Phantom Power

[Ant1981]

Just a quick one about phantom power.

I know that there are 48v phantom power switches on mixers for powering mics. Does this mean i can power my acoustic without the 9v battery too since it has an xlr out?

Is this safe powering loads of stuff with 48v power when they run on 9v batteries or less?

[Algiman]

Interesting idea though that voltage would fry an onboard circuit designed for 9v. The XLR cabling would need to be rewired too as the guitar will not have been wired to receive power through it.

[rgr]

Only if the guitar is specifically set up to do that, my guess is no. Only certain mics (condensers and such) need phantom power, most don't but it doesn't hurt them for it to be on, it just looks like an open circuit to them. I'm guessing you guitar will be the same but you'll want to check the specs on the preamp in your guitar.

As far as power battery operated items with phantom power, no, you can't do it.

[Algiman]

Originally Posted by Ant1981

Is this safe powering loads of stuff with 48v power when they run on 9v batteries or less?

Thought you were an electrician?

[Ant1981]

Originally Posted by Algiman

Thought you were an electrician?

I am, but I know little about phantom power and the requirements of the specific equipment. It's rather different to mains supplies and systems.

[rgr]

Okay, say you take your 120V American toaster, change the plug, and plug into a European wall outlet.

[Ant1981]

It won't get so hot, using power law, the higher voltage on the same size heater coils would draw less current, therefore running cooler and probably not toasting so well.

[rgr]

Buzzzz, wrong. The heater coils would have the same resistance no matter what voltage they were plugged into. So assuming that the heater coils are say 100 ohms (just throwing out a number here), then the current through them at 110V would be 1.1A, but the same 100 ohms with 220V going through it would be getting 2.2A which would probably fry it. The power ratings on the back of appliances is their power draw based on the rated voltage input. So the max draw of the 110V 100 ohm toaster would be 121W but would draw 484W (for a short period of time) at 220V.

*EDIT*I think you assumed that the power draw would be constant based on the rating on the appliance, therefore, if the power draw remained constant, based on a higher voltage input, the current would go down (if power is the constant, this is true). But power would not be the constant here, it is calculated/measured based on the appropriate voltage going into the unit (load).

Make sense,
Roger

[Algiman]

Originally Posted by rgr

Buzzzz, wrong. The heater coils would have the same resistance no matter what voltage they were plugged into. So assuming that the heater coils are say 100 ohms (just throwing out a number here), then the current through them at 110V would be 1.1A, but the same 100 ohms with 220V going through it would be getting 2.2A which would probably fry it. The power ratings on the back of appliances is their power draw based on the rated voltage input. So the max draw of the 110V 100 ohm toaster would be 121W but would draw 484W (for a short period of time) at 220V.

Make sense,
Roger

tick.

[Ant1981]

Not sure where you get your info from, what qualifications do you hold? Cos you are wrong. Higher voltages will draw LESS current, not more. Look up power law.

Anyway, forget bloody toasters, this isnt helping, I wanted to know about XLR phantom power, doesn't matter, I dont think I need it for Shure SM57's and acoustic guitar preamps.

[Algiman]

I think you mean Ohms law:
V=IR (not perfect in this case because in the toaster there will be a little reactance as well, but not enough to make a big difference.

So, V=IR, therefore I=V/R, therefore because R stays constant (for all intents and purposes) double the voltage, you double the current.

Power law? I guess you mean P=I(squared)xR - not really relevent in this case other than working out how many watts the toaster could handle before it caught fire.

Back on topic:

I wouldn't bother with phantom power for an acoustic either.

[Ant1981]

I did mean power law.

This is the confusion I address at a group in one of Londons Engineering Colleges whilst discussing questions for upcoming City & Guilds exams for the new (2003 onwards) electrical installation courses.

We took a group, asked them a similar question on the sample paper and the same thing happened. Some used Ohms Law, some used Power Law. I took it back to City & Guilds, but they're not interested in making much sense at times with thier questions, so why they asked for I.E.E members who are electricians to give some input is beyond me and many others from the same group.

But yes, back to phantom power, as some devices can use battery or phantom power, that's where the electro acoustic guitar with XLR out came into question. But I should think the battery will last. Anyone know the main benfits over XLR as opposed to 6.3mm jack then?

[rgr]

Originally Posted by Ant1981

Not sure where you get your info from, what qualifications do you hold? Cos you are wrong. Higher voltages will draw LESS current, not more. Look up power law.

BS EET, ~20 years in product design and development, okay, at least half of that has been in mechanical design and testing. We've been through this before. I didn't mean for my toaster comment to sound condesending or anything. Everytime someone tries to help you it seems like you do your best to shoot them down or argue with them, so I keep asking myself, why bother... Yet, I still answer, DOH!

http://www.tangible-technology.com/p...r_connect.html (haven't read all of this, I'm at work).

Phantom power is not there to power 9V pedals, guitar preamps, etc... Your guitar is probably not set up to use the 48V phantom power for its preamp. Phantom power is a very low current supply to power condenser type microphones, that's pretty much it. If you plug an XLR with phantom power into the XLR out on your guitar, it will most likely just ignore the phantom power (look like an open to it), can't guarantee it, but if the designer had a clue, that's what it will do. Maybe it can run off of phantom power, it is a good idea, but I don't know of any that do. Same with dynamic mics, you can have a SM57 (or SM58, EV N-Dym, whatever) and a condenser plugged into a board with the phantom power on, the dynamic mic just ignores it, it doesn't need it, the condenser uses the phantom power to bias its diaphragm. In reality, the phantom power probably doesn't have enough current to power a preamp.

The toaster was an oversimplification, but fairly accurate, the supply voltage sees whatever it is powering as a load, a resistance if you will (yes, I know I'm not taking into account phase shifts, reactance, etc... but a heater coil doesn't really care and the preamp is DC Voltage). So, a higher voltage will induce more current for the same load, V=IR or I =V/R, raise the voltage, raise the current. You used I=P/V assuming that the power dissapation would remain the same for a higher voltage. If this were true, why would so many products, amps, whatever have input voltage selectors on them. If you are so sure about your calculations, try setting your Marshall on the 110 setting and plugging it into your 220 wall outlet and playing it for a while (not adviseable). Some circuits are designed to sense different input voltages and deal with them. Those are usually power supply circuits that use switchers, laptop power supplies, the One Spot power supply for pedals, lots of new ones, but your preamp is expecting to only see 9V, for these your power assumption is correct, higher voltage = less current because the power draw is the same. The North American toaster is only expecting to see 110V - 120V.

What would happen to the preamp with 48V instead of 9V would be that you would probably blow some of the electrolytic/tantalum caps which are probably rated at 16V or 25V (plenty of headroom at 9V, not for 48V), and it might toast the transistors and/or opamps, their circuits are designed to see 9V, so if you feed them 5X that the least you would do, if nothing blew, is to send the signal to the upper rails and you would hear garbage, my guess is that they wouldn't be able to handle that much on their Vsupply pins though.

If your preamp will run on phantom power that is a cool feature, and the designers have accounted for that with a voltage divider or something on the power coming from the XLR pin, the circuit is still probably only seeing 9V - 10V, so if you plugged 48V into the 6.3mm plug, you would probably blow the circuit. The advantage of being able to run off of phantom power should be obvious, not battery or external power supply needed. It would probably also have to be a very, very low current draw, which is probably the most limiting factor to a design like this and make it non-feasible.

[Ant1981]

No wasn't shooting anyone down, apologies if it seemed that way.

We were just arriving at different or smiliar things using different laws. It doesn't matter now.

Thanks for enlightening me on the XLR phantom power though. I'm sure the desingers of the electro acoustic have ignored the phantom power pin, so that it just runs off the battery. But yes, you were right in assuming I were thinking that there would be a resistor network to reduce the voltage to 9v.