'Nuther very DUM power supply question

Ok,

So I want to rack up 8 channel strip preamps that draw 25ma from
the plus and minus rails. 200ma per rail total for 8 strips. This does not seem to be such a hefty power supply requirement. I need +/- 22vdc.
(each strip has local 16v regulators). This supply seems to be enough
but I am suspicious that I'm wrong.
http://sound.westhost.com/project44.htm

The 1970s mixer the strips they came from was powered by a MONSTER rack mount supply with big mofo trannys and HUGE can filter caps. Heat sinks run down each side of the outside of the power supply with 2 regulators per side. Weighs about 50lbs and is the size of an old power amp.

The mixer was a 22 channel and there were meters and things to power
besides. I just don't want to under spec my supply as this is the first
supply I would make for a multi module rack job.

Thanks in advance.

That 25mA per strip is the idle current, but it may draw considerably more at signal peaks. They may have designed conservatively.

Over and above that, though, the original console with 22 strips and other stuff was drawing a minimum of a bit over 12W from each leg of the supply, all the time; depending on how much voltage they were dropping across the regulator, how conservative they were being about signal peaks & low line voltage. and how much extra stuff was on the console besides the channel strips, I'd expect it to be a whopper supply needing whopper heat sinks.

So...figure out what your minimum unregulated line voltage is likely to be, figure out what your maximum current draw will be if all your strips peak at once (and how much your unregulated voltage will sag if that happens, even though it's pretty unlikely unless you record taiko drums), and set your nominal unreg level based on that (probably 1.1 x minimum level) and how much drop your regulator wants. Then add 10% to the unreg. voltage and figure out how much voltage is dropping across the reg. under those circumstances, multiply by total idle current, and you'll know how much power the reg will be dissipating. My guess is that it'll be quite a bit, and you'll need a good chunk of heatsink.

Peace,
Paul

They were just using the old, existing console supply.

But it's always nice to overspec your power supply. I suggest a 2x18v toroid with 35VA rating - this just under 1 amp each rail. The no-load raw DC voltage would be around 25v, plenty of margin there.

VA rating = total VA rating of the xformer
Two secondaries, divide VA by 2 (35VA/2 = 17.5VA)
18 volt secondaries: amps = VA/volts = 17.5/18 = 0,97
Smoothed DC voltage = AC * 1.414 = 18v * 1.414 = 25,4

..Or maybe a 2x20v toroid, depending on the mains condition of your location.

Thank you all very much.
I guess I should rehook my DVM and yell into the mic to get
more real world current measurments.

This is gonna be cool though.

Solder on,

:bit.

[quote author="Viitalahde"]Smoothed DC voltage = AC * 1.414 = 18v * 1.414 = 25,4[/quote]

Well, there's the diode-drop thing. For a full-wave bridge rectifier:

Smoothed DC Voltage = (AC * 1.414) - 2.5 = 25.4 - 2.5 = 22.9V

A little low if you want to regulate to 22V; although there are low-dropout regulators that can do it, they won't leave you any slack for low line voltage.

Peace,
Paul

[quote author="bitman"]
(each strip has local 16v regulators)[/quote]

i wouldnt regulate it at all. some rc filtering should do it.

Buy some "open frame" linear supply modules that have specs well-above anything you can imagine and be done with it.

Bri