Brian Roth
Well-known member
It is a bit odd that the VPR Alliance spec says 5 mA max for the 48V rail while the "official" IEC standard mentions up to 7 mA (10mA max) for any given mic or other device. The VPA spec ignores the existance of "hungry" mics. However, most mics draw 5 mA or less.
Keep in mind we have two "branches" for the 48 V rail after it passes through the phantom switch.
The primary path is through the pair of 6k8 resistors which provide power to the mic. The absolute worst case current would be approx 14 mA with a fault condition where XLR pins 1, 2 and 3 were all shorted together (totally crapped out mic cable!).
[Sidebar: The 6k8 resistors will dissipate around 0.3 Watts under the bad fault condition so need to be rated at least 0.5 Watts. I've seen more than a few consoles that had been in service for many years which had random 6k8/0.25 Watt resistors "browned" due the presence of a bad cable at some point.]
The other 48V path in the proposed design (and the CAPI schemo) only makes the lamp illuminate. In this case (10K in series with the LED which will have approx. 2 Volt drop across it), approx. 4.6 mA will flow with the lamp "on". If no phantom flows to a mic, the module will draw 4.6 mA. A hungry mic drawing 10 mA means the total module draw will be 14.6-ish mA. A boogered mic cable (worst case condition) raises the total to approx 18.6 mA. Note that the LED will still light even with the boogered cable.
Rambling answer to @djmiggymigz IMHO, the 4.6 mA draw in the CAPI design may be too high to account for a wimpy 48V power supply and whatever. As has been mentioned in this thread, a high efficiency LED (and a higher series resistance) should be a good solution to CYA.
Bri
PS, for jollies I looked in the JKL catalog for 48V incandescent bulbs. Found one that draws 53 mA! lol
Keep in mind we have two "branches" for the 48 V rail after it passes through the phantom switch.
The primary path is through the pair of 6k8 resistors which provide power to the mic. The absolute worst case current would be approx 14 mA with a fault condition where XLR pins 1, 2 and 3 were all shorted together (totally crapped out mic cable!).
[Sidebar: The 6k8 resistors will dissipate around 0.3 Watts under the bad fault condition so need to be rated at least 0.5 Watts. I've seen more than a few consoles that had been in service for many years which had random 6k8/0.25 Watt resistors "browned" due the presence of a bad cable at some point.]
The other 48V path in the proposed design (and the CAPI schemo) only makes the lamp illuminate. In this case (10K in series with the LED which will have approx. 2 Volt drop across it), approx. 4.6 mA will flow with the lamp "on". If no phantom flows to a mic, the module will draw 4.6 mA. A hungry mic drawing 10 mA means the total module draw will be 14.6-ish mA. A boogered mic cable (worst case condition) raises the total to approx 18.6 mA. Note that the LED will still light even with the boogered cable.
Rambling answer to @djmiggymigz IMHO, the 4.6 mA draw in the CAPI design may be too high to account for a wimpy 48V power supply and whatever. As has been mentioned in this thread, a high efficiency LED (and a higher series resistance) should be a good solution to CYA.
Bri
PS, for jollies I looked in the JKL catalog for 48V incandescent bulbs. Found one that draws 53 mA! lol
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