My LM3886 Stereo Amp. w 12-0-12VAC Transformer - diyAudio
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Old 9th June 2009, 11:12 PM   #1
Andreq is offline Andreq  Canada
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Default My LM3886 Stereo Amp. w 12-0-12VAC Transformer

Hi,

I've been thinking about building this little amp for quite some time and now is the time.

I've got some quite good answer in the PSU board and here's the final verdict :

I'll use my 12-0-12VAC Transformer with 4700uF capacitor for the power supply. It should output about ~17VCD. As I'm using 8ohm speaker, I should get about ~9-12W (as said pacificblue PSU thread) which should be fine for my use.

I'm not looking for anything fancy. I'll use this stereo amp for music/movie in a small apartment with 2 ~30years old speaker. I'm far from being an audiophile, I just want to replace the old ~30 years old FM/AM radio/amp that currently power my "sound system".

For the Amp part of the project, I'm going to build my own PCB using UV exposure method. I've just made my first test PCB with great success. I'm using a UV led box with 3:30min exposure time for my next PCB.

See :

Test PCB
From left to right : 30sec exposure to 5min

Best result
Best exposure time is the one in center : 3:30min

The smallest track is 6mil...which I think is ... great, as it etched perfectly!

Anyway,

for the PCB design, I've search the forum back and forth and found this :


lm3886 PCB
Posted by bigparsnip.

I want to use the minimum required part, but I'd like to know if this PCB need anything more to build my amplifier.

1. Do I need to add an inductance on one of the resistor ?
2. What would be the best possible resistors value for my application?
3. I'll add my own volume control on the input, but do I need any input capacitor as I've read somewhere else on the board?
4. Anything I should be aware of before etching this PCB?


As I'm still waiting for my micro drill bit & LM3886TF (x2) I'll take any advise before building anything.

Thanks a lot !
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Old 9th June 2009, 11:21 PM   #2
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With that low psu voltage and power, why dident you go with the LM1875 instead ? Running a LM3886 @ only a fraction of its power capability is kindof a waste of money.
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Old 9th June 2009, 11:41 PM   #3
Andreq is offline Andreq  Canada
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Yeah I know, but I bought the LM3886s before the transformer.

The transformer was sold as "24VAC 10A"... which I tough would be 24-0-24VAC ... but I was wrong... anyway I don't really want to waste more money on buying another transfo, I'll use this and see if it suit my need. I'll still have the possibility to upgrade later.
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Old 10th June 2009, 08:02 PM   #4
ratza is offline ratza  Romania
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Why don't you go with LM1875? I will play awesome at that power supply and the board will be very small. Something like 1 square inch.

Later edit> I forgot to answer the questions.
1. Yes, on the output resistor you put 10 turns of AWG18 wire (10 ohm/3W in my boards).
2. If you mean feedback resistors, 22k with 1k works fine.
3. Put a capacitor between the cursor of the pot and the non-inverting input.
4. Not necessarily. You can look also for other designs or even design your own.
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Old 10th June 2009, 10:35 PM   #5
Andreq is offline Andreq  Canada
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As I said,

I've already bought the LM3886 and transformer and don't want to spend anymore on this project.

Thanks for the answers.

As for question 2, I would like to know what value should I use for all part on the board.

I've attached the schematic from bagparsnip design.

What would be value for R1 to R4. Same as C1 to C4.

I'm really not the "hardware guy"... My normal task is to program (which is what I'll do for my digital volume control based on a Pic or Attiny)

Thanks again!
Attached Images
File Type: png schematic.png (7.2 KB, 1153 views)
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Old 10th June 2009, 11:24 PM   #6
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DONT use protective earth, this will cause groundloops and other problems, the amps ground should be ISOLATED from mains ground.

You mains ground the chassi to prevent electric shock but isolate the secondary side of the transformer from it.
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Old 11th June 2009, 07:52 AM   #7
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Quote:
Originally posted by Andreq
As for question 2, I would like to know what value should I use for all part on the board.
Everything you need to know is in the datashet

Quote:
Originally posted by Tekko
DONT use protective earth, this will cause groundloops and other problems, the amps ground should be ISOLATED from mains ground.

You mains ground the chassi to prevent electric shock but isolate the secondary side of the transformer from it.

Don't give such advice.

The normal and correct way is that all voltages will be connected with one end to PE in some way. There are only few exceptions, where this rule does not apply, and all of them require safety measures that most DIYers cannot provide. E. g. insulation monitoring, double insulation, protective extra-low voltage, etc.
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Old 11th June 2009, 11:10 AM   #8
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Ive NEVER seen an amp w anything of the secondary side connected to chassi, its always insulated from the chassi to avoid groundloops.

Usually there is no grounding @ all, just a 2prong cable.
Transformers nowdays are so well made that a event of the chassi becoming live is so unlikely that its could be called impossible.

If you make a connection of the secondary side to the chassi it should be a single wire going from the 0V rail to the chassi, preferably with a 10ohm resistor to reduce the risk of groundloops.

Ive fried many transformers and NONE have had either the core nor the secondaries go live.
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Old 11th June 2009, 12:17 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Tekko
DONT use protective earth, this will cause groundloops and other problems, the amps ground should be ISOLATED from mains ground.

You mains ground the chassi to prevent electric shock but isolate the secondary side of the transformer from it.

Quote:
Originally posted by Tekko
Ive NEVER seen an amp w anything of the secondary side connected to chassi, its always insulated from the chassi to avoid groundloops.

Usually there is no grounding @ all, just a 2prong cable.
Transformers nowdays are so well made that a event of the chassi becoming live is so unlikely that its could be called impossible.

If you make a connection of the secondary side to the chassi it should be a single wire going from the 0V rail to the chassi, preferably with a 10ohm resistor to reduce the risk of groundloops.

Ive fried many transformers and NONE have had either the core nor the secondaries go live.
Tekko, STOP.
You have confused double insulated equipment and earthed equipment.
The information you are giving out is not just wrong, it is dangerous.
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Old 11th June 2009, 12:27 PM   #10
Andreq is offline Andreq  Canada
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Quote:
Originally posted by pacificblue

Everything you need to know is in the datashet
I'll take another look at the datasheet. I'm really having hard time reading all the fine prints. I mostly use datasheet for pin layout hehe.

I think I found the important information on page 20 to 22.



Thanks






p.s. Please stay on topic instead of fighting . I was pretty confident the Amp ground need to be connected to the star ground (earth ground).
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