The Truth About Interconnects and Cables - page 4

We hear so much about damping factor, the effect of speaker leads (and how much better this lead sounds compared to an "ordinary" lead), and how amplifiers should have output impedances of micro-Ohms to prevent "flabby" bass and so on. But what does it all really mean?

Before an informed judgement can be made, we need to look at some of the real factors involved. There are a multitude of impedances involved in a typical amplifier to loudspeaker connection, most of them having a vastly more profound effect than the impedance of the speaker lead alone.

For example, my own (tri-amped) hi-fi uses separate amps for the bass and mid with a designed output impedance of about 2 Ohms. This provides a useful extension of the bottom end (I'm using sealed enclosures), without excessive peaking at resonance. Much the same effect is found with most valve amplifiers, which typically have an output impedance of 1 to 6 Ohms.

Ignoring the losses in the speaker lead (which are usually very small), the impedance of the cable is very low compared to that of loudspeaker crossover networks and the like. While there is no denying that some speaker leads do sound different, the important thing is "different" rather than "better".

A double-blind test carried out by an Australian electronics magazine many years ago found that most listeners thought that the really thin figure-8 type speaker cable had better bass than all the more expensive ones. Treble response was generally thought better using a heavy duty 3-core mains cable. No-one thought that any of the high priced cables sounded better than anything else.

Other workers in the field, such as Douglas Self [ 1 ], have determined much the same, so even in the light of some convincing evidence to the contrary, we have reviewers still extolling the virtues of cables costing more than a decent set of loudspeakers.

Generally, resistance and inductance in the speaker lead can (and does) cause minor variations in level, especially with difficult loads. These deviations are likely to be less than 0.1dB for reasonable cable constructions, with inductance less than 4uH. The resistance of a typical cable (perhaps 0.1 Ohm) causes response variations across the band, following the loudspeaker impedance curve, but these are usually even less at around 0.05 dB. Neither variation is audible.

You will even find references in some cases to the cable's characteristic impedance - a value that is only useful if cables are used for radio frequencies, or are many kilometres in length. These are uncommon in audio listening rooms in my experience. The characteristic impedance of a cable has no effect whatsoever on signal frequencies that are low compared to cable length. At the worst (using coaxial cable) a signal travels at 0.8 of the speed of light (3x10^8 m/s).

Assume that for an adequate safety margin we want to be able to pass up to 100kHz through the speaker cable. The wavelength at this frequency is 3000 metres, or in coax, 3750 metres. A typical listening room may require up to 10 metres of cable, so at the very worst case, the cable is 1/300 to 1/375 times the wavelength of the signal. The effect is utterly insignificant in all respects. The signal will be delayed by an amount that is less than that experienced if the listener were to move his/her head by 1mm towards or away from the loudspeaker. This is of course a common occurrence, and often by several millimetres, even while asleep.

Difficult Loads
While it is true that reasonable quality twin cables (figure eight or zip cord) are adequate for nominal 8 ohm loads over short distances, there are a number of popular loudspeakers that are anything but nominal at high frequencies.

Two that a reader advised me about are the AR11 and the Quad ESL (old model). Both of these drop below 2 ohms in the treble frequencies. The AR bottoming out at 5kHz and the Quad at 18Khz (although anything from 15kHz to 18kHz is common). The dips are fairly sharp and so the load impedance is highly capacitive on the way down and inductive on the way up. The frequencies are high enough to not worry good amplifiers but what about the response at these dip frequencies?

Twin wire cables all have significant inductance which increases in proportion to length. With 10 amp rated twin flex over only 5 metres the response was down by 2.5 dB into one Quad ESL at 18 Khz, and 3.5 dB into the other speaker which had 8 metres. This was audible and unacceptable.

The only way to reduce cable linear inductance is to make the two wires talk to each other. Running in close parallel is a start, tight twisting is better but only by using multiple wires for each and interweaving can you really get the inductance down. Several cable makers have done this and sell them as low impedance cables, which is exactly what they are. There are several different cables that use this method, and twin coaxial cable is also used to achieve a similar result.

One construction uses two groups of 72 strands of enamelled wire plaited around a solid plastic core. Using these cables with difficult loads, the droop at either 5 or 18 kHz disappeared and the sound was distinctly better. There would be virtually no other way to solve the problem short of mono amplifiers sited next to each loudspeaker.

One (potentially major) drawback ocurs if you own certain amplifiers that are unstable with capacitive loads. Typical multiple twisted pair cable has about 9nF per metre of capacitance with little resistance or inductance, which causes many amplifiers to go into parasitic oscillation. The fix is simple, wind twelve turns of wire around a pen and put it in series with the beginning of the cable. This tiny coil has far less inductance than even one metre of twin flex.

This description of the possible issues with speaker cables is the first I have seen that makes some sense from a technical perspective. There is sufficient evidence from my own measurements and those of many writers that there are indeed some detectable (and measurable) differences. With this in mind, and wanting to provide all the information I can, I have included this information - and this is the one area where properly sized and well made cables really does make a difference. If you own speakers that present a highly capacitive load, or have deep "notches" in the impedance curve, I would take this information seriously.

Summary
Essentially, the main offenders in speaker leads are resistance and inductance. Of these, inductance is the hardest to minimise, and although usually small, it may still cause problems with some loads (see update, below). Many construction methods have been used, from multiple CAT-5 data cables, with the wires interconnected (usually all the coloured leads are deemed the +ve conductor, and all the white wires - the "mates" - are used as the negative). Because of the tight twist, the inductance is minimised, but at the expense of capacitance. In some cases, the capacitance may be high enough to cause instability in the amplifier, which not only does awful things to the sound, but can damage the amp.

Another popular method of minimising inductance is to use a pair of coaxial leads (e.g.75 Ohm TV/video coax or similar). The inner conductor of one and the outer conductor of the other are joined to make the +ve lead, and vice-versa for the negative. A good quality coax has a relatively low capacitance, and by interconnecting in this way, inductance is also reduced by a very worthwhile margin.

It is widely held that with difficult loudspeaker loads - as presented by many modern speaker systems with complex crossover networks - that reducing inductance can be very beneficial. This is especially true where the crossover causes significant drops in impedance at some frequencies. This also places unusually high demands on the amplifier - one of the reasons that some amplifiers just don't "cut it" with some speakers.

These problems can be reduced or even eliminated entirely by biamping or triamping [ 3 ], allowing the use of good quality but not extravagant speaker leads.

Resistance, which is easy to eliminate, reduces the damping factor and wastes power. With even reasonably robust leads, this should not be an issue.

Bottom Line on Speaker Leads
Use quality cable, but extravagance will buy no more genuine performance. You will be able to obtain far greater benefits by biamping the system [ 3 ] than spending the same amount on esoteric (read "expensive") speaker leads.

Be willing to experiment, using 3-core mains cable (not the types described above, either), and paralleling two of the conductors for the speaker negative connection (or the positive - the speaker will not care either way). Save yourself a fortune, so you can buy more music instead.

I have seen several references on the web regarding the use of Cat-5 network cable and specially wired coaxial cable for speakers. The idea with network cable is to parallel the wires (these cables are usually 4-pair), and it is claimed that the sonic performance is excellent. I haven't tried it, but Cat-5 is relatively inexpensive, and might work quite well. Try it if you want to. Wiring coaxial cables for speaker use is also not too hard, and it is claimed that this can beat most of the really expensive cables.

Before one even considers the alleged benefits of one cable over another, here is something to think about ...

"What does "veiled" mean (in reference to high frequency reproduction), and how is it determined that the veiling effect is caused by anything specific, as opposed to everything in general? This includes state of mind (i.e. good day, kids acting up, wife annoyed about something), health (cold or flu, hay fever), position of listening chair (was it moved to vacuum the floor?), etc."

And, no, these are not trivial questions. They are every bit as important as anything else, and all the more so if we have only a subjective interpretation of the sound, without measured results that show the effect. Have a look at the article " Amplifier Sound " for more info.