Weirdly Specific HUM issue

I have a pair of KEF LS50 Meta's driven by (2) Schiit Audio Lidar amps configured as monoblocks, coming through a Schiit Freya+ preamp. I use an Ompower EM1100 conditioner and Audioquest shielded power and interconnect cables.

I get a slight but noticeable hum in the Left speaker only (farthest from the preamp), but ONLY under these conditions:

  1. Plugged into (powered on) power amp only (no source connection) - NO HUM
  2. Preamp connected (balanced XLR) and powered ON - HUM
  3. Preamp connected (balanced XLR) and powered OFF - HUM persists
  4. Hum only stops when I physically disconnect the input from the preamp (same with both preamp output channels)

Thoughts? Why would the hum persist even with the preamp powered down?

Thanks in advance.

Ag insider logo xs@2xgwilmot

The problem is not due to the fact equipment powered on, rather it is due to the fact it is plugged into the wall outlet, or in your case, the power conditioner.   The problem arises due to the parasitic capacitance causing leakage current from line to chassis.  This leakage current travels to some other component via interconnects that has an earth ground - the third prong on power cords - and that current causes a voltage drop in the interconnect which is considered a "Signal" and amplified.  The leakage current can be quite high, 3/4 mA for two prong units and as much as 5 mA for three prong units yet, still meet UL codes.   If you are wondering why some interconnect and power cord cables sound differently than others, this is one reason.

That leakage current can also leak from the transformer primary windings in the units power supply to the chassis, too; so just because nothing has failed, or you don't have a blown fuse, doesn't mean you don't have a problem.

First, make sure ALL the equipment is plugged into the power conditioner.  Absolutely, no power plugs anywhere else.   

Second, remove any Ethernet and / or CATV cables from your system.  If you later determine these are the cause of your hum, you can get galvanic isolators for them which should cure the problem.

Third, make sure the power conditioner is plugged into a correctly wired outlet.  That is, it has an earth ground that actually goes to earth and a neutral that is tied to the neutral side of the outlet, not the line side.   You will need to contact a qualified electrician to verify that neutral and earth wires are terminated and connected properly.   Make sure they can run that test before you hire them, they will need to test it, not just check it visually.

You can try using an extension cord to power the conditioner to try another outlet and see if that makes a difference.   I suggest you try a few outlets in other rooms, just to get a multiple data points and confirm the same thing happens on all the outlets.

If you are certain that all of the above is correct, then remove the power conditioner and plug everything into one power strip.  Not two power strips!

If you still have a hum problem, then go read this paper by Bill Whitlock.  It is has a very good test procedure it in to determine which component is causing the hum problem.  You may have a problem with the transformer in one of your units.  The little test connectors can be found at DigiKey.   Do understand everyone has this problem to some degree.  The fact is, any hum at all, no matter how tiny, is degrading the clarity and definition of your system, so it pays to dig it out.