mma
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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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