OK on another issue then... Powering a usb sound box from the computer seems to result in a whine on the analogue audio output of said device. One assumes as this changes as things happen in the computer that this is getting in via the psu lead from the computer via the usb. So if one bought one of these oft used powered usb hubs, could I expect this to go, or would I still be plagued by this either due to the signal still getting down one of the digital inputs or by there being a not very well designed wall wart on the remote hub. My first thought would be to buy a well known make and hope for the best, but if this fails, could one open up the sound box and get some nice kind person to put some really big capacitors across the 5v supply? Brian
Nevertheless, you have to be careful of smart phones sold circa 5 years ago with a dedicated USB charger and the warning "only use the supplied charge r". My HTC used to complain on screen if I plugged it into a universal USB charger or it got rather hot, implying that current limitation was designe d into the HTC charger. Since the more recent moves not to sell chargers w ith new phones, current limitation is likely built into the phone and the p roblem has gone away.
When it comes to kit that's designed to be charged from a USB port (whether on a desktop PC or a wallwart usb charger), it's best to stick to such five volt sources. Unlike a lot of SoHo IT kit designed to be powered from a wallwart, Tablets, Smartphones and the like are best confined to the supplied or recommended USB chargers. You can't play 'Fast and Loose' with wallwart voltages here. It helps that the charging cables are USB cables of one type or another in this case - you'd be hard pressed to connect them to a DC voltage source other than a 5 or 5.2 volt one anyway.
The issue here is due to noise ('digital hash') being induced into the analogue signal lines (earth loop or just plain inductive or capacitive coupling).
In the case of a USB powered speaker box which still uses the speaker or line out ports, the problem is simply a 5v supply polluted with 'digital hash' not being sufficiently screened or decoupled from the analogue signal lines. A larger decoupling capacitor across the 5v supply might improve the situation but have no effect (or might even aggravate the problem) if the issue is coupling between the supply wiring and the analogue signal wiring.
If the USB sound box has its own DAC sound chip built in to convert the digital sound stream via the USB data lines, the problem is the result of a badly filtered 5v supply in the amplifier circuitry. In this case, using a powered hub may alleviate the problem, assuming an analogue regulated wallwart supply or a high quality smpsu wallwart.
A poor quality smpsu wallwart may well introduce different but similar whining noises into the audio output. However, a capacitor upgrade in the amplifier may well provide effective relief, depending on how the interference is being introduced (amplifier supply itself or coupling into the analogue line between the ADC and the amplifier).
Any 'digital hash' coupled into the USB data lines is going to be extremely low level and its effects nulled out by the use of the balanced line (D+ and D- wires). Even if such interference to the D+ and D- wires was to reach a sufficiently high level as to cause trouble, the symptoms would be data corruption of the stream leading to random pops, crackles and bangs or just complete drop outs. Such interference wouldn't account for 'whining' noises which are only heard by virtue of their being induced into the analogue signal paths[1].
Assuming the usb sound box appears as a 'device' in Device Manager (ie it has its own DAC built in), there's a good chance that a powered hub with either an analogue or high quality smpsu wallwart will eliminate (or, at the very least, modify[2]) the 'digital hash' whining noises you're currently experiencing. An alternative to modifying the innards of the USB sound box in this case would be to get an in-line filter module made up for the wallwart to powered USB hub feed (basically something as simple as a 1F 6.3v capacitor across the 5v line :-).
[1] A properly implemented DAC chip will prevent the 'digital hash' that is the data stream arriving at its digital input port from being coupled into its analogue output port. However, badly laid out circuit boards can negate the DAC chip designers' best efforts at preventing such 'cross- talk' issues so it's best not to assume that every implementation will be free of this problem.
It has to be said, however, that it takes a certain level of incompetence for such cross-talk afflicted abominations to pollute the market (but please bear in mind the *years* we had to wait before the PCI soundcard analogue line input clipping issue was resolved).
[2] Even if such a change merely modifies the form of the interference rather than eliminate it, that still helps you in pinning down the culprit so you can concentrate your efforts on more productive measures. In this case, a change in the characteristics of the unwanted noise, rather than elimination, may simply indicate that a better quality wallwart is all that's needed.
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