DIY Factory

The Soft Start Phantom Project
A project brought to you by Charlie.
Updated 3 November 2004

rev 1.0

General Description

This was a circuit designed by my good friend and mentor Jeri McCord. It came about as a result of as yet unfinished project of racking up a couple of Ward Beck Mic Pre modules. I wanted to add phantom power and wanted to use a soft starting circuit instead of the regular circuits available for slightly more "professional" operation (no POPs!). This is actually a portion of a power supply design for a tube mic preamp to come.

The schematic is sorta divided into two sections, the "regulator", and a switch that soft starts. The regulator actually holds the phantom voltage at a relative constant, the switch section ramps the voltage up or down depending on the switch position. The coolest difference between this and the LM317 type app circuits I've seen, is that the ramp in this circuit is not dependent on the load!

The LAB has a related thread :
http://www.prodigy-pro.com/forum/viewtopic.php?t=3079

Some basics for the Newbs
At the far left of the schem, we have your basic full wave bridge that converts an AC voltage to DC. This is smoothed out by C1. The regulator (a zener shunt style with pass transistor) is formed by: Q1, D7, D8, R1, and C2. This topology was chosen for its (arguably) lower noise figure over a 3-terminal regulator. If you prefer, you could substitute a 3-terminal regulator here instead. Also, see Mods section below. The combination of D7 and D8 can be any combination of zener diodes that will give a 48V output. For the prototype, I used 3 zeners. Be careful using too many small zeners; lower voltage zeners can have leakage problems. R1 can be adjusted if necessary to give greater regulation for higher current draw. The proto was tested with this value and a load of 10mA with no problems. D5 is a reverse protection diode to prevent damage to Q1 during power up/down.

The Ramp !
Q2 and Q3 form an isolating switch to turn the regulated voltage on at the load. Q3 is the actual current carrying device, while Q2 turns it on and off slowly to create the ramp function. Ramp speed is set by C3 and R3 (or R4) for power up (or power down). C3 = 10uF gives a ramp time of about 6 seconds. Reducing this value to 1uF drops this to less than 1 second. You can adjust to your liking...as stated earlier, the ramp time is completely independent of the load. R2 is just a bleeder resistor to drop the voltage from the mic.

Mods !
Happy, Happy DIY! If this isn't cool enough for you here are some options to customize the circuit for your application! Multiple outputs? No problem! Just add more of the "Switch" sections at the output of the regulator. More than two outputs and you should probably upgrade the Q1 to a TIP41C or equivalent. This means you'll probably need to adjust R1 and increase C1 as well. How about an indicator for "Phantom ON"? Just replace R2 with an LED and a 6.2k/0.5W resistor (you might need a different value here if you use a high brightness or blue LED!!)

Building the Circuit
I built the prototype on one of those little PCBs from Radio Shack with the copper rings all over it. The layout is not critical at all and nothing here should need a heatsink!

Parts List
Q1, Q2, Q3 - MPSA06 or equiv (TO-92)
D1-D6 - 1N4004 or equiv
C1 - 1000uF/100V
C2 - 47uF/100V
C3 - 10uF/100V
R1 - 510/0.25W
R2 - 47k/0.25W
R3, R4 - 100k/0.25W
S1 - any SPDT

When choosing a transformer for this project, keep in mind that about 10mA is the generally accepted maximum for most condensor mics. Check your mics to see!

Downloads

just what you already see

Pics

none yet

ERRATA

So far so good.  Check back for any updates in this area.

Contact :
Cofox (at) cowboy (dot) net
or post a question in the LAB

Peace !
Charlie

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