Krell KSA 200s ground hum

@jea48 

“Are you using the OEM power cord that came with the Krell Amp?

Is there a ground loop hum with only the preamp connected to the Krell amp? (Nothing connected to any of the inputs on the preamp. No other type of ground wire(s) connected to the preamp either.

If yes there is hum with only the preamp connected to the amp, (nothing connected to any preamp inputs. No external ground wires connected to the preamp... For a test install a ground cheater on the power cord of the Krell amp only. Hum should be gone... Yes?

Just curious are the shorting jumpers, pins, installed on the XLR inputs?”

Not using the power cord that came with the Krell. It did not come with it unfortunately. Using an older large gage probably 1 grand cable from its time. Can’t recall the brand offhand and it’s worked flawlessly with other amps. 
 

shorting pins were not connected, again I did not get them. I did buy some XLR cables to go from dac to preamp and from preamp to amp and same thing. 
 

Preamp with nothing connected to it causes same issue. 
 

thanks

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.

In recording studios we used twin screen, connected RCA pin to +ve, and -ve & screen to the shell at the source. At the amp XLR, +ve to +ve and -ev ONLY to -ev. Screen unconnected at amp.

If that does not work, there is an issue with the source earth.

If you are going to use cheaters plugs, ground the amp and lift everything else.

@mofojo said:

Preamp with nothing connected to it causes same issue.

Did you install a ground cheater on the krell power cord like I asked? If yes, still a loud hum? Ground cheater on the Krell amp only. Nothing connected to preamp inputs...

I assume the preamp and front end equipment using the other amp you were using before worked fine. No hum?... No ground loop 60Hz hum?

I asked about the power cord because I would bet the AC wiring polarity is correct on an OEM power cord. That’s not always the case with an aftermarket power cord. The only way to know the cord is wired correctly is to check it. If wired wrong the mistake is usually at the female IEC connector. The Hot and Neutral will be reversed.

The Krell KSA 200s has an usual AC mains wiring configuration. Not sure what would happen if the polarity, Hot and Neutral, were reversed. The soft start circuit is different as well. I tried to see if there was a component on the AC mains side of the amp before the primary winding of the power transformer that may be leaking a small amount of current to the chassis.

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@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.