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Old 9th August 2012, 11:19 AM   #1
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Default LM3886 optimisation and power supply review

Hello all!

I need an amplifier for bedroom or small room to listen CDs and TV. I've decided to build a LM3886 based small amplifier.

Very good sound quality is a must, but I am not willing to sacrifice everything for the best in class. The size of the amp (and also the cost) was on very high scrutiny. :-)

After reading some posts here, the result is on the next two diagrams:
lm3886_additional_components -> optimization of the original
power_supply -> passive and simple enough

Based on experience, I would like your help to review the above schematics and also Id appreciate any advices or ideas.
Thank you
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File Type: jpg power_supply.jpg (491.8 KB, 816 views)
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Old 9th August 2012, 11:44 AM   #2
AndrewT is offline AndrewT  Scotland
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Cin is in the wrong location. It must be after the pot.
The 1uF // to the 470 nfb cap is not required.
The 1uF on the PSU are not required.
All the 100nF caps around the secondary and the rectifier must be converted to R+C snubbers.
The 1nF caps on the primary must be Y rated.
You can add an X rated cap from N to L.
Cs can be simplified back to the National recommendation. That works well.

The 100r//100n could kill you, or another operator/user if left in the location shown.

You must ensure that the route from Main Audio Ground can pass mains Fault current to Chassis and then to Protective Earth (PE)
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Last edited by AndrewT; 9th August 2012 at 11:50 AM.
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Old 9th August 2012, 12:10 PM   #3
ddd is offline ddd  Bulgaria
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Quote:
Originally Posted by AndrewT View Post
Cin is in the wrong location. It must be after the pot.
Why? How +in will be DC zeroed then?
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Old 9th August 2012, 12:13 PM   #4
AndrewT is offline AndrewT  Scotland
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then fit a Rin value that suits.
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Old 9th August 2012, 01:06 PM   #5
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+1 on Andrews comment on the 100R//100n see "loop breaker" here Earthing (Grounding) Your Hi-Fi - Tricks and Techniques for a safer way to do it. (read the whole page whilst you are at it)

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Old 9th August 2012, 03:23 PM   #6
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Thank you Andrew for your help: some answers below:

Quote:
Originally Posted by AndrewT View Post
Cin is in the wrong location. It must be after the pot.

- please elaborate, what is the difference? the input impedance would require bigger Cin

The 1uF // to the 470 nfb cap is not required.

- it is there for the second variation, the one with polarized cap

The 1uF on the PSU are not required.

- You're right, it its a general PSU with the 1uF there

All the 100nF caps around the secondary and the rectifier must be converted to
R+C snubbers.

- as above, space is a premium, R at this power needs some cooling

The 1nF caps on the primary must be Y rated.

- they are Y

You can add an X rated cap from N to L.

- it is already there

Cs can be simplified back to the National recommendation. That works well.

- Ok, I;ll try the simple way

The 100r//100n could kill you, or another operator/user if left in the location shown.

- aware of that, this will implemented only if the case will be metal. On the other hand, space for a bridge....

You must ensure that the route from Main Audio Ground can pass mains Fault current to Chassis and then to Protective Earth (PE)
Do you have any other ideas or advices?
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Old 11th August 2012, 04:16 AM   #7
ddd is offline ddd  Bulgaria
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Quote:
Originally Posted by AndrewT View Post
then fit a Rin value that suits.
My question still remains. A C after the pot will destroy totally DC regime.
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Old 11th August 2012, 06:54 AM   #8
Marc Vi is offline Marc Vi  Netherlands
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See my thread "LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage". You can save an electrolytic capacitor and good ones are expensive.

The pot must be placed before Cin. The DC regime is not destroyed because of the R of 20 Kohm after the Cin.

There is no need for Cc of 220 pF. See my thread "LM3886: Stability at high frequencies and output level".

Marc.
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Old 11th August 2012, 07:44 AM   #9
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Thanks for feedback, I would like to point out that the cap in the feedback is not electrolytic, most likely is a non polarised and very affordable price. Could be used MUSE 100F - 1,60 or MUSE 470F - 2,70 .

Condensateur radial audio MUSE - 25V - 100F - Condensateurs

Condensateur radial audio MUSE - 25V - 470F - Condensateurs


Quote:
Originally Posted by Marc Vi. View Post
See my thread "LM3886 without....". You can save an electrolytic capacitor and good ones are expensive.
.........
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Old 11th August 2012, 07:45 AM   #10
ddd is offline ddd  Bulgaria
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Quote:
Originally Posted by Marc Vi. View Post
The pot must be placed before Cin. The DC regime is not destroyed because of the R of 20 Kohm after the Cin.
That's what I was trying to suggest (secretly).
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