Why manufactures don´t burn in their amps and ...

We do 90% of the burn-in here in our shop. The last 10% takes the longest time- up to 6-8 weeks depending on the number of hours a day. Especially when you have amps or preamps built up with custom options, doing an extended burn-in is just plain impractical!

As to power cords, we acknowledge that power cords are going to make a difference. That is why we install IEC connectors on our products. The problem is, what we think is the best power cord may not be what you think is the best power cord. In fact we have seen a power cord work great in one installation and make no difference in another. You have to take into account that variables of house wiring, cleanliness of the AC power, neighborhood line voltage and other system anomalies that will affect what the end user thinks is the 'best'.

Guess what? No-one can do predict all that- so that is why we supply our gear with IEC connectors. Our stuff is known for being really transparent- and we invite you to experiment. That is the only way I know of to sort out what is going to work in the area of power cords. It would be frustrating as an end user to realize that a $200 power cord had been included with the unit, only to find out that in your system its not the right combination!

For those who wonder how a power cord can make a difference, consider this: an amplifier has a power transformer, rectifiers and filter capacitors in its power supply. Normally, the filter caps are charged by the rectifiers as the transformer makes voltage.

When the filter caps are nearly charged, the only time that the rectifiers will conduct is at the peaks of the 60Hz input waveform. So all the energy going into the amp is concentrated at the peaks. This is the point that the diodes turn on, as the voltage on them is higher than the voltage on the filter caps.

As soon as the wave is past peak, the rectifiers will turn off, when the filter cap voltage is higher than the voltage from the rectifiers.

So the energy going into the amp is often doing **only at the peaks**. The difference between a cheap power cord and an expensive one, in terms of its ability to dump large amounts of current during these peaks is only about 3%. It is that 3% that audiophiles hear.

A second phenomena is AC voltage drop across the cord. We have measured nearly 3 volts in some cases. In a 100-watt amplifier that can account for a nearly 50% DROP in power!!!

If you think these two phenomena are not audible you're missing a bet. The bigger the amp (IOW the more power drawn), the more audible power cords become.

We figured some of this stuff out nearly 20 years ago, which is why one of our bigger amps (the MA-2) actually runs two power cords per chassis (since there are 2 AC circuits in the amp for 2 power transformers) to limit the audible effects of the power cords. Back then there were not nearly so many exotic cords as there are now! -so we had to make do and adding a power cord worked out very nicely.

A voltage drop on the power cord is caused by two things: the gauge of the wire and the quality of the connections.

If your power cord is heating up at the connectors, here is an example of why things like the Porterport or medical-grade connectors can make a difference!

The wire itself will heat up if it is inadequate gauge.

If it is built improperly, it can resist the high current/high frequency surges required at the peak of the waveform.

I think its a good idea to have a shield too.

IMO, building a cable carefully with good connections at either end and a heavy gauge will hit about 95% of what is important. We built a few power cords just for fun and they turned out quite well. Some of our gear does draw some power and they seemed will up to the task. After you build a few and add up the costs, you start to see why some power cords cost what they do. Not all though :)

Magfan and Al, you want to keep in mind that the power surges are the ones where the power supply rectifiers commutate (that is to say they turn off and on) at the peaks of the AC waveform. As soon as the AC waveform drops below the value where the caps are charged in the power supply, the rectifiers turn off- please refer to my earlier comment regarding this.

The bottom line is that there are indeed HF power surges occurring in the power cord (yes, 60 times/second), unless you have another way of describing a current spike a few milliseconds wide :)

The rectifier commutation is often responsible for a great degree of radiated HF noise. They have to be properly bypassed to reduce it. However, the power transformer core has its own reaction to the ON/OFF load! If you don't believe me Google 'spark coil' and look at the principle of operation.

A fair amount of this noise manifests on the primary side of the transformer. That is why I think its a good idea to have a shielded cord :)