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Old 12th March 2005, 05:12 PM   #1
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Default What is correct method for shielding?

What needs to be shielded when building an LM3875 chipamp?

Shoud just ransformer be in metal enclosure, or should power supply pcb be in same enclosure as well? DOes anything else need to be shielded - or can all else just be ouside the transformer shielded enclosure, and in, perhaps, a wood enclosure?

I have a Bud aluminum enclosure, and would like to put apropriate parts in it, then other parts outside of it, than everything ins a wood enclosure.

What goes in the aluminum box?
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Old 12th March 2005, 06:40 PM   #2
mcs is offline mcs  Denmark
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You don't nescesarrily have to sheld anything. My LM3875 amp works fine without any shielding.

Best regards,

Mikkel C. Simonsen
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Old 12th March 2005, 07:03 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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That depends where you live. Some areas have very bad RFI problems. Where I live I find it's generally more desirable to shield things than someone living out in the middle of nowhere. How good your supply rail filtering is has an effect on how much or little shielding you need.

It will probably be alright unshielded, but if you hear a lot of noise or start picking up radio stations then you know what to do!
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Old 12th March 2005, 08:19 PM   #4
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What do you mean ? Shielding against RFI or against induced hum ? Both are quite different questions.
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Old 12th March 2005, 08:27 PM   #5
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Default Shielding for RFI and Hum

Let me rephrase my question. What is the recommended approach to shielding for RFI and hum? Would like to build a chipamp that can be used without mods even when I move to a new location, so want the best bet approach.

I understand in many cases one is ok without any shielding, and in others it is mandatory. What is recommended?
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Old 12th March 2005, 09:09 PM   #6
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It isn't question, which can be answered by a few words. But only basicaly : a) hum : use only correctly designed PCB and give amp as possible at bigger distance from mais transformer. Use internal wiring by twisted wires ( all - supply and signal ). b) RFI : use for ALL apparatus " oversized " ( declared for much higher current than is nominal consumption of apparatus ) mains filter. Much more better results give in this case insulating transformer. That's all, don't worry.
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Old 12th March 2005, 10:56 PM   #7
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Default That's helpful...

OK, so twisted wires, amp boards away from tranny, best to shield tranny, but may be fine without. What about the power supply pcb? Keep it close to transformer, in same metal enclosure, or away from transformer?
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Old 12th March 2005, 11:02 PM   #8
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PS PCB ? It is quite free - but remember that all grounds must lead to GEOMETRICAL middle between main elyts .
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Old 13th March 2005, 02:40 AM   #9
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Default OK I'll give it a try

Thanks Upupa - I'll try it and see how it turns out...
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Old 14th March 2005, 01:47 AM   #10
homer09 is offline homer09  Canada
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Quote:
Originally posted by Upupa Epops
PS PCB ? It is quite free - but remember that all grounds must lead to GEOMETRICAL middle between main elyts .
can you elaborate on this? what are main elyts?
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