DJ mixer earthing with a 2-pin power supply

[Lotus 7]

In this case the "earth" terminal is not a safety ground to the power supply ground reference. It is a "earth" reference to tie the turntable cartridge shell, tone arm and usually the mechanical chassis to the same reference level as the chassis of the phono preamp input. It's in addition to the shielding on the phono input cables and is mainly designed to prevent any possible slight voltage difference between the conductive turntable components and the preamp ground reference and to prevent any "ground loop" current from flowing in the output cable shields. Remember, the turntable leads are "single-ended" so any current flowing in the shields will generate an offset voltage (hum and noise) in the actual cartridge signal. A "short as possible" length of 16 or 18 Ga. stranded wire is usually fine for that connection.

Having a "turntable ground" post on phono-input equipped typical consumer stereo receivers and preamps is very common.

Also, for the best signal to noise ratio, it's always a good idea to be sure that the turntable motor is being powered from the same AC (mains) source. In other words, don't plug the turntable into one wall outlet and the DJ mixer in to a long extension cord going to another wall outlet.

For a good S/N ratio, it's not necessary to have an actual "earth" (ground) connection, but it is imperative that all the system parts have the same reference voltage for the low side of the signal cables (in single-ended analog systems). That's one of the reasons that more complex, professional signal patching always uses balanced connections which eliminate problems caused by slightly different ground reference levels.

[tjhertz]

thanks for the info... i think i'm missing something though. if the AC supply to the mixer is non-grounded and can be connected either way round, how does the mixer know which of the two pins is neutral and which is live?

in other words, what's to stop the mixer connecting the ground post to the live wire?

[Lotus 7]

Originally Posted by tjhertz

thanks for the info... i think i'm missing something though. if the AC supply to the mixer is non-grounded and can be connected either way round, how does the mixer know which of the two pins is neutral and which is live?

in other words, what's to stop the mixer connecting the ground post to the live wire?

The power input to the mixer is "floating" neither of the wires going to the IEC socket are ever used for a ground reference. Although one may be "low" and the other "high" it doesn't matter and has nothing to do with you signal "ground".

On 2-wire power supplies, the power input is fully isolated from the chassis ground.

With everything connect properly, and presuming that your turntable has a 2-wire line power cable like the mixer, the "ground" reference for your whole system is "floating" in reference to the power supply ground (as it should be). The "low" side of the power outlet is not actually at true ground potential because of the return current, and may be at as much as a few volts above true ground. That's why it's never to be used as a signal ground reference.

It's entirely possible that with a high input impedance voltmeter or a oscilloscope you might measure a difference of several volts between the chassis of your mixer and a true ground (a copper rod driven into the earth or a cold water pipe "earth"). That doesn't matter, as long as all of your components are at the exact same reference level.

Signal grounding and safety grounding are entirely different situations.

[JohnRoberts]

Originally Posted by tjhertz

thanks for the info... i think i'm missing something though. if the AC supply to the mixer is non-grounded and can be connected either way round, how does the mixer know which of the two pins is neutral and which is live?

in other words, what's to stop the mixer connecting the ground post to the live wire?

2 wire line cords are double insulated. Neither gets connected to internal product circuitry. 2 wire plugs are often polarized with different sized blades, but this is not generally used as a safety ground.

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[joke]
To properly "earth" a DJ mixer be sure to dig a deep enough hole. If the soil is not conductive enough consider watering it personally. [/joke]
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Seriously any ground terminal is for turntable chassis ground (usually a separate green wire) to reduce hum and is unrelated to mains power ground other than providing a path for mains related noise picked up by turntable structure. This DJ mixer ground does not really return noise current to earth, just clamps the voltage to the mixer electronics chassis and local reference 0V. Note: the local DJ mixer ground may find real path back to earth ground through output wiring to a power amp, if it is properly grounded.

JR

[tjhertz]

right, i think i get it, but only partially. i understand that the signal ground can be floating in relation to mains ground. but if the signal ground is the same potential as the chassis (is it?), then how can the device be double insulated? what's to stop the mixer from putting out high voltages at the audio terminals?

feel free to point me in the direction of relevant literature if i'm asking something embarrassingly basic....

[JohnRoberts]

Yes this is pretty basic..

The mains wiring and transformer primary are insulated by two layers of insulation, to prevent any possible electrical connection...

Two layers of insulation means no shock from that path.

JR

[Lotus 7]

Originally Posted by tjhertz

right, i think i get it, but only partially. i understand that the signal ground can be floating in relation to mains ground. but if the signal ground is the same potential as the chassis (is it?), then how can the device be double insulated? what's to stop the mixer from putting out high voltages at the audio terminals?

feel free to point me in the direction of relevant literature if i'm asking something embarrassingly basic....

Ideally, the mixer "ground" would be at true earth ground, but it doesn't have to be to work.

The fact that it is "double insulated" is what keeps the mixer chassis (and front panel) from being at any dangerous voltage level. There is no source for the dangerous voltage, UNLESS the isolation transformer insulation fails or some other source creates a short to the high line side. That's why we have 3-wire line cords. They provide a true ground return, so in the event of a short between the high side and a chassis, the current is shunted and a circuit breaker or fuse blows.

If you don't trust the "double insulation" then replace the 2-wire IEC socket with a 3-wire socket and a 3-wire mains cord. Or install an independent wire to the "earth" pin on a tested 3-wire wall outlet.

As stated before, signal grounding and safety grounding are two different subjects.

[tjhertz]

got it now... i'd kinda forgotten how transformers and AC power worked. it's been a while. thanks for the help.

[Lotus 7]

Great!

And remember, It's always possible for the "double insulation" to fail AND a 3-wire ground to fail, so if you really want to be safe, wear rubber gloves* and keep one hand behind your back at all times when DJ'ing.

* Always use protection, and look both ways before crossing the street.