quick question, for a beginner ;)

[tragikremix]

hi guys,

let me start by saying i admire you all here, with your awesome knowledge.

i want to build a mic preamp, but that's not the problem.
i've been researching plans for a power supply for the +48v phantom power, using the search here.

i've come across a few plans,

my question for you's is how do i know how much amperage i need the PSU to push out? the have plans for 10mA, 50mA and 100mA, but i can't be sure as to which i would need, to my knowledge.

i did notice a small footnote on the mic pre schemo that said 'quiescent current', so i googled it, and it said it was the amperage running through the circuit with nothing else, just it's standing as is, turned on power.

is that quiescent current the number i would base my PSU on?

one had said 25mA, which would i go with in that case, the 10 or the 50, or just look for another one?

or am i completely wrong

thanks for your help guys, i'm here to learn from some of the best.

 

[Guest]

I would say 5 mA is enough for one microphone.

 

[pstamler]

Assume a +/- 10% tolerance on your 48V supply. At the upper edge of its range, that would be 52.8V. Now, 52.8V running through two 6.8k resistors in parallel to ground is about 15.5mA -- that's the absolute maximum the phantom circuit would be called on to supply. So if you budget 15.5mA for each microphone input you can't go wrong.

Peace,
Paul

 

[ruffrecords]

Well it is usually fed through a pair of 6k8 resistors, So on a single channel, even if you shorted the phantom supply to ground it would take less than 15mA.

Ian

 

[tragikremix]

ok, so we are saying that 15mA would be fine?

http://web.telia.com/~u31617586/#48%20volt%20phantom%20power%20supply

i wouldn't go with any of these then, if i just want to power one single pre amp.

i guess the part that makes you guy so helpful is your ability to understand how different valued components affects the product as a whole.

anyone with a good link for me to start at?

that's all i really want out of this project. i don't care too much about the mic pre, but i want to learn these things, so i can put them to use later when i build an 8 channel replica neve console, eq, comp and all, with added MMC and total recall

tell me that wouldn't be one hell of a project!

ok, so back to learning... going to prowl the search funtion and use some google!

 

[tragikremix]

another question;

i would assume that too much current from the PSU would be harmful, correct?

i know too little wouldnt necessarily be harmful, but it would operate correctly.

is phantom power one of those things where i can deviate, where it is not such a big deal and these typical assumptions do not apply?

 

[Guest]

from the man himself (Bo) - I aksed this same type of question myself when I came to groupdiy about three years ago..

- he is talking about the 100mA PSU on that web page

"around 15 to 25 VA per winding is a more normal choice for this size of phantom PSU (RS 201-7454 or 201-7527 or 201-7606)

(this PSU can delivery more than 100 mA, it is becomes on the transformer, rectifier, the first capacitor (470 uF) and heat zink for the regulator)

You can run a lot of condenser microphones on 100 mA, a normal microphone to day draw aprox. 3 mA (33 microphones= 100 mA) and a Neuman U87 old types draws 0,5 mA and 1 mA for the new Ai types, so you can use 100 of these.

Put a small heat zink on the regulator if you shall use the PSU near 100 mA output current. "

 

[pstamler]

No, there won't be any harm in designing a power supply that will do more than 15.5mA per channel. The specifications on supplies are for the maximum they will put out when asked, but you won't be asking for more than 15.5mA. So even if the supply is capable of 1A, you won't actually see 1A coming out of it unless you hang something onto the +48V bus that draws 1A. Which you won't unless you accidentally short it to chassis with a screwdriver.

If you're doing a single channel I'd say design for max current of 30mA. That leaves you a 2:1 ratio against actual maximum draw, which is nice and safe.

In practice, a TL783 will regulate your phantom voltage nicely, if you can find one that's not surface mount.

Peace,
Paul

 

[tragikremix]

thanks for the replies.

sorry to keep asking questions but i keep thinking and developing ideas!

one more for ya's.

is the phantom power supply also the power supply for the whole circuit? or is that a completely different component in the system?

i'm coming along here..

so here's what i am thinking,

PSU changes 120VAC home electricity into 48VDC- (through a transformer, rectifier, smoothing and regulator, source :http://www.kpsec.freeuk.com/powersup.htm)

from there, the 48VDC is now the power for the mic pre amp, and somewhere along the circuit, it also powers the microphone (phantom power)

therefore, a mic pre amp consists of two peices, the PSU and the pre itself. there isnt like a 2nd psu that is differnt for the circuit correct?

that being said, the mA output of the PSU is not a big deal, as long as it is enough. 50 mA output of a PSU won't kill the entire circuit, but will power the circuit and the microphone, and everything will live happily ever after. being that i'm looking to be just one channel for now, technically any PSU will deliver what i need.

will that PSU schematic work for my needs, with any old mic pre schematic, or does someone have something else they'd recommend?

thank you all so much for sticking with me.

 

[bcarso]

[quote author="tragikremix"]another question;

i would assume that too much current from the PSU would be harmful, correct?

i know too little wouldnt necessarily be harmful, but it would operate correctly.

is phantom power one of those things where i can deviate, where it is not such a big deal and these typical assumptions do not apply?[/quote]

EDIT: posted before seeing recent contributions of Stamler et al---but for the sake of redundancy and belabored points:

You need to do some reading on basic electricity. There are some good posts in this forum and other places. But in particular:

Make sure you distinguish between the current that a given load (microphone in this case) will pull when connected to your power supply, and the current capacity of that supply.

When we talk about power supplies, most of the time we specify an output voltage. When we add the specification on output current, that refers to what the supply could provide if the loading were such as to require it, not some current that the supply "wants" to pump into the load.

If we put a low enough resistance across this supply, so as to ask for more current than the supply can provide, typically the voltage starts to drop. It can no longer regulate to maintain the output voltage.

Think of a circuit in your house. It may be rated to supply 20 amps, but it's not trying to push that current into anything you happen to plug in to the outlet (and a good thing for it!).

We could (but rarely do, in the context of power supplies) specify a supply that attempts to maintain a given current in a load, regardless of the voltage across that load. Then we would say it was a constant current supply with a specified voltage compliance. If that voltage compliance was, say, 100 volts, then a load that required more than 100V for the current to flow would be one for which the supply couldn't regulate. But again, this is an unusual supply---the vast majority of power supplies attempt to provide a more-or-less constant voltage, within a range of load currents.

Getting back to the pros and cons of how much current for phantom, there's nothing wrong in principle with having a huge maximum current capacity; but if you happen to short-circuit such a supply you could do some damage to things, and as well the supply gets big and bulky and expensive.

 

[tragikremix]

thanks.

thats what i needed, the correlation to a house rated at 20A. the dots connect now!

so now it's time to research and build?

20 hours on the search function will do wonders.

the current is there, but it doesnt have to be used

ahh now i'm slapping my forehead and say duhh! :roll:

no more questions i think! but feel free to add anything you guys might want to mention. i've learned more in the past 12 hours than i have the last 6 months in school..

 

[Guest]

Have a look at the threads about multiple output PSUs (one of my fello Englishman drew a circuit based on a mixing desk - have a look at Rob Flinns ideas)
Basically - if you are going down the big PSU equals everything you could look at sending unregulated DC down into the modules - where he module regulates it's own supply
There is a mixer project somehwere on the web with a PSU exact what you are after - soundwest as a host - some aussie guy - rings a bell)

EDIT - here it is
http://sound.westhost.com/project30c.htm

I came here with the same idea as you three years ago - and have settled for a more modest outcome

 

[Guest]

[quote author="tragikremix"]
PSU changes 120VAC home electricity into 48VDC- (through a transformer, rectifier, smoothing and regulator, source :http://www.kpsec.freeuk.com/powersup.htm)

from there, the 48VDC is now the power for the mic pre amp, and somewhere along the circuit, it also powers the microphone (phantom power)
.[/quote]
Close - but most preamps will survive on +15, +18 or +24v depending on the design
SO you will find that the PSU will supply a small current 48V for the phantom, the "hefty" +15 etc... on separate lines (ever wondered what all those pins on a PSu that connects to a mixer are for - different voltages)
Then there are the "negative" voltages to consider (they aren't really negative - just relative is how I think about it)

Also check the meta
http://www.groupdiy.com/index.php?topic=2665

 

[tragikremix]

perfect, guys.

so the PSU is just there. it does it's thing, and can have multiple output voltages.

48v goes to phantom.

then there are other output voltages on it, (much like a computer PSU, every component gets a different voltage), which would be somewhere on the schematic, as for instance +15v as a line drawn into the circuit, -15v somewhere else along the circuit. and then there could be something else mixed in as well, ive seen that before.

and that's how i would know what PSU i need, based on what the voltage requirements of the pre, i would determine what and how to make it. some pre's take +,- 15, or 17, or 24 etc. and that is the art of building a PSU eh? determining exactly what you need and how to get it there.

awesome.

 

[Guest]

you got it
then there is the current requirement of each module
each module will have a number of active components thatrequire voltages to work - like a transistor or an amp chip (help me out guys if I have got this wrong)
Each of these consumes current - add up the current requirments and there you have your mA for each module
I think simply - because I am just a newbie here..
Some modules will hav a lot of active components - some only a few...
Have a look around the metas and you will see somewhere mentioned the current requirements of each project and the std transformer VA rating for each
Now you start to get into problems - (or opportunities - blah) because some DC regulators only work up to a specific current...
Enjoy

 

[tragikremix]

i just checked out that link.

does a PSU have one transformer that can split into multiple voltages, or do you use multiple transformers that each make one voltage?

and i get it, the pre amp or whatever cirtcuit itself requires different voltages, but then that is usually taken care of within the circuitry of the pre amp, and doesnt affect the psu. (correct?)

 

[Guest]

Tragicremix, if some mic wants more than 5 mA it means something is wrong with him, don't giv him more!

 

[Guest]

re: the transformer - you can have one huge hefty thing with multiple output windings (I once saw Cyril Jones tap his own output line of a transformer - scared the life out of me when he said it was scrap and did iI want it ... cheers mate but errr.. can I pass on that)
Or you can have a big hefty one for the +24/-24 (as Roger pointed out above) and maybe a smaller one for the phantom and maybe another for the relays if you plan to use true bypass switches - somebody sells boards for these around here (for the relay true bypass)

 

[tragikremix]

ok, so i checked out some stuff, particularly the green pre setup.

http://1176neve.tripod.com/id10.html

i could replace the 'gain rotary switch' with a pre made pot no? what value would that be?

and the preamp schemo is here
http://1176neve.tripod.com/id11.html

where would i find out what voltages the pre needs? or does it not matter? the PSU gives you an alternative either 15v or 18v, which would would i use? or is either one fine?

 

[Guest]

Going to have to point you in the direction of the metas and search on that one I am afraid..
http://www.groupdiy.com/index.php?topic=2665
Peter (the owner of that site) is a member here and sells boards