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Old 1st September 2012, 09:48 PM   #1
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I just finished a JLH1969 amp and was thinking of building a LM3886 chip amp. I bought a couple of LM3886 pcb's from china a while back. I transferred it to a schematic program, (first attachment). It basically looks like the TI app note, (second attachment). I found a circuit posted by Apex of an LM3886 amp, (third attachment). I figured out a way to transfer the Apex schematic to the chinese pcb without to much trouble. I was wondering if sonically the Apex schematic would be better than the stock chinese pcb circuit even though it is slightly different than the app note. Or would the app note circuit sound better worked around on the chinese pcb than either of the other ones? I think the Apex circuit might be a Yorkville design. I am looking to end up with something that is fairly HI-Fi and not so much PA oriented. I would like to use the pcb's if I can. (the fourth attachment is my idea of the Apex schematic on the chinese pcb). There may be no way to answer these questions without building all three and listening but I was hoping someone might be able to look at them and say, based on components and layout, that one would be better than the other. Thanks.

Attached Images
File Type: jpg LM3886 PCB (1).jpg (47.8 KB, 56 views)
File Type: jpg LM3886 App note (2).jpg (25.1 KB, 55 views)
File Type: jpg APEX schematic (3).jpg (60.1 KB, 56 views)
File Type: jpg Apex Yorkville (4).jpg (69.5 KB, 56 views)
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Old 1st September 2012, 11:21 PM   #2
Jay is offline Jay  Indonesia
Join Date: Feb 2003
Location: Jakarta
The Apex circuit is trying to play safe. Some components are for compensation or for safety in case of oscillation. If there is no oscillation tendency, they don't have good effect on sound but on the contrary they make sound worse (there is no free lunch).

Better sound will be achieved with lower gain (R3/R4 in first pic) but with higher possibility for oscillation. Minimum is i think 10x or 12x. Datasheet example show 20K/1K==20x. Apex uses 15K/1K==15x. I have used 10k/1k without compensation but it is too dangerous as LM3886 is not as stable as LM3876 imo.

Compensation or safety measures are:

1) small cap between pin9 and pin10. Too high will give worse treble but better stability. I have never used any. Or try lower value such as 100pF.

2) C8-R6 in Apex circuit. You can exclude it or use lower C and higher R such as 10R-47n

3) C2 in Apex circuit (680pF). The value is too high and the (treble) quality will suffer.

Apex do this because the gain is relatively low (15x), but also to save inexperienced builders who do not know what can cause instability. I would suggest 12k/1k and use little compensation.

Just check what the Chinese PCB has on board. The stability is a factor of PCB design and output/input coupling.

EDIT: I don't want you to ruin your chip, so go with at least 15K/1K. The compensation? Hmmm... imo you can reduce a little bit. I think 15k/1k with too much compensation might sound worse than 20k/1k without compensation. The RC at output has little damage to the sound so you may want to use that.

Last edited by Jay; 1st September 2012 at 11:35 PM.
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