noob question of the week

[Ptownkid]

When sending a test signal from a generator into a mic pre, what two pins of the XLR are you supposed to use?

 

[Sender]

2 & 3. Pin 2 is 'hot'. 1 is ground.

 

[Ptownkid]

See, i would have though it would be 1 and 2...

Can you elaborate as to why it's the way you say?

 

[Sender]

The EIA Standard RS-297-A for balanaced audio states that 1 is ground, 2 is hot, 3 is inverted (cold).

If you look at an XLR, at least on Neutrik Chasis connectors, there is a tab right by pin 1 that grounds the barrell as well..

 

[Ptownkid]

Haha, sorry, i should have been more specific...I do know what pins are which on an xlr.

What I meant was that I thought that I would have to use hot and ground, as i assumed that the black lead coming from a signal gen was ground, but it appears my reasoning is wrong.

I thought maybe you'd be able to further explain why I'd send the signal in through the normal and inverted pins of an xlr and ignore the ground. Whenever we used gens in school we always had to hook the black lead to ground, so I'm a tad confused.

 

[Mailliw]

1 Chassis ground (cable shield)
2 Normal polarity ("hot")
3 Inverted polarity ("cold")
(borrowed from wikipedia)

A balanced output from a microphone will have a normal polarity signal (one side of the dynamic mic coil), an inverted polarity (the other side of the dynamic mic coil) and a ground (the metal case of the mic).

The mic preamp has differential input, it amplifies the difference between the normal polarity signal and the inverted polarity signal.

Normal polarity signal: 1 V
Inverted polarity signal: -1 V
difference: 2 V

add +1 V of noise onto each signal
difference: still 2 V

Noise is rejected.

But your signal generator doesn't have an inverted signal. Forget about the noise rejection. The generator signal goes into pin 2 ("hot") and the signal ground goes into pin 3 ("cold"). The difference between pin 2 and 3 is simply the amplitude of signal 2 (since signal ground is 0 V).

Normal polarity signal: 2 V
Signal ground: 0 V
difference: 2 V

Hope that helps,
William

 

[pstamler]

It's worth noting, though, that some signal generators' output ground really is hooked to AC safety ground, which means one leg of your mic preamp's input is grounded (assuming the preamp has a properly grounded plug). Depending on the input, that may or may not be an issue, but at the very least you should make double-triple-certain the phantom power is switched off.

What may be safer to do is to use a 1:1 transformer between the generator and the preamp. Or a good DI, if you have one.

Peace,
Paul