| ransom peek |
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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| mcs |
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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| Mr Evil |
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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| Upupa Epops |
| What do you mean ? Shielding against RFI or against induced hum ? Both are quite different questions. |
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| ransom peek |
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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| Upupa Epops |
| 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. :cool: |
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| ransom peek |
| 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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| Upupa Epops |
| PS PCB ? It is quite free - but remember that all grounds must lead to GEOMETRICAL middle between main elyts :cool: . |
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| ransom peek |
| Thanks Upupa - I'll try it and see how it turns out... |
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| homer09 |
| 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 :cool: . |
can you elaborate on this? what are main elyts? |
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| Upupa Epops |
| Main elyts = capacitors in PS. |
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| rabstg |
| quote: | Originally posted by homer09
can you elaborate on this? what are main elyts? |
Electrical ground (pun intended) zero.
Point between the +/- mains Electrolytic caps.
Wires twisted, sheilded and centrally grounded. |
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| rabstg |
Here is a better picture of the shielding, but I can't reduce the size here @ work so it is zipped.
Under the central wires is a ground point that all the sheilding and the amp grounds tie to. If I were using a center tapped transformer it would be grounded there also.
Edit: Even the zipped file is too large. I guess the small amount of compression didn't gain anything. Maybe later from home. |
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| ransom peek |
Are you saying to use this as example of proer shielding? That would be helpful.
Is there any hum caused by not having metal plate between transformer and rest of circuit, especially since it is all so close?
You advise that if you used center tapped transformer... Arent all chipamps that use single xfrmr having center tap to get + - voltage?
Looking forward to the photo... |
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| rabstg |
| quote: | Originally posted by ransom peek
1. Are you saying to use this as example of proer shielding? That would be helpful.
2. Is there any hum caused by not having metal plate between transformer and rest of circuit, especially since it is all so close?
3. You advise that if you used center tapped transformer...
4. Arent all chipamps that use single xfrmr having center tap to get + - voltage?
Looking forward to the photo... |
1. No way! I don't "witness" things and I don't "set examples" :D . You only need to screen the wires if you HAVE RF inference. That was one way to solve RF issues.. I would advise you to build a good LOGICALLY laid out amp then try BEFORE trouble shooting for issues (this case RFI).
2. No hum what so ever. I would be more concerned with rectifier-induced noise over Toroidal transformer induced.
3. BrianGT's kits are set up for dual secondaries, NOT CT.
4. No. Center tapped are dual secondaries with tied leads. Ask off line if you have questions and I would be glad to explain. I did a post a while back showing how to split a CT transformer into a dual secondary.
http://www.diyaudio.com/forums/show...10&pagenumber=1
Edit: Found the post. |
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| homer09 |
| quote: | Originally posted by rabstg
I would advise you to build a good LOGICALLY laid out amp then try BEFORE trouble shooting for issues (this case RFI).
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What is one trying to achieve when logically laying out an amp? What distances should be the shortest/equal/irrelevent? im laying out my amp now, and im just shortening signal path as much as possible, but what else should i pay attention to especially with regards to grounding. (its a brian LM3875 integrated amp, passive dual mono volume control, input selector switch, two PS boards, one transfo) |
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