Home > Studio Recording DIY Projects> Sony MXP-3000 Mic Preamp I did some preliminary testing with a signal generator and oscilloscope, just to see if it will pass signal. IT WORKS! Output is about 13Vpp (or +18dBu). If I use a Sowter (600:600) wired as 1:1.5 output transformer, I can get about 17Vpp (or +21dBu). Problem is the Sowter transformer doesn't fit in a 1u case! AAHHHHH!!!! I wish I used a 2u case, but then it's going to be too big. I need to find an output trafo that will fit inside a 1u case. If not, I might just use my 1:1 MCI 416 transformer. If I use the MCI transformer, I'll have to leave with the +18dBu output. I connected my MXL condenser mic and did some testing...again, it works! :) The EQ also works, when I press the EQ button, the light lits up, and the EQ becomes active. (No smoke! :) Which means the reverse engineering of figuring out which pin is what is correct!) I can tweak the sound with the EQ and the filtering is nice. It's semi-parametric, meaning I can adjust the levels and center frequency, but not the "Q" of the filter. There are 2 buttons though for a "peak" function for the low frequency and upper mid frequencies. Mic Preamp The Mic preamp has a button called "PAD". With the PAD engaged, the volume varies gain from 5dB to 45dB. If the PAD is disengaged, the volume varies gain from 25dBu to 65dBu. Last minute issues: While everything works, there are still some issues I need to address. 1. I need to wire the "power lamp" to indicate if power is ON. 2. The transformer is 18-0-18 VAC, which after rectification becomes 27.1 VDC. This is then regulated down to +18V.... Problem is the regulator gets too hot! (at least for my own comfort). So I ordered some transformers and will be replacing the toroid transformers with a lower rating... that way, it won't get too hot. I could add a heatsink but there was no place to put it. In the future, I'll redesign the power supply. 3. Need to figure out what to do with the outputs... Use the MCI416 transformer or find a replacement? Either way, the output will be balanced 1/4" TRS. 4. The phase reverse switch on the front panel is inverted... when the switch is up, the phase is normal. If the switch is down, the phase is 180-degree reversed. I'm thinking of doing it the other way around... i.e. switch down (normal), switch up (phase reverse). Final Update Well, I decided to just use my 1:1 transformer but I added a "TRIM" knob to fine adjust the gain of the mic preamp. Connected the preamp to my studio and it is so QUIET!!! The addition of the output transformer really helps makes things quiet, sort of like a built-in Hum Eliminator. Here's
a block diagram of the whole project:
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