Posted by Wayne Parham [ 66.139.43.222 ] on March 28, 2005 at 23:42:35:
In Reply to: Stoetkit mods posted by Wayne Parham on March 15, 2005 at 17:54:03:
After finding the choke didn't do much, I decided not to use it. I looked for some alternative power supply capacitors, but didn't really find much that would provide any improvements. The voltage across the caps is about 230VDC, so I'm not comfortable with using caps rated less than 350V. That makes the capacitors required physically large. I could mount capacitors externally, but that doesn't appeal to me for the same reason having an offboard choke wasn't attractive. It's a kludge that doesn't offer much in the way of noise reduction. So I elected not to do anything to improve 120Hz ripple. The Stoetkit does pretty well in this regard anyway.
The place where noise reduction is significant is in the 8kHz switching artifact. There are two easy mods that really reduce this down to dead solid black quiet. First, solder a 0.022uF capacitor across each diode. Second, remove the 150 ohm resistor R21 between earth ground and signal ground and tie them together with no resistance instead. These two things reduce the 8kHz switching artifact significantly. In fact, it goes away completely. The switching pulse becomes so small you can't even see it with a scope. Naturally, the tiny buzzing sound disappears too. After this mod, you won't even hear it with your head in a compression horn tweeter.
I added a 0.1uF capacitor across each of the power supply electrolytics too, C6, C7 and C8. These didn't make a noticeable improvement, but I have a ton of mylar 0.1uF caps and it was easy to mount them on the etch side of the PCB so I did it just 'cause. Can't hurt to put mylars across electrolytics.
I highly recommend adding the 0.022uF caps on the diodes and shorting the 150 ohm resistor R21 between earth and signal ground. Those two things will take the Stoetkit from being very quiet to being dead black quiet. It's certainly worth doing, and the cost of this mod is next to nothing.
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