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Old 29th March 2010, 10:05 PM   #1
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Default First chip amp - lm3886

Well after building my first electronic project as per this group buy - http://www.diyaudio.com/forums/group...roup-buys.html

I wish to start off on amplifier's.
So i picked chip amps, it's the future after-all .

So i looked around and i like the look of the lm3886, I want something simple to follow, i don't mind p2p, but if it would fit a breadboard i'd prefer that, even if i have to p2p the rear legs.

So a simple circuit for the lm3886 is all i'm after. I want to to push as much power as it can into a 4 ohm load. I will be making a pair of these, for stereo use.
What's following this circuit like?
http://www.circuit-projects.com/cimg...conductors.gif

Can i just do that? Or are there significant gains over that now?

Here is what i'd like to do i think, i don't need dc protection caps i guess, having a tube pre.
http://dogbreath.de/Chipamps/ThreeRe...reeResAmp.html
I will not have the patience to solder that close though, so does it matter if i spread the components about a little more, if only leave the resistor legs ideal length so i can solder more easily?

Sorry for so many questions...

Last edited by agithegreat; 29th March 2010 at 10:12 PM.
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Old 29th March 2010, 11:46 PM   #2
tomchr is offline tomchr  United States
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Take a look at the "typical application" schematic in the data sheet. Use a good quality cap for the 22 uF cap labeled Ci and connect a 100 nF polypropylene in parallel with it. That's probably as simple as it gets.

~Tom
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Old 30th March 2010, 11:52 AM   #3
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Thanks,

I have a rough idea of what to go for now then.
I need help with the powersupply though.

I have a single diode bridge here, left over from my dac.
It supports upto 700v and 20amps.
Now my question is, do i have to do a split system and create an artificial earth as it were for my amp from the power supply?
Or can i just connect the -ve and +ve from the bridge?

Basically i would like to know if i can have just a single diode bridge run this amp, or do i need a common earth (common to both the PS and amp)??

Thanks
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Old 30th March 2010, 12:34 PM   #4
AndrewT is offline AndrewT  Scotland
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there are two quite separate issues with grounding.

1.) all external conductive parts must be permanently connected to Protective Earth.

This is for SAFETY of the user and his/her household. This rule must never be contravened. This Safety connection does not improve the Audio Quality. It often makes Audio Quality worse.

2.) the Audio Ground (sometimes called Star Ground) for circuits that require a reference ground to operate correctly. Many circuits require this for Audio Quality.

You will need both the Safety Earth and the Audio Ground.

As a beginner, do not use the circuit you posted in #1.
This is the stripped down version from the datasheet and can result in poor performance and damaged chips/speakers.

Use an AC coupled design with all the National optional components in place.

Go to decibel dungeon and to ESP and read read read.
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regards Andrew T.

Last edited by AndrewT; 30th March 2010 at 12:37 PM.
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Old 30th March 2010, 04:50 PM   #5
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Thanks andrew, so I should go for the complete design then.
It's impossible to use a single bridge then i assume, as there is no audio ground?

When you say an ac coupled design, you're refering to the amplifier?

For a power supply then: 2 windings, 2 bridges in series then using the middle pins (-ve of bridge 1 and +ve of bridge 2 joined to form the audio ground), is that the best method?

The protective earth will always be used, thats basically to the earth pin of the iec socket, yes?
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Old 30th March 2010, 05:06 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the Power Supply (PSU) is usually a dual polarity supply. This by definition must have a zero volts reference.
With a centre tapped transformer, one must use a single bridge rectifier and the junction of the two series connected smoothing caps and the centre tap become the PSU zero volts. This junction must be taken to the Audio Ground. Do not bring the Audio Ground to the smoothing capacitors.

A dual secondary transformer can use a dual bridge rectifier.
The junction between the series connected smoothing caps and the common point in the two rectifiers becomes the PSU zero volts.

Have you downloaded decibel dungeon or ESP yet?
Have you started reading?
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Old 30th March 2010, 05:35 PM   #7
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Quote:
Originally Posted by AndrewT View Post
Have you downloaded decibel dungeon or ESP yet?
Have you started reading?
No and ermm.... no...

I'll get to it, but reading is boring, making is much more fun, even if it goes pop, bang or smelly (underwear included).

If i were to have a dual secondary transformer, could i used this in anyway on a single bridge rectifier?
Or could i just series them and assume them to be one the same, then regard the centre join as the middle tap and use that for the ground?
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Old 30th March 2010, 06:50 PM   #8
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Have yourself a read, you are a long time dead if you get it wrong.

John
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Old 31st March 2010, 12:32 AM   #9
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What can i say, i have no choice other than to read then. The whole idea i thought of a forum was to help those less knowledgeable than you...
Not shove people off to "go read".

I'm an utter electronics noob, i have no qualifications, but i do have a-level electronics, which consisted of mainly theory about safety of electronics and some basic logic circuits. The whole reason i didn't do a degree was due to theory getting right on my nerves about how everything is so tightly regulated.

I come onto here after a practical circuit i can make, i have a go at finding my own starting points and what do i get?



/Here endeth the rant (I'll go search some more and just knock up anything).
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Old 31st March 2010, 01:06 AM   #10
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Nobody is saying "go away we don't want you" What is being said is that there is enough electric in there to kill you, or your loved one twice. There is a base level of knowledge that you need for your own safety.

You describe yourself as an utter noob, would you fancy trying to build a car or hang glider without a reasonable idea of what to do and why? If enough people kill themselves with diy hifi the safety police will see a whole new avenue of regulation that had slipped under the radar. That said, with a-level electronics you are far more qualified than I. I am a car mechanic by trade.

If I caused any offence to you, it wasn't meant that way.

John
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