Why do some amps don't have a ground pin in the power cable?

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rg12

Member
2010-08-10 3:52 am
I actually have two questions:

First one, why do some amps including mine doesn't have a ground
pin in the power cable that goes to the socket?
Is it ok that i already added a cable WITH a ground pin to my amp
while modding it?

Second question, how come my amp and many other devices can
work with the power cable connected either way and the polarity
doesn't matter?
 
The amps without ground are specially made in a safe way called double insulated. Their purpose is so that you don't have ground loops in the sound. You are fine connecting the amp case to mains ground (earth) but you may find you get ground loops if you connect other equipment that is also grounded.

Because the mains just feeds a transformer which works the same whichever way round it is wired. Do however always observe the correct live and neutral wiring convention though as there may be an implication to the fusing which could be dangerous.
 
The amps without ground are specially made in a safe way called double insulated. Their purpose is so that you don't have ground loops in the sound.

This is certainly true at DC. But since mains is 50Hz AC then there's nowhere for the AC to go when the amp is ungrounded (transformers have capacitance to the mains). If you connect to all other ungrounded equipment that's fine, but as soon as one grounded piece is introduced you'll get current flow to the ground. The current is small, usually less than 1mA but isn't limited to being just 50Hz - it contains all kinds of RF hash too.


What was it about ground loops that you didn't get rg12?
 
This is certainly true at DC. But since mains is 50Hz AC then there's nowhere for the AC to go when the amp is ungrounded (transformers have capacitance to the mains).

Eh?

If you connect to all other ungrounded equipment that's fine, but as soon as one grounded piece is introduced you'll get current flow to the ground.

Yes. So?

Unless you have another grounded piece in the system, what exactly is the problem?

se
 

What did I explain wrong here? Just point it out and if I've made an error, I'll correct it.:D


Yes. So?

Unless you have another grounded piece in the system, what exactly is the problem?

I didn't say there was a problem. Just something to be aware of, particularly when choosing cables - if your cable screen has appreciable resistance or inductance, there will be some noise in series with your signal.
 
What did I explain wrong here? Just point it out and if I've made an error, I'll correct it.:D

Just trying to figure out what you were trying to say when you said "This is certainly true at DC. But since mains is 50Hz AC then there's nowhere for the AC to go when the amp is ungrounded."

So what do you mean by "there's nowhere for the AC to go" and what exactly is the consequence of that?

I didn't say there was a problem. Just something to be aware of, particularly when choosing cables - if your cable screen has appreciable resistance or inductance, there will be some noise in series with your signal.

How so? What exactly is the mechanism?

se
 
SE, it's a simple matter of a leakage current or noise source having to be sunk remote. Even if none of the equipment is grounded, if you have one singing chassis, it's driving everything else, which could have the oddest loads including re-radiation points. The problem gets worse the higher frequency gets. When it gets to be a significant problem would be anyone's determination.
 
SE, it's a simple matter of a leakage current or noise source having to be sunk remote.

Having to be sunk remote? What do you mean by "sunk remote"?

Even if none of the equipment is grounded, if you have one singing chassis, it's driving everything else, which could have the oddest loads including re-radiation points.

What I was querying about was abraxalito saying that if you have one safety grounded piece of gear in the chain you'll get current flowing in the safety ground.

I was wondering how that would result in interchassis currents if none of the other components were safety grounded.

se
 
Just trying to figure out what you were trying to say when you said "This is certainly true at DC. But since mains is 50Hz AC then there's nowhere for the AC to go when the amp is ungrounded."

So what do you mean by "there's nowhere for the AC to go" and what exactly is the consequence of that?

OK, I agree I was using a handwaving style of discourse. So then, let's be a bit more precise. Mains supplies are referenced to earth at the substation - that's my understanding (which could be wrong as I'm not an electrical engineer). So with all double-insulated components connected together, there's the potential for the phono socket shield (assuming they're all interconnected by unbalanced cables) to be considerably above earth potential. This comes about because of the capacitance to mains provided by the transformers in the system. In this condition, there is a common-mode voltage (measured relative to earth) on all the exposed metal contacts, but no common-mode circuit and hence no way for the common-mode potential to drive an appreciable current anywhere. That was my meaning of 'nowhere for the AC to go'. Any clearer now?

So to answer your question about the consequences - they're that there's a common-mode voltage on all the cables. If there's a grounded component in the system, this common-mode voltage will be considerably reduced, but not of course totally eliminated.

How so? What exactly is the mechanism?

The noise appears across the cable for the simple reason that there's an AC current loop - an AC circuit, part of which is the cable screen. Given an EMF and a finite conductance, a current will flow. A current flowing with a finite conductance develops a potential difference.

Just for fun I measured some cables I have lying around. The first one was a freebie which came with one of my 99RMB DVD players (see my blog if you'd like to learn more). This is about 1m long and when the screen was passing 1A DC it measured almost exactly 1V end to end. So it has 1ohm screen resistance.

The next one was a 1.5m length of video cable - this dropped 80mV, hence 80mohms. The last was a 5m twin phono - stereo jack cable, measured back to back at 800mohms. So each 5m length contributes 400mohms in screen resistance.

The freebie cable is more than an order of magnitude worse than those other two cables. Quite simply - its been cost-engineered and doesn't have enough copper in the screen.

Now if this cable were used between my 99RMB DVD player and an earthed amplifier, it would be carrying the common-mode currents back to earth. I seem to recall measuring about 200uA of leakage to earth from the player - this becomes 200uV of noise (remember, the cable screen is 1ohm) and is in series with the wanted signal (2V max). The SNR is therefore only 80dB, not very impressive for a purportedly 16bit system. Actually, it would be 6dB better because we'd have the two screens in parallel, still underwhelming though.

If the amp though were double insulated, we might get a much better figure. It would probably be limited by the common mode RF performance of the amp and the cable inductance (which I don't have the means to measure).
 
I was wondering how that would result in interchassis currents if none of the other components were safety grounded.

Those would happen only at higher frequencies, certainly not at mains frequency where the wavelength is way longer than the wire lengths involved. At RF, the potentials at the mains side of the transformers won't be identical between two interconnected units. To minimise this effect, plug all interconnected units in together at the same distribution block. Which is what you'd do anyway to reduce low frequency or DC ground loops.:)
 
Mains supplies are referenced to earth at the substation - that's my understanding (which could be wrong as I'm not an electrical engineer). So with all double-insulated components connected together, there's the potential for the phono socket shield (assuming they're all interconnected by unbalanced cables) to be considerably above earth potential.

Yes that's a pretty accurate description of the situation.

In this condition, there is a common-mode voltage (measured relative to earth) on all the exposed metal contacts, but no common-mode circuit and hence no way for the common-mode potential to drive an appreciable current anywhere. That was my meaning of 'nowhere for the AC to go'.

Everything has the possibility to be at some voltage above ground, this is not a problem though. There is no need to "bleed" this "nasty above ground voltage" off, it's simply an arbitrary potential.

Steve Eddy seems to have the right idea :)
 
Everything has the possibility to be at some voltage above ground, this is not a problem though. There is no need to "bleed" this "nasty above ground voltage" off, it's simply an arbitrary potential.

A sudden blinding flash of insight and I feel I've got where the misunderstanding came from. When I said 'the AC has nowhere to go' I wasn't implying that it needed to go anywhere at all. So that statement wasn't a veiled criticism of not grounding equipment, it was just an observation of what's so. I agree, no need to bleed the 'nasty' voltage off.

But I do have a question about your original statement that the purpose of double insulation is to avoid ground loops. What is it that leads you to consider that's the reason behind having double insulation, rather than other marketing issues? As far as I recall from designing amps with ground connections, there weren't complaints from customers about audible grounding issues. From memory, I think the reasons behind double insulation had more to do with compatibility with different countries electrical systems. UK has only a 3-pin system but many other countries have 2-pin and 3-pin circuits side by side. Having a ground pin in those locations requires more different plugs for different countries, complicating the distribution.
 
As far as I recall from designing amps with ground connections, there weren't complaints from customers about audible grounding issues. From memory, I think the reasons behind double insulation had more to do with compatibility with different countries electrical systems. UK has only a 3-pin system but many other countries have 2-pin and 3-pin circuits side by side. Having a ground pin in those locations requires more different plugs for different countries, complicating the distribution.
If the customers weren't complaining about "ground loops" it is because they weren't using engineering language to discuss hum problems. Hum occurs any time there is AC power in the building and a ground loop couples current flow in that loop to a sensitive amplification component. Induced current is the product of the flux reading times the area of the loop, which is why bands have a lot more trouble with hum than individuals, their loops are bigger. Double insulating a component decouples the case (which if metal must be safety grounded by US & UK law) from the signal ground. If two components (guitar amp, keyboard) have signal grounds coupled to the 3rd pin safety ground, a ground loop occurs including the safety ground, the power cords, and the signal ground cable shields. Thus current can flow causing hum. Some bands use transformers at the end of all signal cables, or "DI" units which use op amps to reject hum. If your amp or preamp doesn't require a safety ground by design, not having safety grounds in the power cords eliminates one source of problems. If your amp needs a safety ground, then if hum occurs some sort of star groundiing scheme (no loops) must be implemented by the sound man to eliminate hum and still keep people safe from shock.
 
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abraxalito, I see that you're in the UK and I know nothing about the UK power mains system.

Here in the US, power is delivered to our homes via a 240 volt center tapped transformer up on the pole (or underground in newer neighborhoods). The center tap is called "neutral" and we get two phases of 120 volt power and a single phase of 240 volt power (which is typically only used for things like electric clothes dryers, central air conditioning, etc).

At the service panel (where the power from the pole enters and gets distributed through the house), the neutral line is connected to an earth ground rod for lightning protection.

The 120 volt distribution line consists of three conductors. A hot (which will be one of the two 120 volt phases), a neutral and the safety ground.

Neutral and safety ground are effectively one and the same as they are both tied together at the service panel. The safety ground lead is only there to provide a fault return path back to neutral should the hot lead in the chassis fail and come into contact with the metal chassis.

Why the safety ground causes problems with interconnected audio systems is because the hot lead couples to it capacitively resulting in leakage currents in the safety ground. Because the safety ground has resistance, this leakage current results in a voltage drop between between any two points where there is leakage current flowing.

This is what can cause the classic ground loop in interconnected components which are safety grounded.

If the component's signal reference ground is connected to the equipment chassis, which is subsequently connected to the safety ground, then any voltage appearing between the safety ground pins on the power cords of those two components will result in interchassis currents which flow through the grounds of the interconnects.

And because the interconnect grounds have a non zero resistance, there will be a voltage drop across it which appears as noise at the input of the load component.

The sole purpose of the safety ground is safety. It serves no other purpose beyond that except to cause headaches for people trying to set up nice, quiet audio systems.

That's why, as in rg12's case where he has a component that doesn't require a safety ground, I don't recommend that he create one when modding the power cord.

Things may be different in the UK. As I said, I don't know exactly how the AC mains system is configured in the UK.

se
 

psdiy

Member
2010-09-10 5:16 am
I'm an electrical engineer specializing in communication systems. I think that I can provide some clarification.

The function of a grounded power cable is to provide consumer protection against an accidental leakage connection between the device's AC input supply and an external "close to ground" path. (Example - AC connected radio in a humid bathroom.) If such were to occur, the power cord presumably would be the better path and the supply mains fuse would either blow or the GFI would trip.

In operation the "grounding" conductor of the power cable is capacitively coupled to the power conductors. If you were to connect an oscilloscope from the "grounding" conductor to a high quality ground (metal water supply pipe), you'd see all kinds of noise on this connection. The noise is created by inductive devices like refrigerator motors, and capacitive devices like CFLs. Intemittentr sikes and dips are introduced by inrush and collapse currents as these devices are randomly turned on and off.

When we want to use "ground" as a shield we are looking for a nearly noise free source, and in most cases we'll try to use the Earth by connecting into the water table.

Problems arise when the electrical path back to this water table becomes long enough to contain appreciable resistance. Audio industry practice is to maintain this connection well below 0.1 ohms.

Here's a practical example of how electrical ground and signal ground can interact in unanticipated ways.

In Chicago we have a light rail system that runs about 15 miles from a northern suburb to the central business district. I worked for a company that had a contract with the transit company to provide a private telephone/intercom sytems to connect the stations along the this line.

When we connected the telephone switching equipment at the second station we experienced distinct hum in the audio. We measured the connection to the local earth ground and found it satisfactory - 0.03 ohms

But at each station that was further from the northern terminal the problem grew worse- regardless of the quality of connection to local ground.

After some thought we disconnected all the stations equipment from the transmission lines, tested the lines themselves and reconnected the stations starting at the business district end. The problem disappeared until we connected the northern station.

The problem was that the northern-most station and its electrical substation were built in an area with a thick layer of clay. The clay trapped an isolated pocket of ground water, creating in essence, a giant capacitor with a high resistance path back to "earth".

This wasn't a problem for the electrical motors of the trains, but was creating havoc on out audio system. What we had been using for our signal ground had a DC potential of more that 15 volts and AC "hash" envelope of almost 100V p-p.

To resolve our problem required driving steel beams as grounding rods down through the clay until we penetrated the limestone aquifer.

I've since witnessed similar issues in digital communication systems in hospitals. One utilized multiple power supplies in one closet for various subsystems. The signal grounds terminals were connected to the mains grounding line. The system worked erratically. Suspecting a power issue, I metered all the connections and lines. I found that the mains protective ground path had excessive resistance.

We notified the building maintenance department that they had an electrical safety issue, but rather than wait for them to resolve it, we separated the signal connections of our system from the mains grounding line and ran several heavier conductors back to where the water main entered the building.

Moral of these stories - mains protective "ground" is not the same as an Earth connection, and not a reliable source for a signal ground or shielding connection.
 
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