pietro_moog said:
thanks
i know Jim had, i should mail him.
"There's a couple of ways to do an easy balanced output. You might want to pick up one of jlmaudio.com's DI boxes."
i have 2 JLM fet DIs. what do you mean? i just connect a unbalanced line at -10 in a Di (connected in a preamp) and i will find the output
balanced? i guess i could get 14db of gain..
is it a correct way to do it? will i have problems of some sort?
thanks
Joe's FET DIs and Opamp based DIs are unity gain. They don't add any gain or take it away. So if you put in -10 dbish signal you get that out of the DI plus added current (which makes your signal strong and durable). If you put +4 in you get +4 out and so forth in as much as the signal does not clip the circuit. I imagine with 48v or effectively 24 +/- should give you reasonable room before clipping. Joe would know specifics.
The outputs from the DI are 'balanced.' By that I mean they are balanced per AES definition and Bill Whitlock (from Jensen transformers). The positive and negative outputs are the same impedance compared to ground. This is where you get your noise rejection from. Other balanced outputs (transformer or electronic) are balanced in this manner and also have an inverted polarity signal on negative (pin 3 or ring of a TRS).
My understanding is that the polarity inverted signal has nothing to do with noise rejection and more to do with maximum signal strength and a handful of other things that I can't remember right now.
Look at the top of page 3 of this white paper by Bill Whitlock -
http://www.jensen-transformers.com/an/an003.pdf
Here he advocates a way to have such a balanced output if you know the manner in which your output on your unbalanced equipment is constructed.
Joe's opamp DI is kinda an adaptation of the application note (figure 9 page 12) of this
http://focus.ti.com/lit/ds/symlink/opa2604.pdf and obtains the impedance balanced output much like Bill Whitlock described.
So you have a couple of ways to do this. You could make your own thing with a balanced line sender (kinda a DI on a chip) plus a power supply, a transformer, an existing DIY product (like Joe's stuff - which I think are great and use my self), or if you know the output manner of the unit you are trying to make a balanced output from you could use Bill Whitlock's suggestion.
The nice thing about impedance balanced output is you can plug in a mono jack and not potentially damage the thing. You have to be careful with various electronic balanced outputs depending on how it is constructed.
Good luck.
CC