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more on cable seletion...

Posted on February 28, 2010 at 1:02 AM

more on cable seletion... This is some of ESP's comments that I find interesting and agree with from my past work in Communications, RF and engineering.

Interconnects

All well designed interconnects will sound the same. This is acontentious claim, but is regrettably true - regrettable for those whohave paid vast sums of money for theirs, at least. I will now explainthis claim more fully.

The range (and the associated claims) of interconnects is enormous. Wehave cables available that are directional - the signal passes withless intrusion, impedance or modification in one direction versus theother. I find this curious, since an audio signal is AC, which meansthat electrons simply rush back and forth in sympathy with the appliedsignal. A directional device is a semiconductor, and will act as arectifier, so if these claims are even a tiny bit correct, I certainlydon't want any of them between my preamp and amp, because I don't wantmy audio rectified by a directional cable.

Oxygen free copper (or OFC) supposedly means that there is no oxygenand therefore no copper oxide (which is a rectifier) in the cable,forming a myriad of micro-diodes that affect sound quality. The use ofOFC cable is therefore supposed to improve the sound.

Try as I might (and many others before me), I have never been able tomeasure any distortion in any wire or cable. Even a length of solder(an alloy of tin and lead) introduces no distortion, despite the resinflux in the centre (and I do realise that this has nothing to do withanything - I just thought I'd include it :-). How about fencing wire -no, no distortion there either. The concept of degradation caused bymicro-diodes in metallic contacts has been bandied about for years,without a shred of evidence to support the claim that it is audible.

At most, a signal lead will have to carry a peak current of perhaps200uA with a voltage of maybe 2V or so. With any lead, this current,combined with the lead's resistance, will never allow enough signaldifference between conductors to allow the copper oxide rectifiers(assuming they exist at all) to conduct, so rectification cannot (anddoes not) happen.

What about frequency response? I have equipment that happily goes toseveral MHz, and at low power, no appreciable attenuation can bemeasured. Again, characteristic impedance has rated a mention, and justas with speaker cables it is utterly unimportant at audio frequencies.Preamps normally have a very low (typically about 100 Ohms) outputimpedance, and power amps will normally have an input impedance of 10kOhms or more. Any cable is therefore mismatched, since it is notsensible (nor is it desirable) to match the impedance of the preamp,cable and power amp for audio frequencies.

Note: There is one application for interconnects where the sound canchange radically. This is when connecting between a turntable andassociated phono cartridge and your preamp. Use of the lowest possiblecapacitance you can find is very important, because the inductance ofthe cartridge coupled with the capacitance of the cable can cause aresonant circuit within the audio band.

Should you end up with just the right (or wrong) capacitance, you mayfind that an otherwise respected cartridge sounds dreadful, withgrossly accentuated high frequency performance. The only way tominimise this is to ensure that the interconnects have very lowcapacitance, and they must be shielded to prevent hum and noise frombeing picked up.

At radio frequencies, Litz wire is often used to eliminate the skineffect. This occurs because of the tendency for RF to try to escapefrom the wire, so it concentrates on the outside (or skin) of the wire.The effect actually occurs as soon as the frequency is above DC, butbecomes noticeable only at higher frequencies. Litz wire will notaffect your hi-fi, unless you can hear signals above 100kHz or so(assuming of course that you can find music with harmonics that go thathigh, and a recording medium that will deliver them to you). Even then,the difference will be minimal.

In areas where there is significant electromagnetic pollution(interference), the use of esoteric cables may have an effect, sincethey will (if carefully designed) provide excellent shielding at veryhigh radio frequencies. This does not affect the audio per se, butprevents unwanted signals from getting into the inputs or outputs ofamps and preamps.

Cable capacitance can have a dramatic effect on sound quality, and moreso if you have long interconnects. Generally speaking, most preampswill have no problem with small amounts of capacitance (less than 1nFis desirable and achievable). With high output impedance equipment(such as valve preamps), cable capacitance becomes more of an issue.

For example, 1nF of cable capacitance with a preamp with an outputimpedance of 1k will be -3dB at 160kHz, which should be acceptable tomost. Should the preamp have an output impedance of 10k, the -3dBfrequency is now only 16kHz - this is unacceptable.

I tested a couple of cable samples, and (normalised to a 1 metre length) this is what I found

Single Core Twin - One Lead Twin- Both Leads Twin - Between Leads

Capacitance 77pF 191pF 377pF 92pF

Inductance 0.7uH 1.2uH 0.6uH NT

Resistance 0.12 Ohm 0.38 Ohm 0.25 Ohm NT

NT - Not Tested

These cables are representative of medium quality general purposeshielded (co-axial) cables, of the type that you might use for makinginterconnects. The resistance and inductance may be considerednegligible at audio frequencies, leaving capacitance as the dominantinfluence. The single core cable is obviously better in this respect,with only 77pF per metre. Even with a 10k output impedance, this willbe 3dB down at 207kHz for a 1 metre length.

Even the highest inductance I measured (1.2uH) will introduce anadditional 0.75 Ohm impedance at 100kHz - this may be completelyignored, as it is insignificant.

The only other thing that is important is that the cables are properlyterminated so they don't become noisy, and that the shield is of goodquality and provides complete protection from external interferingsignals. Terminations will normally be either soldered or crimped, andeither is fine as long as it is well made. For the constructor,soldering is usually better, since proper crimping tools are expensive.

The use of silver wire is a complete waste, since the only benefit ofsilver is its lower resistance. Since this will make a few micro-ohmsdifference for a typical 1m length, the difference in signal amplitudeis immeasurably small with typical pre and power amp impedances. On thedown side, silver tarnishes easily (especially in areas where there ishydrogen sulphide pollution in the atmosphere), and this can become aninsulator if thick enough. I have heard of some audiophiles who don'tlike the sound of silver wire, and others who claim that solidconductors sound better than stranded. Make of this what you will :-D

The use of gold plated connectors is common, and provides onesignificant benefit - gold does not tarnish readily, and theconnections are less likely to become noisy. Gold is also a betterconductor that the nickel plating normally used on "standard"interconnects. The difference is negligible in sonic terms.

There is no reason at all to pay exorbitant amounts of hard earned cashfor the "Audiophile" interconnects. These manufacturers are rippingpeople off, making outlandish claims as to how much better these cableswill make your system sound - rubbish! Buy some good quality audiocoaxial cable and connectors from your local electronics partsretailer, and make your own interconnects. Not only will you save abundle, but they can be made to the exact length you want.

Using the cheap shielded figure-8 cable (which generally has terribleshields) is not recommended, because crosstalk is noticeably increased,especially at high frequencies. That notwithstanding, for a signal froman FM tuner even these cheapies will be fine (provided they manage tostay together - most of them fall to bits when used more than a fewtimes), since the crosstalk in the tuner is already worse than thecable. With typical preamp and tuner combinations, you might get someinterference using these cheap and nasty interconnects, but thefrequency response exceeds anything that we can hear, and distortion isnot measurable.

hope this stuff helps debunk the snake oil salespeople!

here's some good selection info:





cheers

geo


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