how to regulate an unregulated 9vdc wallwart without too much of a voltage drop?

[iandy4]

Hi guys,

I'm building a circuit which only has a few mpf102 jfet transistors drawing current.  I have been using a 12Vac wallwart going through a 7809 regulator to get it to 9Vdc. I want to power my circuit using the same 9Vdc (center negative) supply as all my other pedals so I can daisy chain mine with them.  I thought I could get away with using an lm317 regulator set to output 7.8 volts (far enough below the lm317's forward voltage drop in combination with the 9Vdc wallwart actually only putting out 8.8 Vdc).  Here is the schematic I used (added a 470uf cap to ground on the input before the .1 uf cap shown only because I always see it in power supply schematics):

http://www.lm317-circuits.com/images/lm317-circuit.png

The problem is, I get a low oscillation sound and some high pitched hum which is definitely coming from the power supply.  ( I checked it against my well used 12Vac to 9Vdc power supply I usually use as a reference.  If anybody could point me in the direction of how to regulate an unregulated 9vdc wallwart without too much of a voltage drop I would really appreciate it!

[G. Hoffman]

What is your power supply?  Because, most of the comercial PS that you can daisy chain together are already regulated, at least to some degree.  If you are using, say, a OneSpot, that is QUITE well regulated.  What I mean is, do you really need to regulate it?


Gabriel

[iandy4]

The one I have is unregulated.  I'd also like a solution that accounts for unregulated 9Vdc supplies.

[PRR]

Forward drop of LM317 is over 1.5V. You really need to be over 11V raw to have a hope of 9V regulated.

If the LM317 is on the edge of dropout it can be pretty unstable.

> the same 9Vdc (center negative) supply as all my other pedals

The circuit runs on 9V? You have 9V? What's the problem? I'm not understanding.

[iandy4]

I see that now.  I'll stick to the 12 Vac supply for now.

[iandy4]

Anybody have links for how to double a dc voltage? Ive seen a few examppe based on diodes and caps and others using transistors. Ill be searching in the meantime

[PRR]

> how to double a dc voltage?

Convert to AC.

Double.

Convert to DC.

[gritz]

Hi, there are modern regulator chips with a much lower dropout voltage available - many of them are surface mount though so be prepared to do a bit of searching. There are also pcb mounted DC to DC step up converters available. I probably wouldn't be too happy using them in a single ended circuit like your jfet app though (especially if it's an overdrive / distortion) because although these coverters work at typically 30 - 50kHz any ripple that gets onto your circuit's supply rail may mix with your audio signal and produce audible sidebands. It's possible though that a small dropper resistor and low esr electrolytic etc. on that power rail will quieten it down nicely. Mebbe convert 9V ->12V then use a linear regulator to produce quiet 9V.
Is your jfet circuit particularly sensitive to supply voltage?

[DavenPaget]

Hi, there are modern regulator chips with a much lower dropout voltage available - many of them are surface mount though so be prepared to do a bit of searching. There are also pcb mounted DC to DC step up converters available. I probably wouldn't be too happy using them in a single ended circuit like your jfet app though (especially if it's an overdrive / distortion) because although these coverters work at typically 30 - 50kHz any ripple that gets onto your circuit's supply rail may mix with your audio signal and produce audible sidebands. It's possible though that a small dropper resistor and low esr electrolytic etc. on that power rail will quieten it down nicely. Mebbe convert 9V ->12V then use a linear regulator to produce quiet 9V.
Is your jfet circuit particularly sensitive to supply voltage?

When you boost a 1044 , it gives you 60kHz .
It doesn't matter if you use a LDO or not , any regulator near or at dropout is useless .
And his wallwart already puts out 8.8v .

[gritz]

...And his wallwart already puts out 8.8v .

Aha, thanks, I missed that. Looks like a new psu might be the most pragmatic approach as the old unregulated one seems to be running flat out.

[Tubebass]

You do have a rectifier and filter between your 12vac wallwart and the regulator chip, right?