Clarification on what travels best on long cable lines, line level or mic level.

[esumsea]

This question was addressed by remoteness and others in a thread entitled analog or digital snakes Multipin Snakes: analog or digital?. I was told that line levels were best, but this seemed contrary to what I had learned. It also goes against the use of D.I. boxes. I was confused (this happens a lot ). Then I came across this technical note on whirlwind's site:

"High impedance lines are more adversely affected by the inherent capacitance that is present in the cable itself. This capacitance combines with the impedances of the source and destination to set up a filter. As the impedance increases and/or the capacitance per foot increases, the active frequency at which the filter comes into play gets lower. The frequencies above this point actually begin to "short out" across the cable's conductors before they ever get to their intended
destination. Keeping impedance low and using quality cables can be important issues for maintaining wide frequency response in long lines."

This seems to suggest that it is best to have a mic signal travel a long line than a line signal. If this is true, why would anyone want to have mic pre's on stage instead of at the recording location. Can anybody explain why you would want to do this? Please help clear my cobwebs.
Let me know. As always, it is appreciated.
Best Regards,
Mario

[pkautzsch]

Mic levels *are* higher impedance than line levels, and so theoretically are more susceptible to long cable runs. In practice, one will seldom have cable runs long enough, since a general rule says that modern mics take cable runs up to 900 ft without significant loss of treble as long as it's quality cabling.
German broadcasters often have their mic snake going to the desk's preamps in the truck outside the venue (some use Stagetec Nexus systems with the remote controlled pre/ad right on stage and just one thin cable out to the truck).

[esumsea]

I just did some checking and I guess Mic pre-amp levels are lower impedance than mic outputs. I don't know what I was thinking, Possibly instrument output impedance. D'oh!
Thanks for the response to the stupid question.
Regards,
Mario

[Barnabas]

There are two different kinds of "line level" signals, and a world of difference between the two. You mention the high-impedance, unbalanced line level, such as that coming out of a guitar or keyboard. These high-impedance, unbalanced line level signals should not be run for more than about 20 feet. This kind of signal is usually run through a DI to convert to a balanced mic signal to run long distances.

The other kind of line level is the 600 ohm, balanced line signal. This is what you usually have coming out of a mixer going to the amplifier, or coming out of a separate preamp going into the line input on a mixer. This is usually terminated with 3-pin XLR connectors or TRS connectors and the cable is the same as mic cable. This balanced line level signal can be sent for very long distances, even longer than mic lines. The 600 ohm, balanced line signal is the basis of the current telephone system, and we know those signals travel a long distance.

So to answer your question, high-impedance line-level signals are the worst for long distances, mic signal can go hundreds of feet without problems, and balanced line level signals can go the farthest.

BTW: The impedance of a balanced microphone is around 200 ohms, which is “less than” the 600 ohm impedance of a balanced line.

[Remoteworld]

Let me go one step further...

I'm not an expert on transmission line degradation, but there are a couple of other things you should be aware of...

1. Generally, "balanced lines" are preferrable to "unbalanced lines" because they offer much greater immunity to "RFI" - "radio frequency interference." This is the MAIN reason that microphones (usually) output a balanced signal (two conductors, twisted, with a surrounding "ground" - or "shield" - or "drain".)

2. Most low impedance dynamic microphones want to "see" a "termination" of about 250 ohms - but not always. Most ribbon mics (I'm thinking along the line of a RCA 77 or 44, or a Shure 300) want to "see" a much lower termination - somewhere around 50 ohms. The difference in termination has a VERY profound affect on the linearity of the transducer.

3. In the case of "line level" signal transmission (I'm thinking, here, of the output of a recording console, or the output of piece of signal processing gear), the output impedance of the device is typically VERY low (something around 40 ohms, or less) and is NOT terminated at the other end (a recording device, for instance.) Rather, the receiving end impedance is typically VERY high (around 10k ohms, or higher.) This is called a "bridging" situation. The "load" presented to the transmitting device is very low.

The other comments about capacitance, etc. are correct. All common professional audio recording devices (microphones, preamplifiers, etc.) transmit low impedance signals. Signal degradation (linearity) is not a severe issue unless you are running fairly long cables, BUT RF issues are another thing.... despite the shielding that runs around a mic cable, RF can be a BIG issue. Having the ability to run "hotter" signals down a long piece of cable means effectively increasing the "signal to noise ratio." Clearly, if the RF "noise" induced on a cable carrying a mic level signal demonstrates a 30 db "signal to noise" ratio, the same cable carrying a level 60 db hotter will now demonstrate a signal to noise ratio of 90 db!! That's a big difference!!

There are volumes written over these issues... this just scratches the surface.

Phil Edwards (remoteworld)

[esumsea]

Thanks guys! Here are a couple of questions. Reading the jensen white paper on mic splitting they say that most mics have a low impedance output @ 150 ohms. They say that it is best if the input impedance is at least ten times higher, or 1.5 kilo ohms. The result would be a 91% voltage transfer and very low db loss. According to them, the higher the input impedance the better. This goes against what was written about ribbon mics by remoteworld. So if that could be explained more I would appreciate it. You could also point me to a thread or white paper that would explain this, especially when it comes to linearity. The problem I have is that I bought a bunch of JT-MB-es which are a 4 way split. Jensen (Dan Hill) told me that the mic sees all the connections in parallel and therefore the impedance drops which each connection. They say this is "hard" on the mic. I wish I would have known this because I am currently planning to use these as a two way split. I just thought it would be wise to have the ability to do a 4 way (in case I had to feed my live sound FOH, monitor and two different sets of recording outs (I know, I know ). Now I find out that if you leave the any of the tranny windings unloaded you may get some ringing so I will have to present them a load somehow to avoid this until I connect this. Either way, I keep lowering the input impedance which I am learning is not a good thing.

About balanced line outs, I guess the hotter level of the signal makes up for the fact that the signal has 3 times the resistance, but does that (the hotter level) not only affect the s/n and not the high frequency roll off? I know both are important. Maybe the high frequency roll of does not occur until you run hundreds of feet, but how many? Is there a formula for this?
Any more input, comments, criticisms, etc are appreciated. Thank you for everything !
Regards,
Mario