Help I think my AC is polluted

After months of trouble shooting, I've come to the conclusion that my AC is the culprit of a very annoying hum. The thing that's odd is it only occurs every couple weeks, and only for two or three days at a time. The other thing that's strange is it happens on the weekend and ends usually by Monday night.

Let me give a few details about my system.
- I recently installed two dedicated 20 amp lines. This did not change the intermittent hum.
- I've tried cheater plugs during the hum and they had no effect.
- I'm in the process of auditioning two power conditioners. The Equi=tech Son of Q Jr. and the Furutech e-PT609. Niether of them has had any effect on the hum when it's happening.

I've put my ear right next to each component during the hum and found the Equi=tech transformer humming. I turned off all other components and the Equi=tech continued to hum, even in standby. When the hum is gone, the Equi=tech is dead quite. The hum doesn't change whether the powercords are plugged into the wall or either power conditioner.

I'm going to give the power company a call but I don't expect much help from them. I may try putting in an isolated ground and see if that helps. According to Equi=tech, balanced power should take care of most all ground loop issues, so I don't think an isolated ground will help much with this problem.

Has anyone else ever experienced this? Any ideas on a solution?

Can you put a system on battery when this occurs? The disconnection from AC would confirm that the problem is AC-borne and not induced.

Does this phenomenon occur regardless of what source(s) you are listening to?

I'm wondering if this is a situation where an actual power regenerator (as opposed to a power conditioner) would be warranted. I'm thinking of the ExactPower EP-15A.
Do you get hum when it happens with the Equi=Tech plugged in but nothing running through it and not when it is not plugged in?

If you can, put a scope on the line both when you have the hum and not. I would expect what you see would be different and that your problem is outside your home.
I have had similar experiences that occur when line voltage is low. I have 2 Arcam amplifiers and came home one day after dark around Christmas when all the houses were lit up with Christmas lights. Both of the amplifiers transformers were humming loudly. Scared the heck out of me. Unplugged the equipment and hooked it back up the next day. No hum in the transformers, everything fine. Later in the day when all the Christmas lights started coming on, back came the transformer hum.

Talked to the local dealer who sold me the amps and they had experienced the low line voltage problem when they first opened. After some research they had isolated the low line voltage and were able to make the local power company adjust the transformer feeding their shop. In our area (Cincinnati) the power company has responsibility to maintain a minimum working line voltage.

In my neighborhood the power transformer that feeds our house blew up shortly after that, and I never got to confirm the low voltage with meter readings or to request the power company check the voltage. I haven't experienced the transformer hum problem again with the replacement by the power company.

Not sure it is the same issue you are having, but just something I had experience with.

Jim S.
since you have the system on dedicated circuts shut off all the other breakers in the house one at time (when the hum is active and with your system on) see if you can locate the hum to something in the house.
imho i think the problem is coming from outside the house
Thanks for all the quick responses. I'll try to answer all your questions in order.

Kr4 - Presently I don't really have a way to run the system on battery power. If I go in that direction, is there a product on the market that can power my entire sytem?

Nsgarch - Yes. It occurs whether using the CD player or tuner as the source. A power regenerator may be the answer, although I'd rather go to the source of the problem. I've also read that a power regenerator may inhibit current flow and dynamics depending on the system. What did you find with the ExactPower.

Tbg - When I have the hum, nothing I do can change it. What makes me pretty sure it's dirty AC from outside the house is yesterday I disconected everything from the Equi=tech and still had the hum when I put my ear up to the conditioner while powered up and on standby. Today it's gone and the Equi=tech is dead quite. I'm not quite sure how to scope a line. Can you give me more info on that?

Stilljd - You may be right about low voltage. I've been living here for six years and we used to get quite a bit of power surging. I would often see the lights brighten and dim at night. I haven't noticed this recently though. I will complain to the power company and see where it gets me. The odd thing is the hum happens on the weekend and that's when the power grid should be cleanest. Unfortunately, it's also when I have most free time to listen to music.

Hotpanley - I will try your suggestion but given the two week intervals between this problem, I'll be surprised if it's something within the house.

I appreciate all your responses to help solve this problem.

Happy listening, Mike

All transformers will hum to some extent, since they are being energized with AC. Audio tranformers are designed for 50/60 cycle operation and are designed and mounted to be as quiet as possible. One would expect those with iron cores mounted on a heavy chassis to be the quietest, as opposed to those without iron cores mounted on a light chassis.

The transformer/chassis combination is subject to resonance just like any material. If the external power is contaminated with large amounts of repetitive noise, one would expect the transformer noise to increase, especially if it gets near a resonant point. Also, sources of external AC noise can be very intermittant, such as air conditioners, industrial pumping, maintenance cleansing etc., which can explain your symptoms.

A really good solution to this is an isolation transformer on the AC mains ahead of the fuzes to the dedicated lines (e.g., a 4-5kva Topaz). The iso has the advantage of reducing common and transverse mode noise by up to 120db, eliminating any DC on the line, and blocking large voltage spikes. Some iso's also have additional taps to raise or lower the output voltage, if that is a problem in your area. Consider this approach as well as the power conditioners you are looking at.
Zargon - Thanks for the informative post. What is the cost of an isolation transformer such as the Topaz? How much work is it for an electrician to install? I had one hell of a time getting an electrician to do two dedicated lines. Would this be a better/more cost effective solution than a power regenerator?
I recently bought 2 - 4kva Topaz iso transformers at an industrial surplus supply for under $300 each. You can find them on the internet and ebay as well. It's best to get one locally, since they weigh in around 75-80 lbs. Topaz was a major supplier in the 60s-80s and many were installed during that period. These transformers are built like tanks and buying used should not be a problem.

You would need to pick a size that would accomodate your systems peak load plus a safety factor of 50-100% to avoid saturation of the core and distortion of the wave form.

The iso goes between the main panel supply and your dedicated lines. In effect, it creates a new source. If your dedicated lines are installed in the main panel, you would want your electrician to add a second small new panel to house those breakers. Installation is straightforward (hot, common and ground on both sides), however, he would need to ensure the proper wire sizes to code, and ensure the polarity of the lines are maintained across the iso (this is usually marked on the case).

You might have to search a bit for an electrician who has worked with iso's or is comfortable taking it on. Try a larger contractor that works on industrial sites or in hospitals or laboratories where iso's are common. Also, there are a number of us around that have done this and could provide additional assistance. Try looking in the archives as well.

Mootsdude posts;

The thing that's odd is it only occurs every couple weeks, and only for two or three days at a time. The other thing that's strange is it happens on the weekend and ends usually by Monday night.

Is your house fed by a utility pole transformer?
If so go outside and see if you can count the number of houses that are fed from it. Sometimes 4 to 6 houses
Next thing to check is your neighbors that are connected to the same transformer. Any weekend body or machine shops. Any Arc Welding on weekends?

Next time it happens check the voltage at the power Amplifier's receptacle outlet with the Amp on and a source playing.

Take a walk outside and listen for any abnormal noises coming from any neighbors garage.

I've put my ear right next to each component during the hum and found the Equi=tech transformer humming. I turned off all other components and the Equi=tech continued to hum, even in standby. When the hum is gone, the Equi=tech is dead quite.

Read the specs on the Equi=tech, bet it has a toroidal iso transformer in it. This louder hum than normal is caused by DC offset voltages on the utility AC line. Do a search on Google for more information, what causes a toroidal transformer to hum loudly.
I recently installed an iso xfmr for my Arcam Alpha 9 CDP. The Arcam has a toroidal power xfmr and at times the toroidal transformer was mechanically noisey. Connected to the iso it is quiet.

You also mentioned in your post that the lights in your home would get bright and dim. Brighter than normal? Do light bulbs burn out quicker than normal?
"Dirty AC" will not cause transformers to hum, but direct current in the AC line will (transformers are AC devices). It sounds like you're getting excessive DC in your house on those days. You can complain to the power company or you can buy DC blockers like the ones sold by PS Audio, Xantech, etc.
Fascinating to hear that undervoltage can lead to hum. That conforms to my experience: our AC line hums most in winter, when heating is on and power draw would be highest.

Humming toroids eh? My loudest hummer is a Chinese integrated with an 800VA toroid in it.

My BAT amp's 1000 KVA toroid transformer hums loudly when my girlfriend uses her hair dryer. Scared the heck out of me when it first happened; I thought that something had gone bad in the amp. But it was low-voltage. I just turn the amp off now before she turns the dryer on.
It appears that an iso transformer may be the answer to this problem. From what I've read, the iso transformer will take care of external AC noise, voltage fluctuations, and DC finding its way into the house. I'm first going to call the power company and see if they can do anything. Does anyone know if there is a maximum allowable voltage swing that the power company must maintain within established limits for residential applications? If the problem is DC finding its way in, can I hold the power company responsible for rectifying the problem on the assummption that its a safety hazard? I'm guessing this is a somewhat unique problem given that it occurs intermittently.

Thanks for all the helpful responses.
Jgreene, wow your electrical service is third world. Is it an old house or an old apartment?
Jmho excessive mechanical noise of a toroidal transformer is not due to low voltage. It is due to DC offset voltage on the AC power mains.

Here is a link for the industry standards for power companies. The nominal AC power voltage they should maintain to the customer shoud be within plus or minus 5%. Usually a + or - 10% range for equipment.

Thanks for the discussion and information. It points out a misunderstanding I had about causes of transformers humming - not low voltage - but DC offset voltage (see stilljd above, I started the low voltage stuff). Now I have to understand what DC offset voltage is.

Also explains why my Alpha 9's CDP transformer hummed occasionally at home but not at work.

Appreciate the information and I didn't mean to post erroneous info, just hoping to learn more myself. My apologies.

Best Regards,
Jim S.
I seem to have a similar problem. Is there an easy way to measure DC offset on the AC line? I have access to a scope, but no skill in using one.
Also I discovered when I added dimmers to the dining room light that the electrician had not run a ground to the switches (single pole). I get hum even when the dimmer is off.
Any advice?