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Old 13th December 2006, 09:59 PM   #1
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Default Please check my LM1875 PCB layout

Hi,

I'd be really glad if anybody would check my LM1875 PCB layout!

I tried to approach a "star ground", didn't realize exactly that, but my board shouldn't be much worse, is it?

The circuit is very basic, maybe I'll add an output filter for the high frequencies (small cap and 1R resistor across the output).
The low current part is on the left side, you recognise the high current part by the trace widths.

Especially, I wanted to ask if that layout should be ok in terms of stability.
Any other improvements (while keeping the circuit small and simple) are welcome!

Here comes the schematic...

Cheers,
Dominique

PS. The circuit will probably be changed slightly, but I'd like to have a functioning base.
After building a tiny stereo amp with an attenuator and two of those pcbs, I'm planning to use several of these mono circuits to power active loudspeakers...
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Old 13th December 2006, 10:00 PM   #2
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here's the PCB...
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Old 13th December 2006, 10:19 PM   #3
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Looking good! You could solder R3 directly to the pins of the chip for an even shorter feedback path.
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Old 13th December 2006, 10:39 PM   #4
BWRX is offline BWRX  United States
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I would route the power traces such that the pads for the electrolytic caps are in the middle of the trace. Also be careful about the location of C4. Depending on its height you could have trouble fitting a screw driver or other tool in there to tighten the screw that will hold the chip to whatever heat sink you're using. Of course that's assuming you're going to use a screw instead of a clamp.
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Old 13th December 2006, 11:17 PM   #5
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@pinkmouse
I'm glad you rated it not that bad!
I also thought about soldering R3 directly to the pins, and if it can make a difference (Was there any report on stability improvement by someone who tried both?), I'll do so!
But I want the PCB also to have the R3 pads, for the "nicer looking" option.

@BWRX
Oh dear! Bad error! You've seen that well!
Thanks, I'll fix it somehow...

Big thanks, mates! CU later!
Dominique
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Old 13th December 2006, 11:27 PM   #6
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compared to: http://www.national.com/images/pf/LM1875/00503002.pdf

Your voltage gain will be about 3 db down from the National design ... but that would be fine for a headphone amp ... changing the feedback resistor to >> 20K will bring it into almost exactly the National recommendation for small power amp =
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Old 13th December 2006, 11:42 PM   #7
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@FastEddy
Yes, gain is quite low. I'm not sure for the gain yet, but 10 is sort of minimum (also from the datasheet specs).

At the moment, I'm running a point to point soldered chipamp with only a gain of 2.7 from a CD player...
Ok, that was because my old amp was broken and I wanted to listen to music, so I quickly soldered something together whiile didn't finding the right resistors , but that's really loud enough for my small room.

@everyone
1) How important do you rate that high frequency filter across the output?
(1 ohm and 0.22uF is recommended in the datasheet / look up FastEddy's link)

2) Except space requirements, does it make a difference if I place the 1000uF filter caps on the PS board and use sth. like 100uF on the amp's board, along with the 0.1uF?
Then, I wouldn't have to put the big caps left and right from the LM1875 in order to be able to reach the screw of the cooling pad...

Cheers,
Dominique
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Old 13th December 2006, 11:57 PM   #8
paulb is offline paulb  Canada
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Quote:
Originally posted by Dominique
[B@everyone
1) How important do you rate that high frequency filter across the output?
(1 ohm and 0.22uF is recommended in the datasheet / look up FastEddy's link)
[/B]
Very. It's not a filter, it makes the amp stable. So it won't oscillate.
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Old 14th December 2006, 12:02 AM   #9
BWRX is offline BWRX  United States
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You could easily shift the large caps over to the right side of the board to obtain easy access to the mounting screw. You'll have to reroute the traces a little but nothing complicated.
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Old 14th December 2006, 12:05 AM   #10
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" ... 1) How important do you rate that high frequency filter across the output? (1 ohm and 0.22uF is recommended in the datasheet / look up Nationals link) ..."

If they say it should be so, it must be true.

My Carapolla: This prevents high freqs from getting through to the load, the speaker. If your speaker is a voice coil type, you probably don't need it as "stray capacitance" of the speaker and leads is about 0.01 nF ... and voice coil speakers resist changes above about 25k Htz. Piezo electric types (ala some cheap headphones) and some electrostatic speakers would need it as they may respond to 50K Htz or more ... it all depends on the load. You could install these directly on your speaker terminals ... then cut 'em out of the loop at a later date if you think they are not worth the trouble. (I would still leave the solder pads available on the circuit board ... as some here will have alternate suggestions.) ... ignore this as it is mostly not applicable in this case.

" ... 2) Except space requirements, does it make a difference if I place the 1000uF filter caps on the PS board and use sth. like 100uF on the amp's board, along with the 0.1uF? ..."

Yes, leave the solder pads on the board, well placed and big and properly spaced so you can add more as needed / if needed. You may at some future point want to add some plastic "snubbing" caps here as well (extra solder pads in parallel with the big guys) = most here believe plastic caps result in better sounds. Also BWRX is right: " ... Depending on its height you could have trouble fitting a screw driver or other tool in there to tighten the screw ..." so, having the big caps off the board for tweaks is a good idea ... then final assembly might allow you to add them back ... or not. BWRX is also correct here: " ... You could easily shift the large caps over to the right side of the board to obtain easy access to the mounting screw. ..."

!! Ooops ... Paulb above is right = the small cap is to keep the op-amp from oscillatting ... best to leave it on the board !!

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