The safety earth is connecting point for the electrostatic shield. But, safety earthing have very poor HF draining characteristic - thin, often corroded metal and PVC dielectric. So much of the HF noise find its way to transformer secondary. Shield is cure, but it is always better to avoid disorder than to cure it. Only way to do it is to use EI transformers if it is possible. R core transformers are better than toroids in blocking mains garbage , but still have two times more capacitance between primary and secondary than same wattage EI types. They are very similar , almost identical to C core transformers but without cut cores.
I have bought recently a few used R cored transformers for very low price at the flea market. I destroyed a faulty sample to see its construction. Contrary to many prejudices, primary and secondary are not wound on the separate bobbins. Half of the primary is wound on one bobbin and another half on the another bobbin but in different direction ( balanced windings) . The same goes for the secondary winding. Stray field of R core transformers are ten times lower than EI, but mains filtering ability is less than EI, but better than toroids and identical to C -core.
R -core are ideal for applications which need transformers inside the enclosure
such are power amplifiers , but for remote power supplies for low power circuits, separate , split bobbins Ei , especially low profile models are optimal solution. Countless of split bobbin Ei transformers can be found at the very, very low prices
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John,
Way back you mentioned you used Signal brand transformers. As they used to make their transformers in Brooklyn, to match the competition's cost advantages, they had to be clever. Their most noteworthy design is the widely copied flat pack transformers. The three interesting features were the use of an "O" core, two bobbins and winding the primary and secondary next to each other!
The dual windings allowed the transformer's turn ratios to be more flexible. But for audio the big improvement was the side by side windings. They did this because it allowed easy customizing and faster winding times. For audio it meant much lower leakage capacitance from primary to secondary.
This winding design also carried over to the E-I cores they made. The advantage of an E-I core is the extremely efficient use of core material. Punching 2 E's leaves two I's!
The side by side coil design is however less efficient than the standard concentric winding method. This shows up as less "regulation" or the difference between open circuit and full load voltage. This is usually modeled as a resistance in the transformer circuit.
Most manufacturers of transformers rated theirs for 50 to 400hz. That is because aircraft started to use 400hz. AC supplies in the 30's and 40's to reduce the weight of the magnetic material.
A normal low power transformer, as I have measured, passes signal well into the 100's of khz. The Signal design does not work as well as traditional designs even at 400hz. A bonus for audio.
The concentric designs would often have a shield between the primary and secondary windings. This shield should be connected to the safety ground wire from the source side in a proper installation.
Off topic, I have a system going in where the audio system transformer was mis-spec'd as a 3 phase 25kva model instead of 2.5kva. The problem is when there is no load on the transformer the leakage from the primary can raise the secondary voltage due to the leakage capacitance. As the primary is above 4kv, this might lead to surprises! There is a shield which is grounded to the electrical system's safety ground. As the safety ground is also connected to the common or neutral lead at the building's entrance, this should provide protection. But only as long as everything is correctly wired and working. (Yes, this is where "surge protectors" are mandatory.)
Back to topic. this type of transformer often used for small power levels is called a loosely coupled transformer. Due to the higher losses it tends to get too hot in higher power designs.
Some people worry about stray magnetic fields, although toroidal transformers are better, in most transformer designs there really is not very much compared to the core fields. Of course you used a separate chassis so distance and lack of a continuous flux path work well for you.
One big note, when you hear a transformer buzzing it often means that the core is saturating. This can be caused by either a DC offset on the primary or secondary or not enough magnetic material in the core for the operating frequency and current. That is why some do not use dual bobbin transformers as a center tapped version, but use it as dual DC supplies.
The issue of diodes is not as simple as it seems. When you are using a transformer of this type, the time when the diodes are forward biased and conduct is extended. That is because until the diodes conduct the secondary voltage floats higher than it would be under load. As the diode begins to conduct it lowers this voltage just enough to keep the current flowing. The inductance of the secondary winding usually keeps this from being oscillatory. As the input voltage peaks the diodes continue to conduct as the secondary floats up again as the current load drops. Soft switching or fast diodes reduce the noise at this point. However all diodes have switching transients.
You mentioned you also put inverse parallel diodes in the common leads. I assume you do this to drop any EMI leakage that is passing through the transformer when there is no current flow.
Also mentioned was the use of a common mode choke after the diodes. As long as the DC current is balanced here a small core can be used for this as the balance between power and return will cancel out and only the common mode EMI and diode noise will be filtered.
One other issue is the line cleaning capacitor. You mentioned you have used a capacitor straight across the AC line. That works, so does a tuned RC network. A better solution may be a capacitor across the secondary of the power transformer. This requires a larger transformer to allow for the circulating currents, but has the advantage of the transformer's inductance helping the filtering action.
I assume filter capacitors are next?
Way back you mentioned you used Signal brand transformers. As they used to make their transformers in Brooklyn, to match the competition's cost advantages, they had to be clever. Their most noteworthy design is the widely copied flat pack transformers. The three interesting features were the use of an "O" core, two bobbins and winding the primary and secondary next to each other!
The dual windings allowed the transformer's turn ratios to be more flexible. But for audio the big improvement was the side by side windings. They did this because it allowed easy customizing and faster winding times. For audio it meant much lower leakage capacitance from primary to secondary.
This winding design also carried over to the E-I cores they made. The advantage of an E-I core is the extremely efficient use of core material. Punching 2 E's leaves two I's!
The side by side coil design is however less efficient than the standard concentric winding method. This shows up as less "regulation" or the difference between open circuit and full load voltage. This is usually modeled as a resistance in the transformer circuit.
Most manufacturers of transformers rated theirs for 50 to 400hz. That is because aircraft started to use 400hz. AC supplies in the 30's and 40's to reduce the weight of the magnetic material.
A normal low power transformer, as I have measured, passes signal well into the 100's of khz. The Signal design does not work as well as traditional designs even at 400hz. A bonus for audio.
The concentric designs would often have a shield between the primary and secondary windings. This shield should be connected to the safety ground wire from the source side in a proper installation.
Off topic, I have a system going in where the audio system transformer was mis-spec'd as a 3 phase 25kva model instead of 2.5kva. The problem is when there is no load on the transformer the leakage from the primary can raise the secondary voltage due to the leakage capacitance. As the primary is above 4kv, this might lead to surprises! There is a shield which is grounded to the electrical system's safety ground. As the safety ground is also connected to the common or neutral lead at the building's entrance, this should provide protection. But only as long as everything is correctly wired and working. (Yes, this is where "surge protectors" are mandatory.)
Back to topic. this type of transformer often used for small power levels is called a loosely coupled transformer. Due to the higher losses it tends to get too hot in higher power designs.
Some people worry about stray magnetic fields, although toroidal transformers are better, in most transformer designs there really is not very much compared to the core fields. Of course you used a separate chassis so distance and lack of a continuous flux path work well for you.
One big note, when you hear a transformer buzzing it often means that the core is saturating. This can be caused by either a DC offset on the primary or secondary or not enough magnetic material in the core for the operating frequency and current. That is why some do not use dual bobbin transformers as a center tapped version, but use it as dual DC supplies.
The issue of diodes is not as simple as it seems. When you are using a transformer of this type, the time when the diodes are forward biased and conduct is extended. That is because until the diodes conduct the secondary voltage floats higher than it would be under load. As the diode begins to conduct it lowers this voltage just enough to keep the current flowing. The inductance of the secondary winding usually keeps this from being oscillatory. As the input voltage peaks the diodes continue to conduct as the secondary floats up again as the current load drops. Soft switching or fast diodes reduce the noise at this point. However all diodes have switching transients.
You mentioned you also put inverse parallel diodes in the common leads. I assume you do this to drop any EMI leakage that is passing through the transformer when there is no current flow.
Also mentioned was the use of a common mode choke after the diodes. As long as the DC current is balanced here a small core can be used for this as the balance between power and return will cancel out and only the common mode EMI and diode noise will be filtered.
One other issue is the line cleaning capacitor. You mentioned you have used a capacitor straight across the AC line. That works, so does a tuned RC network. A better solution may be a capacitor across the secondary of the power transformer. This requires a larger transformer to allow for the circulating currents, but has the advantage of the transformer's inductance helping the filtering action.
I assume filter capacitors are next?
Well said, Kamis and Simon. For me it is also better to avoid the problem in the first place by separating the windings, and only allowing that magnetic coupling to be significant. Free filter, you see? I think that we use R core in the Parasound JC-2, and it works well. In the CTC Blowtorch, designed about 10 years ago, we found the E-I high isolation transformers from Signal to do the job, BUT the stray magnetic field could be a problem IF we had put them in the same box. That is why we used 2 Boxes, one for the power supply, and one for the preamp electronics. This works OK so long as the 2 boxes are separated from each other.
Typically, they are all one and the same node.
But as simon7000 said, for the safest implementation, it would go to the safety ground. Which will be tied to the chassis, and the chassis tied to power supply and other grounds (though some attempt to isolate power supply and signal grounds from the chassis with diodes, thermistors, RC networks, etc.).
se
I generally don't rely on 'safety ground' for anything. I don't even have it in my living room. I use other techniques such as power line filtering, good AC isolation, and high isolation transformers, just like I did, when I was young, and 3'rd wire grounds were not legislated in to all home outlets. I am always trying to make the idealized equivalent of a battery. Sometimes, in my home systems, I use just batteries.
In the US, "safety ground" is the same as the "neutral" AC lead. The two are tied together at the service panel. So I don't see that the safety ground is any more a source of RF and industrial interference than AC neutral given that they're at the same potential.
The reason for tying the electrostatic shield to the safety ground is purely a safety issue, in the event a failure in the transformer causes the hot lead to contact the shield. This would allow the fault current to flow back to neutral and prevent potentially lethal voltages on the equipment chassis.
Safety ground's a problem because it's the source of chassis leakage currents due to capacitive coupling with the hot AC lead. The safety ground by code must be connected to the equipment chassis. And signal grounds are also commonly connected to the equipment chassis.
Thus, if you have two pieces of equipment connected to each other, particularly unbalanced equipment, the leakage currents in the safety ground can be picked up due to the IR drop across the two points at which the equipment's safety grounds are connected.
se
Pavel,
Safety ground is just that, used for protection against primary insulation failure. It may not be required when other safety measures are used. Grounding for signal shielding as most every one has learned is hard to do properly and rarely useful. A Faraday cage is a better approach.
Bonsai,
The CT cap is used to meet EMI emission requirements. It is to reduce radiation by balancing the switching noise to common. Reducing send noise sometimes uses different methods than reducing received noise.
To All,
All noise has the source, the path, and the receiver. Three areas of concern!
Safety ground is just that, used for protection against primary insulation failure. It may not be required when other safety measures are used. Grounding for signal shielding as most every one has learned is hard to do properly and rarely useful. A Faraday cage is a better approach.
Bonsai,
The CT cap is used to meet EMI emission requirements. It is to reduce radiation by balancing the switching noise to common. Reducing send noise sometimes uses different methods than reducing received noise.
To All,
All noise has the source, the path, and the receiver. Three areas of concern!
Pavel,
There are differences. In the US AC power is distributed as three phase at high voltage at least 4160 often 12-14kv. These cables run at the top of the power poles. Often the neutral is the top most wire but not always. This is in some systems stepped down to 120-130 volts with one side being connected to the same neutral as the HV line. This is then grounded at the pole base. Two or three of the 120-130 volt lines are then fed to the customer depending on if they get single phase "220" volt service or 3 phase "208" volt service. The step down transformer does have taps to get the exact voltage closer to what is expected. In solely residential neighborhoods the transformer may be a single center tapped "220" volt unit fed from a single phase of the three phase high voltage line. The CT is grounded and also used as the neutral.
The safety ground is tied to the neutral in a proper installation only at the buildings electrical service entrance. It is not tied at each panel or sub-panel.
The shield in a consumers equipment should be tied to the safety ground. In a low noise transformer design there will be a second insulated shield that is used for noise prevention. This should go to the Faraday cage potential.
The US used this system because at first there was no neutral wire used, just the actual earth ground. This saved on copper wire!
In some safety ground retrofits the safety ground is tied to the neutral at each panel. That effectively places the safety grounds in parallel with the current carrying neutrals and creates hum and noise on any gear connected between the panels AC feeds.
ALL EARTH GROUNDS HAVE NOISE! Just stick two ground rods in the earth anywhere and your spectrum analyzer will show you that. The farther apart you place the ground rods the greater the voltage.
If your transformer only has one shield that should be connected to safety ground. The transformer can then be mounted with case insulators. That way you can float your chassis and use it as a Faraday cage.
If you do not have a second isolated shield the center tap can be coupled to the chassis. If using dual supplies then you can use 4 capacitors to get the same effect.
I do sound systems of many KW. I do not get hum and meet code requirements.
There are differences. In the US AC power is distributed as three phase at high voltage at least 4160 often 12-14kv. These cables run at the top of the power poles. Often the neutral is the top most wire but not always. This is in some systems stepped down to 120-130 volts with one side being connected to the same neutral as the HV line. This is then grounded at the pole base. Two or three of the 120-130 volt lines are then fed to the customer depending on if they get single phase "220" volt service or 3 phase "208" volt service. The step down transformer does have taps to get the exact voltage closer to what is expected. In solely residential neighborhoods the transformer may be a single center tapped "220" volt unit fed from a single phase of the three phase high voltage line. The CT is grounded and also used as the neutral.
The safety ground is tied to the neutral in a proper installation only at the buildings electrical service entrance. It is not tied at each panel or sub-panel.
The shield in a consumers equipment should be tied to the safety ground. In a low noise transformer design there will be a second insulated shield that is used for noise prevention. This should go to the Faraday cage potential.
The US used this system because at first there was no neutral wire used, just the actual earth ground. This saved on copper wire!
In some safety ground retrofits the safety ground is tied to the neutral at each panel. That effectively places the safety grounds in parallel with the current carrying neutrals and creates hum and noise on any gear connected between the panels AC feeds.
ALL EARTH GROUNDS HAVE NOISE! Just stick two ground rods in the earth anywhere and your spectrum analyzer will show you that. The farther apart you place the ground rods the greater the voltage.
If your transformer only has one shield that should be connected to safety ground. The transformer can then be mounted with case insulators. That way you can float your chassis and use it as a Faraday cage.
If you do not have a second isolated shield the center tap can be coupled to the chassis. If using dual supplies then you can use 4 capacitors to get the same effect.
I do sound systems of many KW. I do not get hum and meet code requirements.
In my country , also, neutral lead and safety ground are tied together at the main panel. You can measure mains voltage either between live and neutral or live and safety ground, results are the same. But local safety earth at so called Shuko sockets are also tied with green/yellow wire to water and central heating pipe for additional protection.
Only signal ground of one piece of equipment in audio system( power amplifier) should be directly tied to safety ground. Others components will be protected via power amplifier safety ground terminal. Metal enclosures of
preamp, cd player etc. should also be tied to safety ground , and its signal ground must be connected to safety ground via 4.7K , 11W wirewound resistor.
It is value recommended by Ben Duncan, a well known British audio expert.
To be honest, I do not fully understand why such a high values are needed, but even with high gain MC input turned up to maximum , there are no any hum and buzz in my system.
Almost all commercial CD players are delivered with two wire cable, and their metal cases are not tied directly to safety ground , but to signal ground. A three wire cable should be installed , metal enclosure grounded to safety ground, and signal ground connected so earthing point via am. resistor.
Ben Duncan mentioned PCB track evaporation (!) if signal ground remains unconnected. Let us trust him!
Simon, my point is that connecting a shield of a toroidal transformer to a safety ground does not help much to protect from intereference coupling into the instrument. I do not discuss here differences in power distribution networks, BTW I have installed plasmatron systems in US, Japan and Europe and I am aware of these differences.
There are other, more efficient design principles, like double shielded box, or transformer with more shields than one.
I support John here, EI transformer has MUCH lower capacitance between primary and secondary windings and brings less troubles with interference throughput from mains to the instrument.
There are other, more efficient design principles, like double shielded box, or transformer with more shields than one.
I support John here, EI transformer has MUCH lower capacitance between primary and secondary windings and brings less troubles with interference throughput from mains to the instrument.
No. There's absolutely no reason to install a three wire cable. If the equipment chassis meets Class II specs, consider yourself lucky that it doesn't need a three wire cable.
I haven't much faith in Duncan.
Or are you being sarcastic?
se
Moving ahead, it is good to try to make the audio device as free from the power line as possible, since the power line is so full of extra interference. Batteries are the best, but expensive and unreliable. Good isolation can be made with an E-I or an R core transformer, avoid toroid power transformers if possible. With a good transformer, a common mode choke could be very useful and they are cheap and easy to buy. Where to put the common mode choke can depend on the situation.
In this 'intellectual' atmosphere, I am not sure if I can go further forward on this subject. I don't get paid to teach on this website, so the attitude of the 'class' has some importance to me. Now that I know that SY has direct moderation of this thread, bodes serious problems for me to try to give my honest and successful, I might add, opinion as to how to make better audio products.
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