how can a line cord affect frequency response ?

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.

Spatial King,
I was describing the very basics of amplifer power supplies, paying particular attention to the rectifer output/resevoir cap.

I know that the power bandwidth of most amps easliy extends beyond 20 kHz.
Perhaps I did not articulate my thoughts precise enough, but I was not suggesting that an ampllifer's bandwidth is the same as the transformer.
I was purposely not getting into too much detail.
The question that Mr Tennis asked was how can ac-line cords affect frequency response. My simple answer is: I think that if there are ferrites or other inductive material involved in the construction of the cord, the inductance of this particular ac-cord may impede sudden fluctuations in current.

At the crest of the rectifed pulse, ie the charging pulse, the secondary of the transformer is now dumping current to the cap and the output device. It has to be this way or Kirchhoff's current law is full of hot air, which is not the case. The point I was trying to make is that during this brief moment the required current must ultimately come from the ac-line. If the ac-line is heavily inductive the instantaneous current performance may suffer.
I have never tested this theory on a real amp with different power cords. I have done circuit simulations in PSpice and i'm just throwing this out there as one way high frequencies or dynamics may be affected by PC's.

The engineering types are the ones who give you your music Ronniekoh.
The marketing types love your paranoia.

My Friends,
If any of my posts came across as arrogant or whatever, I am sorry. I am a music lover and audiophile, but alas, I have a degree in EE. That being said, having an EE degree does not automatically make someone an authority on power supplies and audio amplifiers. There are many paths one can take in this field but designing high performance audio amps, for instance, is a rare path indeed. My job involves digital designs with FPGAs for software radios, but my real passion is the analog stuff.

Although I am in the more objective camp, I have never stated that power cords can not make a difference. Rather, if you follow my previous posts, you can see I made a feeble attempt to describe how such a thing might be so.

One thing that I will never give up is my fundamental belief that if a real change is heard, however subtle, and repeatable, say between one power cord vice another, then there must be an underlying electrical change to manifest such sonic deltas.

If I did not believe this, then it really does boil down to faith in something else.

I do believe that there are material-electrical differences among pcs, and if in the right high resolution system with great acoustics, then these deltas may manifest. I think this can and does get out of hand and folks have to be somewhat realistic about expectations.

The longer I think about this, the more i'm inclined to believe that it's not simply reducible to the large amount of romex, nor the junction box, nor the long lengths from the step down xfrmr on the pole a bazillion feet away, nor the miles of 100 kV lines back to the distribution station. No. It must be the interface to this network. Why/ how I do not know yet but i have some ideas.

I listen to music alot, but I also love to measure things. I love to understand the physics behind something. In my view one reinforces the other.

A basic experiment to test currents on the ac-line:
A high power series resistor with the hot lead. The resistance will have to be low and this is not ideal, but it could work. A safe box of some sort will have to house the plugs and power resistor.
A basic ocsilloscope with a differential probe across the resistor (do not use the ground clip as you will be connecting the hot directly to ground via the scope cable--not good).
A test signal, like some continuous tones ala stereophile cd-2. Of course you need a trigger for the scope. Simply use the 60 hz ac line setting. This way you capture the events you want to see based on the power cycles of the ac.

You plug your amp pc into the thing you fabricated that houses the power resistor. Fire up the amp.

At this point, with the volume low, you might see some activity on the scope that relates to the charging of the filter caps, aka 120 hz (full wave rectifed).
If you can trigger the scope just right,and play with the time base, and vertical gain, sample rate, etc i bet if you crank up the input signal you will see, at the peaks, fluctuations that are directly correlated to the amplified signal.
Here is where i think power cords can make a difference.
The current is pulled sharply from the power amp at these peaks. We know that abrupt changes indicate high frequency. This is direct from theory and confirmation over hundreds of years.
(This IS not equal to whatever high frequency content in the signal to be amplified)

This is one fairly simple test that can be run. I currently do not have a working personal scope and probe of reasonable quality. I would love to do this however.

Of course this may be all wrong, but I like to offer possibilites to understanding our beloved hobby.

Later!

Unfortunately this has turned into a freshmen 101 philosophy discussion.

Can we please stay on topic? please, can we let the pure technical treatment of this question carry forth?

I am one engineer who does not try to reduce the world around me to equations and mechanistic operations.
But, in this case
I do think there is a technically legitimate answer... This topic is a matter of electrical interaction, nothing else, pure and simple.

This thread is another example of how we audiophiles talk about hearing subtle differences/shadings, sure they are important, but then we rant and rave about this and that being so and the "well if your system, ear, room, etc are not up to resolution " blah blah blah...with no real technical discussion based on facts. It happens and will continue to just be so.

Why such an unwillingness exists to tackle the problem they way it should be dealt with is just lazy.

It's really ok to talk objectively. Objective science and engineering are what brings forth the awesome equipment we love and your music will still sound great to your ears if you choose to bury yourself in the technical details.
It's a matter of choice.

SpatialKing,
Sure that could work too. Do you plan to trigger on the ac-line frequency and run some test signals into a power amp, or do you have some other arrangement in mind. If you have a typical amp and speakers in your lab that would be best of course, to correlate the fluctuations in the musical signal to the changes in current as picked up by your inductive probe.

Please let me know how it goes.

I also agree that BIG expensive power inductors can be used to stretch the charging pulse and therby reduce ripple. It is rare to find this on most amps out there. I think Nelson Pass discusses this very technique on his DIY website devoted to the designs of his Aleph knock-offs.

Thanks for not giving in to the entropy.

TWL: What is a negative feedback power cord?
I also stated that music as well as test tones, perhaps used only to calibrate the measurement technique, be used.

The "dummy load" was not a dummy load across the output of the amp.
It was in series with the Hot line of the outlet, for example. I suggested this. Someone else mentioned using an inductive pickup, and while that may work fine on unshielded cables, the majority of high-dollar aftermarket PCs employ HEAVY shielding. Therefore the inductive "kick" will be attenuated if not totally reduced to noise.

If one were to pull the outlet junction box out of the wall and expose the romex, which is most likely unshielded, then the pickup can be placed appropriately.

Please note, any of this is potentially dangerous and should not be attempted by any individuals not qualified to do so.

Shadorne,
Thanks...
Now we have a start for the "my electrical station is superior to your electrical station" thread.

You just made PS Audio very happy. Everyone quick flock to PS Audio website and drool over the latest line conditioners.

That was hilarious Guidocorona.
Perhaps it's best to split this thread now into two distinct subjects:

1) the line cord frequency thingy
2) proposed new branch:
"How a technical discussion affects the mental status of Audiogon members" included we have a cast of mechanical froggies, psychologists, lawyers, philosophers, fisherman, and the impatient

If you are looking for good natured humor choose path 2.
If, however, you don't mind technical details and perhaps dry humor choose 1.

once again we are on path 2...

Wonder why my 0.02 about inexpensive cable solutions got squashed by the MODERATORS?

as an alternative to 1000.00 / meter PC
I could simple squirt Spray and Wash on my speaker cones to provide the audio windex to the same degree the silly overpriced electron wielding garden hose of the month (pc) does.

Once again this thread proves that AudioPhiles typically have more dollars than sense.