35v to 18v

[purpf]

Hi,
What's the best way to drop the voltage of a PSU from 35v to 18v? Im thinking about a shunt voltage stabilizer.
What's your opinion?

 

[mjf]

hello.
do you want to use the psu with a preamp.......or something else?
current consumption?
greetings..........

 

[acid_k2]

Other questions:
Do you need stable voltage? intermittent or continuos load? low ripple? low noise? low heat dissipation? small space available? other specification?

 

[sawreyrw]

There is no way to give you a good answer without knowing your requirements: power level, regulation, noise and ripple, overload protection, efficiency, etc. Nonetheless, a current mode buck regulator would be a good choice.

 

[Conrad Hoffman]

Going from 35V to 18V is a 17V drop. If you want to draw even a measly 200mA, you'll dissipate 3.4W. Maybe that's ok, but the suggestion of a switching buck regulator is a very good one if you need any significant amount of current. There's a reason you'll almost never find a shunt regulator in commercial equipment (non-audio) other than as a voltage reference- they're a bad choice in almost any imaginable application.

 

[jackinnj]

There's a reason you'll almost never find a shunt regulator in commercial equipment (non-audio) other than as a voltage reference- they're a bad choice in almost any imaginable application.

Except to save yourself warm in upstate NY winters.

 

[purpf]

I'm thinking of using a 2x24v toroidal core transformer with 2 PS units. 1 PSU to power an LM3875 chip amp and the other to power a preamp. Unless I can find a preamp powered by ~35v, I need to lover the voltage to about 18v.

 

[purpf]

I need stable DC current, the simplest solution is always the best

 

[Conrad Hoffman]

Though I don't have schematics handy, I think there have been a variety of discrete preamps over the years that ran on 35 and higher voltish single ended supplies. The claimed advantage was headroom on transients.

 

[wwenze]

Hmm? I know of op-amps that can handle +-17V. You can get a rail splitter to split the +35V to +-17V

 

[star882]

If the current draw is low, just a pair of common linear voltage regulators (7815 and 7915 will give you +-15v) should work.

 

[pacificblue]

A regulator in a TO-220 case can dissipate up to 20 W at 25 °C. With the right heatsink that should be able to feed common DIY preamps even at 18 V voltage differential.

The LM3875 works fine without active preamp by the way.

 

[Juergen Knoop]

grummel 7815 grummel 35V grummel grummel!
regards

 

[pacificblue]

Is "grummel" the correct English translation of the same word in German?

And yes, the 7815 would not be my choice either. I wonder about National's AN-1192 in that respect, where they even use the TO-92 version to step down 42 V to 15 V.

 

[Juergen Knoop]

Is "grummel" the correct English translation of the same word in German?

don't know...frankly speaking, I was to lazy to search for a translation.
But it should be quite self-explanatory.

And yes, it is bad practice to feed that poor reg with 42V and should be avoided in educational publications even if the part survives.
regards

 

[AndrewT]

get rid of the active pre-amp in front of the chipamp.
The high input impedance of the non-inverting input does not need a buffer nor an amplifier to condition the signal.

The source may need a buffer to drive the cables that follow it or after the volume control.

 

[acid_k2]

a good solution for too high voltage input:
dirty cheap 22V zener + medium power transistor, followed by 7815 (or your favorite low-noise stabilizer)

 

[ChrisA]

I'm thinking of using a 2x24v toroidal core transformer with 2 PS units. 1 PSU to power an LM3875 chip amp and the other to power a preamp. Unless I can find a preamp powered by ~35v, I need to lover the voltage to about 18v.

The normal solution is a a third winding on the transformer, two secondaries.

I 've got the same problem with an amp I want to build. The power transformers I have is 481V and I need 320V. Dropping 160V with a resistor is not going to work. I'll just have to buy another transformer and save my big 481V transformers for another project.

 

[acid_k2]

OT (@ ChrisA)
320Vdc is about 1500W rms @ 8 ohm. Are you sure?

 

[pacificblue]

OT (@ ChrisA)
320Vdc is about 1500W rms @ 8 ohm. Are you sure?

Tube amp!