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10/25/14 at 20:07:55


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Zkit 1 Hum fixed (Read 857 times)
mma
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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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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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Raduschka
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Re: Zkit 1 Hum fixed
Reply #2 - 06/06/14 at 05:07:51
 
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.

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schebb
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Re: Zkit 1 Hum fixed
Reply #3 - 06/06/14 at 16:47:49
 
Raduschka,

As you can see from my post above, adding large capacitances to the CRC filter of the ZKit had little effect on the voltages of my amp but a huge effect on reducing hum. I added a 40 uF and an 80 uF for total capacitances of 73 uF and 113 uF and the voltages only went up 5 to 6 volts, well within spec. It should be noted that the original caps left very little ripple voltage on the supply. Adding more capacitance, even huge amounts, cannot cause the voltage to climb beyond the peak of the source less the series voltage drops.
Having said that, my amp is using a solid state rectifier and you have tube rectification. That can and will cause differences. Tubes generally drop more of the supply voltage leaving reduced voltages supplied to the amp compared to solid state rectifiers. One can also create issues with excessive inrush current with large capacitors which may exceed the current rating of solid state rectifiers. This is less of a problem for tube rectification I believe.
Before adding any more capacitance, measure your voltages across the first two capacitors in the CRC filter circuit (very carefully) and compare to the given specs of the amp. Make changes in steps and monitor voltages and general operation as you make changes.

Steve
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