Unshielded RCA cable is bad design

Here is a bit long article about HIFI system interconnect cables. There are pages promoting use of different unshielded cable constructions for HIFI systems interconnections. But the truth is that unshielded RCA cable is bad design and should be avoided. This article tells the truth about unshielded RCA cables, why they are bad and how many pages promoting them are wrong.

Nearly any signal that uses RCA connectors are considered line level. These are very weak signals (0.1-2V) and will pick up noise easily (unbalanced, output impedance can be up to 5000 ohms). RCA connectors are also used to carry sometimes phono signals, those are even weaker and will pick up noise even more easily than line level signals.

Interconnects with RCA plugs should be made with coax cable or shielded twisted pair cable to work well. Typically interconnects with RCA plugs are made with coax cable. The coax encases the signal wire in a pipe that protects it from interference.

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Unshielded cable, like ordinary speaker wire or CAT5 UTP, is unsuitable cable for RCA interconnections. It will pick up noise very easily. This noise is usually heard as  a low level fuzz (less than totally silent background)  and obvious hum.

Your speaker level wires (with about 10-30 volts signal typically 8 ohm load impedance and fractions of ohms amplifier output) are much less sensitive to radiated noise than the RCA signal wires. That’s why they can live well without shielding.

Line level cables use considerably lower signal voltages and very much higher impedance, so they pick up noise more easily. For interconnects shielded construction is the way to go for line level RCA interconnects. Shielding on unbalanced interconnects does make a big difference in the background noise level.

Unshielded twisted pair cable, like CAT5 networking cable, is bad idea for RCA interconnects. It is true that due to its characteristic CAT5 and similar have low capacitance, but that’s where the good news end.

utp

You might have read that that UTP cable construction has good noise rejection characteristics and can carry signals well without shielding. CAT5 cables used in a computer network system are renowned for their ability to reject unwanted noises. It is true that twisted pair geometry works for the phone companies who run miles of cable and for Ethernet LANs very well without noise problems. Unshielded twisted pair works and rejects noise in those applications well because it is used in them to carry balanced signals.

Cat 5 and similar unshielded twisted pair (UTP) cables only work effectively when run as balanced lines where there are common-mode noise rejection properties built into the system. That common mode rejection is easily accomplished when the receiving circuit is passive (headphones or loudspeaker), transformer coupled, isolated and battery operated, or otherwise not referenced in any way to the transmitting-circuit common (either capacitively or resistively connected). For best performance the signal signal sending end should have same impedance from both signal wires to ground.

When you wire unshielded twisted pair to unbalanced signal source and receiver RCA connectors, most of the good properties of the UTP cable noise rejection are lost.  Unbalanced RCA interfaces found in normal HIFI equipment do not have common-mode noise rejection properties. The coupled noise will get to the signal on the cable. Therefore UTP is not effective for unbalanced home/consumer applications.Read Cat5 cable VS RCA cable for car stereo use article if you want to see test results.

There are also special unshielded braided unshielded cable construction made for HIFI interconnects. Those suffer from the same basic problems as the unshielded twisted pair cables, although some designs might offer a slightly better noise rejection characteristics than twisted pair. But they are still poorly shielded and should be avoided.

4play_plugs

(image from 4-Play)

Unshielded unbalanced cables are very sensitive to capacitive coupling especially when circuit impedance is high (impedance mostly determined by signal source output impedance in HIFI systems). For example power cords radiate a magnetic field and an electric field that is prone to carrying the interference that rides on all AC power lines to some extent. Shielding is an essential part of a proper RCA audio cable.  It is true that shielded cable has higher total cable capacitance than otherwise similar unshielded cable, but that’s the price you have to pay for the noise free sound.

The cable capacitance can affect the sound going through the cable. The most typical effect is that the cable capacitance together with signal source output impedance forms a low pass filter that more or less attenuates the highest frequencies. How much cable capacitance effect depends very much on the signal source output impedance (output impedance is typically in 30 ohms to 5000 ohms range, much bigger variation range than in capacitance of different audio cable types). If the signal source badly engineered, then the cable capacitance affects the signal source output driver amplifier stability, causing distortion and risk of oscillation at high cable capacitance. A well designed HIFIi equipment should be designed to cope with the normal cable capacitance. If the capacitance of normal shielded cable has considerable effect on the sound you get, there is something wrong with your signal source!

Some HIFI manufacturers (especially cable manufacturers that make such cables) say that unshielded unbalanced cables generally sound better than shielded (if your area does not have a lot of RF energy). If you are so lucky that you don’t have such noise sources nearby and like playing then you can try unshielded cable if you wish. Anyways in today’s high tech world  it is hard to find places without much RF energy nearby because for example cellular phones are very common and cause lots of noise if placed nearby unshielded cable. All well designed interconnects sound identical. Some interconnects are badly designed and do indeed sound different.

In today’s modern world you should always use shielded RCA cable. A coaxial design with a braided mesh shield is a good choice and braid shielded twisted pair is also good for making RCA cables. All well designed interconnects sound identical. Some interconnects are badly designed and do indeed sound different. Unshielded RCA cables are conceptually bad designs and should be avoided. Unshielded cables are OK only for applications that use balanced signals (professional audio), but even in those applications using shielded cables is the preferred.

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  35. SoBizarre says:

    It’s not always that simple. There are equipment combinations which REQUIRE twisted pair RCA. Here’s explanation:

    http://www.youtube.com/watch?v=QOagVDZLQnA

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    • tomi says:

      The video you pointed to and I can agree things said on this.

      In case the amplifier has differential RCA inputs (that are pretty rare), using twisted pair cabling with then would be better than coaxial cable. But again here shielding would be beneficial: properly shielded twisted pair is better than unshielded twisted pair.

      Unbalanced RCA interfaces found in normal HIFI equipment do not have common-mode noise rejection properties, and using unshielded twisted pair cable with gives poor performance – said on my original article and on the video.

      This was my short comment. I might some later time write some longer comment on this on my blog…

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  36. Twisted pair RCA cables again « Tomi Engdahl’s ePanorama blog says:

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  43. James Whitehouse says:

    Hi, good article thanks! I already have made a few coaxial cables, and want to try shielded twisted pair (which I already use for balanced cables with XLRs) with unbalanced RCA connectors. Could you comment on what you feel is the best way to wire the shield in this configuration, as there seem to be differing schools of thought (e.g., some suggest +ve to centre pin at both ends, -ve + gnd twisted together and joined to shield only at the source end, -ve only at destination end, etc.)?

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    • Tomi Engdahl says:

      Generally for short runs (few meters or less) approach where one wire in the pair is for signal ground and other for signal is best, and the shield of the par gets grounded to signal ground at the end with bets ground (generally signal source, but with typical audio mixer setup the end that connects to mixer).

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