'Quad' cables used in balanced audio - Page 1
Forum section: electronics / beginners

author: petzel
author_rank: Newbie
timestamp: November 26, 2013, 06:19:25 am
content: Hello!This is my first post so I'll try to not sound too much like a newb. I have been a hobbyist (off and on) for 30 years. I am a System Administrator for an Internet Service Provider in my day job.I'm asking about 'quad' cables used in balanced audio from an electronics perspective, not an audio perspective. At least I hope my question is taken that way. I'm wanting the understand the engineering (or lack of it) behind this concept. I know asking subjective questions like "do these sound better" would be inappropriate for this forum.I've searched the forum for some information on this topic. I found a couple references to 'quad' cables but no discussion on my questions about them.I've recently heard about so-called 'Quad' cables used for balanced audio. Here's a link to what I'm talking about:http://www.mogamicable.com/category/bulk/microphone/quad/Supposedly this type of cable will reduce RF and EM interference but as compared to what? The marketing material is vague, saying, "Double conductors quad cables are more effective in canceling noise that can get past even the best of shields..." It's my understanding that in balanced audio, interference rejection comes from differential signaling not shielding, so the comparison seems invalid.Looking at the specifications for this cable, it seems like all 4 conductors are twisted together instead of 2 sets of twisted pairs. These 4 conductors are then soldered to an XLR connector based on like-color to like-color. Would there be some advantage to this quad-type of twisting? I don't' even know for sure if this cable is quad-twist or 2 twisted pair.One other thing I've read about these 'quad' cables is their lower capacitance. I don't know much about how capacitance effects cables but isn't capacitance going to be an issue for MHz or GHz or higher frequencies? Shouldn't 20 KHz and below be too low of frequency to have capacitance issues (with a standard balanced audio cable) unless you have mile-long cable runs?I really would like to know if I'm wrong. I know that many things in the audio world are marketed with either outright lies or extending a truth beyond it's practical application's limit.I'd like to know if the concept of a 'quad' cable for balanced audio is valid or just hype.

author: PA4TIM
author_rank: Super Contributor
timestamp: November 26, 2013, 08:26:35 am
content: I see no single advantage to 4 wires if you take two wires together so they no longer act as transmission lines. You only lower resistance but then you better take two wire cables. That is more copper in the same outside diameter.I do not see any advantage in XLPE. At audio frequency the dielectric constant is not realy something to worry about and it is still just PE.That form of shielding is indeed much better, if you transport RF signals and use them as transmission line. A twisted pair on itself is allready transmission line (100-600 Ohm depending the construction and long enough, what will be a problem for audio).Braided shielding is indeed not as good as foil shielding but for audio and up to a few huunderd MHz a good breaded shield will be enough to minimize radiation loss of your signal (and not to prefent signals coming in becauase twisted pair on itself allready has a good common mode rejection. Radio amateurs use non shielded open parallel balanced lines. They are not shielded and a few cm apart. I use that too on one antenna (balanced antenna and balanced tuner) and this gives me less noise and common mode problems as the coax I use on my other antenna. But then you are talking 3.5MHz and higher. RFI that is a few MHz should not be a problem for decent audio and balanced connections. Twisted pair are also used to decrease crosstalk between pairs, but if you take them together they are not twisted anymore and if you connect only one channel there is no case of crosstalk.Every transmission line has capacitance and self inductance. Together they make the characteristic impedance (Zo). The capacitance is cancelled by the selfinductance and you are left with a resistive Zo. That is, as they form a transmission line. The weird thing is they even seem to advise it single ended use. I see not a single advantage in that. Bob pease once wrote, about coax for speakers. A friend showed him the resuts of a study of speakerwire. the inductance of two wire (20 feet) gave a phaseshift of around 10 degrees at 10 kHz. Even for the unobtainium made by virgin elves cable. But flat 40 lead ribbon cable and connected so it forms 20 parallel pairs paralleled the selfionductance was lower by a factor 10. He wrote that taking cheap 75 Ohm parallekl cable and 10 of those parallel would give you a perfect very cheap and less bulky speaker cable whit a flat response. It would form a 7,5 Ohm transmission line too.http://www.epanorama.net/documents/wiring/twistedpair.htmlHere more info on twisted lines from an electronics and transmission line point of viewI think it will be great for balanced connected headphones. A twisted pair for each channel, canceling out common mode signals and extra shielding. Not that you will gain much over normal cable.

author: madires
author_rank: Super Contributor
timestamp: November 26, 2013, 03:12:47 pm
content: A TP cable would make sense for differential/unbalanced signals and each pair within a multi-pair cable has to be twisted separately. Also the pairs should have different twist rates. The idea is to reduce EMI and to be able to transport the signal over some larger distance. Ethernet is a good example. Cables for balanced signals should be shielded and be kept short. IMHO the "quad" cable would be nice for unbalanced audio if it's a real TP cable (two pairs, each twisted separately). And for overengineering things each pair could be shielded too :-)

author: sync
author_rank: Frequent Contributor
timestamp: November 26, 2013, 04:24:21 pm
content: Quote from: JohnnyGringo on November 26, 2013, 02:11:11 pmNote this is microphone cable. And here cable quality matters. Low voltages and long cables in a harsh environment. It costs 4€/m. This is too cheap for audiophool BS.I don't understand why a quad cables should reduce noise. But even if it doesn't it adds redundancy which is good.

author: JohnnyGringo
author_rank: Regular Contributor
timestamp: November 26, 2013, 04:29:40 pm
content: Quote from: sync on November 26, 2013, 04:24:21 pmInteresting. Wouldn't it be cheaper if the microphone had a mini-amp built it (signal booster)? I always thought that was why female news reporters always used the hand mics with the big fat handles. Maybe that was just to make their hands look smaller. *grin*

author: VintageTekFan
author_rank: Regular Contributor
timestamp: November 26, 2013, 04:58:03 pm
content: A few links, albeit from manufacturers.http://www.soundonsound.com/sos/nov09/articles/qa1109_4.htmhttp://www.belden.com/blog/broadcastav/How-Starquad-Works.cfmHaving used it in field, I have noticed a better rejection (no more noticable HUM) of common mode noise sources - especially around SCR dimmers and high current power leads. This is with properly connected cables in new condition. More notable on better sound boards.

author: sync
author_rank: Frequent Contributor
timestamp: November 26, 2013, 05:02:05 pm
content: Quote from: JohnnyGringo on November 26, 2013, 04:29:40 pmThere are microphones with build in amplifiers. But dynamic microphones (often) don't have one. These have a only few mV max. output. So microvolts of noise matters. Thing about concerts. A few microphones with long cables (30m) on a several kW PA. I only have limited experience in this area. But you hear when a wrong/ low quality cable is used for microphones.

author: petzel
author_rank: Newbie
timestamp: November 27, 2013, 05:41:20 am
content: Thanks everyone for all the feedback! I really appreciate it.

author: JuKu
author_rank: Frequent Contributor
timestamp: November 27, 2013, 09:40:43 am
content: - Starquad absolutely works and the Belden page explains the reasons better than I could.- Built-in amplifiers in microphones are rare in professional use. They create more problems than they solve. In particular, no batteries! Murphy and experience says that when you have tens of mikes on the stage, some will go flat in the middle of a show if running on batteries.(- Microphone cable quality is crucial and difficult. The cable is routinely coiled for and against the natural coil), it is stepped over and pulled, yet it must be very reliable, give no handling noise and protect against interference.- On live situations, it is not the PA that is the problem, it is the light rig. And most often, it is the substandard wiring of the venue. (Free tip that has saved several lives in cases I know(!), and undoubtely, many that I don't: Never, ever plug in anything at a new venue without checking the wiring of the outlets! The checker is inexpensive.)- In home equipment cables matter as well, at least in the high-end. I've measured tenfold increase in distortion from cables alone, and of course, there is no upper limit in noise protection. (And don't tell me that -110dB and -100dB distortion difference is inaudible. It might be on your equipment and the music you listen to, but...)- And psychology plays a part too. Often, it is the peace of mind: High quality cables, well built equipment etc. ensure that the performance is as good as possible, within the budget and the corcumstances. Any single thing might or might not make a difference, but everything adds up. And you know there are no weak links in your system. Just like higherformance car: you might never use the top hp, but it is assuring to know it is there.

author: VintageTekFan
author_rank: Regular Contributor
timestamp: November 27, 2013, 03:46:35 pm
content: Quote from: JuKu on November 27, 2013, 09:40:43 amI wouldn't say they're rare - at least in my experience. There is a magic switch on most real sound boards called "phantom power". It provides DC coupled +48V (nominal @~4-8mA) to the microphone, and retrieves the AC coupled sound signal. Of course, this can cause it's own problems (like sudden charge/discharge on a flakey connector).Phantom Power WikiExmaples I've used are the Shure PG81, Shure SM137, Audio-Technica AE5100, and a few more I can't remember.Quote from: JuKu on November 27, 2013, 09:40:43 am

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 27, 2013, 04:54:29 pm
content: Yes, we use Star Quad in the BBC for mic level circuits, and it's definitely good stuff. Much better mechanical construction, and a much tighter twist that holds up better during handling than the standard stuff.When it comes to balanced audio, the key objective is that both signals (hot and cold) areequallyaffected by interference, so that the differential input stage can reject the noise. Normal twisted pair cable has a much looser twist (and inferior overall screening), so it's no wonder it's not as good as SQ.But despite being the best for low-level analogue circuits, it's hopeless for AES digital audio - the characteristic impedance is circa 35 ohms...

author: JuKu
author_rank: Frequent Contributor
timestamp: November 27, 2013, 07:34:35 pm
content: Quote from: VintageTekFan on November 27, 2013, 03:46:35 pmThose are condenser mics. The basic idea is a condensator, the other plate moves with the air. These require DC voltage to pre-charge the condensator. This is what the phantom power is for. Microphones with built-in amps are rare - you get much better results with mains power.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 27, 2013, 08:16:44 pm
content: Quote from: JuKu on November 27, 2013, 07:34:35 pmFunnily enough, you're both right in a wayThe signal from a condenser microphone capsule is so very, very small - and at such a high source impedance - that a pre-amp is required, and this is indeed usually powered using phantom power from the sound desk.The output from such a microphone is typically at "mic level" (-60dBu nominally), so imagine how low it is at the capsule itself.

author: G7PSK
author_rank: Super Contributor
timestamp: November 27, 2013, 09:18:48 pm
content: I have been on stages installing equipment where dozens of radio mikes and mikes with pre-amps have been in use, and seen buckets filled with AA batteries removed from the mikes to be replaced with new ones between every performance, so batteries are used and no one is worried about them going flat as they only get the one use.

author: Monkeh
author_rank: Super Contributor
timestamp: November 28, 2013, 10:55:27 am
content: Quote from: Mark Hennessy on November 27, 2013, 08:16:44 pmI've often wondered why there aren't inline preamps with a bit more gusto available. It'd be incredibly helpful on a long run, and the supply is there..

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 28, 2013, 12:54:52 pm
content: Quote from: Monkeh on November 28, 2013, 10:55:27 amYou don't get much from phantom power, and good quality audio op-amps tend to be a bit thirsty (8mA for an NE5532, for example).The trend I see is towards stageboxe systems. These accept the analogue signals, digitise them, and send them back to the remote end over a rugged connection - often fibre. Fibre is nice in an OB scenario, as it breaks the electrical connection between the venue and the OB vehicle, which eases the vexed matter of electrical safety.Another approach - which I first saw in the Royal Albert Hall - would be microphone splitters. These are powered boxes that buffer the signal and feed it to 2 outputs - so the venue sound crew can easily share their microphone signals with a visiting broadcaster or recording operation.

author: ciccio
author_rank: Frequent Contributor
timestamp: November 28, 2013, 01:16:46 pm
content: Quote from: JuKu on November 27, 2013, 09:40:43 amI agree, remembering one dance room where the metal stage structure was wired to PHASE (but with a green/yellow cable) and the room floor, covered with lovely STAINLESS STEEL tiles, was wired to ground.My body still remembers the the shock when I tried to shake hands with a customer that was on the stage.... I was there to investigate a big hum problem: the mixer was on the floor and the mics on the stage.Returning to the OP, my experience confirm that star quad cable works.I will not explain the reason here, it is clearly explained in the Belden document quoted in a previous postBest regards

author: Monkeh
author_rank: Super Contributor
timestamp: November 28, 2013, 03:33:47 pm
content: Quote from: Mark Hennessy on November 28, 2013, 12:54:52 pmNot that an NE5532 should be used for a mic preamp. I see no reason 10mA should be insufficient for a reasonable amount of gain on a dynamic mic.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: November 28, 2013, 05:28:06 pm
content: Quote from: Monkeh on November 28, 2013, 03:33:47 pmWhy not? Still takes a lot of beating, especially at the price. Still widely used in pro (and consumer) gear.QuoteDo bear in mind that it comes from a 6k8 source impedance per leg, and if you load it down too much, you might not have enough voltage to polarise a microphone capsule in a preceding condenser mic. Even if the mic is dynamic, you're pushing your luck rather. If it were possible without significant compromise, it would already be done.All the best,Mark

author: Monkeh
author_rank: Super Contributor
timestamp: November 28, 2013, 05:42:07 pm
content: Quote from: Mark Hennessy on November 28, 2013, 05:28:06 pmCan't say I've ever seen one used for mic preamps. Usually a discrete transistor stage, or a purpose designed chip.QuoteFair points. I still intend to have a go at it some time, when I can afford to play a bit.