Hi,

A few of things to remember while you are back there cleaning things up a bit ...


1) Use Care in Bundling Cables.

Don't bundle the low level signal cabling with high power low impedance noise source cabling. This would be any of the Audio RCA cables with the speaker output or power cabling. It might also be an issue with the RCA Video cabling as well. The HDMI cabling may also be susceptible based on shielding characteristics of the cable.

Don't allow long runs of parallel cables.

Bundle the data cabling separate from the power and audio cabling. The split loom works well for this.


2) Separate Power Cords from all other cables.

Try to separate the high power AC power cords away from other cables by 3 to 4 inches or more. This would be any Power Amplifier or high power receiver power cables. Noise coupling from low impedance high noise sources is coupled magnetically at the distance cubed.

Make sure that you keep the cables away from power cables in the wall. Remember, there may be a local house power cable inside the wall that you can't see.


3) Clean Connectors makes a quieter system.

Make sure your connectors in and out of components are clean. Shielding effectiveness of the cable system is inversely proportional to the transfer impedance of cabling at low frequencies. The lower the transfer impedance, the higher the shielding effectiveness.

The noise transfer within the system is based on the *system* transfer impedance. The cable has an inherent transfer impedance based on the DC resistance of the shield and to a some degree, the optical coverage of the shield (how much in % of the internal conductor is covered). Think of it as a resistor at low frequencies. The resistance of the connectors are part of the overall resistance of the system. If one of the connectors is dirty or doesn't make good contact, it presents a series impedance to the cable shield and the overall low transfer impedance of the shielded system is compromised and localized noise increases.

Keeping connectors clean reduces resistances and provides lower noise. Salts and moisture cause oxides which accelerate corrosion between mating surfaces. Gold-on-gold connectors is best. Gold on nickel or tin promotes galvanic couples to be high and degrade performance.

By-the-way, braid is better than foil at lower frequencies below 100kHz. The copper spiral wrap is generally a poor shield, and tin coated copper braid is best due to improved ferrous shielding of the tin. The shorter the cable, less DC resistance, better shielding effectiveness.


4) Bundling Excess Cabling.

If you have to shorten the length of cabling, it is better to Z-fold it (loop long - 12" sections back onto itself to use up the excess length), than to roll or coil it up. A coil is a natural inductor and will tend to transmit or pick up noise. It also creates a larger loop area for magnetic coupling noise to/from other cabling. The Z-fold is better because it makes current flow across the shield in the opposite direction on each loop-back and opposing currents cancel. The other thing it does is reduce the visible electrial "antenna length" of the source/destination antenna to other local fields.

Be mindful of the minimum bend radius of cables. It's best not to be less than 5x or 10x the cable diameter (ie... 1/4" diameter cable should have a minumum of 2.5" bend radius. Too small a bend radius can open the braid shield - on the inside of the loop. If it is a foil shielded cable, it may crack the foil rendering the shield useless (it becomes a dipole antenna with the crack being the center of the dipole). Check manufactures information, some are much more liberal on bend radius, I just gave you a general rule-of-thumb.


5) Twisted pair signal cables.

The twisted shielded pair cabling works by conductors being twisted with each other. The effectiveness of the cable is governed by impedance, twist proximity, and twists per inch. Each twist is a small shield within itself - as above, the current flows in opposite directions and cancels in each twist. It can provide great shielding for localized fields - for both high and low frequencies. The more the twists per inch, the higher the rejection of noise as the currents are kept more localized to each twist. Sometimes twisted pair cabling is the best way to control low frequency noise in a system. Many display power cabling is twisted to provide low noise and immunity.

Those that have an additional shield improve things even more. A foil shield once again provides electric filed (E-field) shielding - not particularly helpful for audio, but good for RF, video, digital control, & HDMI. A braid shield provides magnetic field shielding (H-Field)- good for Audio, video, digital control, & HDMI and RF high frequency. Foil and braid is a good combination IF they terminate the foil properly (a drain wire is not a good termination).

The shield is typically terminated on the source side and the receive side not terminated - that's why they have an arrow showing direction of signal travel. This is generally to prevent ground loops (alternate return paths for noise to travel). Make sure you have the arrows oriented correctly in the system cabling, it does matter. The arrow always goes to the source side connector (like Audio Out).


6) Cable Impedance & Capacitance

One of the problems with Shielded Twisted Pairs (STP or TSP), is that they have a capacitance line-to-line and also line-to-shield capacitance. Pick the lowest capacitance cabling if you can.

Some RCA cables can have quite high capacitance from the conductor to the shield. If driven from a weak source impedance, and if the cable capacitance is too high, it can degrade the high frequency response of the system. Pick low capacitance cabling where possible. I once checked a bunch of cables and it is amazing how they vary from manufacturer.


7) Balanced cables.

Balanced cables are generally 600 or 150 Ohm fully shielded twisted shielded pairs. The shield is generally terminated on both sides of the cable. Differential driver and receiver sets provide loop isolation and you get the opportunity to have a fully shielded interface. See #5 information.


I hope that some of this may be of help to prevent or correct a problem.

Ross