The Power FAQs
Straightforward answers with no spin!
Q: What causes transformer hum?
There is nothing more annoying than having perfected a high end audio installation only to have the world’s most perfect sweet sounds forced to compete with a low hum emanating from inside the amplifier cases. The only more annoying thing is to install a brand new PurePower+ AC regenerator and get a tantalizing taste of the wonderful dynamics and soundstage improvements that come with pure regenerated AC and have the pleasure overridden by amplifier transformer hum.
Believe me, we sympathize.
The humming sound you are hearing is simple vibration of the transformer coils - a purely mechanical noise. Something is causing the transformer to act as a buzzer.
Hum may appear mysterious and counter intuitive. I must admit it took us months and years to track down some of the multiple causes - but when all is said and done it comes down to figuring out what electrical characteristics can cause a transformer winding to vibrate. Transformer hum is a mechanical vibration brought on by an asymmetrical condition that can be electrical or physical.
The most common culprit is DC offset, sometimes called “DC in the AC”. It appears on a scope as an asymmetric sine wave. For instance the voltage sine wave may vary from +121 to -119 instead of being +120 over -120. That is enough to cause the windings to respond to the offset by vibrating. That is why many experts suggest a DC blocking circuit to cure DC offset and forever banish transformer hum.
Unfortunately, removing DC offset is often ineffectual. We know because we thought it would work ourselves. Yet when we redesigned the original PurePower unit to remove DC offset we only cured transformer hum in about 50% of the cases. Powering an amplifier from a PurePower+ regenerator will rule out DC offset as a cause of hum.
The obvious conclusion was that DC offset is not the only way to create transformer hum
Thus we were forced to hunt down other causes of hum. The second candidate was excessive harmonics in the AC sine wave. 5% and over can cause hum and overheating. That also is cured by a PurePower regenerator because the PurePower dramatically reduces utility harmonic levels. That led to a continual search for more possible causes.
We quickly discovered poor transformer construction can cause hum. Although we expect audio power supply transformer quality to be high, faulty construction nevertheless needs to be considered. We have actually heard humming audio transformers in which the number of turns in the windings was causing the asymmetry. We also tested one customer’s humming torroids and found the bolts attaching the transformer pots to the case were loose.
Ground noise is yet another candidate to affect transformer hum. Here the PurePower does not offer a cure. PurePower units generate a brand new AC source, but do not renew the grounding system or address gound problems within the system. Ground faults can thus be a cause of hum. The cure is to find and remove ground faults. (Another entire topic)
If an asymmetrical voltage sine wave can create humming – so can an asymmetrical current waveform. The nature of the power supply can result in a current draw that is larger on one side of the waveform than the other, again making the windings vibrate. In this case the “cause” is the amplifier’s own power supply design. This is the most difficult to control. We incorporated a circuit in our earlier designs that allowed the user to “dial out” the current waveform distortion with a trim pot. It was possible to watch current waveform distortion reduce on a scope while listening as the transformer hum faded.
Before we can hope to cure transformer hum it must be accurately diagnosed. That can’t really be done with simple “home tests” of guesswork. The best way is to test the amplifier in operation with an oscilloscope or power quality meter. The actual cause will usually be readily revealed.
When we first began to install PurePower AC regenerators in 2004 we ran into transformer hum very quickly in as many as 20% of amplifier models. As we learned, we kept discovering new causes and providing remedies until we had almost no amplifier brands that hum when powered by a PurePower regenerator. That means we were able to support more and more amplifiers.
Hum incidents became fewer and fewer. When we released the PurePower+ we considered amplifier hum to be thoroughly defeated. But they continue to pop up here and there. (So far we have only seen one new case in 2013..
We are committed to help our customer defeat each new case.
There is one solution that PurePower fervently wishes all amplifier manufactures would avail themselves of. The fact that transformers represent a hum waiting to happen should cause all good audio designers to adopt the perfect mechanical solution to what is, after all, a mechanical problem. Many of the best amplifiers brands already do. Epoxy (or 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 confirm that the power transformers be preemptively treated to prevent hum. The cost is small – and I can’t imagine why all high end products shouldn’t have it
As far as we understand, the electrical performance characteristics are not affected by epoxy potting, especially if the manufacturer specifies it as part of the initial design. Existing transformers can also be potted – but the materials and techniques must be correct.
When is the Premier the best choice for your AV system?
What about current limiting?
Long term, short term and instantaneous power.
Voltage drop and circuit “intelligence”
Does PurePower offer variable frequency control?
Will balanced power technology do what PurePower does?
Will an isolated receptacle provide clean AC power?
Won't an isolation transformer make a new clean power supply?
Why aren't the PurePower receptacles isolated from each other?
When you see the term “isolated outlets” in receptacles in the same enclosure or on the same power bar you should take the term with a grain of salt. Many power conditioners have an added low pass filter – basically a small coil and capacitor – attached to each receptacle. The theory is that this will filter high frequency noise generated by a device plugged into one receptacle and prevent “cross contamination” to other devices.
inexpensive filters may have some efficacy, but there are 10 different
“power gremlins” that can reduce AC power quality, and a filter can
only affect 1 of the 10, so perhaps “1/10 isolated receptacle” might
be a better descriptor. If you plugged your amplifier into one outlet,
and a light dimmer switch into the other, some of the high frequency
noise from the dimmer switch could be attenuated by the filter. If you
own a CD player or preamp that makes noise like a dimmer switch, it
could likewise attenuate that noise. (Our advice to all audiophiles is
to not let a dimmer switch come anywhere near your house, and if you
bought a piece of high quality audio gear that radiates noise like a
dimmer switch, send it back.)
are two flaws in the filter plan. 1: most audio system component power
supplies either do not radiate such noise into the AC line, or they are
so small a contributor as to be negligible. 2: The distortion they do
tend to generate is harmonic distortion. This distortion is completely
unaffected by a low pass filter.
engineers considered adding low pass filters to our output receptacles,
but discarded the idea. We believe it is just as likely for the filters
to reduce current flow and diminish sonic performance as it is to remove
harmful high frequency noise. We think they are of small benefit –
other than to use as a marketing point.
We do continually test customer systems to look for examples of cross contamination. So far we have not detected any, but we will keep an open mind.
Q: Can an ordinary, off the shelf, computer UPS product do the same thing as an AC regenerator?
Computer UPS systems such as the APC SmartUPS do not perform full continuous power conditioning, but pass through the AC power from the wall outlet, turning on their battery powered AC inverters only in low voltage (usually <95 volts) or blackout conditions. Their battery life tends to be limited. They also tend to be noisy and intrusive unless housed in a separate room or electrical closet.
APS will completely remove all dimmer switch noise from the ac line supply to your system.