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Old 25th January 2013, 06:05 PM   #1
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Default Practical considerations building LM3886 amp

Ok first off sorry if i accidentally make a double post, my internet connection is unstable.
I am working on a stereo LM3886T stereo amp (schematic below although i think everyone knows it, seems to be popular IC) but not in the classic and easy i think dual supply mode. I am limited by the power supply itself since using my biggest transformer would only deliver 2x18V (before apply signal).
Click the image to open in full size.

1. i know signal ground must be separated from power ground or else because interaction sound quality would be mediocre ( i already experienced that) but which components? I do believe everything except Zobel and mute cap is that correct?

2. if i use a twin shielded cable for input do i connect each shield to each signal ground via the small resistor that must be used to separate power and signal paths? or do i connect them straight to signal grounds and each "separation" resistor goes to the power gnd of it's IC?
in the first case i believe it could possibly be beneficial (and i've seen some local enthusiasts do that but with dual supply circuit) since the path will still end in the ground but that of the preamp which is tube so there is very little current flowing there so it is quiet.

3. the mute resistor? if i judge by the datasheet it should be about 5.6K (keep in mind the voltage at pin 4 is 0) but i used 4.7K and seems to work just fine.
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Old 25th January 2013, 08:58 PM   #2
djoffe is offline djoffe  United States
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The three most important points to single point ground...the bottom of the input pot, the bottom of Ci, and the grounded side of the speaker.
Just behind that, RA and CA should be there...

The biggest issue is managing the large half wave rectified currents from the negative supply pin. You need to keep those out of the signal path.

Last edited by djoffe; 25th January 2013 at 09:00 PM. Reason: add info
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Old 26th January 2013, 12:16 AM   #3
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Not sure i follow. From what you are describing it sounds like star grounding (?). The issue is these are 3886 T not TF, that means the power ground is in contact with the heatsink. That brings the problem of a possible ground loop between the tabs and the input ground. That is why perhaps i should rephrase: am i in this setup allowed to break ground loop by a small resistor as i would in the dual supply setup? The fact that there is no audible hum does not mean there is no current induced and as the psu is switching, the "hum" is not audible but may still superimpose on the signal and impair (especially the highs)
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Old 26th January 2013, 06:58 AM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
my biggest transformer would only deliver 2x18V
That means that you *do* have two rails.
Go for the simpler split supply configuration.
Which will solve 2 problems:
1) your back flange will be insulated from ground.
2) more important: by definition, out of the 3 high current paths you have there, 2 (+/- rails) will *not* be mixed with ground and the 3rd, the speaker ground or return, can use its own wire straight to the filter caps centerpoint.
Remaining ground is audio ground, not a difficult task since currents are very low, simplifying your task.
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Old 26th January 2013, 08:06 AM   #5
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Perhaps i should, although that wouldn't provide the speakers with half of what they can take. Then there's the problem of preamp power. Since the transformer mentioned is large, the switching supply won't fit any more so then a second transformer is needed for preamp. And now none of the supplies would be regulated. To make matters worse, the em fields of transformers interact and cause the case to hum, or more accurately the entire desk hums, extremely unpleasant.
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Old 26th January 2013, 08:17 AM   #6
shaju is offline shaju  Bangladesh
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well... i'hv already build 3xLM3886 + 3xLM3886 amp. it's nice. but what size of speaker should i use??? is it possible to use 15" speaker as woofer + 10" speaker as mid + 3" tweeter.... plz reply....
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Old 26th January 2013, 09:32 AM   #7
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well i don't really know since you did not mention the rating of the speakers or what power supplies the LM3886. but if the speakers are of no known brand, the power rating on the back will be a lie, so the answer then is probably no, they will not take that much power.
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Old 26th January 2013, 09:36 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bloodmist View Post
Perhaps i should, although that wouldn't provide the speakers with half of what they can take. .............
A 18+18Vac transformer will allow the same maximum power output as a 36Vac transformer.
The single secondary version uses the single polarity schematic as you posted.
The dual secondary (or a centre tapped secondary) can use the same single polarity schematic, or the "normal" dual polarity schematic.
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Old 26th January 2013, 09:55 AM   #9
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a 36Vac transformer would result in a voltage of up to 40 after rectification/filtering. the output from my current power supply is 60V regulated so there is a little bit of a difference.
as for my transformer, the 18+18V is not the ac output but what is obtained after rectification, so it will drop even lower when there is signal. the transformer actually delivers 14+14Vac. that is not the main problem since i don't plan on going to insane levels (my room is rather small) but then i have to add the transformer for the preamp as well and it's not even the same 180V as i use now, but 300V so i have to redo the whole preamp as well and that's not particularly nice.
as a solution i will start rummaging around for any piece of mica i can use to insulate the ic's from heatsink so i'll just need to insulate the shields of the input cable from the ground by resistors as mentioned and the preamp will receive ground for power via a separate wire from the power supply. that should take care of any possible ground loop. As for the grounds in the output stages, i suppose i don't necessarily need to use resistors there, but just connect them by a thin trace/wire and that way the schematic of these stages will be unchanged electrically.
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Old 26th January 2013, 06:25 PM   #10
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ok forget the loop nonsense, now the problem just made a sharp turn for the worse. i realised pin 5 was not connected out of my neglect. i assumed it would be internally connected but it is not. now that it is connected, the ics heat up momentarily and of course then the power supply protection trips. before it trips the voltage across the ic is only 25V (out of 60) that indicates something really bad just not sure what.
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