To DVH Group, Important information about use of the Ithaco preamps: The 1201 Ithaco preamps (the black differential preamps) should be plugged in at all times. They have batteries that allow them to be run unconnected from the AC line to reduce 60Hz noise, but the batteries also help to stabilize the output. When plugged in and set to charging (the normal mode --- middle position on the rear switch), they will constantly charge --- the battery LED indicator on the front should light on both polarities when they are properly charged. I even prefer they be left on to keep them warmed up --- that reduces dc drift --- but at least they should always be left plugged in. We have six of these. I found yesterday that several were NOT plugged in and only one shows a proper battery charge. Some may recharge in time, but the batteries may need to be replaced --- that was last done in 1995 by Brian Yanoff. The batteries do eventually go bad, but their failure is accelerated when they get completely drained. The batteries are probably $300 each to replace. We have quite an investment in these --- they are about $2500 each --- and we rely on them a lot. They are instruments we really want to keep functioning properly. Please help to maintain them and report any malfunction so we can get them working well. FYI, they are now made/maintained by DL Instruments (Ithaco no longer exists) --- the data sheet is here http://www.dlinstruments.com/products/pdf/1201.pdf . Dale