

An Easy way to solve the problem is to use two op-amps to form the
balanced low-Z output, but this solution does not provide any
galvanic isolation between keyboard (or whatever plugged) and
mixer. If no tight requiremets are demanded and some hum, snap,
crackle and pop sounds (formed by ground loops) can be tolerated,
the op-amp solution is just what you are looking for! (It is
cheap...somewhat $10/DI-box).
Not sure, but Craig Anderton did introduce one DI-box project
in Guitar Player mag years ago (transformerless).....


Hope this helps. Email, if more details wanted....


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Heikki Paananen                           heke@stekt.oulu.fi
The University of Oulu                  
Department of electrical engineering      -Just a student
Finland