[strangelove audio hardware design]
+4dBu balanced input and output modification for Revox B77 tape recorders.
Project ID on printed circuit boards is 20-12.01.00, title is "+4dBu balanced I/O for Revox B77".
PCB mounted on some B77 decks that underwent full restauration by strangelove audio. The PCB replaces the original RCA connectors with TRS jacks and provides electronically balanced inputs and outputs at a +4dBu nominal signal level. The internal nominal level remains 0dB, in accordance with the original Revox design.
The output is driven by a cross-coupled output stage and can drive single-ended loads just as well as balanced ones, in which case it is recommended you tie cold and ground together (i.e. by using a mono TS plug to connect to the B77).
The output driver is the THAT1646 IC and can easily drive 600Ω or long cable lengths.
This project only modifies the B77 i/o connector housing. No other changes are made to the tape recorder.
Input impedance : 16kΩ balanced, 8kΩ unbalanced
Output impedance : 50Ω
For removal of the Balanced I/O circuit board, please follow these steps:
- Unplug the output connections on the right hand side of the Audio Interconnection pcb assembly (1.177.210). These are the black, green and white wires located on the bottom right hand side).
- Unplug the RADIO and AUX input connections on the Audio Interconnection board. These are the green, white, yellow and brown shielded wires.
- Remove the Capstan Speed Control board and remove the cable tie behind it that secures the Remote Control and Varispeed wire loom.
- Remove the four M3 PH2 screws that secure the I/O connector housing.
- Turn the housing around and unplug all 3 Molex connectors on the Balanced I/O board.
- Remove the screw that secures the right angle mounting bracket on the top of the Balanced I/O board to the connector housing.
- Remove the nuts of the audio TRS jacks. These are not the typical threaded nuts, but only need a quarter turn to unlock. They then can be pulled away.
- Lift out the Balanced I/O board, taking note of how the wires pass between the TRS jacks.
- For reassembly, follow these steps in reverse, but do not forget to put back the plastic sheet underneath the PCB, to isolate it from potential short circuits with the other components mounted on the connector housing block.