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Old 17th October 2010, 05:38 AM   #1
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Default LM3886 on 1sided PCB design

A fair amount of optional components here.

I may end up taking out the Zobel network as I doubt I'll be buying highly capacitive speaker wire and am driving efficient Warfdale 8 ohm bookshelf speakers.

While star earthing is unnecessary at this level, I thought, 'what the hell' and tried it anyway. It was certainly a challenge.

I could probably drop the 1uF decoupling MKT capacitors as they seem superfluous.

Comments, suggestions and corrections very welcome...
Attached Images
File Type: gif schematic.gif (12.4 KB, 1039 views)
File Type: gif lm3886 main pcb.gif (33.1 KB, 1028 views)
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Old 17th October 2010, 07:10 AM   #2
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Default PSU design

Regulated power supply.

I'm dubious MKTs will be useful when the electrolytic capacitors are low ESR. I'm aware there's some controversy about bypassing bypass capacitors with smaller ones.
Attached Images
File Type: gif gainclone psu circuit drawing.gif (47.1 KB, 957 views)
File Type: gif gainclone psu pcb drawing.gif (41.0 KB, 907 views)
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Old 17th October 2010, 07:20 AM   #3
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Default Mains RF EMI filter

Pre-transformer RFI filter stage for this amp. Standard sort of design. Safety link between Earth and power-ground copied shamelessly from Power Supply for Power Amplifiers

The PCB diagram looks a bit weird because at that point I hadn't worked out how to use Altium (trial edition) properly!

I've built this and run preliminary tests.
Attached Images
File Type: gif gainclone rfi filter circuit.gif (38.4 KB, 899 views)
File Type: gif gainclone rfi filter pcb.gif (33.5 KB, 425 views)

Last edited by random007; 17th October 2010 at 07:22 AM. Reason: Added images
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Old 17th October 2010, 07:46 AM   #4
ampimp is offline ampimp  United Kingdom
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Hi
Im working on a lm3886 pcb at the moment . The biggest problem im finding is with the psu decoupling caps at the ic the ones you have marked C6 and C9. it is easy to get C6 close to the ic but i can find no way to get C9 closer. i had tried C9 in the same location as you a and after some reading find it is best to solder the cap directly to -v pin and using a cap with very small pin spacing 2.5mm would be good 5mm is easier and over 10mm could be problems when you add the length of the track into the equation it would lead to a high chance of being affected by inductance wich will render this decoupling cap as quite usless. Im quite new to this but im sure the imfomation i give is correct as it came from a very good source. Also it is a good idea to mount the feedback resistor to the ic as well to help keep the fb path ultra short. I wanted my pcb to contain all the parts in a nice neat way but if that will affect sound quality then it is a deff no for me. ps it is not over the top to use a star ground with this ic in fact id say it is law.
Best regards ian
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Old 17th October 2010, 07:49 AM   #5
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I should point out the mains input is a 10A IEC connector with integrated fuse, the transformer is at the moment a 160VA 30+30V but once testing is complete I'll buy a 300VA model. The tranformer has two 4n7 capacitors wired across its secondaries as per Tuning a PS Transformer on a GainClone. (replaced with 8n2 when I get the 300VA).

Yet to finalise a buffer PSU - this amp will include a discreet FET buffer from STEREO FET BUFFER FOR DAC, PREAMPLIFIER, TAPE REC, ... - eBay, Other, Computers Networking. (end time 06-Nov-10 03:41:04 AEDST) - with some component substitution, it's got some good reviews both on and off these forums.
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Old 17th October 2010, 08:13 AM   #6
ampimp is offline ampimp  United Kingdom
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is pin 7 supposed to connect to power ground and not s ground and inputs and outputs should be kept as far away as possible from each other. maybe Andrew t would like to comment as it will give me a chance to see if im learning anything and to help us both build good sounding amplifiers.
best Regards Ian
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Old 17th October 2010, 08:31 AM   #7
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Quote:
Originally Posted by ampimp View Post
Hi
Im working on a lm3886 pcb at the moment . The biggest problem im finding is with the psu decoupling caps at the ic the ones you have marked C6 and C9. it is easy to get C6 close to the ic but i can find no way to get C9 closer. i had tried C9 in the same location as you a and after some reading find it is best to solder the cap directly to -v pin and using a cap with very small pin spacing 2.5mm would be good 5mm is easier and over 10mm could be problems when you add the length of the track into the equation it would lead to a high chance of being affected by inductance wich will render this decoupling cap as quite usless. Im quite new to this but im sure the imfomation i give is correct as it came from a very good source. Also it is a good idea to mount the feedback resistor to the ic as well to help keep the fb path ultra short. I wanted my pcb to contain all the parts in a nice neat way but if that will affect sound quality then it is a deff no for me. ps it is not over the top to use a star ground with this ic in fact id say it is law.
Best regards ian
Thanks. Soldering components to the underside of the PCB is still neater than pin-to-pin construction and seems perfectly reasonable to me.

You need to use a star ground between amp units, power supply, input and output etc but many, many chip-amp PCB designs use what could be charitably called 'rail topology' without problem, though I still prefer star ground on principle.
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Old 17th October 2010, 10:22 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
where did that 3886 schematic come from?
It is so difficult to read.

The PCB layout is unlike anything I have ever seen.
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Old 17th October 2010, 10:24 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by random007 View Post
the transformer is at the moment a 160VA 30+30V but once testing is complete I'll buy a 300VA model.
Hi,
if you get that far.
30+30Vac is very high and is very likely to take the supply pins well outside Nationals' recommended voltage range.
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Old 17th October 2010, 10:30 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by random007 View Post
Pre-transformer RFI filter stage for this amp. Standard sort of design. Safety link between Earth and power-ground copied shamelessly from
R1 does not need to be 5W and there is no advantage in using anything over 500mW.

The maximum voltage across the resistor will never exceed Vf of the diodes when passing Fault current.

Can your diodes pass Fault current until the fuse blows and the arc extinguishes?

Back to the resistor.
If Fault Vf=2Vpk the Pr~400mWpk for a few ms (or us).
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