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Old 14th May 2008, 02:00 PM   #1
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Talking Beginner's Gainclone, LM3875, The Amplifier Board

This is for the purpose of an accessable hifi gainclone amplifier, it is in reference to Commercial complete Gainclone kit for a beginner? and it is by request.

Without further ado, here's LM3875 by hand, at low cost, and easily made by following the "play by play" photographic format.

Since we're not shipping PCBs through the mail on this project, here's a photo of LM3875 that shows the hookups we're going to use. The purpose is wide spacing for easy soldering on phenolic board, which is available anywhere.
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File Type: jpg lm3875tf.jpg (30.2 KB, 1241 views)
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Old 14th May 2008, 02:13 PM   #2
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Here's a photo showing how to fit the chip into phenolic board.

This is the only "hard part" and if you can do like this photo, then the rest is a breeze.
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File Type: jpg lm3875 into phenolic 002.jpg (75.3 KB, 1262 views)
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Old 14th May 2008, 02:24 PM   #3
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Following along with the directions in http://www.national.com/ds.cgi/LM/LM3875.pdf, we're supposed to put 100nF capacitors, at the rails, and close to the chip.

100nF is also known as 0.1uF.
100nF is also known as code 104, and there's probably a "104" printed on your capacitor.

You can use ceramic or polyester, or polypropylene at any price point or brand as long as it has a "104" printed on it.

See photo for their location:
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File Type: jpg lm3875 into phenolic 100nf.jpg (67.6 KB, 1191 views)
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Old 14th May 2008, 02:41 PM   #4
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Let me introduce you to your new best friend for instant success with soldering:
Electronics grade flux. For example:
http://www.radioshack.com/product/in...entPage=search

You get a small, inexpensive toy paint brush and apply a thin coat wherever you expect the solder to stick.
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Old 14th May 2008, 03:42 PM   #5
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Lets add a pair of either 470uF or 330uF.
These are the onboard capacitors for power.

See picture.
Marked in pen is where the wire will go.
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File Type: jpg lm3875 into phenolic power circuit.jpg (93.6 KB, 1187 views)
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Old 14th May 2008, 03:53 PM   #6
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Hookup of the power circuit:

We get 3 pieces of hookup wire.
That's generally a 24ga solid copper.

These 3 pieces are as long as the whole project.

The one ground wire, down the middle, is twice as thick.
It "stops" at the centerpoint between the little 100nF caps.

The two "hot" lines (V+ and V-) are given a tiny little "hook" on the end, so that there's certainly a secure connection to the V+ and V- pins of the chip. Reference post#1 for their location, just to be sure.

See picture.
It isn't necessary to make the sort of mess that I did here. Just the general hookup will work fine.

In case your board doesn't have the pads, just bend the capacitor pin over top of the hookup wire for a firm connection prior to soldering. That technique is shown at the centerpoint of the two 470uF caps here.
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File Type: jpg lm3875 into phenolic power circuit2 001.jpg (65.1 KB, 975 views)
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Old 14th May 2008, 04:28 PM   #7
gychang is offline gychang  United States
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Quote:
Originally posted by danielwritesbac
Lets add a pair of either 470uF or 330uF.
These are the onboard capacitors for power.

See picture.
Marked in pen is where the wire will go.

thank you, thank you.

this is the kind of pictorial assay that is tremendously helpful for us total dummy newbies.

gychang
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Old 14th May 2008, 04:44 PM   #8
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Its time to install the feedback resistor.

What it does:
This is one of a pair of resistors that make up the NFB.
The NFB, Negative Feedback Loop sets the gain of our amplifier.

Do it clean:
Lets take a moment to rub the end of the soldering iron with some steel wool, or you can use a damp sponge.

The part:
Lets get a 27k resistor. Its color code is red, purple, orange.
EDIT: Shown is ordinary 1/4watt carbon from the variety bag at the local Radio Shack. Of course you can use the blue (metal) if you like. The important part is that both NFB resistors (27k and 680R) be made of the same materials.

Prepare:
Have a look at the photo.
Paint metal with flux so the solder will stick.

Directions:
Make a little hook with one of the resistor leads (see photo) and trim it short.
First, hook this over pin3 and solder into place.
Second, just stuff the other end down the hole that's right in front of pin8, and solder it to pin 8.

See photo.
*Allow a bit of space so that the resistor doesn't touch other pins.
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File Type: jpg lm3875 into phenolic feedback resistor 003.jpg (62.4 KB, 926 views)
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Old 14th May 2008, 04:58 PM   #9
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There's a typo above.
The NFB resistors are supposed to be
27k and
820R
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Old 14th May 2008, 05:18 PM   #10
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Quote:
Originally posted by gychang
thank you, thank you.

this is the kind of pictorial assay that is tremendously helpful for us total . . . newbies.

gychang
Oh, you're welcome indeed!

The design on this thread has some answers to the many questions that I've been asked in public and in private.

That includes goals of general compatibility with speakers and sources, as well as a non-botique approach that will allow it to work just as well with recycled, free, or hobby-shop parts parts as it does with name brand parts. Use what you have and what you can find.
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