OK, so I've been messing about and it seems the USB side of things does cause FAR more noise than even a hashed up mini jack to mini cable.
On my mixer, I have L and R balanced output (1/4") and I've just been using a single mini jack lead, with a 1/4" adapter on one end, plugged into one of the two MAIN outs and into the rear line input on my PC.
Obviously this is makeshift, but I presume the 'right' connection would be two mono 1/4" jacks to a stereo mini jack?
I also can't find a way to make the ASIO drivers playback, so I'm just using Wave drivers and they're working just fine with no latency issues that I can notice (it was more my timing previously I think! What can I say - I'm a drummer!)
Remember that signals from a microphone, and instrument, are always a mono signal. If you are recording with one mic, then you don't need stereo going to the computer. You just need the mono channel the mic is on. You can get away with a cable that is 1/4" mono at the mixer to a 1/8" mono to the Line-in on the computer. Lay each instrument track down as a separate mono track then convert each track to stereo in the recording software at the final stage.
If you need to record two channels at the same time, two mono 1/4 jacks (or RCA jacks) out of the mixer to a Y stereo mini jack would work. A company called "Comprehensive" makes some very reasonably priced cables that still sound good. Not sure if you can get the brand in the UK.
Generally using the Line-in at the computer is the safest choice. It helps to avoid creating multiple gain stages in the signal chain, which can introduce noise. The mic input on the PC will have an amplifier in it and it will introduce noise if the signal has already passed through preamps before reaching the computer.
With digital recording, recording tracks in mono, and then converting them to stereo in the software isn't really a problem as long as you are laying down each instrument as a separate track. You can work with the pan feature in the recording software to "place" the various instrument tracks where you want them on the virtual music stage in the final mix.
On a lot of consumer grade equipment the mono signal defaults to the left channel. With a mixer you can use the pan knob to determine if the mono will be left or right channel. Then in your recording software you select the appropriate input channel to be the main mono channel.
With higher end gear you are often able to split channels to be individual mono channels independent of each other.
Good for your for finding which input method is the quietest! Please excuse me if the stuff below is something you already know. I'm also posting it because these threads get read by other folks who are trying to learn.
More on USB noise below, but first... You are now beginning to work to address something called the "noise floor". In recording the term noise floor describes the level of noise, in decibels, that your recording setup (equipment), and space (the room) generates and hears when all the input levels are turned up to the proper recording level and no music, signing, etc. is being played.
How low a noise floor you need depends on what your are recording. Very quiet and delicate acoustic playing, and single spoken voices, require a low noise floor to have all the nuances of the playing and speaking sound great. Blaring high decibel metal rock with distortion doesn't require a low noise floor. Proper room treatment goes a long way to reducing noise floors.
Mics have something called "self noise", this is the amount of electrical noise that the microphone itself will introduce into the signal path. This appears in the recording as hiss. The lower the self noise of a mic in decibels (dB) the less noise/hiss the mic introduces into the recording. Condenser mics in the $100 - $200 range will often have a self noise around -20 dB. Mics in the $1,000 range can have a self noise around -12 dB or less. These numbers are important because the less self noise a mic has, the more actual signal you get into the recording from your instruments and vocals.
The noise with USB tends to go away as the quality of your gear improves (better attention to design and specs). When building a PC used for recording taking the time to research which motherboards are well shielded and grounded is worth the extra time.
All recording chains can pick up something something called a "ground loop hum". This is a hum in the signal chain that is due to an issue with electrical grounding somewhere in the entire electrical/wiring setup (it's a nightmare to try and fix in a home). It is present with equipment using three prong plugs and AC current. If your computer uses a two prong plug and has a DC power supply (laptops, Mac mini, etc.) then usually ground loop hums aren't an issue with the computer, but any equipment using AC can still pick them up (studio monitors, mic preamps, mixers, etc.) and introduce them into some part of the signal chain.
A company called Ebtech makes several solutions for eliminating ground loop hums. I use their Ebtech HumX three prong plug filters. They work great, but each one is limited to 6 amps max. I have one on the power chord of each of my studio monitors, and one plugged into my mic preamp. When I was using a Mackie mixer I also had one on the mixer's AC plug.
Lastly, mic preamps and mixers often have a "Low Cut" filter. This is a button that will reduce the signal below a certain frequency. Typically the frequency is either 80 or 75 Hz - low end rumble. Engaging this will help cut down rumble from HVAC units, electrical hum, etc. and lower your noise floor a bit. However some instruments (bass guitar and bass drum) may need those frequencies so you have to begin to understand what frequencies you want captured by what mic at what time.
Enjoy.
