Power ?

[anneflyr]

Hey

I am buliding a recording channel and I need both 2x24 volt and 2x15 volt ac.

One solution is to buy to input transformers, but I wonder......

Is it possible to buy one 2x24 volt transformer and then put in a small transformer after this one and step the 24 volts down to 15 volts?
I think this will give me less problems with noise, is this true

I have seen this solution on computer powersuplyes. What are these transformers called? Are tehy easy to find?

Thanks

Andreas

 

[SSLtech]

[quote author="anneflyr"]I have seen this solution on computer powersuplyes[/quote]
I doubt it, most computer PSUs are switch-mode.

How much current do you need? -If it's not a lot, just regulate the 15V rails down from the already-regulated 24V rails.

Cheap, easy, tidy.

Keith

 

[alk509]

I do believe he said AC, Keith...

Having said that, what DC voltages are you ultimately shooting for? What Keith suggested is how I would try to do it, with maybe a voltage dropping resistor in between the two regs, if current requirements are on the high side...

Peace,
Al.

 

[Svart]

it is not efficient to do. most PSU trafos are either designed to be pulsed ala switchmode PSUs or given a steady LINE input voltage like 120/240.

feeding the trafo something other than the designated LINE input voltage will give you an output but they are designed to be energized by the line voltage. anything else will be inefficient at best.

Computer powersupplies are ALL switchmode and they are special trafos built for this purpose. without getting into detail they won't work for you.

 

[SSLtech]

[quote author="alk509"]I do believe he said AC, Keith...[/quote]
Right you are... I should read closer!

Mind you, There's not a lot of AC power used in audio that isn't turned into DC eventually, so I still reckon my reply might be worthy of consideration.

Keef

 

[ioaudio]

psu for tore seem strip?

 

[anneflyr]

Thanks for comments

It is a DIY combo

Green pre
calrec eq
g1176 comp

The problem is that the G1176 that gets the 24 volts converts it into + 24 on one side and - 10 on the other

I need to use a 2x24 transformator for this.
The green pre and the calrec need 2x 15 volt. I have made peterc's psu for those two.

If I connect the 2x24 transformator directly on the 15 volt rectifier smoothning caps and regulators, the regulators burn out. I have tried to use two 220 ohm resistors after the transformator, but then the voltage gets very unstable.

Suggestions??

A

 

[alk509]

[quote author="anneflyr"]If I connect the 2x24 transformator directly on the 15 volt rectifier smoothning caps and regulators, the regulators burn out.[/quote]

Well, you're asking the regulator to dissipate a lot of power there... Keith's suggestion was to skip the rectifier and filter caps and feed the +/-15V reg directly from your +/-24VDC supply... Which would make good sense, except that now you told us your rails aren't symmetrical. So back to feeding the 15V regs from the 24-0-24 transformer: as you correctly figured out, you will need voltage dropping resistors...

[quote author="anneflyr"]I have tried to use two 220 ohm resistors after the transformator, but then the voltage gets very unstable.[/quote]

First of all, make sure you measure the regulated DC under load. Regulators don't like to regulate into an open (or almost open) circuit. If that doesn't help, keep reading...

Now, how did you arrive at that 220 Ohm value? My guess is you got a bad calculator there... :wink:

IIRC, the Green pre pulls about 20mA/rail at idle, plus all those LEDs when there's signal through it, plus whatever the Calrec draws... Let's say 80mA/rail as an estimate: .08A*220R=17.6V. Now:

24*1.4=33.6Vpeak at the transformer secondary.

33.6Vpeak-two diode drops=32.2Vpeak after the rectifier (assuming full-wave).

32.2-1Vp-p(ripple)=~31V after smoothing caps.

31V-~2V for the regulator to work right=29V.

And then,

29V-17.6V being dropped by the 220R resistor=11.4VDC!!!

Break out the resistors bin, and try some smaller resistors in there, making sure that they're big enough (in physical size, not resistance) to safely dissipate whatever they're dropping. And make sure your regulators are solidly attached to big heatsinks.

Good luck! :thumb:

Peace,
Al.

 

[anneflyr]

...Then I was on the right track.

I just newer was able to figure out how many amperes the pre and eq were pulling.

I connected the calrec to the psu (with the two 220ohm 2w resistors) I had about 8 volts dc in on the calrec. That’s not what we want, is it :roll:

That’s why I thought that the resistor solution was the wrong thing to do.

Thanks for setting my head straight. This tells me that the calrec pulls about 100 mA so plus the ~ 30 mA/rail on the Green Pre and the LEDs (estimating 2,5 mA on each LED, is this correct?) I get 130 mA

Lets say I drop it 12 volts to 17 volts. To get it down, but still high enough to keep a stable voltage when everything is pulling. Then I get

12/0,13= 92,3

The store I use have 82 or 100 ohm 2watt resistors. If I go for 82 I will drop 10,66 volts. Will this be fine?

And again thanks for this lecture!

Andreas

 

[anneflyr]

bump

 

[Sammas]

Doesn't the JLM Power Station do this with the addition of a 48v rail also? Perhaps have a look at Joe's Schematic for inspiration...


or you could just buy a JLM PSU and be done with it :wink:

:thumb:

 

[anneflyr]

Thanks for the tip, but no.

It gives +/-2 to 37 volts x2.

I need 15x2 and 24x2. So , this takes me just as far as I am.

I am going to try a 24 transformer and the 82 ohm resistors.

Will be back with report.

A

 

[JLM Audio]

The JLM power station will run the 3 DIY's you mention at once with one 2 x 24vac power transformer with ease and give 48v phantom power rail as well.

Start with a list of the dc voltage rails needed and the current each draws and work backwards.

 

[bevins]

Why not use a mosfet? Gate it for whatever you need and a zener to protect the gate.

Or just use a zener.
How much current are we talikg here?

Bob