USB is a digital data bus. audio cable is an analog signal? I'm just trying to figure out how you think that those USB monitors are somehow better than a $50 RADIO with RCA input from say Radio Shack? Aside from the DAC on them that might be mildly better than the soundcard that came with your computer. Not that those things are a lot of cash, but you could probably get $30-ish for the radio when you sell it. Where those monitors being specialized would have many folks asking what is it, and then concluding that they've lived without it for 40 years and saying no thanks. Do you see my point?
You seem to be trying to find a need for the device. Instead of having a need and a device that satisfies that need. i.e. A headphone preamp could take your TRS output from the keyboard and forward that to your existing USB powered and cheap PC speakers. Or a pair of headphones, but your keyboard probably already has a headphone jack if it outputs an analog sound signal and not just MIDI only events. The headphone preamp can be had for $50 or less including 1/4" to 1/8" adapter and 1/8" to RCA cable. And IMO, the quality would be roughly the same. Granted more plugs and cables and not as pretty perhaps.
Samson Audio - StudioDock 4i
StudioDock 4i - Active USB Monitors
http://s3.amazonaws.com/samsontech/related_docs/StudioDock4i.pdf
A bit more than I thought on them.
An iPod dock --- ohhhh
20W of power each --- ahhhh
65Hz - 23kHz frequency response --- ewwww
(human hearing is considered 20Hz - 20kHz)
(i.e. Hope you're not into BOOM BOOM music)
16 bit and 44.1Khz or 48kHz --- ehhhh
(kind of the norm for USB stuff)
Doesn't even appear to have TRS or XLR inputs. Just RCA or USB. With speaker outs (i.e. wires with crimps). The image shows an Aux input on the front that appears to be TRS. But who knows what it actually is. unbalanced / TS? It'll probably work.
Just to keep perspective, here's an audio sweep. Starting tone is 65Hz, the BOTTOM spec of those monitors. Your existing speakers should reproduce this pitch and perhaps most of the rest of them too. But according to spec, those monitors WONT reproduce the swept tones. Not that it actually wont, but they got the specs from somewhere. The clip is 2 seconds of 65Hz sine wav. 3 second null / pause, then 0.5 seconds for 65Hz to 20Hz per whole number Hz. I probably should have used pluck versus sine, but oh well. Just to illustrate the LIMITS of that which you are considering buying. To my ears, the bottom couple of keys on an 88 key keyboard is well within that range of frequencies. Which might drive you bonkers to be playing the note, having your gear generate it, but the speakers DON'T. Simply maddening...
