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finished kit; question on noise (Read 1359 times)
mma
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finished kit; question on noise
04/16/12 at 13:48:20
 
Hello:

I just finished a ZKIT 1 build, doing this on a slab of oak.  I used the EDCOR transformers, and the recommended Russian tubes.  I also ran a wire from ground to the case of the board mounted volume control.  I am using the board directly; that is, the inputs, outputs and volume control are all on the board, per the circuit's original configuration.

After measuring voltages at points A-D, and ensuring all is in order (and it was), I went to a listening test.  I am using this amp to drive the upper end of a two-way horn system.  I found the noise levels to be rather high to my ear, and they seemed to vary in intensity and character with the volume setting on the amp.  I then used (2) 100 ohm resistors, one on each leg of the 6.3 heater supply, and tied the other ends of the resistors together at a ground point on the board.  This reduced the noise somewhat, and made it simply vary with volume, with the same character regardless the volume setting.  This was suggested by another poster, Kalonar.  The EDCOR transformer does not have a center tap on the heater side, and that is why I added the resistors.

The character of the noise is alot like the noise I remember hearing on an AM radio.

I tried lifting the power ground from the board, to see if I had a ground loop issue; no change in character or intensity.

Is there any other thing I can try to reduce this, or should I run the amp at a volume setting of say, 10 o'clock, and drive it harder on the signal input?

Or, do I begin testing tubes to see if it is a tube issue.  The noise is about the same, Left or Right, so I guess I would begin with the input tube...

Any thoughts, ideas, recommendations?

Thanks,

Mike
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dank
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pair of dual 18
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Re: finished kit; question on noise
Reply #1 - 04/17/12 at 00:09:31
 
mma

Read through the 3 pages of the post "Hum Problem" starting with my post dated 11-12-09 and read through at least to my second post on 11-26-09.  Should fix you right up.  here's a link:
http://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1257041114/0

DanK
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mma
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Re: finished kit; question on noise
Reply #2 - 04/17/12 at 13:12:39
 
Thanks for the tip.  I will re-read that series of posts.  I had _thought_ that that series was targeted at those who mount their volume pot and RCA inputs, outputs elsewhere on some sort of an enclosing box.  I did not realize that the thread was applicable to the board level kit, as built.

I will make the mod as you suggested, and see what I get...right now, I am not able to get enough output free of hum to really experience what this amp is supposed to be capable of.  

I have used the exact same connection scheme to this amp as I had been using for a T-amp (which is what I bought this ZKIT to replace).  That amp ran fine, no hum or noise (except at the very top of gain, but, I run closer to 11-12 o'clock on the gain, and it's dead dark there).  I am willing to accept a bit more hum as the price of tubes, but, I was not prepared for quite this much!

I think I may just short the RCA inputs and see what this produces in terms of hum, along the way, too.

Thanks!
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dank
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Re: finished kit; question on noise
Reply #3 - 04/17/12 at 13:32:09
 
mma

Be sure to use soild core wire (like Romax), not stranded (like lamp cord).  It makes a big difference.


DanK
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mma
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Re: finished kit; question on noise
Reply #4 - 04/19/12 at 13:24:58
 
Thanks once again.

I used 12 gage solid copper and made one run each from both RCA input jack lows, then I tied both of the filter caps ground sides together, which is where I terminated the RCA runs, and I also replaced the case ground on the volume control with a large gage run, all tied at the filter capacitor grounding point.  It looks somewhat similar to your photo in the noise/hum thread you linked me to.

This seemed to improve matters, but I still had the hum/noise.  Going further, I removed the inputs to the amp altogether, and guess what....the noise went away!  Hmmm.  I feel rather foolish now.  With the amp disconnected at the input side, the noise/hum level was really quiet, so much so that I could turn the volume control to max and then only get the most faintest of sound, and then only if I was directly in front of the horn.  Mind you, I am using 107db commercial compression drivers, so, if the signal's there, I hear it.  So, I think this amp is sufficiently quiet!  I won't know if the mods I did further improved the noise or not, as I am not inclined to remove the 12 gage runs I've installed.  I DO know that the addition of the resistors on the 6.3v supply lines made an immediate difference.  Regardless, the noise/hum looks to be very acceptable.

What I need to track down now, is, just where is this coming from?  Seems like it is direct from the preamp (Hafler), but, I don't hear it nearly as much as when I simply interchange the ZKIT with a T-Amp.  Anyway, this problem is not the ZKIT, but rather, some other grounding/supply problem.  

In the end, two changes were needed on this amp in my case: 1) adding (2) 100 ohm 2W resistors to the 6.3V heater supply lines, where the resistors are tied toegether at the other end and then tied to ground, and 2) making the changes that dank has elsewhere documented.

I wonder, dank, if the changes you had to make circa 2009 are still needed on the 2011 revision level of the circuit board?  The 2011 board is quite different in many respects to the earlier board (at least the one pictured in the assembly instructions document).

Either way, I am quite pleased with the noise performance at this point and consider the case closed.

Thanks for the help!

-M
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