Understanding the “Wires”
Thereby Knowing the System
Part Two

So by now you must have some concept of what a cable is and how it may be constructed. Let's now look at some uses in amplified Celtic harp music.

Balanced Microphone Cables (XLR) - From the Pre-amp to the microphone - Very low voltage (in the low milliamp range) with very low distortion and low hum generation. Suitable for very long cable runs of up to 300' or more. My mic cable run in concert is 150' with zero audible noise.

Balanced Line Level Cables (XLR or TRS Phone Jack) - Cables that run between the "boxes" with voltages of around 1V. Used as interconnects between pre-amps, mixing desks, equalisers, processors and amplifiers. The fact that they are balanced means that the amount of hum generated by the close proximity of 120 V powered audio gear is minimal. Most of these cable runs are 3 to 6 feet in nature. The exception may be the cable run from the back of the concert venue to the amplifier(s) on stage. In my case that is 125'.

Unbalanced Line Level cables (1/4" TS Phone Jack) Not balanced so very susceptible to hum generation. Used on stage from instrument to a small instrument pre-amp. May be used as an input directly into an amplifier or an in situ sound system. Cable length is best kept to 25' or less. Must not be placed in parallel with 120V or speaker cables. Cheaper consumer level gear use unbalanced interconnects and lengths should be kept to 3' or less. Consumer Note: If all the inputs and outputs on a piece of audio rack gear are only unbalanced TS then it is not professional quality. The converse of all balanced XLR/TRS MAY denote better quality. The two interconnect standards are not necessarily compatible.

Snake Cables - A bundle of balanced cables that run from the back of the hall where there is a mixing desk to the stage where there are microphones and amplifiers. Snakes vary from 50' to 100s' in length with 6 to 36 separate balanced connections. Mine is 125' with 20 balanced connects (14 from the stage and 6 to the stage)

Speaker Cables - Unbalanced heavier gauge cables that run from the amplifier to the speaker. Up to 70V. Very reminiscent of the cord that comes out of your toaster. Great hum generator and best kept separate from and not closely parallel to all the low voltage cables used above. Should cross these cables at close to right angles. Lengths are best kept short (25'-50') as they are very susceptible to hum generation from mains power cables and internal cable distortion/attenuation of the higher audio frequencies. Also these cables can give you a bit of a nasty "snap" if you short yourself across them. Consumer Note: Some speakers have amplifiers already built in. This means that you can dispense with the speaker cable but must add a balanced line level cable and a 120V mains cable directly to each speaker. This has pro's and con's.

Mains Power Cables - Just like what you get from the hardware store only hopefully of better quality. Never use a cable with the round ground prong removed. Always test the circuit for correct wiring and phase use a three prong 120V circuit tester. Refuse to use such a circuit- find another. If all circuits fail - have such an eventuality written in your contract. Many musicians are electrocuted. This is how it happens. Use a GFCI shock protector. If you keep blowing a circuit breaker immediately, you have a ground fault. If you lose power when there is a lot of amplifier use, you need to remove other power hogs from the circuit or change circuits. Mains power cables are great hum generators so keep them away from the rest of the cabling.

Hum Loops - An unexpectedly large amount of hum in the system is usually caused by 120 volt considerations. Move the 120V mains cables from close parallel runs. Make sure all the mains are plugged into the same outlet/circuit. Use a professional RFI power conditioner (Furman). Monster Cable makes a good non-pro unit. Keep the amplifier powered directly from the mains and not through the conditioner. When all else fails you have 4 choices. Turn out the fluorescent lights, turn completely off the wall mounted light and spotlight dimmer packs (common in churches), or find the fridge or electric motor sharing your circuit (sporadic crackling gives them away - use the conditioner). The first two "turn off" options are the most common sources of hum in a new-to-you venue. Changing circuits cures the third. The last fourth choice you do not want. That is where you have a hum loop caused by an imbalance of audio grounds in your rack gear. Deal with this by doing a "Google" search for "hum loop prevention" as it is out of the scope of this Series. The solution will be simple but the detective work will be very time consuming.

With the knowledge gained from these pages and some hands on experience of touching some gear in a music store, a lot of what the salesman says will start to make some sense. But I still do not recommend buying yet. Do your homework, ask lots of questions. The internet is a wonderful tool for this - find a mentor.

I will add your knowledge and experience to the “mix” if it can add to the effectiveness of these articles. The rest of the Celtic Harp Amplification Series is available here.

Stephen Vardy
Harpsound Audio
[email protected]