some basic audio-transformer questions....

hi,

I have a few questions about transformers and especially the wireing:


A) if a transformer has the same value on both sides (1:1)
does it play a role, which side is secondary and which side is primary??


B) I just made a basic wireing diagram for the connection of the
transformers (input and output)

is the connection of each transformer to the xlr's and to the circuit
correct??

C) what about the ground?? should I connect pin1 on both XLR's ???






thanks,
mat

A: dosen't matter for audio. There can be some issues with capacitance-to-shield in special cases though.

B: looks right.

C: Yes, unless you want groundlift, in which case you leave pin1 floating.

thanks,

do I have to connect gnd from pin1 to "circuit ground" ??

mat

yes

You can also connect a 1K resistor paralleled with a .01 uf capacitor from pin1 (Gnd) of the XLR before going to ground.

From my experience, it eliminated noise without having to do a ground lift.

Concerning grounding:

If this is a passive box, then what Gyraf said and you assumed is the correct answer. Pin 1s are connected. Most often this is done by joining each to the metal chassis where the connector receptable is mounted.

If you've got an active box with a power supply in there then you'll probably have this type of scenario:

The meains earth connector (ground from the AC outlet on your wall) is attached to the chassis. The circuit ground is connected to the chassis through some sort of passive device i.e. 10ohm resistor at one place only - Usually the 0V connector of the power supply. The purpose of this passive device between them is to provide a small degree of isolation while maintaining a strong connection. Remember, path of least resistance... we want our circuit's 0V reference to be physically grounded but we don't want all of that noise and harmful stuff from the mains earth creeping into our audio signal path so we add some resistance. Now you can connect the XLR pin 1s (or TRS sleeve) to either the circuit ground or the chassis ground - ideally at the same point that the two grounds are meeting. In my experience/opinion it makes more sense to connect to chassis ground so that cable shield noise stays out of the audio path, but people seem to have their personal preferences.

There are a number of good articles about grounding on the internet if you want to learn more. Just do a google search for "grounding and shielding". I particularly like the one from Rane. In addtion, Jensen and Lundahl have some papers/schematics on their websites which will help you learn more about both transformers and grounding.

In general a 1:1 transformer is used for isolation and/or to balance an unbalanced signal (many audio circuits do most of their work unbalanced) or vice versa. While not stepping the voltage up or down it is still very much having an effect on the sound and serving two useful functions.