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Old 29th December 2013, 07:50 PM   #1
TimNJ is offline TimNJ  United States
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Default Grounding and Earthing- What and where?

Hi all,

I'm sure this is worn out topic and I've looked at several other threads but I have not been able to deduce a solid answer.

I'm building a low powered LM386 guitar amplifier. It's going to be mains powered.The enclosure it's in is wooden, but the switches and input jacks are all going to be fixed to a steel panel.

The steel "chassis" will be connected to earth.

1.) Should the transformer housing be connected to earth?
2.) Should the power/signal grounds from the circuit be connected to earth?
3.) When running a twisted pair-shielded cable from the 1/4" input jack, should I attach both the shield and the ground wire to the jack, and leave the shield lifted at the circuit...or should it be vise versa..or neither?
4.) Since the output power is so low, I was thinking about running the same shielded cable to the speaker. Is that a bad idea? If not, should I follow the same lifted on one end strategy?
5.) Should I try to use star grounding when building the circuit?

Thanks for any advice, even if you can only answer one!
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Old 30th December 2013, 01:20 PM   #2
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If you're going to the trouble of mains powering, honestly I wouldn't bother with one of those wimpy noise and distortion generators known as LM386. Saving a few pennies on a half-decent amplifier IC and heatsink is just not worth it.

5) is a no-brainer - of course!

2) would depend on whether you want an IEC Class I (w/ safety earth) or Class II (double insulated) device.
In a Class I device you would typically connect signal ground to earth in exactly one spot (often near star ground); connections elsewhere would be at RF only (using caps in the 1-10 nF range).
Class II devices usually don't even have a safety earth connection, and even if they do, signal ground and safety earth are not connected anywhere. (In return, some more care is required in internal construction.) A lot of commercial hi-fi equipment is built like that, and for good reason - connect two devices that have their signal grounds firmly tied to safety earth, and you get the best ground loop imaginable. And isolation transformers tend to be either expensive or not particularly hi-fi.

If your amp is going to be standalone, you should be able to go Class I just fine.

3) My suggestion would be "both". Instrument cabling is unbalanced anyway.

4) It's not likely to matter very much, since your cable probably won't be particularly lengthy anyway. A grounded metal chassis would also keep a lot of RF out.

Finally, take some time to familiarize yourself with the local electrical safety guidelines for mains-powered equipment. (You don't want to end up with a potential fire or shock hazard. A few guitarists have lost their lives due to electrocution on stage, and the sensation of a nasty shock is not unknown to many more. If you built it, it's your responsibility if it burns your house down.) Choose an appropriate mains fuse.
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Old 30th December 2013, 07:46 PM   #3
TimNJ is offline TimNJ  United States
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Thanks a lot.

I know it sounds rather ridiculous to mains wire an LM386. I thought the same thing, but I think I spent less than $10 on the power supply components, and that it will definitely cost less than all the 9V batteries I would have to go through.

I would build a higher power amp, but I have yet to find a good "in-between" amp, say 3-10W or so.

I have designed the amp so that it is very modular. I should be able to pop in a new amplifier board if I am so inclined to at some other point. The only controls it will have is gain and volume so nothing too unique.

But on grounding and earthing.

I think it's going to have to be a Class I device based on having some exposed metal. I'll make sure to keep safety at the foremost.
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Old 31st December 2013, 12:54 PM   #4
gootee is offline gootee  United States
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Old 31st December 2013, 11:56 PM   #5
Jsixis is offline Jsixis  United States
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yep, as important as grounding is one would think there would be a sticky.

I gave up asking about grounds, the amps work with no hum so I guess I have it right and so far I have not been shocked.
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Old 1st January 2014, 02:04 AM   #6
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Decoupling close to the LM3886 is vital to stop oscillation.
I would add the zobel too.
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Old 1st January 2014, 02:16 AM   #7
TimNJ is offline TimNJ  United States
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Thanks guys. Nigel, this is about the LM386. Did you realize that. On a side note I have built a LM3886 amp and I do agree, they are very important!
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