DownunderAudio.com - Design of Interconnects

Design of Interconnects - By Joseph Kendall

My take:

After further internet walkabouts I also found another great article by Reference Fidelity Components.
This doesn't touch upon all aspects of cables but is a good general rule.

I recommend further surfing the internet if you are or become passionate about audio cables but definitely take any and all advice with a grain of salt. Specially if there is no hard scientific evidence just pure speculation and their "opinion".

Likewise you do not have to agree with everything below but for the better part I personally support this technical article in the message that is trying to be portrayed. Cables are cables, buy quality but crazy esoteric cables I'll never go near. I'd rather upgrade other more meaningful components, even better spend my money on true live performances and quality cd's/sacd's/hdcd's/blu-rays.

Design of Interconnects Written by Reference Fidelity Components.

So what’s in a cable and what does it do?

Interconnects come in various types, the main ones being:

Balanced: RFC Reference Saturn

Balanced:

Designed for balanced audio circuits and consisting of a “hot” (positive), “cold” (zero Volts) and “Ground”. The two signal conductors have a signal which is symmetrical in respect of ground with the “hot” wire being the positive signal path and the “cold” wire being the signal return. Since these two conductors have equal impedance relative to ground, and the circuit only measures differences between them, then any noise induced into the wires is cancelled out at the receiving end. Unlike unbalanced (or single ended) connections, the ground conductor between the pieces of kit being connected carries no signal, so any measurable voltage differences between their chassis cannot be added to the signal as ground noise current. Noise which can be induced by RFI (Radio Frequency Interference from strong RF transmissions) and EMI (Electro Magnetic Interference induced by the signal passing through magnetic fields) is cancelled out via common mode noise rejection Balanced audio leads are terminated in three pin XLR plugs.

Unbalanced: No-name budget RCA cables.

Unbalanced:

Most hifi circuits are of this configuration where the signal is what’s known as “Single Ended”, ie it has only one conductor and is referenced to a zero V signal return, or ground. The shield or ground couples the kit being connected and any voltage differences between the kit chassis are carried in the ground as noise current. Since the shield, or ground wire has finite resistance, the result is a voltage of magnitude “x” which exists along the ground. Voltage “x” is in series (unlike in balanced connections where it is not in series) with the signal voltage and it’s equal to the common mode current multiplied by shield resistance. Since it is in series with the signal, it will add directly to it at the receiver so has no mechanism for common mode noise rejection. By reducing the shield or ground resistance, the Signal to Noise ratio can be increased thus mitigating the effects of that noise on the signal. Typical arrangements include twisted pairs, Litz Braid and simple coaxial unbalanced cable terminated in RCA phono plugs. Whilst some external RFI and EMI noise can be slightly reduced or mitigated by twisted pairs and a Litz type braid configuration, a lot (most) will still get through to the signal, so the best type of single ended configuration is the coaxial shielded cable. More on this below:

Effective Unbalanced: RFC Reference Pluto

Effective Unbalanced Cables:

The most effective form of unbalanced cable by far is the shielded coaxial (coax) cable. This uses asymmetrical conductors, typically a central signal conductor wire, surrounded by a braided or spiral shield. In order to be effective at reducing external noise, the ground or shield must have very good coverage and for reducing common mode noise impacts, must have lower impedance than the signal wire (ie be more conductive) There is a common misconception that a simple un-shielded twisted equal pair (sometimes referred to as the “perfect” twisted pair) somehow cancels noise in an unbalanced circuit, but since there’s no common mode noise rejection the signal will pick up noise as well as raising capacitance which is a factor of each conductor’s diameter and distance apart (plus the insulation between them). So twisted pair signal cables in unbalanced circuits and high fidelity are not to be confused! Litz braid usually employs a greater number of strands for the ground return than the signal so the theory is that the ground return will pick up more noise than the signal and result in “quieter” interconnects. This configuration is arguably more effective at reducing common mode noise than a twisted pair. The signal will still be left very vulnerable to induced noise since the ground wires coverage of the signal is poor, and adding yet more conductors to the cable results in even higher capacitance. For low noise environments, a simple 3 braid Litz is fine for lengths of a metre or so, but for all other applications, there’s no substitute for a properly shielded cable.

Types of Shielded Unbalanced Cables:

There are two types commonly employed for audio signal leads: single core coax and pseudo balanced coax. The former is a simple single core arrangement but suffers from one problem, in that the ground return needed to complete the signal also carries any additional noise from RFI and EMI which is additive to the noise induced into the signal. One way of mitigating this is to use twin core shielded unbalanced cable. The shield is connected at one end only (usually the source end) and acts effectively to pick up and shunt unwanted RFI and EMI noise to the source component ground without being induced into the signal or ground return. That’s the theory; in practice there’s limitations of this design as some noise inevitably will be picked up (as it won’t reduce common mode noise any more effectively than a single core coax) but it can be more effective at preventing ground loops. It also raises capacitance, but providing cable lengths of a few metres or so aren’t exceeded, the capacitance will be within acceptable limits for most line level sources.

So what matters?
1. LCR Parameters

The term “LCR” is commonly used to describe Inductance, Capacitance and Resistance (or in the case of AC signals, Impedance). This is principally what matters for all audio signal cables. Interconnects differ from speaker cables in that they are principally in a low current, voltage driven circuit and speaker cables are in a low voltage high current driven circuit. Whats the difference? Well in low current systems of milliamps but with driving voltages of a magnitude greater, the signal is less prone to inductance effects, including those of self inductance. Inductance is the ability of a conductor to store energy in a magnetic field, with the property that in doing so it creates an opposing voltage proportional to the rate of change of current in a circuit. In effect, the current flowing down a signal conductor creates a tiny electromagnetic field but in signal applications the value is so low as to be negligible in terms of inductive reactance (not so in speaker cables). Inductance can be managed to some extent by varying the size of the cable conductor. So with tiny currents at audio frequencies, inductance isn’t that important, nor is resistance (or impedance) since audio systems are usually designed on the rising impedance principle (output impedance of a source component may typically be 10 times or more lower than the input impedance of a receiving amplifier) so this just leaves us with Capacitance. Many of you may have read that capacitance at audio frequencies is not important, but it is, very important, especially where phono signals are concerned. High cable capacitance leads to part of the signal being lost as energy stored in the dielectric (insulation between signal and return conductors) and lost as heat or part of that signal returned to the signal conductor when the electrostatic potential increases. The effect can be a rolling off of higher frequencies at extremely high capacitances of several hundred picoFarads or more (pF) and with phono signals, MM cartridges like to see a load generally of lower than 400 or 500pF. Taking the phono stage input into consideration (typically 150 to 250 pF) you can see the effects of adding capacitance from a cable of say 3m long with 100pF/m capacitance. It may have a marked effect on signal by rolling off the HF. For this reason, single core coax is best suited to longer runs for phono leads since it’s capacitance is lower.

Another factor in signal propagation is how that propagation (and the propagation velocity) is affected by high capacitance. Use of dielectric insulators with high dielectric constants (PVC being one such example) can lead to unacceptably high capacitance in coax cables. This, at a very low level, can affect signal propagation efficiency and low level information loss can result at very high frequencies, but for most part, it’s the circuit overall capacitance relative to the source and the amplifier that matters. There are lots of manufacturers who may try and claim other serious affects but for the most part much of this is misinformation. There are a small handful of very reputable cable manufacturers though who pride themselves in their attention to detail in design, and go the extra mile to seek perfection for total signal integrity. Whether you’ll ever notice the difference between such cables and simpler designs I’ll leave for you to decide, suffice to say that those cables are generally very expensive and may be worth consideration if your system is ultimately revealing and your pockets very deep!” Avoid pseudo scientific claims and anything subjective in terms of claimed design benefits. If there’s no evidence to back it up, it becomes a matter of opinion and marketing.

2. Construction Quality – unbalanced cables

This matters. If poorly shielded cable with poor dielectric insulation is used and the connectors in particular are of a poor quality, then noise will enter the signal, the signal will be degraded and where the transfer of that signal matters most (ie at the connector’s junction with the receiving phono socket) then further losses will result. The standard of termination is important too. Where the conductors are joined to the plug, that joint must be secure and not increase unduly the impedance or the capacitance plus the use of shielded non-magnetic plugs is always preferable to the use of unshielded plugs which may be magnetic. The best type of solder to use for soldered joints is solder alloy with a low eutectic melting point (so that excess heat doesn’t damage insulators or alter the physical properties of conductors) and high conductivity. The use of a lead/copper/tin/silver solder (silver content is typically 4 to 5%) is recommended. All of my leads use Cardas Quad Eutectic Solder for this reason. Keeping the joints neat and free of cold joints is vital too, so its best not to attempt soldering your own connections unless you are confident in your soldering skills! Mechanical joints are better still, since a cold weld can be formed via crimping, but there are very few plugs on the market suitable for this type of joint, although some of the better factory made cables are terminated in this way. So, when considering a cable purchase, is it better to just use the freebee cables that came with your system or invest in decent quality cables? You have to decide that, but bearing in mind the above, a good quality cable will always have benefits over a low quality cable of similar design and many enthusiasts are prepared to pay a small premium for quality, which is a sensible thing as with many other purchase decisions in life

3. Construction Quality – Balanced cables

This matters for the same reasons as for unbalanced cables, but the quality of the XLR plugs in particular can have a marked effect on performance. Whilst most if not all noise may be rejected by common mode noise rejection, it is still preferable to utilise non magnetic plugs with high build and contact quality. The results using the same cable but when comparing very poor quality connectors with decent ones can be quite marked. Worth considering the pin quality too, as repeated removal and insertion can lead to wear on the conductors and a loss of signal integrity. High quality plating, as with RCA phono plugs is important for longevity and reliability.

What Cable Do You Need?

The answer depends on your intended use.

1. Unbalanced Cables:

For Phono Applications it is recommended to use a quality single core coaxial cable to minimise capacitance and ensure sufficient shield coverage of 95% or greater of the cable together with low capacitance non-magnetic plugs for best results. Keeping the cables to a minimum length matters, so aim for 2m or less if possible. If this is not possible, you may need to find out what your MM phono preamp capacitive loading is and then work out the cable capacitance (total) needs to be to keep things under around the ballpark of 500pF for the circuit (this is less important for MC cartridge capacitive loading which tends to be higher).

For Line Level Applicaitons either a single or twin core cable which satisfies similar criteria as for phono leads but usually as these are less concerned with capacitance (up to a point) you can get away with longer cable runs without serious degradation of the signal. If budget’s an issue, go for a high quality single core coax. This will yield decent results. If you think you have a problem with ground loops then a high quality shielded twin core may fit the bill and this may also be more effective at rejection of RFI and EMI from the signal.

2. Balanced Cables

Pretty much as for unbalanced cables in respect of capacitance issues on phono leads, but otherwise you can make these as longer (generally speaking) without serious effects on the signal. 5m or more for line level applications. Things to consider are the use of good quality connectors.

3. Power cables:

One thing matters above all else and that is SAFETY. ONLY ever use power cables with high quality fused plugs and high quality IEC’s. NEVER use a mains cable which is not fused or of questionable quality. Ensure the plugs and IEC’s are from a recognised and reputable source and if possible, carry the Kitemark symbol or CE certification. For amplifier leads, using unshielded power cable is fine. Most decent quality amplifiers have power supplies that deal well enough with any EMI or other noise, bearing in mind its not a signal cable and not directly connected with the signal. Low impedance and cable damping may also be said to be important. I only use Furtech plugs and IEC’s by choice as they are amongst the highest quality components for this application and there are some additional benefits using the IEC’s with their patented ground jumper technology.

For power cables to components, I would advocate, particularly where CDP’s and DAC’s are concerned, using a screened cable, as these sources have high EMI emission which risks being induced as unwanted noise to surrounding components signal cables. The shield is normally connected to ground at the component end in the case of grounded components, and to the mains plug end in the case of double insulated components.

Disclaimer:

This review and technical article is purely the opinion of RFC and it's not specifically direct towards any specific Brand or company. Purely an insight into some practices rampant in the industry.