Newbie Chris, here.
As some of you may know, I have an Ampex 351 electronics "toob" preamp I am hacking, err... "modifying" for use as a microphone preamp (don't worry, it was incomplete and trashed before I got my grubby little hands on it, so I didn't quite start with a pristine vintage piece).
Anyway, I am modifying it according to notes by Greg Norman (Steve Albini, go here for his document from this page. Refer here for original schematic). He says that they use an outboard source for phantom power. However, I was hoping that there could be a way to supply phantom power from the stock power transformer. Because I was already planning to run all the 12A*7 filaments on the 12.6 VDC secondary, I had an idea to utilize the power transformer's 12.6 VAC secondary for a phantom power supply after looking at Jakob's G9 mic pre schematic here. I was hoping to create a "voltage quadrupler" to get the voltage up to 48 VDC. The thing is, I understand that I'll need anywhere from 10 to 20 mA for phantom power. I'm not sure ('cause I'm a newbie :wink, but I remember reading that for some applications four 12A*7 tubes (from original schematic) draw on average about 0.3 mA each (or 1.2 mA total). But, I am not sure if the power transformer in this preamp *isn't* overdesigned to deliver a minimum of 10mA. My questions are 1) is 10 ? 20 mA a reasonable requirement for phantom power, 2) does anyone know what the current ratings are on this particular secondary and, more importantly, 3) is there a way to calculate this info so I don't have to resort to asking the forum everytime and moreover so I can try and learn something for a change? :grin:
Finally, is there any harm or benefit regarding using DC for the filaments on all the preamp tubes instead of what's shown on Norman's schematic? I guess he based his modified schematic on the original schematic as V4 and V5 originally used AC on their filaments. I thought I remember reading something somewhere about preamp applications benefitting from DC filaments as it keeps the noise/hum down.
Thanks,
Chris
As some of you may know, I have an Ampex 351 electronics "toob" preamp I am hacking, err... "modifying" for use as a microphone preamp (don't worry, it was incomplete and trashed before I got my grubby little hands on it, so I didn't quite start with a pristine vintage piece).
Anyway, I am modifying it according to notes by Greg Norman (Steve Albini, go here for his document from this page. Refer here for original schematic). He says that they use an outboard source for phantom power. However, I was hoping that there could be a way to supply phantom power from the stock power transformer. Because I was already planning to run all the 12A*7 filaments on the 12.6 VDC secondary, I had an idea to utilize the power transformer's 12.6 VAC secondary for a phantom power supply after looking at Jakob's G9 mic pre schematic here. I was hoping to create a "voltage quadrupler" to get the voltage up to 48 VDC. The thing is, I understand that I'll need anywhere from 10 to 20 mA for phantom power. I'm not sure ('cause I'm a newbie :wink, but I remember reading that for some applications four 12A*7 tubes (from original schematic) draw on average about 0.3 mA each (or 1.2 mA total). But, I am not sure if the power transformer in this preamp *isn't* overdesigned to deliver a minimum of 10mA. My questions are 1) is 10 ? 20 mA a reasonable requirement for phantom power, 2) does anyone know what the current ratings are on this particular secondary and, more importantly, 3) is there a way to calculate this info so I don't have to resort to asking the forum everytime and moreover so I can try and learn something for a change? :grin:
Finally, is there any harm or benefit regarding using DC for the filaments on all the preamp tubes instead of what's shown on Norman's schematic? I guess he based his modified schematic on the original schematic as V4 and V5 originally used AC on their filaments. I thought I remember reading something somewhere about preamp applications benefitting from DC filaments as it keeps the noise/hum down.
Thanks,
Chris