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Modifying vintage pre amp Mc Curdy to have phantom power
Old 11th March 2009
  #1
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Red face Modifying vintage pre amp Mc Curdy to have phantom power

Hello everyone,

Im a student in audiovisual and I have a big project to do so I would need a little help here..First of all, I'm a french canadian so please give clear information because my english is good but not so good hehe. Thank you.

So I have a DA501 pre amp from Mc Curdy and I need to do some modification on it to have a Phantom Power 48v+. To do that I need to do some modification on the power supply to have a 48v+ output. We have 47v not regulated at the end of the power supply but we need 48v+ DC.

At some point,my teacher give us a hint by telling us we need a LM317T high voltage I think. Also he told us we need to know how much voltage is at the transfo to adjust the value of the resistor so we can get 48v+ DC.

I need to do some research but I'm actually very lost about this so if someone could help me out a little, it would be really appreciated. Maybe a explanation of what job needs to be done to have 48v+ DC and explanation of the concept.

If you can give me some hint also of where to search, it would be usefull because I need to gather information but I'm completly lost in that project.

Thank you
Old 11th March 2009
  #2
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kafka's Avatar
Well, I'm a nice guy, but I'm not nice enough to do your homework for you. So, I'll hint you the obvious part:

power transformer -> voltage doubler -> voltage divider -> voltage regulator

Now, there's one important detail I left out, without which this won't work straight up. And that, as they say, is left as an exercise to the reader ...
Old 11th March 2009
  #3
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Well Im not sure to understand clearly what your are saying.
And for the homework part well I have to do it whit the gear actually..We have to install a phantom power on preamp so even if you are nice to me...I will have to understand and do it myself hehe

So please help more if you can

Ty
Old 11th March 2009
  #4
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Martin Kantola's Avatar
 

Download the LM317 data sheet and and look at application hints and examples. Look at how much voltage the regulator needs to produce a clean 48V. Can you get enough voltage from the transformer? Do you need a voltage doubler? If so, google that.

Martin
Old 11th March 2009
  #5
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nosebleedaudio's Avatar
 

47V is good enough, dont forget the 1% 6.8K resistors, I match mine to more like .1%.
All regulators need a minimum 3V higher than the output...So for 48V you need 51-52V on the input..
A 7824 with a 24V zener between ground and the middle pin gives a clean 48V...
It should be WELL filtered and decoupled..
Old 11th March 2009
  #6
A Texas Instruments TL783 high voltage regulator is far more reliable, they operate up to 120 volts.

Jim Williams
Audio Upgrades
Old 11th March 2009
  #7
CDA
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CDA's Avatar
 

Quote:
Originally Posted by kafka View Post
Well, I'm a nice guy, but I'm not nice enough to do your homework for you. So, I'll hint you the obvious part:

power transformer -> voltage doubler -> voltage divider -> voltage regulator

Now, there's one important detail I left out, without which this won't work straight up. And that, as they say, is left as an exercise to the reader ...

ooooh oooooh ask me ask me!

It's the switch! heh

btw, 47 or 48 volts.. In this scenario. No difference really. It's not a matter of current capacity. The voltage is just there to provide potential.

By the time you put a regulated 48 volts into a long as hell mic cable, you're going to get less out the end anyways...
Old 11th March 2009
  #8
CDA
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CDA's Avatar
 

Quote:
Originally Posted by Jim Williams View Post
A Texas Instruments TL783 high voltage regulator is far more reliable, they operate up to 120 volts.

Jim Williams
Audio Upgrades
Hey thanks!

You just gave me an idea...
Old 11th March 2009
  #9
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Its nice to know about that!!! Thank!!

New info: I took the measur of the voltage at the exit of the transfo and it give me 46.5 V
So if I wanted a regulated 48v+ well I cant because to be regulated, we need more voltage in the input of the regulator that the output.

Somebody have a suggestion???????
Old 11th March 2009
  #10
Gear Maniac
 

Hello,

You mention that the "exit of the transformer" is 46.5v. If it's really at the transformer then you're measuring AC, and the rectified DC will be higher. If that's the case you can build a small rectifier/filter/regulator circuit right off the transformer that will give you close to 60v with no load, plenty to do regulated phantom with it's low draw.

If you actually mean 46.5v at the output of the rectifier/filter circuit, DC, then here are a few options -

ONE - Use the existing unreg 46.5v. This is easy but will give you less than great results because I'm willing to bet it's straight off the rectifier/filter circuit, meaning it has most but not all of the 120Hz ripple removed. This residual ripple will cause noise problems because of the amount of gain a preamp does. Yes it's common-mode in phantom power, but it will still affect hum levels. You can minimize that ripple with an RC network before the two 6.8k resistors but it will still be there. The fact that it's a lower voltage won't matter that much. If you really want you can reduce slightly the 6.8k resistors to give you more current at the slightly lower voltage.

TWO - Use a voltage regulator on the existing 46.5v. The advantage of this is you get huge ripple reduction. The disadvantage is a lower voltage. Most voltage regs have a 1.25v voltage drop, so to maintain adequate regulation as your unreg voltage varies, you may have to set the output for 40v or so to make sure you always have the regulator working. You can compensate for this by reducing the 6.8k resistors to 5.6k, very closely matched of course. Not an ideal solution but workable for most mics.

THREE - You already have heard this in a previous post - voltage doubler, then regulator. Do a google search on voltage doubler. This requires you to go upstream of your existing rectifier/filter circuit to the output of the power transformer, then build your own doubler/filter circuit. These are easy to do and worth it if you have a small current draw. A single channel of phantom is under 10ma with anything less than a short circuit.

FOUR - Dc/Dc converter. Not recommended for your level of expertise. Expensive and requires more electronic knowledge. Mentioned here for completeness.

Good luck
Hugh
Old 11th March 2009
  #11
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kafka's Avatar
Quote:
Originally Posted by Jim Williams View Post
A Texas Instruments TL783 high voltage regulator is far more reliable, they operate up to 120 volts.
That's what I would have gone for, but the instructor specified an LM317.
Old 12th March 2009
  #12
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Thanks you guys for your time and answers.. this will help a lot
Old 12th March 2009
  #13
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ulysses's Avatar
If you have to use an LM317, you should use the LM317HVT, which is the "high voltage" part. It just means it can take an in-out voltage differential a little higher than the 34V specified for the standard part. You still have to protect it against a shorted output though, since it might see 60V coming into it and a short means 0V coming out of it, which is going to be more than even the LM317HVT can handle.

I've got a circuit I use instead that provides reduced noise, is more bulletproof, and still does its job even when the input voltage is low. It uses one transistor, one zener diode, a resistor, and a few capacitors.
Old 12th March 2009
  #14
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wildpark's Avatar
 

why dont you buy a external 48 box cost you around 20 euro or 30 dollar and your ready
Old 12th March 2009
  #15
Quote:
Originally Posted by kafka View Post
That's what I would have gone for, but the instructor specified an LM317.
Looks like an opprotunity to "teach the teacher". I always enjoyed that myself, until they put me up in front of the class to do their job for them. Just be aware many in the profession are ego sensitive. They assume they are the final word on truth. Those open minded will give you extra credit, those that are little dictators will probably reduce your grade, especially if you do this in front of other students.

Jim Williams
Audio Upgrades
Old 13th March 2009
  #16
CDA
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CDA's Avatar
 

Quote:
Originally Posted by Jim Williams View Post
Looks like an opprotunity to "teach the teacher". I always enjoyed that myself, until they put me up in front of the class to do their job for them. Just be aware many in the profession are ego sensitive. They assume they are the final word on truth. Those open minded will give you extra credit, those that are little dictators will probably reduce your grade, especially if you do this in front of other students.

Jim Williams
Audio Upgrades
Isn't this the truth.

The way I see it is, I'm pretty good at what I do.

It's fun to think you're the best.

But then, right about the time you start doing that. You blow something up.
Old 17th March 2009
  #17
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Ok we have 96V from the output of the transfo

Here is the schema of the power supply. in is original form(1960)stike

So if somebody would be nice to explain how it would be possible to have phantom power at the exit of the power supply whit this schema...
TY
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Old 18th March 2009
  #18
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ulysses's Avatar
That 800-ohm resistor is wasting nearly 3W of power for no other reason than to pull the transformer's voltage down a bit. Really a silly approach, though I suppose in 1960 electricity was cheap and nobody had any concern for greenhouse gas emissions.

With the diodes, capacitor, and resistor in place, it says you're supposed to get 47VDC, though you can expect to see a whole lot of ripple on that. Remove the resistor, and the voltage will pop up - possibly higher than that 75V capacitor can handle. So did you remove the resistor and then measure 96V DC at the output of the rectifier?

I think it would be much better to have a higher-voltage capacitor, and then a regulator to reduce the voltage, rather than dumping 3W into a resistor just to pull the voltage down.

If you really want help from us that will be accurate and useful, you need to tell us very specifically what you're measuring and where. Where did you connect your voltmeter when you measured 96V? Were you measuring AC or DC? Were all of the components (as shown in the schematic) in place when you measured?

It's easy to install a regulator circuit of some sort at the end of a DC power supply, but only if you know exactly what the AC and DC voltages will be coming into the regulator. They can vary with a number of factors (input voltage and output load, primarily). So you need to know what the largest and smallest DC voltages are that your regulator will see, and also what the maximum possible leftover AC ripple might be that is superimposed on top of that DC voltage. You also need to know the minimum and maximum current draw of your load circuit. Once you know these things, at least to a first-order approximation, then we can help you determine the necessary regulator circuit to do the job. Start with some measurements and be specific.
Old 18th March 2009
  #19
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by Number Juan View Post
Ok we have 96V from the output of the transfo

Here is the schema of the power supply. in is original form(1960)stike

So if somebody would be nice to explain how it would be possible to have phantom power at the exit of the power supply whit this schema...
TY
96V on the AC scale of your VOM?
Its only using 1/2 the voltage, 96/2=48V or close.
Without the resistor the max will be around 67V NOT 75...

Its not using the whole 96V, only half, its a center tap trans...
The resistor is there for a reason, to keep a load on the cap and to keep it from max. voltage...
There are better ways to do this, BUT it depends on the rest of the supply circuit, is there a regulator circuit?
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