Need schematic for Behringer Vintager AC108 amp.

SOURCE: I have a behringer bx4500h.

Behinger normally doesn't give schematics... However here is a link to a BX1200 schematic at a foreign site. OFTEN they use the same circuitry within a series of equipment.

http://elektrotanya.com/behringer_bx1200_sch.zip/download.html

Note it is a zipped file of both F and H revisions.

Please note that the symptoms you describe are NOT usually something you will find in the preamp, but rather in the interconnections. It appears that the BX1200 uses TRS inputs at the jacks... not sure what your BX4500 does...

Make sure ALL your interconnected equipment is powered from the SAME receptacle to avoid ground loops and hum. Use balanced lines everywhere possible.