Monte McGuire said:
JohnRoberts said:
You lost me at subjective, but got me back at flying rails. I did a lot of late night scribbling about that.
...
In a raise the bridge or lower the water exercise, it might be simpler to leave the mic preamp grounded and tweak the phantom power. Namely drive the phantom power resistors with modest voltage, then drive pin 1 to a negative voltage using a servo to keep inputs at 0V.
Sorry to backtrack, but this concept from John Roberts is subtle and brilliant, but perhaps hard to deploy in a commercial device. The big problem is that a box of preamps might be asked to work in a context where the pin 1 voltage of each mike can't be reduced to achieve the 'P48 drive' to the attached mike, simply because pin 1 is common to a whole lot of other pin 1 connections in the room; each pin 1 can't be servoed like that.
Not brilliant, but outside the box thinking.
Indeed pin 1 needs to be a low AC impedance to ground for effective shielding, even if biased at some negative DC voltage. This could be a nightmare for general applications where pin 1 grounds are rarely segregated. Further a singer touching a mic body could likely get energized to that negative voltage potential, perhaps causing issues elsewhere. The voltages and currents are not dangerous to human safety but could interfere with other sensitive inputs (phantom power involves mA level power).
However, for a DIY or specialized application where the user of such device might be able to arrange for each mike line to have its own isolated pin 1 connection, this is actually a brilliant solution to the P48 coupling problem. Further, servos like this are not tough to design, since their noise output will appear as common mode to the mike channel, easy to reject, and easy to 'finesse' by over-design.
Again, a tip of the hat for sharing this gem, which I know you could probably not use in your previous commercial works without generating endless support calls. However, here in this polite land, I think it's worth examining this idea, understanding that (DC) isolating pin 1 to each mike input is all that you need to make this work.
If I had a better handle on rolling my own very high performance A/D convertor, I would float the mic preamp and A/D up at whatever voltage it needs to be to keep the mic happy, and then optically couple the digital output.
Again I see this as mostly cosmetic (perception bias), since modern caps can be pretty clean when well used (i.e. keep signal currents low).
I scribbled plenty but never melted solder because I do not expect a significant audible improvement. There is probably a niche market among cap haters. ;D
JR
PS: It was so long ago I don't even recall what I settled on. Probably two variable power supplies. A positive rail feeding the two 6.8K phantom resistors (0V to maybe +30V), and then a negative rail (0 to -48V) that when added to the positive rail sums to 48V total. So this would all be transparent to the mic.
The servo would regulate the preamp input to be 0V, by varying either the + or - rail, while they would track each other with the 48V spread. A non-phantom mic wouldn't drop any voltage in the phantom resistors so the + phantom rail would be 0V and pin 1 would be at full -48V DC.
Have fun, this is mostly a science fair project.