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Effects of power cords - Page 2  

post #16 of 56
Thread Starter 

Being connected to something 120 times per second is a lot more connection than 0 times per second.

post #17 of 56
Quote:
Originally Posted by markheadphonium View Post

Being connected to something 120 times per second is a lot more connection than 0 times per second.

It's only connected for a very brief amount of time every 1/120th of a second. A time much shorter than 1/120th of a second, such that for the vast majority of time it's simply not in the circuit. And during that vast majority of the time, the circuit is drawing power from the power supply caps, not from the AC mains through the power cord. Further, and with most everything except power amplifiers (and even with a few of those), the power supply will include voltage regulators which will greatly reduce any perturbations on the supply rails. And last but not least, any reasonably well designed circuit will have a good deal of power supply rejection of anything that might have slipped through the regulators.

So I fail to see how a power cord can effect anything that would result in any meaningful change at the circuit's output. Yet people talk about power cords in the exact same fashion as they talk about interconnects and loudspeaker cables which are carrying the actual audio signal.

I don't know, maybe some people are of the belief that the signal is passing through the power cord just as it is through interconnects and speaker cables. I've seen much stranger beliefs held by some.

se
Edited by Steve Eddy - 4/23/14 at 1:36pm
post #18 of 56
Quote:
Originally Posted by Steve Eddy View Post

It's only connected for a very brief amount of time every 1/120th of a second. A time much shorter than 1/120th of a second, such that for the vast majority of time it's simply not in the circuit. And during that vast majority of the time, the circuit is drawing power from the power supply caps, not from the AC mains through the power cord. Further, and with most everything except power amplifiers (and even with a few of those), the power supply will include voltage regulators which will greatly reduce any perturbations on the supply rails. And last but not least, any reasonably well designed circuit will have a good deal of power supply rejection of anything that might have slipped through the regulators.

So I fail to see how a power cord can effect anything that would result in any meaningful change at the circuit's output. Yet people talk about power cords in the exact same fashion as they talk about interconnects and loudspeaker cables which are carrying the actual audio signal.

I don't know, maybe some people are of the belief that the signal is passing through the power cord just as it is through interconnects and speaker cables. I've seen much stranger beliefs held by some.

se

 

 

Letting aside the fact that the power cord is part of the actual audio signal path in some quite common cases, as said earlier, what you're saying is quite true at LF but very optimistic as soon as we're leaving lower frequencies.

 

Regulators will let HF crap in the mains go straight through them, the PSRR of most circuit falls very quickly as frequencies go up and not all designs take care of properly filtering higher frequency noise. Even fewer adress common mode noise on the mains. While I agree that power cables are a strange way to adress those problems, the argument about the transformer being connected for a very short time is pretty much worthless since we still have to address what happens during those short moments anyway.


Edited by 00940 - 4/23/14 at 4:49pm
post #19 of 56

I tried a few power cords in my system and I hear a difference . I replaced 4 power cords in my system and by itself it is not a big change but there is a difference like the lower frequencies are tighter and the sound is more holographic , but when I combine this power cord change with interconnects and fuses - it all makes a big change as well - so its more like trial and error coz every change will shape the sound - good or bad .

post #20 of 56
Thread Starter 

A lot of things in the system have to be right in order for the power cord's sonic characteristics to jump out at you. You need a conditioner or parallel filter cleaning up the power, you need equipment with good resolution and dynamics to start with. 

post #21 of 56
Quote:
Originally Posted by 00940 View Post
 

 

 

Letting aside the fact that the power cord is part of the actual audio signal path in some quite common cases, as said earlier, what you're saying is quite true at LF but very optimistic as soon as we're leaving lower frequencies.

 

Regulators will let HF crap in the mains go straight through them, the PSRR of most circuit falls very quickly as frequencies go up and not all designs take care of properly filtering higher frequency noise. Even fewer address common mode noise on the mains. While I agree that power cables are a strange way to address those problems, the argument about the transformer being connected for a very short time is pretty much worthless since we still have to address what happens during those short moments anyway.

 

Yep, it's all true.

Rectifier diodes have parasitic capacitance so are basically short circuits at high frequencies.

Just sayin', we're on the same page.

post #22 of 56
Quote:
Originally Posted by Chris J View Post

Yep, it's all true.
Rectifier diodes have parasitic capacitance so are basically short circuits at high frequencies.
Just sayin', we're on the same page.

Ok, let's consider about 10pF for a silicon rectifier diode. At 1MHz, capacitive reactance would be about 16k ohms. At 100MHz, about 160 ohms. Not exactly what I'd call a short circuit. How high in frequency do you want to go? At 1GHz, then you're finally down to 16 milliohms, which is much closer to a short. But how much is a power cord going to pass at 1GHz? And while we're talking about parasitics, what about the leakage inductance of the power transformer, which of course is upstream of the rectifier diodes?

se
post #23 of 56
Quote:
Originally Posted by Steve Eddy View Post


Ok, let's consider about 10pF for a silicon rectifier diode. At 1MHz, capacitive reactance would be about 16k ohms. At 100MHz, about 160 ohms. Not exactly what I'd call a short circuit. How high in frequency do you want to go? At 1GHz, then you're finally down to 16 milliohms, which is much closer to a short. But how much is a power cord going to pass at 1GHz? And while we're talking about parasitics, what about the leakage inductance of the power transformer, which of course is upstream of the rectifier diodes?

se

 

OMG!

post #24 of 56
Quote:
Originally Posted by Chris J View Post

OMG!

confused.gif

se
post #25 of 56
Quote:
Originally Posted by Steve Eddy View Post


confused.gif

se

 

Brothers and Sisters,

Let us bow our heads and calculate...

 

Take a 6 ft. power cord with 10 pF/ft of capacitance and 0.155 uH of inductance. It may have a name like, um, Nordost Bue Heaven.

At 1 MHz this is XL = 5.84 Ohms and XC = 2653 Ohms.

Very interesting....

 

The real thing to keep in mind is that HF noise still breaks thru a garden variety PSU if you do not take action to prevent conducted HF noise, i.e. board layout, lead dress, appropriate capacitors, etc.

Take a piece of garden variety audio gear and see if it will pass FCC class A or Class B.

It may take some redesign to get it to pass FCC Class B.

The noise doesn't break thru only when the rectifier diodes are switched on.

In fact, the rectifier diodes switching on and off are part of the problem.

I suspect output transistors switching on and off on a Class B amp may also be a noise source.

 

And then we have radiated noise...

http://en.wikipedia.org/wiki/Electromagnetic_compatibility

post #26 of 56
Quote:
Originally Posted by 00940 View Post
 

 

Actually, in the case of two interconnected hifi devices with 3 wires power cables (and not using ground loop breakers), the earth wire is most probably the audio return for an important part of the signal and is thus continuously in the signal path.

 

If you believe in interconnects, which btw I don't, it's quite logical to believe in power cables.

 

The power cable earth wire is not a deliberate or intended audio return path.

The common wire in the audio interconnect is the audio return path.

post #27 of 56
Quote:
Originally Posted by Chris J View Post
 

 

The power cable earth wire is not a deliberate or intended audio return path.

The common wire in the audio interconnect is the audio return path.


While it's not the intended path, the signal doesn't know that so it will take all available paths to return to its source.

post #28 of 56
Quote:
Originally Posted by Speedskater View Post
 


While it's not the intended path, the signal doesn't know that so it will take all available paths to return to its source.

 

Read my first sentence again, please.

Anyway, a good design will take measures to mitigate this.

Power ground may be isolated from signal ground or common by diodes or resistors or capacitors.


Edited by Chris J - 4/25/14 at 4:24pm
post #29 of 56
Quote:
Originally Posted by Chris J View Post

 

The power cable earth wire is not a deliberate or intended audio return path.

True

 

Quote:
Originally Posted by Chris J View Post

 

The common wire in the audio interconnect is the audio return path.

I repeat:

While it's not the intended path, the signal doesn't know that so it will take all available paths to return to its source.

post #30 of 56
Quote:
Originally Posted by Speedskater View Post

True

I repeat:
While it's not the intended path, the signal doesn't know that so it will take all available paths to return to its source.

I guess you're not really a people person, are you? biggrin.gif
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