Velleman K8084 Earthing / Grounding - diyAudio
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Old 28th June 2012, 01:52 PM   #1
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Default Velleman K8084 Earthing / Grounding


I'm currently building a Velleman K8084 "VOLUME AND TONE CONTROL - PREAMPLIFIER" and am a bit confused about grounding the case and earthing the whole circuit.

I plan to build a wooden enclosure with a copper lining and then connect the lining to a ground terminal on the PCB. However, I'm not really sure how to go about safety earthing the enclosure and the circuit itself. I assume that if I simply connect the earth wire from the wall plug to the copper lining I could set up a ground loop and end up with humming. Do I need to use a loop breaker circuit? What really confuses me is that the manual suggests I don't even need to use a power lead with an earth wire (which doesn't seem too safe to me).

The manual and circuit schematic can be found here:

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Old 29th June 2012, 10:17 AM   #2
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Location: Germany
Building an IEC Class II device isn't rocket science if you know what the requirements are (see linked article). Plenty of audio equipment is constructed like that, otherwise issues with ground loops would be extremely common. And even if the chassis did become live, most homes these days should be equipped with some kind of residual-current device that would trip.

Copper lining would be expected to be connected to star ground. Additional connections using low-inductance capacitors in the 1..10 nF nF range may be established for better RF suppression.

Note that for this kit, the instructions for modifying voltage gain (p.5, R21..R24) are not exactly ideal. If you did this as described, the unit would become needlessly noisy. Instead, aim for RF + RA ~= 10k or so. In terms of noise, RF and RA are in parallel, which is why the suggested stock config is fine in that regard.

If you want to try other opamps, I'd give something like a NJM4560/4565 a shot for IC1 (when not going FET input - and there are good reasons not to - , a low Ib, PNP input part should be safest to use there). The stock TL072 should be fine for IC2. I'm not sure whether isolation resistors R5, R6 are sufficient though, I'd rather play it safe with 47..100 ohms.
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Old 29th June 2012, 11:15 AM   #3
DF96 is offline DF96  England
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For Class II you would probably need to ensure that the transformer uses a split bobbin - many do nowadays. All mains connections would need insulating 'boots'. No single failure (e.g. loose wire) should cause a dangerous situation. Never rely on an RCD. Most DIY uses Class I (grounded case) as that is easier to ensure safety.

If you go for Class I you need to ground not just the case but also the transformer frame, although the mounting bolts may do this for you. Ground loops can be a problem, but if everything is plugged into the same outlet (perhaps via a multiway) then you might be OK.
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Old 29th June 2012, 12:08 PM   #4
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Thanks to both of you for your replies.

So I would need to connect the copper lining and the frame of the transformer (perhaps by ensuring a good connection between the frame and the lining) to the earth wire. What about the circuit itself and the RCA connectors - where should I connect the earth wire to prevent the input and output connectors accidentally becoming live?

Also, would you recommend using a loop breaker circuit to prevent the earth connection causing ground loops? Link below:

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

Also, sgrossklass, would I need to make any other modifications to the circuit to use a NJM4560/4565 as IC1. Finally, were you suggesting I should use R5 = 47ohm and R6 = 100ohm.

Thanks again.
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Old 29th June 2012, 01:17 PM   #5
DF96 is offline DF96  England
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The PCB provides a ground via one of the mounting bolts. That should in turn automatically ground the connectors via the cable screen.
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Old 2nd July 2012, 02:47 PM   #6
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So if I ground the PCB to the case (by using the grounded mounting bolt) I can then just connect the case and transformer to the earth wire and that should prevent the connectors becoming live?
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