Multiturn trimmed to 250uA
Circuit while trimming current (junction soldered together after current set). White visible-crimp BNC is single-ended signal source to transformer (hidden by BNC connectors), other BNC connectors are differential output to Tektronix 7A22 diffamp.
venerable Tek 7623a storage scope w/ 7a22
Signal source & power supply. BNC's on tee are TTL trigger output, triggering the 7000-series scope as well as another scope for other probing, as there was significant trigger jitter on the circuit due to the low-level signals involved.
The key to 63dB CMRR -- the 0.01% Vishay laser-trimmed
Resulting differential gain.