[sdiy] Direct Injection box? (balanced v. unbalanced)

[Alex Dickey]

> A direct box (for bass or guitar) usually converts from single
> ended to balanced output, and provides attenuation to go into the
> microphone input of a mix console. Isolation is usually a 
> feature, as is the 
> ability to separate the grounds. Very often an audio transformer is 
> involved.

i was wondering about this the other day, i.e. balanced inputs vs.
unbalanced inputs.  is the conversion done exclusively with a transformer?  

alex