michell gyrodec power supply
Michell GyroDec Power Supply
Built 1-2009
The stock Michell Power Supply
Above: The stock power supply that comes with the latest (DC motor) version of the GryoDec is a regulated, but low current device.
MPB's High Current Power Supply
Based on my research and experiments, it appears that a higher current version power supply would improve the performance of the turntable. I based the design primarily on the article entitled "Preamp Power Supply" by Gary Gallo in The Audio Amateur magazine, issue 4/90, p.47. In that article, he indicates several key changes from the datasheet recommendations to optimize simple three terminal regulated power supples for audio use, all of which I incorporated into this supply. They include:
1. Use of a much higher current transformer than traditional, he recommends 120VA.
2. Use of very large output capacitors (typically recommended to be much smaller than the input caps).
3. The divider resistor values around the regulator IC are several times the typical value of 100 - 200 ohms, dramatically reducing output impedance and ripple rejection.
4. The use of Linear Technology LT1085/LT1033 adjustable regulators instead of the common LM317/337s.
I used a PCB from "Digi01", available here:
http://assemblycraft.com/regulator.htm
Above: Views of the enstuffed PCB, from Digi01's webpage.
Below is the schematic I used.
Above: Populated PCB, before wiring it up (click to enlarge). I used two Panasonic FC 4700/25V filter caps, one before and one after the LT1085 regulator. Rectifiers are MUR860s. Note the 10 turn pot to fine tune the output voltage, which controls the speed of the 'table. Mine needed 9.430V to be dead on at 33 1/3 RPM. Note that my stock Michell supply was a bit off at 9.46 volts, resulting in a small (but correctable) speed inaccuracy.
Above: Top view, showing general layout. The regulator PCB is lower right, timer on the far right.
Above: Closeup of Avel-Lindbeg transformer, a dual 12 volt at 5A each, 120VA unit.
Above: Rear view of the power supply. I added a Curtis electronic timer (P/N 701FR0011248D2060A, from Digikey) to monitor the hours the power supply is on, as a surrogate for a stylus timer. I also added a separate 12volt regulator, fed by the rectified supply, in parallel with the 'table supply, to power a littl-lite that is mounted next to my 'table.
Above: Finished unit, front view. I used an old Tripplett 10VDC meter to monitor the voutput voltage. There's a power switch on the left, and a switch on the right to turn the lamp on and off. The faceplate is from FrontPanelExpress.com. Just remember to specify that you want the grain of the panel to go horizontally.