Controlling 0V with Power Supplies and Distribution
Sitting away behind the exciting functionality of audio systems are the boring and often ignored power systems, that is until there are problems interconnecting equipment. Hum, buzz, crosstalk etc will all be lumped together here as noise but particularly those which appear only when cables are joined between equipment. The following explores real world examples of the magnitudes of these problems and ways to address them. Diagrams will use the noted representations.
A common design for audio equipment is a power supply internal to the case, connected to the mains supply with a 3 pin cord including protective earth.
Internally multiple PCBs are connected to the same power supply with cables size appropriately by the designer. Ideally 0V and chassis are connected back to a single point with the protective earth connection to isolate the 0V from any external noise. When designed and built well this all works very well with balanced connections which connect the cable shield to the chassis:
Balanced interconnections theoretically cancel any variations or noise in the 0V connection of the output as it appears identically in both signals while the inputs only measure the difference between the two signal lines. Additionally any noise currents flowing in the shield are diverted through the chassis around the sensitive 0V distribution.
Unbalanced interconnects in most equipment instead join both ends of the shield to 0V and measure the signal directly. Any voltage difference between each end of the cable appears directly as noise, and flows into the 0V distribution. While for audio signals transformer isolation can be used to break these currents flowing between equipment, they do not pass DC as needed for CV signals.
But why is there a voltage difference between the 0V distribution in each piece of equipment? Several different effects combine and add to each other such as in the typical example of a Eurorack synthesiser case powered with an external brick:
Working from the mains earth on the left the power supply will have some leakage current which is conducted back onto the protective earth already creating a small voltage difference. But from the output of the brick to the case only a pair of wires carry both the power for the system and the reference back to the mains earth, this drop across the common return 0V adds again. Finally inside the case the power distribution and bus boards carrying the 0V return the power from the modules and adds again. This is all under the ideal situation of no difference between the earth potential at different mains sockets, those problems are often fixed by moving all the equipment to the same power board. But with that done further improvements can be made with a technical earth link:
Here a separate path is created for the 0V reference bypassing the power supply and power cable. This can be back to the mains earth or direct between equipment. Although the power supply has a 3 pin plug and connection to protective earth it has an isolated output, so the mains plug is not an sure indication of an earthing connection. This technical earth works even if the power supply is not isolated as long as the technical earth is much lower resistance than the protective earth, which is typically 0.1 ohms. Many people use this method to reduce noise connecting equipment technical earths with large cables, braids, or metal bars with resistances of 0.01 ohms or less. While this can work well in a fixed setup or studio it is less practical for portable systems. A simple option is to use cables and connectors with lower resistances, typical low cost cables are similar in resistance to the mains connection, 0.1 to 0.3 ohms. Higher quality cables can get to 0.01 ohms or less but again this means the system varies depending on how it is connected which is less than ideal.
Another method to reduce the difference between the 0V of different equipment is to not have a forced reference to mains earth. This is used widely in Hi-Fi products by having them powered with a double insulated supply which has no connection to mains earth:
This works well if there are only a few connections between the equipment and they are physically close to each other. The very small currents flowing are easily handled by the interconnecting cables even for unbalanced connections. This same design can be applied to eurorack systems by using an isolated power brick:
Within the case only differences in the 0V connection are from the power distribution delivering current to supply the modules. The differences between sockets close to each other can be very low but if the distribution is inadequate these voltage can quickly increase across the rack. These differences at the sockets can be measured with a voltmeter or multimeter as a quick way to check the quality of the power distribution, excellent examples have differences of 1mV or less between any two sockets while the worst examples can have 100mV or more. While these can be improved by using low resistance patch cables the system then varies depending on how it is connected, a more predictable improvement is by reducing the 0V distribution resistances. Many vendors offer solutions for this but in the presence of a poor power supply system this alone may not be enough. Low resistances of 0.01 ohms or less needed to control significant 0V currents of 1A or greater are hard to acheive between bus boards with cabling, nessceitating bus bars for larger systems wanting low noise.
Active bus boards with the power supplies integrated onto them remove the additional resistance of the distribution wiring. Combined with lower resistance distribution on the bus board they simplify Eurorack power systems and have been very popular. But most continue to have the problem with the common 0V return if not individually powered with isolated bricks. To simplify power even further the scaleable power supply uses isolated supplies at each board and keeps the power and 0V technical earth separate all the way to the bus boards.
This configuration separately brings out the technical earth to the power connectors for convenient connection on to other cases. By having no combined current flowing in the 0V from the supplies additional boards can be added to a case without affecting the others, scaling to larger systems with reliable and steady performance. The same design can be used instead with an isolated power brick.
The excellent distribution and lower power supply currents from using a higher voltage supply even improve a poor system design with a common power and technical reference. This arrangement is not recommended but included for comparison:
All of the above examples were setup in a studio to compare the current flowing in the cable shield connection to a 0V reference connected to mains earth, this reference point had no voltage difference alone as it was turned off and producing no current. The same eurorack case was tested with many different configurations and upgraded to a scaleable power supply as a direct comparison of the improvement. This was also compared to other studio equipment and a smaller single row eurorack case with a conventional power supply.
|Floating + Technical Earth||0.61||36||1.5|
|Earthed common with power||3.7||56||5.5|
|Earthed separated from power||1.1||1.4|
|Interconnection shield current (mA)|
The smaller eurorack case with low power modules and little imbalance in the 0V distribution current still presents a significant current in the shield when using a power supply with common earth and power. This gets out of control in the larger case with 56mA of current flowing though interconnects to other equipment, 20mA of this was from the common earth and power connection eliminated by using a floating supply and technical earth but the remaining 36mA is mostly from poor distribution. Upgrading to the scaleable power supply earthed either separately through the brick or by a technical earth shows the improvement possible with the almost ideal distribution they provide. This is comparable to the shield current present in other professional studio equipment shown in the last column.
There are multiple effects all adding to the 0V noise. From this information they can be fixed individually by selecting appropriate parts and connecting them effectively, or the scaleable power supply addresses them all in a ready to install design.