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Tell Me About That 48V Float.

SurferJoe46

SurferJoe46

Feb 22, 2010
9,057
763
The Bitterroot Mounts, Montana
#1
I'm not clear on the XLR connex with the third pin - but I know it has something to do with a voltage applied to the mic.

What type of mics actually require this and would any other mic die if it was sent that voltage?

I have other questions about Hi-Z verses Low-Z and balanced mic cables.

Maybe someone can point me to a FAQ area on this site about this stuff? :help:
 
#2
the third pin is the ground. In the case of a device like a di, which can 'lift' the ground, it is not connected.

48v (aka phantom power) carries power in the opposite direction back through the cable. In general, condenser mics require phantom power, dynamic mics (SM58's, etc) do not and you won't hurt them if you accidentally do send 48v to them. However, you will fry Ribbon mics this way. Chances are you'll never run into a ribbon mic outside of a high end studio.

http://tinyurl.com/ye976jv
 
#3
walker rosewood said:
the third pin is the ground. In the case of a device like a di, which can 'lift' the ground, it is not connected.

48v (aka phantom power) carries power in the opposite direction back through the cable. In general, condenser mics require phantom power, dynamic mics (SM58's, etc) do not and you won't hurt them if you accidentally do send 48v to them. However, you will fry Ribbon mics this way. Chances are you'll never run into a ribbon mic outside of a high end studio.

http://tinyurl.com/ye976jv
Click to expand...

TY - and I feel better now that I understand that better.

I have about six or seven of the DR LVX2 mics and that they are cardioid/dynamic makes me a lot happier.

Now I've got to try to understand balanced verses unbalanced cables.

As you can see :
XLR_pinouts.png


I am working on it.
 
#5
SurferJoe46 said:
TY - and I feel better now that I understand that better.

I have about six or seven of the DR LVX2 mics and that they are cardioid/dynamic makes me a lot happier.

Now I've got to try to understand balanced verses unbalanced cables.

As you can see :
XLR_pinouts.png


I am working on it.
Click to expand...


Walker Rosewood is spot on.

But your picture and post made me lol. Good day to you, kind sir.
 
#8
The deal with balanced/unbalanced is like so:

An unbalanced cable has a ground wire and a signal wire, and the signal it carries is an amount of voltage in reference to the ground.

A balanced cable has a duplicate of the signal wire. This duplicate carries the exact same signal, but 180 degrees out of phase. The device on the receiving end takes its signal (the audio you want) from the voltage difference between the two signal wires.

Wires are also antennas, and they pick up noise and hum. The longer the wire, the greater the chance to pick up noise and hum. However the two signal wires pick up the exact same noise and hum as each other, so when it all reaches the receiving device, there is no voltage difference between the noise signals carried by the two wires! Since there's no voltage difference there, the receiving device doesn't "hear" it as a signal at all. The noise and hum ceases to exist as far as the audio path is concerned.

For short distances, balanced is not necessary, but for longer cable runs, especially around noisy stuff like appliances and computers and neon signs, balanced is the way to go.
 
#9
Thanks, bongomania - that makes sense.

I can totally understand the canceling effect with two matching noise signals coming into the amp via both legs at the same time. It only 'hears' the differential signals when the mic generates a signal(voltage).

Makes sense to me - if I used the correct terms.

I worked in a cryogenics lab where we made low voltage transistors work at extremely higher base voltage levels by just mounting them in the voltage and then they did their thing and further jacked up the signal - at least until they blew through their dielectric barriers. We used to call it 'a cascadant' voltage amplifier I think. If they didn't recognize that they were at higher than livable levels, they just worked fine. So I can see how that differential policy works on the balanced cable.​

One last thing for tonight: Why would a dual latching pedal need a balanced stereo cord though? I can understand the 'stereo' part - but why 'balanced' too?
 
#11
bongomania said:
The deal with balanced/unbalanced is like so:

An unbalanced cable has a ground wire and a signal wire, and the signal it carries is an amount of voltage in reference to the ground.

A balanced cable has a duplicate of the signal wire. This duplicate carries the exact same signal, but 180 degrees out of phase. The device on the receiving end takes its signal (the audio you want) from the voltage difference between the two signal wires.
...
Click to expand...
The signals on pins 2 & 3 are not out of phase; each is inverted vs. the other. Phase is a function of time; inversion is a reversal of polarity. Each is an exact "mirror-image" of the other in terms of amplitude.

If the signals were truly out-of-phase, one would be an exact duplicate of the other, delayed (in the case of 180 degrees) 1/2 the length of the signal's frequency. There are 2 main problems with doing this:

1. Real-world audio signals contain simultaneous multiple frequencies that are constantly changing. "180 degrees out-of-phase" is therefore a "moving target". Since the wavelenth is constantly changing, the time represented by 180 degrees out-of-phase would also be constantly changing, therefore the peaks & valleys of the signal (distances between them, not amplitude) would be constantly changing as well. This would cause an extreme distortion of 1 signal vs. the other & would be very undesirable except possibly as an effect.

2. A pair of out-of-phase signals would not be immune to noise like a pair of inverted signals are, which defeats the whole point of balanced signals. Suggested research: Look up "common mode" & "differential mode"; understand the difference & what role balanced signals play in noise immunity.

To answer your original question: 48V "phantom" voltage is applied to both pins 2 & 3, respective to ground.
 

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