Power Cables

How long has this (thread) been going on?

Not remotely close to as long as many other cable-related threads/debates :-)

Regards,
-- Al

Sisyphus, thanks for providing the link. However, firstly, let’s not use the term "pulses." That would imply a squarish waveform, which is not what is being referred to. What is being referred to are the portions of a 60 Hz sine wave that approach and include its positive and negative peaks.

Secondly, and more significantly, the reference you provided does not address what happens **after** the rectifier bridge. As noted earlier by you and also in one of Ralph’s posts that I quoted there are capacitors, which smooth the waveform into a close approximation of DC and also store energy, that stored energy being what powers the downstream circuits.

And as Ralph explained earlier the rectifier diodes will only conduct when the instantaneous voltage of the AC supplied by the power transformer exceeds the voltage stored on the capacitors. Or, more precisely, when the instantaneous AC voltage exceeds the voltage stored on the capacitors by the relatively small amount that is necessary to turn the diodes on (which is approximately 1.4 volts in the case of a typical full-wave solid state bridge rectifier circuit). And assuming the rectification circuit is full-wave, that will only happen when the AC voltage supplied by the power transformer is approaching either a positive or a negative peak. In other words, for just a relatively small fraction of the 60 Hz period.

During the rest of the 60 Hz period the rectifier diodes will be back-biased and unable to conduct. Therefore AC current will not be drawn continuously, but rather for just a fraction of each cycle. In other words, the AC current which restores charge to the capacitors is drawn in narrow "spikes," with "spikes" being defined per the first paragraph in this post. As Ralph, I, and Elizabeth have all maintained.

In any event, thanks for providing the chuckle of the day with the "duty calls" cartoon :-)

Regards,
-- Al

Sisyphus51 6-14-2018
A IR voltage drop of two volts across a very long power cable will not cause an equal 2 volt drop in filament voltage in a vacuum tube amp.

Of course not, and no one has said that it would. In designs in which filament voltages are unregulated the drop will be essentially proportional. 2 volts/120 volts = 1.67%.

In tube datasheets I’ve seen which provide +/- tolerances for filament voltage (and many of them don’t) the specified tolerance is usually +/- 5% or +/- 10%. And I see no reason to expect that performance would be identical or even particularly similar throughout those tolerances. 1.67% is not an insignificant fraction of those tolerances.

Regarding your other comments, Ralph was of course not saying that the extreme example he cited represents typical performance for most or even many designs. He was citing that example in support of his explanation of why power cords can make a difference. And if a person like Ralph says that "depending on the amp this can be pretty profound. and I have seen it with my own eyes," that’s good enough for me.

Regards,
-- Al

Sisyphus51 6-13-2018

Why do folks insist that "Power Cables" need to have a bandwidth equal to that of a very high-spec amplifier’s audio bandwidth? Are you people INSANE? We are talking about 60Hz AC here!

If your power cords are carrying Khz range signals or noise there is something horribly wrong with your amplifier that needs to be corrected immediately, if not sooner because you are essentially operating a Radio Transmitter in violation of Federal Law!

Not sure you are realizing that if current is only drawn during a small fraction of each 60 Hz cycle, as Ralph/Atmasphere clearly explained in the post I quoted, spectral components are present at vastly higher frequencies than 60 Hz.

Which, btw, is the main reason I said in my post earlier in the thread that:

Almarg 6-12-2018

Power amps can generate significant amounts of high frequency noise that can be fed back into the power cord and affect other parts of the system, to a greater or lesser degree depending on the shielding, bandwidth, capacitance and other characteristics of the power cord.

Sisyphus51 6-13-2018
I just measured the drop across the 5.5 foot original power cord on my 1964 Scott 222D Integrated Amp.....

Among a number of other vintage pieces I have an H. H. Scott 299C integrated, ca. 1961, that I sometimes use in a second system. 7591 power tubes and a 5AR4 rectifier. Very nice piece. I sometimes use it in conjunction with a 1952 Radiocraftsmen 10 mono AM/FM tuner, the combo producing very lush, rich, and beguiling sonics on FM.

Regards,
-- Al

And from this thread, in 2010:

Almarg 9-15-2010

Power cords: a 2V drop across a power cord can rob a tube amplifier of as much as 40% of its output power!

Ralph, could you provide a technical explanation of why that would be so? I don’t doubt your statement, but I’m interested in understanding why that would occur.

Re your other points, all of which strike me as excellent, I think that it should be stated that none of those points NECESSARILY mean, to cite an example, that a $2,000 power cord will outperform a $200 power cord in any given system.

Atmasphere 9-15-2010

Al, the reason a power cord can have this effect is simple. If there is a 2 volt drop in a power cord, the filaments of the tubes will run cooler and the B+ will be reduced. Since this is a voltage, the result is we get less voltage output out of the amp. Less voltage=less power. Depending on the amp this can be pretty profound. and I have seen it with my own eyes. I do agree though that that does not justify a $2000 power cord, but it **does** justify one that has decent connectors and conductors that will not heat up at all. That has to cost something, probably not $2000 though. One thing about audio is that if there is a phenomena, there is also snake oil for it.

Almarg 9-15-2010

Thanks, Ralph.

That would also seem to say that the value of the ac line voltage at each listener’s location can be a very significant variable in the performance of a tube amp (assuming it does not have regulated filament and B+ supplies).

Which in turn emphasizes how easy it can sometimes be for extraneous variables to lead to incorrect sonic assessments.

Regards,
-- Al

I think that the following lengthy post that was provided by Atmasphere a few years ago, in a post dated 6-3-2014 in this thread, is worth quoting here in its entirety.  It reinforces some of the things that were said above by Steve (Williewonka), and by me earlier in this thread.

For those who may not know, Atmasphere (Ralph Karsten) is the owner and designer of Atma-Sphere Music Systems, which manufactures unique and very highly regarded audio amplifiers and preamplifiers.  He is also, IMO, one of this forum's most knowledgeable, experienced, helpful, sincere, and level-headed members.

With power cords its all about voltage drop across the cord. Some of that is at 60Hz, and some of that is much much higher- well above 30KHz-100KHz depending on the power supply in the unit with which it is being used.

I've seen a 2 1/2 volt drop rob an amplifier of about 30% of its output power. The cord was rated for 10 amps, and the draw was about 6 amps. This measurement was done with a simple 3 1/2 digit Digital Voltmeter.

The more insidious problem is high frequency bandwidth. The power supplies of most amplifiers have a power transformer, a set of rectifiers, and a set of filter capacitors. The rectifiers only conduct when the power transformer output is higher than that of the filter caps. So:

When the caps are fully charged the amp is able to play. As it does so, the caps are discharged until the AC line voltage waveform gets high enough again that the rectifiers in the power supply are able to conduct. Depending on the state of charge of the filter capacitors, this might only be for a few microseconds or it might be a few milliseconds. Either way, the charge is a spike which has very steep sides- and requires some bandwidth to make it happen.

If the power cord has poor high frequency response, it will current limit on these spikes. This can result is subtle modulations in the power supply or even a sagging power supply voltage.

Romex wiring found in many buildings actually works quite well. So it really becomes all about that last few feet and also how well the power cord is terminated- molded cords generally are not terminated very well. If the ends of your power cord get warm after a while, you know you have a problem!

This can be measured, its quantifiable and also audible as many audiophiles know. Anyone who tells you differently probably has not bothered to do any measurements- please refer them to this post.

I can go into more depth but this is it in a nutshell. Incidentally, Shunyata Research is refining an instrument that does a more in-depth analysis of what this is all about. At the link you will see that their tests essentially confirm what I have said here.

http://www.theaudiobeat.com/visits/shunyata_visit_interview.htm

Regards,
-- Al 

Power amps and preamps are of course very different in their internal designs, regardless of whether they are from the same manufacturer or from different manufacturers.

They also draw very different amounts of current through their power cords, of course.

And in most cases they draw currents which fluctuate very differently as a function of the dynamics of the music, fluctuating essentially not at all in the case of preamps, while fluctuating widely in the case of power amps operating in class AB or class D. Consequently they may have very different sensitivity to the bandwidth of the power cord.

They have very different internal voltage regulation, none at all in the output stages of most power amps; very tight voltage regulation in the case of most preamps. Consequently they have very different sensitivity to voltage drops that may occur in the power cord.

Power amps can generate significant amounts of high frequency noise that can be fed back into the power cord and affect other parts of the system, to a greater or lesser degree depending on the shielding, bandwidth, capacitance and other characteristics of the power cord. Preamps generally do not do that, and even if they do (perhaps due to digital circuitry they may contain) the noise would have very different technical characteristics than the noise generated by a power amp.

Given all of those differences, from a technical standpoint it can be expected that preamps and power amps will react very differently to power cord characteristics. And therefore cords from the same manufacturer would seem to be no more likely to be an optimal combination than cords from different manufacturers.

IMO. Regards,

-- Al