pre amp that is powered by phantom power

[dan slv-tech]

does anyone know if there is such a thing as a pre amp that is powered via phantom power from a desk?

 

Basically what im after is a pre amp (DI box size) that can sit on a stage, has a input/ link out, gain pot, signal/clip LED, Balanced output all powered from the 48V dc phantom power from a desk.

 

Ive tried active DI's, but theres no variable gain (generally +20dB or pad) doesnt really satisfy the need.

 

Im finding that muso's are turning up with acoustic guitars etc but no backline amp for it. Yes you can DI the guitar, send down the core and put oodles of gain on it at the desk end, but the S/N ratio is awfull.

No I appreciate the pre amps on the GL4000 are not the best in the world but a half decent balanced signal would be nice hence the need for the above box.

 

Anyone out there no of anything that hits the spot or is close?

 

cheers

 

Dan :)

[BlueShift]

The difficulty with finding something like that is that many of those little mic preamps supply their own phantom power to an input and so run off of little wall warts. Active DI's on the other hand dont supply their own phantom power and so can run off of desk phantom power, but dont feature proper level controls.

 

Looks like a conundrum to me. Good luck

[Andrew C]

If no one makes one, perhaps this is the niche market for me to exploit!! I do wonder whether P48 would struggle to provide the current needed. Time for some investigation.

[dan slv-tech]

The difficulty with finding something like that is that many of those little mic preamps supply their own phantom power to an input and so run off of little wall warts. Active DI's on the other hand dont supply their own phantom power and so can run off of desk phantom power, but dont feature proper level controls.

 

Looks like a conundrum to me. Good luck

<{POST_SNAPBACK}>

 

phantom power on larger desks will supply 200ma, thats a lot of when you think an active DI consumes 10ma or less.....

I found one box by a company called hi-tech but cant find there web address, so maybe its not around anymore?

[david.elsbury]

But is that 200mA per channel, or 200mA over the whole desk?

Some condenser mics will suck a lot of power ( the big ones IIRC) and that means that many smaller boards will not keep up.

 

I once wished that I could make a radio mic reciever run off phantom, no more nasty wallwarts- but then I found out the amount of power it (doesn't) supply.

David

[dominicgross]

are we baring in mind here: that all Pre's outputs are at line level: NOT mic level?

 

Therefore: sending Phantom down a line input might cause issues.

 

If your muso's have decent guitars, they should have decent pre's built into the body: I know that all yamaha's, martins and almost every semi-accoustic and electro-accoustic guitars have gain control on the guitar.

 

If you are getting a bad s/n ratio then this sudgessuggestsyour guitarists aren't outputting a hot enough signal. It doesn't matter where your amplifiing the signal: S**t in S**t out!

[J Pearce]

Try a DI box with a higher input impedance, I never have any trouble with the MTR DI2s we use at church, and I borrow them for all sorts. Not a signal they can't cope with.

Although as stated above they will only output what you put in in terms of quality.

[Solstace]

Some guitars I've come across use ceramic or piezo pickups, which sound very heavy on the treble and tend to be very quiet. I found the only *quality* answer last time I was presented with this conundrum was to tell the guitarist - get your own preamp, or get a better pickup, or get out!

 

Okay, bit extreme perhaps, but I can't see why so many bands lack their own gear and expect us technicians to have enough time and kit lying around to get it all working in a hurry.

[dominicgross]

as far as I'm conserned: our job is to amplify the musicians: if they dont have the basic tools to enable them to play a gig, then they shouldn't be playing a gig.

 

I've done too many gigs when a guitarist has turned up needing a lead, a non working effects pedal, severe deafness ect...

 

I think the best one is when a bassist turned up the other day without an amp:

'I thought we could DI it' he says

'Whats your point?' I say

'Well my amps heavy, and if your going to be plumming me into the PA anyway?'

'Do you like to be in control?'

'Yes'

'Tuff. NEXT'

[dan slv-tech]

The desk is Allen & heath GL4000 thats 200ma across the desk, and from what I can make out from the circuit diagram each channel can supply a fair amount of that (bear in mind only using 10 or less of the 40 channels for pahntom supply (mics,DI)

I have found glorified DI's with switchable EQ but nothing with variable gain structure.

A company called 'Hi Tech' did do one, tho I cannot seem to find any information about the company and or the DI that they made which did have a variable gain and signal LED.

 

SO its back to the forum......

 

 

 

Dan

[bruce]

Is there a standard for the current available from a phantom supply? Or is it just an arbitrary decision by the manufacturer? 200mA on (theoretically) one channel seems rather high - 200mA@48V is about 10 watts, which could do serious damage in a short-circuit situation.

 

Most of the phantom sources I've come across seem to have resistors in circuit to limit the short-circuit current to around 14mA.

 

Bruce.

[j_b]

The difficulty with finding something like

phantom power on larger desks will supply 200ma, thats a lot of when you think an active DI consumes 10ma or less.....

How do you figure that?

 

at 48V, in order to be able to supply 200mA down a pair of wires... (hot and cold, earth return)

 

it becomes 100mA per wire, which at 48V (source, i.e. no load) means a source resistance plus load resistance of 480 ohms. For maximum current, this would be direct to earth, (giving 0V in the load) but it would mean 4.8W dissipation in the desk, and thus 2 x 5W resistors (with heatsinks) per channel, and a serious power supply for a many channel desk!

 

I havn't yet seen a desk with such a high current sourcing capability, and I would expect most microphones to struggle to get a decent signal into such a low impedance.

 

Typically, most desks that I've seen source through a 6k8 resistors, giving a maximum current supply of about 14mA per channel into a short sircuit, or about 10mA at 14V on the load- plenty to run a mic preamp or condensor, and just about enough to light up a DI box too.

[dan slv-tech]

Had another look at the circuit diagram there is indeed 6k8's in place over the mic line so this would allow around 14ma as per previous posting.

If any one has a mo go to

http://www.concerttrade.com/scan/fi=ct_ite...ory/rf=%2A.html

and you will see the sort of thing im after.

I feel like im asking for the world and whilst telling a guitarist to bring his amp or he wont be heard, we are all professionals and have to deal with whats thrown at us.( ive had bass players turn up with no amp....oh the look of suprise! )

 

Dan

[bruce]

I feel like im asking for the world and whilst telling a guitarist to bring his amp or he wont be heard, we are all professionals and have to deal with whats thrown at us.( ive had bass players turn up with no amp....oh the look of suprise! )

 

If they turned up without their guitar, would you provide that as well?

 

Just playing Devil's Avocado,

 

Bruce.

[dan slv-tech]

I think id have to draw the line there, suppose they want me to play it aswell........