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Zkit 1 Hum fixed (Read 330 times)
mma
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Posts: 8
Zkit 1 Hum fixed
08/04/13 at 13:32:32
 
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
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schebb
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Posts: 10
Re: Zkit 1 Hum fixed
Reply #1 - 09/15/13 at 22:02:37
 
Folks,

I have had the same experience as mma. Investigating the annoying hum that could be heard from the listening position I found that if a real time analyzer was used with a mike in front of the speaker, the hum was at 120 Hz, not 60 Hz as you would probably find ground loop hum. The 120 Hz hum would have to be associated with the full wave center-tapped power supply. I added a couple of motor run capacitors in parallel with the existing 33 mfd capacitors. I added a 40 mfd in parallel with the first cap in the CRC filter and an 80 mfd cap in parallel with the second cap in the filter. This only increased my voltages from 418 to 423 and from 345 to 351, perfectly acceptable. Slight hum can only be heard with ear right up against driver.
Now running two amps in bridged mode which creates a whole other set of ground loop issues, but by themselves the amps are very quiet and sound wonderful.  

Steve
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