I am interested in modifying my M200s to accept a differential signal via an XLR cable, instead of the (factory) RCA jack, similar to the mod performed by Underwood HiFi.

I recognize that performing this mod would void the warranty and, depending on the complexity, I may not even attempt it. But, the equipment I'm using upstream of the M200s provides the differential outputs, these outputs provide better noise immunity, and I prefer the reliability of XLRs to RCAs.

Are instructions on how this could be accomplished available on the web somewhere?

The Underwood mod (discussed in this thread and this thread ) is the only way I know of to convert the M200 to a fully balanced configuration - which not only requires adding an XLR input but also changing the internal topography. I don't know of any sort of DIY instructions, and I would expect it to be a serious undertaking.

Gonk -

Thank you for the quick reply. I'll just leave "well-enough" alone.

Unless you have a problem with hum and noise pickup from extremely long interconnects (over 25 feet), a balanced or differential input has the potential to degrade sound quality and add noise because of the added electronic stages the signal must pass through. The only reason for the existance of balanced circuitry is to cancel interferrence picked up by the connecting cables - if you have no interferrence from this source, you don't need balanced connections.

If you really need balanced runs, perhaps consider a professional or high-quality DIY outboard balanced-to-unbalanced converter adjacent to the amp. DIY designs for such are not hard to find, and if you use high-quality components, you should do quite well.

I don't have to use differential. The Behringer DCX outputs may be strapped for single-ended operation.

Use of differential would allow me to place my amps very near the speakers, away from the electronics rack. If I did that, then I'd have line level signals running 15 ft or so, and noise pickup could become a problem.

This positioning is appealing because the speakers are tri-amped, so there is a lot of unsightly 12-ga wire running across my listening room. If I used the differential line levels, I could tuck small guage wire under/behind my baseboard moulding.

15' is not a length that would be prone to pickup of noise or hum. I would check carefully that you actually need to run differential before doing so. In most instances where there is pickup of hum, simply re-routing the cables takes care of it. Most of my interconnects are over 20 feet in a tri-amped configuration and I have no problems whatsoever with noise or hum pickup.

Agree with Soundhound here. Buy yourself a proper cable with good sheilding and you will have no issue with a 15 foot run. Try the sub-woofer RCA cable from Parts Express, like this one, http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=181-644
they have 12 and 25 footers.

I use two of the twelves and have not issues at all.

The other reason for balanced in professional installs is so that they can run lower voltage signals over the longer lenghts. So for single ended, your voltage is much higher and therefore your noise immunity is higher as well, and you will notice how RCA cables usually use bigger conducting wire partly for this reason.

Actually, you have it backwards. The "nominal" voltage for professional balanced lines is 1.23 VAC, or +4 dbu for "0" db (usually taken as being -20db of full scale digital). For full scale digital (full output for a digital signal) which is +20 db above this level, the voltage would be 9.75 VAC. So the voltage in a balanced line is significantly higher than in a consumer unbalanced line.

Professional balanced microphone lines run at very low voltages, but we're talking about line level here.

Single ended consumer lines usually run at an approximate nominal voltage of around .3 VAC (300 millivolts - it's not a hard and fast specification). At full scale digital for this type of cable, the voltage would be around 3 VAC. The standard output level for consumer CD players is 2 VAC for full scale digital, or .2 VAC (200 millivolts) for "nominal" level.

The size of the conductors in low level interconnects has no effect on signal transmission loss since the load impedances are always very high (over 10,000 ohms) and there is very, very little current flowing (thus no real voltage drop with long cables). In speaker cables, the impedances are much lower (8 ohms usually) and there is substantial current flowing, so the resistance of the cable must be very low. This means the cable must be very large so that long runs don't present significant resistance.

Line level cables can therefore be just about any gauge you want. Professional line level wiring in studios is usually 24 gauge, which is quite small.