Krell KSA 200s ground hum

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?

CAUTION:Use only one input to the amplifier at a time. The KSA is shipped
with shorting pins in the XLR inputs. These pins should remain in the XLR
inputs if you are operating the KSA in the single-ended mode. When the
shorting pin is inserted, pin 1 and pin 3 are shorted together. The shorting
pins must be removed to connect the KSA for balanced operation.

.

@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.

@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.

.

@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.

.