Krell KSA 200s ground hum

@ieales said:

Verify that XLR pin 1 is connected to the line earth. It may have a low value resistance like 10Ω if there is a loop break.

As for the Krell amp, yes pin 1 is grounded to the chassis through a resistor. Seems to be only a 2 ohm resistor though. Not sure what a 2 ohm resistor would do. It’s not unusual to see a 10 ohm resistor used in the circuit.

You can look at the schematic wiring diagram here: You will need to log in to open the PDF.

https://www.hifiengine.com/manual_library/krell/ksa-200s.shtml

Check out the mains AC wiring diagram. I have never seen a resistor placed in series with a mains neutral conductor before. Also checkout the soft start circuitry.

Earth ground vs. chassis ground. Uck. 

Back in the 70s Yamaha made a revolutionary pro live sound mixer called the PM-1000. Very clever audio design and totally modular so any of the 16, 24, or 32 input channel cards could be serviced separately. Cool. But, earth ground and chassis ground were separate, and ground from the mics brought in on Pin 1 through the modules then to earth. A floating ground. Not only was this hum-prone, if a grounded mic case ever contacted a hot AC source (it happens) it would blow every ground trace off every module on its way to earth ground. After having to same-day air freight ($$$$) an entire 32-ch mixer in a 3' X 6' road case yhalf way across the country to save a sold out 20,000 seat show, lesson learned. I was one a a small handful of techs in the country who was allowed to go in and modify the PM-1000s by building a proper ground bus on the inputs before anything could get to the modules. Looks like Krell may have made a similar design choice/mistake. That grounding scheme only works when everything else also has a floating ground. And that's a big assumption. If an upstream connection earth-grounds Pin 1, you have an instant ground loop, and a differential impedance which means voltage, which means hum.

I REALLY appreciate the ideas! You guys are kinda talking over my head right now. Not sure what was said in the last few posts that I can physically apply or check for a resolution. I need something like :

take your meter, put it here..

look for this…. 

if this is this then it’s probably this… lol

Cool, I had that Amp, the hum was unreal, it would hum Loud when it was off on standby and kick up in the morning when the Mr. Coffee came on. It blew fuses and Tannoy drivers with abandon. Go to eBay and find those amp fuses now. Kicked ass tho.

@strawj said:

I had that Amp, the hum was unreal, it would hum Loud when it was off on standby and kick up in the morning when the Mr. Coffee came on.

If I understand your post correctly the loud hum you heard from the Krell Amp was a mechanical hum from the large Toroidal power transformer in the amp. Large Toroidal power transformers are very susceptible to DC offset on the AC power mains. It doesn’t take much DC on the Line to cause the Toroid transformer to mechanically vibrate loudly either. Big boys like Pass Labs, Bryston, and others manufacturers install DC Blocker circuitry in their power amps that use toroid power transformers... Krell does not. At least they never used to...

Easy fix, buy an outboard DC blocker and plug the Krell amp into it. That should block the DC from getting on the primary winding of the toroid transformer in the amp. Well maybe it will block the DC IF the DC blocker is sized for the VA rating of the toroid power transformer in the amp.

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