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When I accidentally connected a faulty mic cable to my beloved Fireface UFX+ the other day and switched on phantom power on that channel, I blew not only this channel but the whole interface with it - smoke and smell of fried components etc. The interface is in for repair for two weeks now...
This accident made me think: I'm using my SL24 Series III mixer in a live environment where faults like this are much more likely to happen than in the studio. Are there any precautions against phantom power failures in StudioLive Mixers? As described for example here (PDF file). I understand that these measures won't necessarily protect against all possible fault conditions and obviously, whatever RME is doing, wasn't enough - but well.. how does Presonus deal with this? Did something like that happen to you guys? |
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I have never seen a mixer damaged by a problem with a phantom powered microphone. Most likely the microphone simply does not work. It has also never been a problem to plug and unplug microphones with phantom power, except the big noise happening then, if you did not mute the channel...
One thing you should NEVER do is to save phantom power "on" with your project/scene. Since decades it has a good habit to switch phantom power on and off when needed and not activate it automatically. When the microphone is set up and the cable connected, then is the time to activate phantom power. The first thing to do after a concert is to mute all channels and deactivate phantom power. It's true that in a live environment a lot of things can happen. Electronic equipment (as well as musical instruments) has to be treated in an appropriate way. The manual activation and de-activation of phantom power belongs to that theme. |
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Odd. 48V phantom power is supposed to be resistor-limited to 7mA short circuit current per pin (14mA per channel). No single cable fault should be able to blow up an input, let alone the entire interface. You do however need to be careful not to feed dc power into a mic input (dmx cables, or when splitting mics to multiple mic inputs with phantom power capability).
As far as I know PreSonus gear doesn’t get damaged due to cable faults plus phantom power. In the first generation ‘classic’ StudioLive mixers the power to the lamp socket was sourced from the same PSU output providing the 48V phantom power for the desk. With light bulbs that could lead to insufficient phantom power. I don’t know if the newer desks are still wired like that but with everyone on LED lamps it’s pretty academic anyway. Hope that helps  NB. Thinking about that Fireface, the only way I can think of a failure like that happening is the current-limiting resistor overheating and failing into a short. With properly dimensioned resistors it should be impossible to overheat them with 7mA, so an external voltage must have been involved I guess. |
wahlerstudios wroteThe manual activation and de-activation of phantom power belongs to that theme. Absolutely! Having an analogue live mixing background, this is second nature to me. Unfortunately it's not what happened here. When I checked the offending cable after the incident, its cold wire was broken internally touching the shield. Maybe something heavy ran over it... SwitchBack wrote48V phantom power is supposed to be resistor-limited to 7mA short circuit current per pin (14mA per channel). No single cable fault should be able to blow up an input, let alone the entire interface. I know, but it's not about the resistors in this case. As can be read in the AES paper I linked to earlier, it's the blocking capacitors, that - under normal conditions - block the 48 volts DC from getting into the input stage. When one or even two wires from the mic cable accidentally touch ground, they drag the voltage on the capacitor's 'back side' to approximately negative 48 volts for a given time - and that can do a lot of damage to input transistors, ICs, whatever (unless there are precautions being made, hence my question). In the case of Presonus' XMAX preamps with their presumably discrete nature, this would most likely be input transistors. The fault current can get pretty high - depending on the gain setting up to 3 Amps! |
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