Cables - interconnects, wires, leads

Some common misunderstandings and misconceptions clarified or debunked.
Analog vsdigital, record vs cd, valve (tube) vs solid state (transistor) and so on.
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Cables - interconnects, wires, leads

Post by superphool » Sat Aug 24, 2019 8:23 pm

The biggest controversy (aside from LP vs CD) in HiFi components always seems to be over the interconnecting cables.

Cables range from the 'plain vanilla' type that come free with all sorts of items to some types that cost £200 per metre or more.

All sorts of claims are made for various cables, some are reasonable and some are not.

Usually the emphasis is on such things as LC-OFC (Linear Crystal, Oxygen-Free Copper) and Gold plating.

Fact: Virtually all copper wire manufactured world-wide for electrical use is Linear crystal, oxygen free copper.

Fact: In many applications, gold plating is irrelevant. Whether it's useful or not depends on the mechanical details of the connector.

This first section applies to Analog Audio cables - Digital interconnects are listed further down.

Analog interconnects:

One fundamental problem is that virtually all domestic type HiFi interconnections use "unbalanced" signals - a signal wire plus a 0V (aka screen, signal ground or common).

That means any difference in the ground voltage between two pieces of equipment is in effect added to the wanted signal at the input. This is a possible source of noise or commonly "hum".

By way of contrast, in professional & studio sound gear virtually all interconnects are done with balanced line cables, usually XLR connectors, and it's all generally driven by low impedance sources, often around 600 Ohm impedance.
The input circuits use only the difference between the two signal wires, so ground offsets and electrical noise between two pieces of equipment (even up to several volts, in some cases) have no effect on the wanted audio.

For home audio, using good quality cables and connectors and keeping cable lengths to a minimum will reduce the ground circuit resistance between the connected items and minimise any possible difference in voltages and reduce stray pickup.

That does not mean expensive cables - quality and price are almost independent.

In connection with ground voltages, another problem when connecting between different items is a "hum loop".
Hum loops can occur when there is more than one possible signal ground path between different items of equipment, allowing unwanted interactions between signals in different things - and commonly pick up of main power frequency from the building wiring - causing 50 or 60 cycle "hum".

There are other hypothetical causes of problems, but the one that is most likely to be experienced is microphonic cable!

*Imagine having a microphone feeding back to the amp at low level, probably well below the point of feedback. It's about as useful as standing a turntable on top of a loudspeaker.

All cables have capacitance between the conductors. If there is a DC voltage present on the cable, due to the design of the equipment it's connected to, any change in this capacitance will modulate the voltage.

Cables that are constructed so the conductors are physically stable in relation to each other are unlikely to cause a problem, but if the wires can move or float within the cable jacket it can cause all sorts of problems!

This is a very common problem and easy to detect; just select each input in turn, then with the source paused or muted and the amp volume at a reasonable level, tap the cable! If you get a click or thump from the speakers, the cable is microphonic.

Note that I mention above that a DC voltage is needed on the cable for this to occur. If the voltage is eliminated, the microphonic effect will also vanish. As any voltage is is dependant on the connected equipment, the same cable can be fine in one application & rubbish in another...

It's also possible that the voltage on the cable could vary with time or the order in which the source and amp are turned on etc... If both end of the cable are capacitively coupled, with no discharge path to earth, the results are unpredictable.

The simple solution is that all capacitively coupled inputs should have a high value resistor between signal & screen, to prevent any voltage build-up on the connecting cables. 1M Ohm would probably be reasonable.

That's one of the "secrets" of some ludicrously expensive cables - a resistor that costs about 0.05 in each of the connectors at one end of the cable, to eliminate any residual voltage.

Another cable trick is to use two core screened cable, with one core for signal and the other as ground/common. The outer screen is only connected to the signal ground at one end of the cable.
That's claimed by some to give less noise pickup that the conventional connection of screen as signal common/ground.

For reference, good quality cable as used in recording studios costs only about £1 per metre. You need to add connectors at £1 to £2 each, plus an allowance for assembly and the sellers markup - but puts a fair price for a top quality cable at, say, £2 per metre plus £10 connectors and assembly.

Digital interconnects:

Digital signals have no comparison to analog; the signal voltages are not related to the information content in any way.

The _only_ thing that matters with digital data is that the on or off (high/low, 1/0 or true/false, whichever you prefer) voltage levels are still recognisable as such when the data gets to the next stage of the equipment.

The signal must be reduced or corrupted to the point that the on or off levels are no longer separable - which takes a lot! of added noise or cable attenuation - for the information to be corrupted.

When the signal is affected to that point, the result is not in any way subtle; the corrupt data bits are as likely to represent large level changes in the information as small level changes and the result is massive clicks, thumps and similar noises.

Unlike analog, digital signal do not degrade "gracefully".

Any review or advert that claims one make of digital interconnect gives a better quality or less noise etc. than any other can be assumed to be total nonsense.

It's like claiming that storing your documents on different makes of disc drive or flash memory will subtly change how they read - absolute garbage.

That applies whether it's digital audio, HDMI, Ethernet or any other form of direct digital signal connection.

For digital interconnects, a "pound shop" cable will work work exactly as well as one costing £1000+, as long as both meet the standards for the connection system in use - HDMI or S/PDIF etc.

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