at say 50W really sound louder than a system rated at 2.5W ?

Am I right in assuming the only advantage of these higher rated USB powered speaker systems is that the speakers work more efficiently, and so there is less distortion ?
 

MrSoftware

Joined Oct 29, 2013
2,015
Audio manufacturers are notorious for misstating specs. You'll never get 50w from a purely usb 2.0 powered amplifier. Does it have a secondary power source?
 

Audioguru

Joined Dec 20, 2007
11,249
Manufacturers of amplifiers and speakers who lie about power are talking about Whats instead of Watts.

A 2.5W output without any clipping distortion sounds fairly loud up close. But a 2.5W output with severe clipping distortion sounds a little louder.
When an analog amplifier is not clipping then its output transistors get hot. The heat comes from the power supply. But when an analog amplifier is clipping like crazy then its output transistors are simply on-off switches and do not get hot. The severe clipping distortion power replaces the not-clipping heating.
 

bassbindevil

Joined Jan 23, 2014
171
An audio signal that has a peak power of 2.5 watts will normally have an average power that's very much smaller, like 0.25 watts. So, if the speaker includes a battery, the battery will supply the necessary peak power while the USB recharges it. (BTW, it's not unusual for good quality Bluetooth speakers to have amplifiers with an honest rating of 10 watts RMS, thanks to cheap and efficient class-D amplifiers; that much power helps compensate for the low sensitivity of speakers that are needed to reproduce bass in a tiny enclosure.) Or they could be lying, a bit. 9 watts of clean power might mean 15 watts with 10% distortion, which gives you 30 watts for a stereo speaker. Car head units of all brands routinely do this.
 

Audioguru again

Joined Oct 21, 2019
3,516
A typical "Bluetooth amplifier IC" is a PAM8403. With a 5V supply its output with low distortion into a 4 ohm speaker is 2.5W for each of its two stereo channels or 1.5W into 8 ohms.
It is class-C so it uses only a little power making heat.
 

Audioguru again

Joined Oct 21, 2019
3,516
Volume level?
Our hearing's sensitivity to loudness is logarithmic so twice the speaker sounds only slightly louder. 10 times the power sounds twice as loud.
Doubling the distance reducing the loudness a fair amount.
 

dendad

Joined Feb 20, 2016
3,909
I like amplifiers that have a specified RMS power rating.
It is not unusual to see amplifiers that have an output power rating that is higher that the power input. These amplifiers manufacturers are very clever. They have solved the free energy question ;)
It seems like the power rating on many amplifiers have more to do with the chrome knobs or number of blinking lights instead of real figures.
 

MisterBill2

Joined Jan 23, 2018
9,267
"Instantanius peak power" is one of those creative measurements that can be done with some lab equipment and never ever duplicated inn an actual product. But mostly the fantastic claims are simply lies. "IFM music power" is the power an amplifier could deliver if the power supply had the capability to deliver the required voltage without any voltage drop at all. And on many occasions the original specification is in milliwatts, but when the ad is printed they say "watts." I have seen that in the claims about an amplifier delivering 100 watts while using a 12 volt 100milliamp wall wart supply.
 

Audioguru again

Joined Oct 21, 2019
3,516
A few years ago I bought a Sylvania 2.1 powered speaker system for $24.99 as a "special buy" at a local store. It sounds great, looks good and is fairly loud. I use it for the TV in my computer room and the digital cable TV converter also plays many FM radio stations.
In the ad it says 150W. On the box and owner's manual it says 75W RMS. On the power label it says 75W (so it is 100% efficient!). Inside it has a small power transformer labelled 9VAC/1.1A (9.9W). The amplifier is probably 50% to 60% efficient but might be a modern class-D switcher that is 90% efficient.
Its amplifiers are two 16 pins DIL ICs with a small heatsink glued on.
One IC is probably a stereo amplifier feeding 1.5W to 2.0W real Watts to each 4 ohm satellite speaker.
The other IC is probably bridged and feeds 4.0W to 5.0W real Watts to the 6 ohm sub-woofer.
So the total output power is 5.5W to 7W.
Each satellite speaker has two 1.5" 2 ohm speakers with foam surrounds and the "soft-dome tweeter" is not a speaker but is actually a vent covered with a soft dome.
The sub-woofer is a 5" long-throw speaker with a foam surround in a pretty big wooden ported enclosure. It lights up with 6 blue LEDs.
The amplifier and power supply are built into the sub-woofer enclosure and has a volume control and bass and treble tone controls.
I have played it loudly for hours and it doesn't smoke. I have never heard its amplifiers clipping. I tested its frequency response with my 68 years old ears and it plays from 40Hz to 18kHz smoothly and very well.

The woofer became intermittent. I looked inside and saw many glued-together connectors for the bass and volume controls and I saw that the "5 inch woofer" is not a speaker, it is a vent cone and the real woofer is deep inside and is tiny.
 

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MisterBill2

Joined Jan 23, 2018
9,267
At no point will I imply that an amplifier will not sound good, I am only saying that the claimed power outputs are often non-reproducible. A half watt output amplifier can be very high quality and no distortion, but if the claim is 50 watts that is false.
 
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