Preamp and Chip Amp from same power supply

[fingersoccer]

Mooly, I WOULD SHAKE YOUR HAND RIGHT NOW IF I COULD. I've seen schematics of zener regulated supplies, but they seem fairly project-specific and I don't know how to calculate what values I would need for the components.

[Mooly]

Here's the circuit.
The two zener diodes determine the rails so that's two 15 volt zeners for -/+15 volts.

The resistors are calculated as follows...

Supply =30 volts (substitute what yours actually is). We call that Vsupply
So each resistor has to drop Vsupply minus Vzener. So that's 30-15=15 volts. That's the voltage that appears across each resistor.

If we choose a low wattage zener, say a 0.5 watt then the maximum zener current allowed is I=W/V which is 0.5/15 giving 33 milliamps.

So the lowest value resistor we can use (allowing for the supply to be safe wuith no load) is R=V/I which is 15/0.033 giving 454 ohms. Play safe and choose next highest common value which is 470 ohms.

Resistor wattage is W=V squared/R which is (15*15)/470 giving 0.478 watts. A 0.6 watt metal film would be fine or a 1 watt carbon.

Your preamp uses about 10milliamps. Lets play safe and say it uses 20milliamps. That means that of those 33 milliamps flowing in each zener, whatever the preamp wants will come out of that. So if it did use 20ma that still leaves 13ma for the zener which is perfect. As long as there is a couple of milliamps for the zener the supply is regulated. If the preamp drew more than 33 milliamps then we would have to rescale things.

The caps are around 10 to 100uf 25 volts... not critical.

[fingersoccer]

Thanks, Mooly. I couldn't have asked for a more complete answer. Just the explanation I needed.

[Mooly]

You're welcome

[diarav]

Mooly's circuit should be good enough.

I am taking +15/-15V (for buffers/preamp) from +35/-35V (for chip power amp) using a portion of Rod Elliot's Power Supply for Preamps after passing through Pre-Regulator

[richard.C.]

To back up Mooly's idea, i have also implemented this zener circuit into my own project without fault. The idea of pre-amp, a few board members made suggestion i wouldn't need one for my chip amp, maybe true that, but i did anyway and found i was able to gain a few more cleaner db's out my project.

rick

[Mark.Clappers]

You could consider adding a RC or LC filter after the zeners, to suppress any noise still present and stabilize the supply a little bit more.