Krell KSA 200s ground hum

A common way of wiring the RCA jack on amplifiers is to connect the RCA ground to both the signal ground and the chassis. The chassis ground connection is isolated through a filter with low value cap. The problem then could be the RCA jack is touching the chassis (a worn insulator) or the coupling cap from RCA ground to chassis has shorted. Try gently moving the RCA cable at the amp input and see if the hum changes or goes away.

@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.