Let's review the setup procedure and precautions. This is for bridging the amplifiers within a single box.
- The mono-stereo-bridge switch must be in bridge position
- The input signal must be connected only to Channel 1. NOTHING should be connected to Channel 2.
- The Channel 2 level control should be turned all the way down. Turning it up will amplify common-mode noise.
- The + speaker terminal is connected to the red binding post for Channel 1 and the - speaker terminal is connected to the red binding post for Channel 2. Neither side of the load should be grounded.
- There are no connections to the black binding posts. They should not be grounded or shorted together.
It does not seem practical to bridge or parallel two MT 1200 boxes; it
would be very difficult to get the levels and offsets precisely
matched. If they aren't matched, you will get oscillations or common-mode amplification - that is
your humming noise.The bridging switch makes six changes to the circuitry to get the
amplifiers properly interconnected in one box so this won't happen in the