I have read your entire thread, and hope I can shed some light on the topic. We have spent the last 6 years immersed in the topic of transformer hum in amplifiers, and we have been actively designing our product to remove the causes of transformer hum.
First there are a few statements I can make categorically.
1. The PurePower AC output contains nothing that causes transformers to hum. It does not "induce" hum.
2. The posting from audio.bill that suggest there is a relationship between hum emanating from speakers reported by other users and the Ampzilla hum is incorrect. Hum from speakers is invariably caused by noise entering the audio signal path. Though often difficult to diagnose, it is alleviated by removing ground faults and ground loops.
3. If the Ampzilla 2000 transformers were removed and replaced with Plitron torroid audio power transformers the hum would disappear. This is not a criticism of the Ampzilla transformer design, which the designer considers a key feature of the amplifier, it is simply to say that the Ampzilla transformer design is susceptible to hum. Searching Google for Ampzilla + hum will illustrate the point. It is worth reading James Bongiorno’s comments in his note “singing transformers”.
The noise you are hearing is simple vibration of the transformer coils - a purely mechanical noise. Something is causing the transformer to act as a buzzer.
The most common culprit is DC offset, sometimes called DC in AC. It appears on a scope as an asymmetric sine wave. That is enough to cause the windings to respond by vibrating. Bongiorno and many others suggest a “humbuster” circuit to cure DC Offset and forever banish transformer hum.
Unfortunately removing DC offset is frequently ineffectual. We know because we once thought that ourselves. Yet when we redesigned the PurePower unit to completely remove DC offset we only cured about 50% of the transformer hum incidents. It turns out DC offset is not the only way to create transformer hum
The PurePower DC offset value is almost zero. Thus you can rule out simple DC offset as the problem.
In Roc’s situation the cause of the hum is still not accurately diagnosed. It can’t really be done with simple “home tests”.
The best way to do so is to test it in operation with an oscilloscope or power quality meter. The actual cause can usually be readily revealed. We can safely bet that we can rule out DC offset and harmonics. But that leaves lots of room for the rest.
For a discussion of many of the possible causes of transformer hum – see
http://www.purepoweraps.com/FAQs.htm#Q1Roc represents the first new case of transformer hum since we introduced the PurePower+ models. We have a vested interest in determining what is causing his Ampzilla hum. We hope he will be able to work with us to try and get a definitive diagnosis and a solution.
The fact he hears no hum on utility and it appears when he powers the amp with PurePower is a clue. The PurePower delivers more current than the wall to his amps, and that may be a factor. Utility power also posses a characteristic called “stiffness” that derives from the fact that it has megawatts of generation capacity behind it while the PurePower has 2700 watts. At this point we don’t have a definitive answer.
There is one solution that PurePower fervently wishes all amplifier manufactures would avail themselves of. Epoxy (or perhaps the wax potting preferred by Cary) is the answer.
When a transformer is correctly potted in epoxy the windings are unable to vibrate and the transformer remains silent even when the conditions for humming exist. I would recommend to all audiophiles that they check the specs of their favorite amplifier design and make certain that the power transformers are potted to prevent hum. If they were we would have no reason for this thread and Roc could be enjoying his Ampzilla performance with improved dynamics and reference grade AC power.
