Using LM317 for high voltage supplies

[grapefruit]

In the LM317 datasheet it says "Supplies of several hundred volts can be regulated as long as the maximum input to output voltage is not exceeded. I'm thinking this could be put to good use for a supply for a valve preamp or distortion, allowing the use of smaller value, cheaper, and pysically smaller capacitors.

I'll build a circuit with the usual resistor - Cap filter supply first and measure the ripple. As long as the ripple is way lower than Vin-Vout max of the reg, and the supply does not vary too much maybe this could be an option.

Has anyone tried this?

Cheers,
Stew.

[brett]

Hi
I've done it in the sense of adding zeners or other regulators to the COM pin so that the voltage is high.  For example, adding a 24V zener to the middle leg of an LM7812 to get 36V.
But I wonder about the need for regulation.  For valve pre-amp stages the voltage usually is non-critical (ie +/-10% makes negligible difference).  An in-line resistor and a moderate-sized cap usually do a good job.  Even for transistor power amps, it is unusual to see them regulated. 
For a valve pre-amp you might see a 1k 1W resistor and a 47uF cap doing this job.  The rule of thumb that I use is that with a decent resistor (ie soaking about 5% of the supply), about 2200uF per amp is good.  So a pre-amp pulling 10mA only needs about 22uF to regulate it with a decent resistor.  Extra resistance always helps, but you'll lose voltage and generate heat which can become a problem. My limited understanding is that the ripple should ideally be about 1 percent or less (unlike the power section where 5 is ok).
cheers

[Paul Perry (Frostwave)]

As long as the ripple is way lower than Vin-Vout max of the reg, and the supply does not vary too much maybe this could be an option.

And as long as the Vin-Ground max is not exceeded either - even under no load (that is, before the heater warms up & current flows).

[MKB]

I've used the LM317 this way in quite a few circuits, my current stereo preamp has a LM317 regulating about 200V.  It works very well, but with the higher voltages you have to take further precautions.  The suggested application circuit also has an input transistor configured to shut down the input of the LM317 when the I/O difference goes above a selected voltage.  You also need to be very careful of powerup and power down conditions to keep the LM317 from going boom.  The NS manuals have application data that shows this configuration.

As brett suggested, regulation may not be that useful in guitar effects and amps.  In fact, many tube preamp designs make great use of sloppy supplies; much of the sound of a tweed Fender amp is the underfiltered preamp, and both Soldano and Rivera have an underfiltered tube stage in their distortion channels.  Underfiltering can make a huge difference.

If you want to use a regulator, here's a great high voltage part, good up to 450V: http://www.supertex.com/pdf/datasheets/LR8.pdf