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In Reply to: Re: I'm Not posted by Roland on April 02, 2002 at 07:04:57:
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
Follow Ups:
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
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