Members mikeyen Posted November 27, 2003 Members Share Posted November 27, 2003 Is there a problem with having phantom power on all the time, or is it potentially damaging to certain mics and equipment? Seems like there would be, or else phantom power'd just be on all the time by default... -Mike Link to comment Share on other sites More sharing options...
Members Audioeast Posted November 27, 2003 Members Share Posted November 27, 2003 it will only hurt other mixers plugged in and maybe ribbon mics Also A&H mixwizards have a popping sound if you leave phantom on ant try plugging xlr's in.....go figure! Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 27, 2003 Members Share Posted November 27, 2003 Originally posted by Audioeast Also A&H mixwizards have a popping sound if you leave phantom on ant try plugging xlr's in.....go figure! Many others too. It's because both pins 2&3 do not connect at exactly the same time, so in combination with charging up cable capacitance, creates a small spike that is multiplied by the differential gain of the mic preamp (say 40dB). So a 1mV spike at the input will equal a 100mV signal internal to the mixer's bus structure. Also, the rise time is very fast, so the transient can propigate throughout the front end electronics via the phantom power bus and grounding. Link to comment Share on other sites More sharing options...
Members tlbonehead Posted November 27, 2003 Members Share Posted November 27, 2003 Yep,all the ones I have used will do this. It only takes once to figure out that you need to plug everything in before engaging the phantom power. Or don't turn the power amps on first. Link to comment Share on other sites More sharing options...
Members ezstep Posted November 28, 2003 Members Share Posted November 28, 2003 Many bass amps (my SWR for one) warn NOT to use phantom power when using the amp's XLR/DI. I don't know the exact reason why, but I always check to make sure that the phantom power is OFF for that channel. My Peavey doesn't have that warning. Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 28, 2003 Members Share Posted November 28, 2003 Originally posted by ezstep Many bass amps (my SWR for one) warn NOT to use phantom power when using the amp's XLR/DI. I don't know the exact reason why, but I always check to make sure that the phantom power is OFF for that channel. My Peavey doesn't have that warning. All Genz Benz amps are fully phantom power protected to 50+ volts. I don't know why everybody doesnt do this, there's virtually no extra cost to this! Even our mixer amps outputs are protected. Link to comment Share on other sites More sharing options...
Members RickJ Posted November 28, 2003 Members Share Posted November 28, 2003 My own little straw poll of bass amp internal DI's shows that about half are intermittent or don't work at all. I'm attributing it to the probability that it got zapped by some club's phantom power. GK, SVT (rack mount) and SWR are the main brands that don't seem to have any protection. You'd think they'd put a couple of caps or something in there to block the DC. Another issue with phantom power is that some soundcos reserve some mic lines for DIs and other direct ties with Pin 1 lifted at one end to prevent ground loops. In this case, phantom power won't hurt; it just won't work with those cables. If you have a board that switches phantom globally to all channels and for some reason you don't want phantom power going to a specific source or feed, lift Pin 1 at one end of that source's cable. Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 28, 2003 Members Share Posted November 28, 2003 Originally posted by RickJ My own little straw poll of bass amp internal DI's shows that about half are intermittent or don't work at all. I'm attributing it to the probability that it got zapped by some club's phantom power.GK, SVT (rack mount) and SWR are the main brands that don't seem to have any protection. You'd think they'd put a couple of caps or something in there to block the DC.Another issue with phantom power is that some soundcos reserve some mic lines for DIs and other direct ties with Pin 1 lifted at one end to prevent ground loops. In this case, phantom power won't hurt; it just won't work with those cables.If you have a board that switches phantom globally to all channels and for some reason you don't want phantom power going to a specific source or feed, lift Pin 1 at one end of that source's cable. Rick, This seems correct in theory (you really got us thinking on this actually, since it seems REALLY clever... all of us here gave it some thought actually and you almost got us! Reminder... buy Rick a beer for best attempt to lead a bunch of engineers down the path of "maybe it can work!") The problem with this idea is that even though the ground return path is not being supplied by pin 1 of the audio system, generally pin 1 is connected to signal common within the amp, which is connected to the power ground pin, and thus back to the audio system ground. Therefore the phantom power return will be through the power system ground rather than pin 1. Since the output is always referenced to signal common (Assuming no output line transformer which would make it phantom power protected by default) the 48 volts would still be present between the output and signal common and cause the same problems. Link to comment Share on other sites More sharing options...
Members RickJ Posted November 29, 2003 Members Share Posted November 29, 2003 agedhorse wroteSince the output is always referenced to signal common (Assuming no output line transformer which would make it phantom power protected by default) the 48 volts would still be present between the output and signal common and cause the same problems. I'm no engineer, so forgive any leaps of faith I may state here ... The pic is a typical mixer's balanced mic input stage. (Soundcraft Series 2) Pin 1 is the only connection in the circuit to ground (not audio common). If Pin 1 is lifted at the mic end, the +48vdc circuit is broken. Also note the two DC blocking caps immediately before the initial gain stage amp. Anything that happens after those caps doesn't matter; there is no + DC power there to return to ground. The channel insert (where outboard devices would be connected) is after the initial gain stage. Let's say you leave Pin 1 connected to a mic line and plug it in to the back of an SWR or other built-in DI out of an MI amp. If that amp's output is buffered as a true balanced out, there is no direct connection to the ground (and +48vdc) if Pin 1 is lifted. If that amp's output is actually unbalanced (in which case Pins 1 & 2 are shorted), the +48vdc is shorted to ground -- and load-limited by the series resistors at Pins 2 & 3 in the mixer. Pin 3 may still be supplying +48 because of its series resistor. In the former case, the amp's output is protected by lifting Pin 1. In the latter case, the amp can fry whether or not Pin 1 is lifted, but there is a possibility that the DC is shorted by Pin 2 and reduced to insignificance at Pin 3. That's why I suggested lifting Pin 1. In any case, it's safer to use a transformer-isolating DI rather than those spigots in the amps. Link to comment Share on other sites More sharing options...
Members agedhorse Posted November 29, 2003 Members Share Posted November 29, 2003 Rick, Pin 1 is broken between the console and the amp out, but the diagram helps by showing that pin 1 is connected to console chassis ground, power supply ground, which is connected to earth (safety) ground (sometimes through a 10 ohm resistor, generally not) so the 48 volt ground is picked up on the other end via the AC power earth ground. Therefore, you will measure 48 volts on pins 2 & 3 with respect to the amp's pin 1 ground which is also connected to power supply ground and chassis ground. The problem is that you can not safely defeat the earth ground path. If you did (cut off the AC ground pif for example) then there would be no reference poing of ground to cause the troubles... but and other gronded device connected to the amp (through any jack) would re-establish the ground reference. BTW, this would fall under a somewhat advanced product development topic, and is the type of thing that we discuss and analyze when designing new products. Link to comment Share on other sites More sharing options...
Members RickJ Posted November 29, 2003 Members Share Posted November 29, 2003 As you stated in a previous post: a nice little transformer solves all of those issues. Brings me to the next point (beyond simply lifting Pin 1) ... Tonight's first band had an Ampeg B-series amp with an XLR spigot. Its rear panel label at the XLR spigot reads: "Transformer balanced DI." BEAUTIFUL!! Pre/Post EQ switch, ground lift switch, and a level adjust knob! The very next band that played used the same cab, but a different rack -- an Ampeg SVT rack-mount unit with an XLR spigot and the label: "Active balanced DI." That one didn't work at all. The third band had a GK bass head that forced the muso's EQ on the internal feed. I opted for a passive DI so I could counter his ridiculously EQ'd 2x10 cab into a manageable signal for the 4x18 subs. It's all give & take. I'll still stand by my claim that lifting Pin 1 is the first defense against frying gear with phantom power. Better safe than sorry. (And let's face it ... we're not talking about frying our own gear -- just the poor muso sap who wants to connect his latest, greatest rig to our systems!) Things tend to happen very fast in the festival world, and I only have two eyes. I've also been that "poor muso sap" in the early days. Link to comment Share on other sites More sharing options...
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