The 990 discrete opamp (DOA) designed by Deane Jensen, operating as a microphone preamplifier. A stepped gain switch up to 70 dB. This build has 8 channels packed into a 2U case.
Built following John Hardy’s M1 design using a servo to null offset voltage on Mitsos circuit boards (from groupdiy.com). The servo allows the elimination of the capacitor from the signal path.
This build uses Jensen transformers (JT-16 , JT-11-BM) and handbuilt 990 op amps. The PCBs for the grayhill switches are from classic API.
The gain switch goes from 15 dB to 70 dB in steps of 5 dB, with an input pad (-20dB) and output pad (-10 dB)