I recently purchased a small lot of circuit boards, some with transformers, etc. from GDIY member fluxivity. Included in the lot were four small boards with MCI logos and Beyerdynamic transformers. The boards are labelled "mike pre-amp" and PC2700B0018B. Serial numbers are 13, 15, 17, and 33.
At first I thought they were early 400 series preamps, before MCI switched to Jensens, but the schematics I found for those had pad, phantom, and gain controls. Also, the early 400 series were not on daughterboards. Then I thought about the tape machines - maybe the JH-110 had mic pres? A bit of googling brought up a thread on R/E/P/ (http://repforums.prosoundweb.com/index.php?topic=5000.0) which confirmed the possibility that they existed. Looking at the I/O board in the JH-110 manual I found J8A, P8A, and P14 connectors which matched the boards in front of me!
I drew out the schematic by cutting and pasting from the early MCI 400b circuit. It is copied below. Note that I did not include the DPDT switch in the drawing. It appears to switch between mic and line input modes.
Next I wired up one of the boards (lucky #13) and ran it from a +/-24VDC bench supply (I know it says 18v on the schematic, but the 400 series ran at 24v). Sitting out on my bench with clip leads to wire it up, the preamp gave me +4dBm into 100k load with an input of -39dBm from a 150 ohm source, so 43dB gain. THD+N was -79dBm. Frequency response was -0.6dB @ 20Hz and +0.8dB @ 20kHz.
I have yet to test the other three boards, but if they all work I am considering a four channel MCI "mike pre-amp" build. I have not decided on how to implement the output. I ordered some transistors to use as an output buffer like in the later 400b preamps but then do I run them unbalanced out, buy transformers, or go with impedance balanced? Any input is welcome of course
At first I thought they were early 400 series preamps, before MCI switched to Jensens, but the schematics I found for those had pad, phantom, and gain controls. Also, the early 400 series were not on daughterboards. Then I thought about the tape machines - maybe the JH-110 had mic pres? A bit of googling brought up a thread on R/E/P/ (http://repforums.prosoundweb.com/index.php?topic=5000.0) which confirmed the possibility that they existed. Looking at the I/O board in the JH-110 manual I found J8A, P8A, and P14 connectors which matched the boards in front of me!
I drew out the schematic by cutting and pasting from the early MCI 400b circuit. It is copied below. Note that I did not include the DPDT switch in the drawing. It appears to switch between mic and line input modes.
Next I wired up one of the boards (lucky #13) and ran it from a +/-24VDC bench supply (I know it says 18v on the schematic, but the 400 series ran at 24v). Sitting out on my bench with clip leads to wire it up, the preamp gave me +4dBm into 100k load with an input of -39dBm from a 150 ohm source, so 43dB gain. THD+N was -79dBm. Frequency response was -0.6dB @ 20Hz and +0.8dB @ 20kHz.
I have yet to test the other three boards, but if they all work I am considering a four channel MCI "mike pre-amp" build. I have not decided on how to implement the output. I ordered some transistors to use as an output buffer like in the later 400b preamps but then do I run them unbalanced out, buy transformers, or go with impedance balanced? Any input is welcome of course
