how can a line cord affect frequency response ?

I have followed this thread closely and until now have not felt like saying anything. But the last post by Sp has sparked me to reply.

Over the past ten years or so I have listened to atleast 100 or more power cords in my system. Never satisfied with the results, I started building my own. At last count, probably 30 samples. Every type of wire, geometry , shielding method and terminations imagineable.

None of my final designs, with the execption of (1) are shielded. Shielding, IMO adds an audible coloration that I find quite annoying.

The question, "how can a line cord affect frequency response?"

Based on my experience, the #1 factor by far is the type of conductor material the cord is built of. Everything else is secondary. How?, the same way an interconnect does. Use conductors that restrict bandwidth and the frequency response will be adversely affected.

what about resistance ? suppose you design a poer cord with very high levels of resistance. will that affect treble reponse ?

or, suppose you design a power cord with a very high level of inductance, what then ?

Spatialking,

Not sure if I completely agree with the power cord acting as an RF antenna. Once energized, it has more rejection qualities, and is more susceptible to large electro-magnetic field influences. Even placing two energized cords along side each other, will have little or no effect on the other's audio component. The newer switching class-D amps can inject adequate hi-freq noise into close-proximity sensitive interconnect cabling and component gain stages (even through some cable shielding and metal chassis), but would be hard pressed to have an audible affect when placed several yards up stream, along side another amp's un-shielded PC (via an extention cord for testing purposes). Oh, and can't forget about all the non-shielded Romex behind the sheetrock walls. Mainly what people hear from two-way radio communication, is affecting radio/television receivers or inadequately shielded sensitive components and IC cabling. That's the only time I've ever heard anything during audio playback, and I live next to a private air park where communication takes place throughout the day. I'm open to explanation, though.

Reb1208,

Regarding conductor materials... copper, silver, or gold are capable of frequencies into the extreme RF bandwidth. The audio bandwidth is child's-play in comparison. Copper is utilized everywhere in these applications. Capacitance, impedance, and shielding are the main issues affecting an interconnect's frequency/performance, as they do with RF cabling or wave-guides.

Not real sure how or why you're having issues with shielded cords creating coloration. Any further explanation to help us understand what might be contributing to your findings?

For RF to propogate it not only requires some type of conducting surface, but the physics of the medium (IE the physical environment that contains the RF) play an important role. Effective transmission lines do not power cords make. True certain frequencies of RF energy may attach to the surface of things other than metal, but in our case, audio amplifiers in metal cases, with power cords going in and cables I/O etc...which under normal conditions are shielded enough. I believe that very few home audio amplifers have ever fallen victim to any incident RF of enough energy as to be noticable.

This report of hard or glassy audible results from "RF" ...where does this come from? Have these types of claims been investigated? Have tests been run where an audio amp and cable have been exposed to swept RF sources while listening tests and/or controlled measurements of the device under test are performed? I highly doubt it.

Like the study of speaker cables, etc... it is a hard test to do and there is no shortage of opinions on the subject.

Remember, we are talking about engineered products. Yes we derive pleasure out of them but, there are cold hard facts based on decades of knowledge and years of experience put into the designs we love...
We need to stay true to the basic facts of engineering and device physics to truly have meaningful conversations about it.

Back to the question: How can a line cord affect frequency response?
In a typical audio amplifier power supply, on one side (we will call it the ac-line side) the transformer sees some resistance (from the dozens of feet of ac line going all the way back to the transformer on the pole), some inductance, and some capacitance.
-Aside: homes typically have 12/14 gauge un shielded solid copper wire that is the standard. Increasing the diameter of the power cord to say 10 or even 6 gauge does nothing to the instantaneous current available "out of the wall". SUre current densities may be different on fat and skinny conductors, but the end result will be the same. These lines are in series.

So the transformer, itself made of two massive inductors wound around some magnetic core decouples the amp-side windings from the line-side windings. This is good news. ANy dc offset on the line side, besides having ZERO effect from the power cord, can now NOT "get across" to the amp-side winding.

SO, on the the next stage: the recitifers. So now we have a sinusoidal 50/60 hz signal driving a bridge rectifer of some sort. You have AC going in and rectified AC going out of the rectifer. THis means that, if you looked at the signal out of the rectifer's positive terminal you would see the negative going waves are now inverted and you only have positive ac-bumps, spaced either 50/60 hz apart or 100/120 hz apart, depending on the overall design of the rectifier stage.
To "make" DC the rectifed pulses charge capacitors. With ZERO loading on the caps,(ie output transistors completely off) the charging currents from the rectifer gradually reduce as the caps reach full charge. The caps can only be charged at a rate directly related to the ac-line frequency.
In the typical class A or A/B output stage design...one of the three pins of the output transistors (either FET or BIPOLAR) are connected to this supply "rail" thus allowing useful operation of the device (I left out the other pins to simplify the discussion).

So now, imagine that the output transistors have a job to do and are now driving a speaker load. The current source for the output transistors comes from:
1) The resevoir caps directly -- during moments the recharging pulses from the rectifier are absent
2) The rectifer/power transformer directly -- During moments that the signal to be amplified places its demands on the output device during charging times. Under this condition the power supply current is not evenly split among recharging the cap and output device. Here is where a good stiff power supply carries the day for audio amps.

In the context of number 2-- consider higher frequencies that require amplification... it is very possible that higher frequencies cause related current spikes through the rectifer stages and back to the transformer/power cord/wall etc...
...also consider 'dynamic' signals such as drums requiring current during this window...

If you have a power cord with extensive ferrite beads or other ferrous material, it is possible that under these conditions, the higher frequencies may be diminished, due to increased resistance to the current at these frequencies b/c of the inductive response from the ferrite. This is why, I think, that ferrites in power cables may diminish dynamics or dull the high end.

It is more from a reducing-the-available-current-under certain-conditions effect than an actual designed filter result, though one can mimic the other.

Of course, the transformer itself has a frequency response and this may over-ride anything else by orders of magnitude under the high frequency analysis.

I hope this helps.