A stereo headphone/speaker monitor amplifier system includes a pair of Neumann 473D Monitor Amps in a Siemens military card frame case. The card cage dimensions are correct for the Neumann 473 cards. The third card was blank epoxy with holes drilled for tie wraps to hold the two RDL model STK-600 Passive Combining Networks. (PCNs) The enclosure is like solid aluminum with giant rubber bumpers and if thrown against a brick wall or run over will survive. It can live on the floor. The handle makes it easy to move around and can be used any which way up. The covers align with large guide pins and are secured with two captured hex bolts. 3 pair of stereo inputs on XLR/TRS Combo Jacks feed the left and right passive (STK-600) 600 ohm combining networks. Mix stereo sources together like drums and band with click in one or both ears. Any line level input will work. Good for gigs or practice or recording.

Note: the 473 cards themselves have transformer isolating balanced inputs (and outputs actually, see drawing) but the ability to keep outside sources isolated from each other by transformer is an important consideration for some applications.  A pair of in-line isolation transformers (see pic) are included and make an easy option to mounting them internally. Its easy to spec transformers to fit in the cover.

OUTPUTS

The outputs are 16ohms transformer balanced or 4 ohms unbalanced. They can be used simultaneously. A manual with complete specs will be included. The headphone/speaker outputs are wired to a stereo 1/4" jack with push-to-release tabs to prevent the plug from accidentally pulling out. Currently, the units are wired for 16ohms. The right jack is not wired but could parallel the left. Or from the edge connectors you can pick up the unbalanced 4ohm outputs. The pot shown on the output side is unused and just filling the panel position. (standard1/4” with rotation lock pilot hole) Put a nice 4 deck rotary in there for the inputs to the 473s or an L-pad for the outputs. In the past I’ve used L-pads on the outputs of the 473D for volume.

The male XLRs on the output side are unwired but can be used for loop throughs to forward to a desk or monitor mixer board there's a service loop of wiring for it anyway. Can be changed to a pair of 5pin XLRs for two stereo loop throughs as well i suppose.

Comes with a 24 supply. It's a switcher but these can work amazingly well. Otherwise use any supply at 24vdc. The PCN blocks do not use power.

The idea building this was to have a bullet proof and dependable studio headphone amplifier system for rehearsals or for problem solving. Its all 600 ohm stuff so its easy to work with. Note: manual is in German