I just got this fix in place and found that it solved my hum problem.  If you search this list for posts by mma, you will see my early history on this.

In short, I followed the advice of fellow lister Dank, and used heavy gage Romex to run separate grounds.  This helped but did not eliminate the hum.

Then I read a few threads here and elsewhere on the use of motor run capacitors and their virtues.  I also noted that several ZKit users were increasing the capacitance of the PS caps where the 33uf electrolytics are used.

I replaced the 33 uf and 20 uf caps in the PS section with the following: Genteq Capacitor , 20 mfd x 440 Oval and Genteq Capacitor , 80 mfd x 440 Round.  These are metallized polypropylene, oil filled.  Motor RUN, not START capacitors.

Note that I continued to use the same 20uf on part of this circuit, but I more than doubled (33 -> 80uf) the capacitance in the other section.

This has completely shut down the hum, and I can no longer hear it.  This is a very sensitive speaker, 105+db woofers and commercial compression drivers on tweeters- so if it's there, I hear it.

The biggest problem with making this change...well, they are BIG, and you have to monkey around to find a way to mount them in an acceptable fashion.  They are also a bit pricey.  But, they solved the problem, at least for me.

If you do this change, be SURE to carefully discharge the existing caps before attempting to remove them!

-M

Hi there, I have just rebuilt the zen kit1 inside a metal chassis and it sounds gorgeous with 10inch audio nirvanas, which are so very sensitive that I can hear a slight hum even though I increased the first cap after the 5u4g to 38uf (40 is max for this rectifier) and the second cap from 33 to 66 (going for 80uf tomorrow). I haven't measured voltages yet so I'm worried about the consequences of such a huge jump in capacitance (33 to 80uf). How safe is it? Doesn't that theoretically increase the voltage a lot more than a few volts? I have a hard time understanding this, please help if you can.