Phantom power drop resistor

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Spencerleehorton

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Hi all,

Just wanted to know people thoughts on this schematic and what is the missing resistor or any other changes to make this useable and noise free, I’ve got a few Neve 1290 preamps I’m wanting the 48v to be nice and clean and solid.
 

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That resistor was probably added to protect the LM317 from over voltage on the input. Generally, I don't like that circuit. I would use a proper transformer with center tap, full-bridge rectifier and a regulator that is for higher voltage like TL783.

Actually I wouldn't use linear at all. I would use an SMPS with common mode choke. SMPS would be better, simpler, smaller, more reliable, easier and most important, probably quieter.
 
Spencerleehorton said:
Hi all,

Just wanted to know people thoughts on this schematic and what is the missing resistor or any other changes to make this useable and noise free, I’ve got a few Neve 1290 preamps I’m wanting the 48v to be nice and clean and solid.
There is no sensible explanation for this resistor, except a very bad palliative for choosing the wrong transformer.
LM317 is a bad choice for phantom because the input-to-output voltage should not exceed 40V. There are tricks for that, but they are prone to fault.
LM783 is the way to go for DIY. SMPS is much more difficult, paricularly if you've never done it before.
Power Integrations has a range of DIY-friendly smps controllers, but still it takes some experimentation with transformers.
One can build a 783-based PSU with 100% confidence it will work as expected. This is not the case with an smps.
 
I've used one of these https://uk.farnell.com/xp-power/ecl05us48-t/psu-5w-open-frame-48vdc-0-21a/dp/1765282
in a Neve project recently. Worked very well!
 
Spencerleehorton said:
You wrote:

I would use an SMPS with common mode choke. SMPS would be better, simpler, smaller, more reliable, easier and most important, probably quieter.

Could you explain by a schematic?
Here:

smps48v.png


APC-16-350 is $7 USD on mouser and pretty small. There's also a euro version (APC-16-350E I think). However, this is actually way bigger than you need. If you don't load it properly it will throttle switching and generate a tiny 4dB noise bump at ~1.5kHz. So you'll need to add a 5W 1K resistor in parallel with the filter cap. If you can find a smaller one, you might not need such a high wattage resistor. But I have used APC-16-350 for phantom and more and I know it works very well. It's very quiet.

Also I didn't draw the caps after the 100R of the 48V RC. But presumably those bits already exist in your amps.
 
abbey road d enfer said:
LM317 is a bad choice for phantom because the input-to-output voltage should not exceed 40V. There are tricks for that, but they are prone to fault.
Provided you are sending around 60V in to the LM317T, why would the differential exceed 40V in a phantom supply?  Are you thinking about faults to ground?
 
My standard LM317T circuit wraps a few 1W zener's across the regulator (ala. the Gyraf G9 supply) and I've never had any problems with regulators zapping.

But it is increased parts count/complexity.
 
Matador said:
My standard LM317T circuit wraps a few 1W zener's across the regulator (ala. the Gyraf G9 supply) and I've never had any problems with regulators zapping.

But it is increased parts count/complexity.
I understand it works for non-critical applications. I have never taken the risk since the correct part exists.
 
Spencerleehorton said:
Here is one on evil bay,

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F392470292492

Do I still need anything after this one?
This is a constant-current PSU, for LED strings, totally inadequate for phantom power.
 
Spencerleehorton said:
Here is one on evil bay,

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F392470292492

Do I still need anything after this one?
Yes, constant-current PSU is absolutely 100% perfect for phantom power. Constant-current means that after the load reaches the rated current (350mA in this case), the voltage will start to drop and the current becomes constant. This is not a problem and in fact I have a feeling it actually makes for a quieter supply. Sound Skulptor uses CC led supplies (or at least they used to in their PS2 supply but now they only do 500 series stuff). I have used that exact supply twice and it works very well. The Mean Well supplies are known to work very well.
 
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