Low cost Power Amp PCB from the net (LM3886) - what do you think of the schematic? - diyAudio
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Old 7th April 2011, 02:30 PM   #1
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Default Low cost Power Amp PCB from the net (LM3886) - what do you think of the schematic?

Hi,

Newbee in power amp design and construction here, and still much to learn.
I recently got a PCB kit for two amps and a power supply unit, both unpopulated, that I am planning to solder myself.

As I have absolutely no idea about the different power amp designs and their merits, I thought that I could ask you what you think about these PCBs.
Or, did someone build this same kit?

I am very interested to read you comments.
Again, this may seem as a bottle in the sea, I just do not know where to search.

Regards,
Attached Images
File Type: jpg schematic PCB.jpg (261.9 KB, 824 views)
File Type: jpg PCB.jpg (289.9 KB, 783 views)
File Type: jpg PCB_2.jpg (244.8 KB, 729 views)
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Old 7th April 2011, 05:11 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I think you will find that C4 is connected to the signal ground on the top side and to the power ground on the bottom side.
You must break this connection.
Be careful you still have to solder C4 from the bottom side and rely on the bottom pad and the plated through hole to maintain that signal ground connection on the top side.

The bottom pad may be attached to the bottom ground plane by 3 or 4 bridges from the island pad to the plane. You need to very carefully cut these bridges without damaging the pad.

Who told the Chinese that ground planes are equal to or better than star grounding?
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Old 8th April 2011, 06:15 AM   #3
djQUAN is offline djQUAN  Philippines
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I think there is supposed to be a small value inductor in parallel with the output resistor (R1) according to the datasheet... or was that explained in the kit documentation?
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Old 8th April 2011, 07:11 AM   #4
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normally just some wire wound around it.
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Old 8th April 2011, 10:27 AM   #5
AndrewT is offline AndrewT  Scotland
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oops,
there is no output Zobel fitted.

Fit at least the Zobel or preferably the Thiele Network.
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Old 8th April 2011, 11:57 AM   #6
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Quote:
Originally Posted by AndrewT View Post
Hi,
I think you will find that C4 is connected to the signal ground on the top side and to the power ground on the bottom side.
You must break this connection.
Be careful you still have to solder C4 from the bottom side and rely on the bottom pad and the plated through hole to maintain that signal ground connection on the top side.

The bottom pad may be attached to the bottom ground plane by 3 or 4 bridges from the island pad to the plane. You need to very carefully cut these bridges without damaging the pad.

Who told the Chinese that ground planes are equal to or better than star grounding?
Many thanks for the quick reply.
I am not sure that I understand how to do this.
Which bridges should be cut?
Is the cut (red line) correct?

Regards,
Attached Images
File Type: jpg PCBb_110408a.jpg (274.4 KB, 594 views)
File Type: jpg PCB_2b.jpg (879.1 KB, 536 views)

Last edited by bidibidi; 8th April 2011 at 12:22 PM.
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Old 8th April 2011, 12:01 PM   #7
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Quote:
Originally Posted by digits View Post
normally just some wire wound around it.
I have to apologize for not undestanding the sentence. "Some wire wound around" what?



Quote:
Originally Posted by digits View Post
oops,
there is no output Zobel fitted.

Fit at least the Zobel or preferably the Thiele Network.
__________________
regards Andrew T.
Again, I am just beginning to learn about audio electronics (not even the fist step), I am not sure to understand where and how to "Fit at least the Zobel or preferably the Thiele Network."
May I ask how to do that?

Regards,
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Old 8th April 2011, 12:15 PM   #8
marce is offline marce  United Kingdom
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I would be interested in any real scientific data on star grounds versus solid grounds, as historicly the star, was a reference to a star point where the analog and digital ground met (today even this practice is not common as one contigous ground is best) as in the intrumentation world of electronics and historicly analoge circuitry has a soldi ground plane. In audio the return current is more problematic as the audio frequencies are where returns are moving from path of least resistance to path of least inductance. Also the skinny looks like an arial (and acts like one) ground schemes that apear also add a lot of parasitic inductance. Add to that the problem of EMC, loop area etc, ground planes are best.

Last edited by marce; 8th April 2011 at 12:18 PM.
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Old 8th April 2011, 03:10 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by bidibidi View Post
I am not sure to understand where and how to "Fit at least the Zobel or preferably the Thiele Network."
May I ask how to do that?
Hi,
The output Zobel is a series pair of resistor + capacitor. Usually it is around 10r +100nF, but National show different values on the datasheet. National also show a number of components as "optional". Beginners must never omit them. All are compulsory.

The Thiele Network is an extension of the output Zobel. Or more correctly the Zobel is a cut down version of the Thiele Network.
An extra parallel pair of resistor & inductor go between the output Zobel and the Speaker Terminals. Some of National's datasheets show this. These two components do not have to be mounted on the PCB. I think they are better mounted away from the PCB, as part of the wiring from PCB to Speaker Terminals.

The Zobel, however, should be mounted on the PCB. as near the output Pin and the PSU zero volts as possible.

Your red line.
No. That is the top side (=side with components mounted when using through hole leads).
Look at the bottom side. find the two pads that link to that same lead. Do they have a trace going to Ground? A better photo of that end of the PCB may show us what is there.
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Last edited by AndrewT; 8th April 2011 at 03:18 PM.
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Old 8th April 2011, 03:16 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by marce View Post
historically the star, was a reference to a star point where the analog and digital ground met (today even this practice is not common as one contiguous ground is best) as in the instrumentation world of electronics and historically analogue circuitry has a solid ground plane.
where did you find this historical information?
Balderdash !
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