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Old 2nd May 2009, 03:33 AM   #1
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Default LM4765T pc Board

Hi All:

I have just completed a pc board for the National LM4765T Overture Amplifier, and wanted to get feedback on its design. If there is enough interest, I will post this in the Group-Buys section and take orders.

Basically, this is a twin LM4765T Chip-amp, each chip wired in Stereo, yielding 4-CH outputs. Each output is good for 30W / 8 ohms with ridiculously low distortion. The boards will be double-sided, plated-through holes, Solder-mask and silk-screening. Input is via an 8-pin DIN connector, although RCAs can be wired to inputs that are pinned out on the traced leading from the DIN connector to the two chips. The Chips' MUTE and STDBY functions are also pinned out on the board, and at the DIN connector.

Thoughts & comments, please. This board was done in ExpressPCB. If there is enough interest, I can order 50 boards for ~USD$360.00, coming out to about USD$6.00 board. All I would add to that is shipping to anywhere in the world.

Waiting (nervously) for your replies,
W
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Old 2nd May 2009, 05:54 AM   #2
Bearman is offline Bearman  United States
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I'm in on a board if everyone thinks it looks good.

I'd rather have two separate boards if that's possible? That way you can put the boards together and have one heatsink or separate sinks.

I've already ordered two chips in anticipation
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Old 2nd May 2009, 07:40 AM   #3
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Don't you use the recommended 100 nF bypass capacitors next to the power supply pins?

The supply bypass capacitors should have a low impedance between each other on the ground. If you turn the 220 F capacitors by 180 , you will achieve that. It means retracing the power traces of course.

Then it would be good not to run the power supply and output traces in parallel.

You use Ci, but not Cin? Either both or none. None might give you compatibility issues with certain sources, so both is preferable.

The Cis are quite small. Will bigger values fit in for those who prefer a lower roll-off frequency?

You have two separate ground connections through the power supply, another through the DIN connector and another four through the fixing screws to the chassis. Aren't you afraid of ground loops? And how about the good practice of keeping power ground and input ground apart?

Are the jumpers by wire or is that a double-layer PCB?

Why did you decide for that DIN connector? Is that the mini DIN type similar to S-Video connectors or the bigger one that is also known as diode connector?

I hope this post doesn't come across as discouraging. Just a bunch of thoughts and comments.
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Old 2nd May 2009, 12:16 PM   #4
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One thing I took to heart from the author of the LM4702 Application Note 1490 -- Audioman54 -- if you can avoid using a ground plane, do so. Probably difficult to do with a single-sided board, however.
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Old 3rd May 2009, 07:45 PM   #5
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Blue (& Others),

Here's the board with some revisions addressing pacificblue's concerns about board layout. I was not able to address the separation of power- & signal grounds, but I did revised the power connections to a singe point, and included hi-frequency bypass caps at the chips' power pins.

This schematic was from the National Semiconductor Data Sheets for the LM4765T, and as such, the Roll-off Caps are 22uF. This does not mean you can't use 10uF, 4.7uF, 33uF, or even 47uF. There are no input caps in the signal path because the schematic shows this as DC-coupled. Anyway, thoughts on the revisions, please.

Steve
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Old 3rd May 2009, 07:48 PM   #6
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oops forgot- this board IS double-sided, per my original post. Should I do a Ground plane on the top-side as well as the bottom side? Thoughts/comments?
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Old 3rd May 2009, 08:15 PM   #7
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by N-Channel
oops forgot- this board IS double-sided, per my original post. Should I do a Ground plane on the top-side as well as the bottom side? Thoughts/comments?
no. Remove all ground planes.
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Old 3rd May 2009, 08:17 PM   #8
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by N-Channel
I was not able to address the separation of power- & signal grounds,
you must find a way to separate the signal return line from the decoupling and speaker return line (for each channel).
Particularly since this PCB has 4 channels on it.
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Old 4th May 2009, 03:28 AM   #9
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OK, I'm gonna have to go back and see how I can do this. BTW, the 8-Pin DIN is, indeed, a full-size DIN connector. I have always used the 5-Pin & 8-Pin DINs because they're so easy to work with, and I think they make a better connection than the Mini-DINs. Never did like Mini-DINs.
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Old 4th May 2009, 05:02 PM   #10
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Default Re: LM4765T pc Board

Quote:
Originally posted by N-Channel
Hi All:

...... If there is enough interest, I can order 50 boards for ~USD$360.00, coming out to about USD$6.00 board......

W
Obviously, I need to go back and review my Math 098 Stuff. $360.00into fifty boards is actually about $7.20USD. Doh!
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