The Journey to make a 312 Microphone Preamp for the Sony MXP 3000 Series Console
This console came with the ability to have stereo microphone preamplifiers on each channel, even thought the channels are mono. When my boss had scrapped out a previous Sony MXP 3000 he kept all of the modules from that console, which had these dual microphone preamp. The only issue was that nearly all of them had faulty potentiometers, which are extremely difficult to find and replace. So this lead me to the idea of making a 312 like mic preamp because we have ~20 of the Jensen JT-115K-E.
Circuit and Initial Design
I had captured the schematic for a full 312 a wile back when I was first starting to work on this idea over a year ago, but I stopped due to not having transformers that I wanted to use. As well I was unsure how Paul Wolf had used the output transformer exactly in his design. So when I had to come up with a new project for the PCB design class I was taking at Pasadena City College I thought it was the prefect opportunity to make some of these mic preamps for our Sony MXP 3000. I removed the output transformer as well as the pad and phantom power. This was due to single sided boards that we were using on the Othermill. And finally the board had to shrink in length.
When I first built the mic pre, there were a few issues that I found. First I had the capacitor, that is part of the shunt circuity in the wrong place. It was above the potentiometer instead of following it so that is the red wire that you can see. The second issue that I has was that there was very little gain from the preamp. This was because I had gotten the position of the feedback and shunt resistors in the wrong position. But, once I figured that out preamp came to life.
For the initial testing we are using the Gar2520 opamp kits from CAPI.
Below is a graph showing the frequency response of the my preamp versus the stock transformer coupled mic preamp that came with the Sony MXP 3000. The blue trace is the stock Sony MXP 3000 transformer coupled mic preamp you can see the the transformer is boosting some of the high frequencies that the unit I built is not, I believe that this is due to the zobel network that I have to dampen the transformer.
Rev A PCB
After fixing a the issues that I found with building the Othermill PCB I changed those and added back the phantom power and pad circuitry. Now I was ready to order the PCBs, for this set of prototypes I order the boards from Bay Area Circuits.
Here is the assembled Rev A PCB. Again there were a few issues that came to light mainly around the pad circuity, the resistors are very close to the switch so I had to mount the switch off the PCB slightly. Secondly, the pot works in the reverse manner. Both of these issues I have addressed in the Rev B schematic.
This past weekend I did a session at AEA and got to try out two of the mic preamps. They sound much better than the stock IC transformer-less that the Sony MXP 3000 has. And, just as good as the transformer coupled mic preamps that we already have.
Now that I have done some initial listening test and some measurements it is time for the Rev B PCB. One of the things that I am contemplating about doing is adding a DC servo to the design. If you noticed in the Rev A design I do not have a coupling capacitor on the output to block DC offset. Granted at this time I have not measured the output to see if there is a major offset. Adding the servo does add more complexity to the design but I think it may be a better choice over a coupling capacitor. I plan on adding both and listening to how they preform.