Dealing with Hum in
Counterpoint Gear
 
 

If you hear a hum from the chassis, or from the transformer in your product, click here.

If the hum comes from the speaker(s), click here.

Transformer Hum.

Hum or buzzing noise which eminates from the chassis is usually caused by a noisy transformer. This can be confirmed by removing the top cover and listening to localize the sound source.

Transformer hum has nothing to do with grounding (which will cause hum from the speakers, not the chassis) nor can tubes affect it. It is rarely due to an electrical problem in the preamp or power amp, although a severe problems may cause the transformer to growl, and, usually, blow the mains fuse.

The "normal" hum/buzz is purely mechanical and is caused, quite simply, because the 50Hz 0r 60Hz Mains AC voltage is shaking the steel in the transformer. See "magnetostriction." A well-built transformer is generally solid-enough so that normal levels of magnetostriction will not cause an audible hum. But power line conditions can make ever the best-built transformer hum very loudly indeed.

DC on the power line can easily cause a transformer core to saturate and rattle and buzz. DC is rare, although a switching power supply used in a television or other product which draws current from only one side of the AC waveform can cause asymmetric compression of the waveform. Badly distorted AC waveforms will cause transformer hum. High levels of noise, caused by things like light dimmers or other waveform-switching devices on your power circuits can also result in transformer hums or buzzes.

I have a Revlon electric curling iron here that causes every transformer on the circuit to buzz. The thing must do something horrible to the AC sine wave. A customer turned me on to it because he noticed every time his wife fired hers up, his transformers buzzed.

Two transformers manufactured at the same time, by the same vendor, in the same factory, with the same materials, and to the same winding and build spec might have different degrees of buzz depending on how well they were vacuum-impregnated during manufacture. Impregnation is a step where a vacuum is used to pull resin varnish into all the nooks and crannies inside the transformer in order to force out all the air and "glue" the whole assembly into a rigid block. Incomplete impregnation can cause bits to rattle and buzz.

The bottom line is that if a transformer buzzes or hums all the time and no other transformers in the system do, it is a noisy transformer. If it buzzes sometimes, and no other transformers are buzzing at the same time, then it might be marginal and is responding to power line noise. If it buzzes badly sometimes, and all the other transformers do, too, then the AC mains are a mess.

There is nothing that can be done about a noisy transformer. The area with the air gap is deep within the steel and copper -- it can't be reached without destroying the unit. A marginal or bad transformer can only be replaced.

Speaker Hum: Lose the Ground Loop

All Counterpoint gear was manufactured with three-prong AC cords. This is for safety: if something were to fail in an electrical product potentially dangerous currents would go into the AC receptacle's ground, and not into a person touching the equipment. Few manufacturers took this step. Counterpoint did it not because Counterpoint gear was particularly prone to faults like this (in nearly 20 years of manufacturing, no piece of Counterpoint gear ever failed in such a way that could result in a "live" chassis) -- Counterpoint did it for customer safety.

The safety benefit of having even one component earthed extends to all the other components in the system because they are all connected toether by the interconnects. So if one component faults, the dangerous currents would travel through the interconnects to the earthed component, and drain safely away through its AC cord.

But if more than one component is connected to the Mains ground wire there's a risk of creating a system hum or buzz audible from the speakers. Earthing more than one component causes a ground loop -- a condition where there is more than route whereby the grounds of the components are connected together. Your components are already connected together through their interconnects. If you also connect them together through the third prong of their AC cords, you get a hum. It's usually not very loud, but it's quite audible, and an indication that ground currents are flowing where they needn't be.

The solution? No, not a line conditioner: they don't break ground loops and your power line isn't so dirty that the filters and power supplies in your gear can't handle it; and it doesn't matter how many dedicated lines you had your electrician bring in, or whether you have hospital-grade outlets in the wall! What you need is to have only one component three-wire connected to an AC receptacle (wall or line conditioner). All other components should be two-wire connected only.

To repeat: regardless of where you have your components plugged in -- wall outlets, plug strips, or power conditioners -- only one component should be three-prong connected. All other component cords must be only two-prong connected.

To do this, for testing purposes to see if this takes care of the problem, you can use ground adaptors ("cheaters") installed on their AC plugs. These useful


Use a grounding adaptor on every three-prong AC cord except for one.

little gadgets can be found at hardware stores and places like Radio Shack. Put them on every three-prong AC cord except for one. Let that one earthed component provide the system ground and lift all the other grounds. Don't connect the little green wire or green lug on the adaptors -- that'll defeat the purpose.

Once you've eliminated the ground loop using the cheaters, you'll have a safe and quiet system. But the cheaters are hardly a high-end solution. Some audiophiles deal with this by cutting the third prong off the plug so they can plug it straight into the outlet without needing a cheater. That's what I do.

(I think that the reason that other manufacturers didn't routinely install three-prong AC Mains plugs in the 80's and 90's was for their convenience: they didn't want to deal with customers calling to ask why they have a hum in their system. These days, nearly every good piece of gear has a detachable AC cord, and those are always of the three-prong variety. )

If You've Done All That But Still Have A Problem . . .

. . . do you have a TV cable hooked to your system?

Perhaps through a VCR? That's another, second, ground. That TV cable is also grounded, but it's not a real safety ground. You wouldn't want to count on it in the event of a power fault.

If you have cheated all but one of the AC plugs and you still have a hum, you can disconnect the TV cable and see if the hum goes away. If it does, then you'll need to isolate that cable ground. There are a few ways you can go about isolating the system from the cable TV ground:

  • Mondial makes a cable isolating device call the "M.A.G.I.C." - Modial Antenna Ground Isolating Circuit. It's available from the Audio Advisor at www.audioadvisor.com, catalog number MONMAGIC. This device also adds lightening power surge protection to your incoming cable. It is pretty expensive for what it is.
  • Or use the VR-1FF "Iso-Max" cable TV isolator by Jensen Transformer ( see http://www.jensen-transformers.com/vr1ff.html). Jensen Iso-Max
  • The electrically-clever can make their own by connecting two 75-ohm-to-300-ohm transformers back-to-back.

. . . or do you have your equipment in a metal rack?

If more than one component is in electrical contact with a metal rack, you are providing a second ground path between the components -- the rack itself. You need to insulate the components from the rack so that the only ground connections between components is through the interconnects.Professional musicians and recordists use racks a lot and they have to deal with this problem all the time.

You can check for this condition by using an ohmmeter to see if the chassis and the rack are electrically connected: If you remove your interconnects but still measure continuity (low resistance, under 10 ohms) between your components, your components are probably still connected through the metal rack. The connection is probably through the screws that are being used to fasten the unit's front panel to the rack. You must isolate the chassis from the rack.

One easy way to take care of this problem is to use "Humfrees Rack Isolation Tabs" from www.musiciansfriend.com.

As they say, "Kill those ground loop hums by using Humfrees to electrically isolate rack unit cabinets from each other and from the rack. Easily installed without removing units from the rack."


Alta Vista Audio Home


 

Here's another article about ground loops and hum:

Click here