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In Reply to: Re: I'm Not posted by Haldor on April 01, 2002 at 15:22:45:
Well assume for a start that professional mic impedance is of the order of 150 ohms not 600! I'm not an electronics expert, but I do use runs of up to 150 metres, with absolutely no discernable downside effects. Being that I do a lot of critical Classical recording, and the questioner was looking at a stage snake, I think that belden as against starquad is a nonsensicle argument. In a PA situation RF and EMI are going to be his major problems. chances are that his snake won't even be 100 metres long. Maybe Keith AKA SSL tech would comment, but I'm sure that your maths is incorrect. I don't think that any commercial quality cable even at 500 metres would exhibit HF loss close to that you are suggesting for starquad at 300 metres.Regards
Roland
Follow Ups:
Roland,Why would you assume that 600 ohm is incorrect? The standard mics you see every day in live sound are still the Shure SM57 and SM58 and these are 600 ohm mics. Don't confuse your experiance in recording with live sound reinforcement.
I got the cable roll-off formula from my copy of the "Yamaha Guide to Sound System Engineering". This is actually just the standard calculation for capacitive impedance, the -3dB point occurs when the cable impedance equals the source impedance.
The reality can actually be worse than I stated since some mixer boards don't have particularly high input impedance mic preamps (at one time the standard for these was 600 ohms, but modern consoles are now normally somewhere around 6K-7K ohm input impedance). If your mixers input impedance is on the low side then you will see the effects of excessive cable capacitance even sooner.
I'm not just making this stuff up. Read any audio engineering book and they will agree with me, excessive cable capacitance is not a good thing, especially in a snake cable. My snakes (I have a 150 ft, and a 250 ft snake cable) both have multi-pin connectors which means I can easily daisy chain them in case I ever need an extra-long run. Why would I want to buy a snake cable that won't let me do that?
Phil
Phil,
Certainly, excessive capacitance is not a good thing. -It is the single biggest limiting factor on hf extension. -However, I'm mot sure that I would agree with the line of reasoning that suggests that lower input impedances aggravate the problem...-If you were to exaggerate the effect and strap a 10 ohm resistor across the load, -making the effective load impedance 10 ohms, then the parallel effect of the capacitive reactance becomes less significant, not more. ...Although a 10-ohm load would be ridiculous, since it would most likely sound awful due to the colossal impedance mismatch, rather than capacitive load issues!
Also, 600 ohms for an SM58 or an SM57? -Not according to Shure. -See the attached pdf link for the full shure specification... 150 ohms is in fact exactly a specified source impedance for both models.
Sanity in a Mad, Mad, World! lol. As Keith states, almost all pro mics are around the 150 ohm load, the 600 ohm were more commonly cheap band mics, like the shure prologues and unidynes.regards
Roland
So here's how I have the Neutrik A2 set up:Generator impedance = 200 ohms, (No 150 ohm setting unfortunately)
Generator frequency = 40kHz (as high as it goes for this set!)
Measurement input impedance = 600 ohmsA quick try of a handful of standard values available to me right this minute gives a 2.4dB drop a 40kHz when I strap 27nF across the line. 27nF = 27,000pf which at 150pF per metre, is equivalent to 180 metres.
-At 150 ohms source impedance, the drop off would be lower still, so it definately seems to disagree with getting a -3dB point at 16,xxxHz with only 100 metres of cable...
As for personal use, there are a number of factors governing this kind of stuff. I have always used starquad-type cable for individual mic runs in noisy environments, -Bundling cables produces a de facto improvement in shielding, but sometimes it all gets a bit to big, so a good multipair snake is a better choice as soon as outside dimension becomes a limiting factor.
On the subject of digital cable, -I have a few hundred metres of it (Clark Wire & Cable part # 801 and Gotham GAC-2) here and I've been forced to use it in a pinch for analog connections... -it's about as microphonic as anything I've ever tried. -Now I realise that other people might make a decent 110-ohm digital cable which is softer than this stuff, and may well be a little less ill-suited, but trust me... you wouldn't want to use this for analog connections -ANYWHERE!!!
-If anyone has any other types of 110-ohm cable, I'd be interested to know if they can hear anything when they flick the cable with their fingernail...Technically yours,
Keith
...In fact the math using a more correct source impedance of 150 ohms gives a -3dB point at 67kHz using the starquad at 158pF per metre, and a -3dB point of 246kHz for the Belden. -Now that seems more like it!!!-Certainly, the lower the capacitance the better, but for short runs in hostile environments, the Starquad -or similar- always gets my vote. -We have an account with Clark, so I normally go with their 'mink-4' cable, which is also a quad.
Frequently overlooked in installations (in addition to the very valid point of spacing cables where possible) is the grounding scheme. -Sometimes you have to star ground with far-end shields lifted, sometimes this forms a nice little antenna-array, and you have to take some of the far-end shields to ground with a small capacitor... sometimes you have to connect both ends... sometimes you're connecting unbalanced sources to balanced inputs, sometimes it's the other way round... Some manufacturers (Like MCI!) used to make the decision for you by installing the shield capacitors on the unit!
-An expensive cable will do you no good at all if the grounding scheme isn't properly worked out... -In fact a friend of mine recently invited me over to hear his new home system. -It took me less than three seconds to tell him that the whole thing was 'out of phase'... -We checked the connections... everything looked good... -finally I got the trusty ol' fluke meter out of the car. -Turns out that one of his (rather expensive, hand-made audio-salon type) speaker cables had a polarity flip... -He'd been listening fairly happily to the system for some weeks... -I'll never trust an opinion from him again!!! ;-)
Technically yours,
Keith
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