Giggity said:
OK.......Add the 220Ω 220µF on the P48v (even though I still don't understand what they are for), add the phase circuit after the preamp and drop the EQ. Probably better off doing the Black Lion mod on the old Alesis 3630 and get some use out of it anyway.
Given the components I am working with and my familiarity with the program, I think DIYLC will work just fine for laying out the PCB traces. Will have to work up a BOM so I can get the dimensional specs for all the parts before I start the PCB trace layout.
That RL has to do with pin 1 problem, as you need the front end to be referenced to chassis, pin 1 is connected as directly as it can, but the 48V phantom power is referenced to analog ground, for practical reasons, because it's referenced to all the PS ground which goes to an analog ground, they should be the same, but they aren't since they have some inductance and resistance. The thing is that's ok to have some noise between one and the other, in fact, that's why shields and chassis are, to pick up the noises from external world and keep them out of the signal and signal reference, so as phantom power is referenced to chassis, looking from the mic side which is what we are interested, and your 48V PS is referenced to ground, you need to decouple it somehow, and the easier way to do so is like this, a series resistor from the 48V PS to a cap, the bigger both the better, more decoupling, too big resistor has too much voltage across and 48V are no longer 48V, but at 10mA 220Ω is just 2.2V, which is fine, with 220µF which is a relatively small cap, you have about 25dB of decoupling at 50/60Hz which is something, bigger cap may not make a lot of sense, expensive, big, etc. and anyway the noise inserted at this point will came as common mode and rejected by the preamp. If you have another transformer or winding for the 48V PS you reference it to chassis and avoid all those caps, but for a 2 channel mic pre is cheaper and easier to use 2 more caps than another transformer. Also you can take this to make a slow turn on/off switching before the resistor, I know 300ms is not a lot to charge it but is better than a spark and for discharge you can go from there to another resistor to ground which will make it as slow as you want, you have to be sure that the resistor is able to charge the cap, will limit the power to 10W but that's just the peak, in 1/3s will be around 3W and when a second and a half later pretty much nothing, I think 1W resistor can take that without problems as long as you don't start to play with it on and of all the afternoon.
JS
