common mode currents

back to learn
back to power supplies

post separation
Common mode currents… a modern audio disease

Noise on the mains can be both common mode and differential mode.

In differential mode the voltage/current is measured between Live and Neutral and for secondary voltages (behind a transformer or SMPS) between + and – (5V, 12V or whatever voltage)

Differential mode noise on the mains will also be present at the output of a mains transformer but lowered with the same factor.
Most mains transformers start to ‘drop off’ in output somewhere between 2kHz and 30kHz (depends on make/type/model etc) so higher frequencies are more or less blocked.
When the AC voltage is rectified and perhaps regulated all these differential mode voltages will be gone.

For countering differential mode noise RC (Resistor & Capacitor) / LC (Inductor & Capacitor) filters help as well as regulators (up to a certain frequency). A LOT of differential noise is removed by rectifiers and buffer capacitors.
Bigger capacitors (values) can remove lower frequencies.
Smaller (de-coupling) capacitors (often found in parallel) can remove HF noise. Depending on their value these are most effective in a certain frequency range. This is why sometimes you see ‘compound’ decoupling capacitors. 2 or 3 small caps in parallel say 10μF ceramic + 100nF + 1nF for instance.

Common mode voltages/currents are measured/present between the (PE/ground/safety earth) and either voltage rail. This common mode voltage can not be found between the L and N lines. That’s why it is called ‘common mode’ as the voltage is common (equal) on both lines BUT exists opposite PE/ground/safety earth.
Common mode voltages (and currents) thus cannot be measured across the mains wires nor across the output terminals.
When there is a common mode voltage the voltage is thus 0V between L and N for instance but is present between L(and N) and (PE/ground/earth).
A common mode current can flow between the voltage lines and PE/ground/safety earth or between devices that are powered separately and are both connected to ground in some way. This can be via mains or via the human body (when touching someting) or capacitive.

Common mode currents can be lowered with ferrites. These work optimally when you loop the wire about 3 to 4 times through a ferrite ring.
Another effective part is a common mode choke which has 2 windings in opposite directions where L and N (or + and -) run through.
Could be combined with capacitors, ferrites or multiple chokes + caps and form filters that have a certain attenuation in a specific freq. band.

When you measure the output voltage of a DC power supply you might see a very clean DC with little to no noise on it.
When you measure the + (or -) to (PE/ground/safety earth) you may probably see a LOT more noise.
The circuit it feeds does not really care about these common mode noises as the total power supply varies along with the common mode noise.

This can become an issue when the device it feeds is connected to another device (say a PC to a DAC to an amp) and all of these devices have different ‘leakage’ to mains / ground.
In this case the different currents also can (depends on several factors) flow between the ground/shield/common of the interlink cables.
Currents introduce voltages and these can be amplified.

Consider each device that is connected to mains as an individual power source while each having a different frequency spectrum (which can range from 50Hz to GigaHerz), different voltage levels (ranging from a few Volt to several hundred Volts) and different currents, mostly somewhere in the mA (milli Ampere) range.

With a simple multimeter that can measure AC voltages and AC currents it is easy to measure the leakage currents and voltages of (internal and external) power supplies.

The black lead of the multimeter is connected to a ‘known’ safety ground. This can be a blank part of a central heating radiator, water tap or safety ground pin of a wall socket. NEVER connect the black lead to either mains pin !

Set the multimeter to read AC voltages. The device under test must NOT be connected to anything else than mains. The red lead is held against the screen of one of the RCA’s.


Here we can see there is a 82V AC voltage opposite the safety ground. The voltage itself is a small indicator of how much leakage there is and depends largely on the power supply circuit used. It is the CURRENT that is important and can cause problems. When this current is relatively high this current COULD (depending on other circumstances) create a small voltage drop oven the screen of the interlinks which will be connecting 2 (or more) devices together. How high this voltage will be and how audible this could become largely depends on the resistance of the screen(common) wire of the interlink and internal wiring of the devices in question. To give an idea of the actual currents of a rather typical SMPS that is feeding this amp simply read the scale on the picture below….. 0.2mA.


The difference with linear power supplies (a transformer followed by rectifier and regulator(s) is rather big. Below a picture of an equalzer with linear power supply.


The leakage voltage is much smaller but more importantly the leakage current is many factors lower as seen below. The leakage current is only 1.9μA which is 100x smaller !


below a schematic view of 2 separate devices connected to mains only.

common mode 3Component A could be a CDP(layer) or the soundcard of a PC or DAC. Component B could be an amplifier or other device connected to A. The audio signals pass through the interlinks from A to B and it appears as that there is no ‘ground loop’ as there is only one connection between A and B… the interlink. Alas there is another path that isn’t that visible. Component A AND component B are also connected to mains and mostly even in the same wall socket. When one considers each component, A as well as B or when more equipment that is connected also C (e.t.c.) as having their own ‘voltage sources’ things change. Each component when NOT connected to another component has a ‘garbage source’ as represented in block A and B. Both components have their own different kind of ‘garbage generators’ on board. These garbage generators are the power supplies and in case of digital electronics also the circuitry itself. This garbage is NOT seen in the output signals of component itself and the audio is clean as a whistle. The ‘garbage‘ itself IS present between the ‘common/ reference ground‘ of the circuit and the part of the power supply connected to mains. How MUCH voltage, current and at which frequencies this generated ‘garbage’ is present depends on the power supply itself. WORST in this aspect, the generation of unwanted signals that is, which I call ‘garbage’ is generated by SMPS that is connected to 2 prong mains outlets (the double isolated power supplies) which create considerable amounts of garbage within and well outside the audible band depending on the topology and circuit (components) used. On second place we find SMPS connected to 3 prong outlets that in general produce slightly less ‘garbage’. With some considerable distance 3rd place is for the highly regarded linear power supplies with Toroidial transformer(s) 4th place is for E core transformers with windings on top of each other. 5th place for E cores with separated windings and best in this aspect are transformers with bobbins (coils) far apart such as R core transformers. SMPS have the highest amount of ‘garbage’ on it where mains transformers in linear power supplies only have ‘leakage’ caused by capacitances of wiring close together. This causes a leakage current from the mains into the audio circuit which can become high enough to cause minute currents in the audio-path, combined with high resistances creates voltages IN that signal path which can be heard as a low level background hum that is hard to get rid off. Higher frequency garbage on the mains (from dimmers or other electrical appliances) cuts through these transformers even more so creates louder noises, clicks or buzzes.

When this equipment is used stand alone this id NO problem at all. Things CHANGE, however, when these 2 different ‘garbage’ generators are connected to each other with interlinks and the ‘garbage generators’ each are connected DIRECTLY to each other via the screen of the audio interlink(s). Now these generators have a loop and thus the ‘garbage’ voltages produced by the individual components have a loop in which currents WILL flow.

common mode 2How MUCH current and what frequencies will be present depends on the properties of the ‘garbage generators’. How AUDIBLE these currents become depends on if they are in the audible domain, how (in)sensitive the components A AND B are for these currents. It is also quite possible garbage with frequencies well OUTSIDE the audible band (present in SMPS !) are ‘demodulated’ by component A or B in a similar way an AM radio makes music. Think of a cell phone being close to audio equipment and is being called or switched on. The ‘tone bursts’ you hear are demodulated signals in the 900 or 1800 MHz range ! This loop must be ‘broken’ but is hard to do. Mains filtering can help but ONLY when done properly. What ‘properly’ is differs from situation to situation.
When a circuit is sensitive to cell phone signals (900MHz or 1800MHz) this doesn’t automatically mean it will be sensitive to other frequency common mode signals nor is it the other way around. A circuit can still be susceptible to common mode garbage at lower frequencies but be completely insensitive to cell phone frequencies. So the cell phone test (putting a cell phone on top of the DUT (device under test) and calling that phone) is NO indication it is susceptible to lower frequency common mode CURRENTS as well. Small common mode CURRENTS can create larger common mode VOLTAGES when that circuit has a high local impedance on the PCB which could be narrow band as well. Bad PCB layout and incorrect usage of ground-planes may be the cause of high local impedances at high frequencies.

A way to lower the common mode currents is to add a separation transformer as shown below.

common mode 1This doesn’t CURE the problem it can only lower the currents to a value that isn’t of any significance any more when it drops below the audible limits of our hearing. There will always be common mode currents but those in the audible range will be significantly lower. HF garbage (>10MHz) will pass the transformer by the capacitance between the primary and secondary windings as if it isn’t there and can still cause problems (mostly in digital equipment).

I am willing to bet many people have experienced strange sounds or ‘a strange pressure’  while listening to music which they find hard to explain or get rid off. In the old days problems like hum and switching on/off appliances in the house causing loud ‘ticks’ would be the only problems that had to be addressed, now with the cheap SMPS and digital circuits this common mode garbage thing can ‘fog’ audio as well.

The article below will show you how to minimise those pesky ‘garbage’ currents and might be a cure in case you are experiencing weird noises/sounds or hum problems in your audio gear.

post separation
How to get the lowest amount of interference in your audio system.

This works for all analog as well as digital and is based on asymmetry in leakage currents of all power supplies be them SMPS (switch mode) or linear (transformer)

Remove all interlinks so you have ONLY separate equipment.
Note that equipment that is double isolated differs from those that have safety ground.
Also note that safety ground is NOT a the same as  RFI ground !
In general all safety grounds are extremely lousy RFI grounds.

You will need a either a voltage finder like the one below.  It is a sort-of ‘screwdriver tester’ designed to show which is the Live an Neutral prong in a mains outlet.


When the device under test is connected to mains only an switched on on you can simply measure the leakage (in coarse steps) of that device with respect to ground. You must touch the rubber test-pad on the screw driver to create the ‘ground’ to which it measures. The height of the voltage is an indication of how much leakage there is.

Turn the mains plug 180 degrees in the socket (when possible) and check again. Sometimes there is a big difference in leakage between those 2 positions of the mains plug (rotated 180 degrees or not). If there is little to no difference it doesn’t matter HOW this device is connected to mains. When there IS a difference in voltage the orientation of the plug in the outlet must be marked on the plug and wall socket. The lowest value is the optimal way to connect it to mains.

Another option is to use a multimeter.
It should have AC voltage AND AC current measurement capabilities.

The black lead of the multimeter is connected to a ‘known’ safety ground. This can be a blank part of a central heating radiator, water tap or safety ground pin of a wall socket. NEVER connect the black lead to either mains pin !

Set the multimeter to read AC voltages and set it to at least 200Vac. The device under test must NOT be connected to anything else than mains. The red lead is held against the screen of one of the RCA’s. Note or remember the measured AC voltage.

19VAfter this VOLTAGE measurement set the multimeter to measure CURRENT, this often involves changing the position of the red test-lead in the multimeter. Make sure you ALWAYS plug the leads back in the voltage measurement holes AFTER measuring currents. When you accidentally try to measure a voltage with the leads set in the current positions you are sure to blow the fuse inside the meter OR the meter itself… consider yourself WARNED. In the current measurement mode repeat the measurement above but now for AC-A. Set the meter for at least 200mA for the first measurement and decrease the range selector when needed to get a good reading. Note or remember the measured AC current.


Turn the mains plug 180 degrees in the socket (when possible) and do both measurements again. MIND the test-lead socket positions and the range/mode selector switch of the multimeter.

Sometimes there is a big difference in leakage between those 2 positions of the mains plug (rotated 180 degrees or not). If there is little to no difference it doesn’t matter HOW this device is connected to mains. When there IS a difference in voltage the orientation of the plug in the outlet must be marked on the plug and wall socket. The lowest value is the optimal way to connect it to mains.

You can mark the outlet(s) with a sticker or waterproof marker ‘dot’ for example. When using extension cords or connection blocks mark all the outlets. Use a simple voltage finder to determine the ‘hot’ and ‘cold’ mains pins.

Note that equipment with figure- 8 mains connections on the back also need to be marked as these can easily be set in the wrong position.

When all equipment is tested and mains connectors are set for lowest leakage current connect them with clean interconnects (clean the RCA’s too if dirty/oxidised) and this will be the best situation.

NOTE that this test only works to determine the lowest leakage currents in the LF range (50-60Hz). There can be much higher audible frequencies present as well as frequencies well above the audible range which can also create audible problems as well. In general the ‘garbage’ output is also less when the measured voltage (current) is lowest.

Those plagued with weird sounds or ticks from external  equipment e.t.c. or faint hum or whizzing noises may find their equipment to be less plaqued or even totally silent when all equipment is connected with the mains plugs in their ‘optimal’ position.
If still plagued with unwanted signals then one can resort to ferrites (do little if nothing mostly but could be worth trying), insert mains filters (they may also worsen things !) or separation transformers.

Ferrites only ‘help’ if you are experiencing weird noises in the audio that is caused by RFI common mode garbage.
It is not easy to determine if noises are caused by audiofrequent groundloops or RFI without testing or measuring.
Ferrites can only attenuate RFI a certain amount of dBs which could, if the attenuation is enough to become too low to cause problems (below a certain treshold it has no influence anymore, that treshold differs).
Groundloops where garbage is in the audible range (or just above it and ‘mirrors’ into audio by interference) will NOT be addressed by ferrites at all… NO influence in the audible range but only above 100kHz.

The audio frequency groundloops can also be ‘broken’ or at least lowered considerably in the audible range by powering the one or more pieces of equipment via their own isolation transformer(s).
These transformers do little to nothing against RFI ground loops, only ferrites and common mode mains filters can lower these loops.

When you are not experiencing any weird noises I see no reason to apply all these costly measures myself.

post separation

back to learn
back to power supplies

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.