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Old 19th May 2008, 11:30 PM   #21
Edits is offline Edits  Indonesia
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Hi Dan,
Can not wait the sequel! Compiled those parts, but still miss the 27K feedback resistor. Can I substitute with 20k, 22k? or 33k?
Is there any "Design guide" for LM1875 like the one for LM3886?
Quote:
The actual "middle of the road" transformer selection corresponds to these transformers, with their 18-0-18 vac.
That is the one I'll try.
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Old 20th May 2008, 02:51 AM   #22
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Quote:
Originally posted by Edits
Hi Dan,
Can not wait the sequel! Compiled those parts, but still miss the 27K feedback resistor. Can I substitute. . .
There are many designs possible. This thread is about just one design, which does include a 27k resistor. Feel free to use the PM/Email feature!

The resistor values that I've used are widely available in "variety packs" from retail storefront electronics hobby shops.
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Old 20th May 2008, 03:34 AM   #23
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Default Heatsink.

I had a knife out, scoring some plain veroboard so that it would snap to appropriate sizes, got curious and unpacked the box with the new heatsinks. . .

WOW!! OMG! That heatsink (above) just arrived in the mail. Its whopping huge! Some metric versus U.S. conversion error? Anyway, about half that large will do it.

Its so tall that I'll need to go to the hardware store and buy a metal cutting blade to fit my "Junior Sabre Saw" (we call that a JigSaw, here in the U.S.). Although that's quite inexpensive, if you do it, use a dust mask and eye protection.

You can also get heatsinks from these guys: http://www.apexjr.com/Sinks.htm

At "established" (old) computer shops, you can find good-size "Slot CPU" type heatsinks, probably for free. You can also find suitable heatsinks inside defunct equipment that's sold at "thrift" stores and second-hand stores, also at little to no cost.
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Old 20th May 2008, 06:39 AM   #24
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Default Building it

Here's a photo showing LM1875 with the pins slightly spread and fit into veroboard (plain phenolic), for easy soldering.
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File Type: jpg lm1875 on veroboard1 002.jpg (76.4 KB, 3593 views)
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Old 20th May 2008, 06:51 AM   #25
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This photo shows the pin spacing. Its a view from the bottom side of the board. Note that this is plain phenolic board, without any copper on it.

There's at least 1/8" of pin sticking through the board. And there's plenty of room to get a hook or loop of copper wire around any of the pins, without worries about any connections too close to any other.

See photo.
(click photos to enlarge)
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File Type: jpg lm1875 on veroboard1 003.jpg (20.6 KB, 3223 views)
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Old 20th May 2008, 11:22 PM   #26
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Next step, we get some nice 24ga or 22ga copper wire (or copper core wire) and do like this picture.

These 3 wires are about an inch longer than the edge of the board. Each has a small hook so it can make solid contact with the LM1875's pins.

These little hooks are dipped into the flux jar before soldering. That makes for instant success with the soldering.

Make the end of your soldering iron, bright, silver, clean, by wiping it with either a wet sponge or #0000 steel wool. That helps to make the connections nice and clean.
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Old 21st May 2008, 12:26 AM   #27
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Okay, here's our 27k feedback resistor.
That's its name because its one of the two resistors that make up the NFB. That stands for Negative Feedback Loop, and it sets your gain.

This 27k resistor is red, purple, orange.

After setting the resistor in place (as shown), I looped it around the pin of the LM1875 (as shown).

Because the connection is smaller than my fingers, I used Hemostats, although needle-nose pliers will do in a pinch.

Then I painted the area with flux (for success).
Lastly, I cleaned the tip of the iron and soldered.

I'm proud of this picture. Click on it.
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File Type: jpg lm1875 on veroboard feeback 27k.jpg (50.4 KB, 2512 views)
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Old 21st May 2008, 12:56 AM   #28
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The partner to the feedback resistor is an 820R (820 ohms).
This one is gray, red, brown.

Connect as shown.
(click photo)
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Old 21st May 2008, 03:33 AM   #29
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Before we get much further, I need to say:
The design on this thread, and its schematic, are free for diy, non-profit use.
Okay!

Next, we'll hook up the speaker + wire. And, that step completes the connections to the chip.
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File Type: jpg lm1875 on veroboard feeback loop speaker 1.jpg (93.3 KB, 1425 views)
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Old 21st May 2008, 04:26 AM   #30
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Its time to hook up the main ground lead.
This is thicker than any other wire.

The first of the power caps help hold the ground lead in place.
They're 100nF, which is 0.1uF, and they probably have the number "104" written on them.

You can use the plain 3 cent ceramic caps or something that looks prettier. See photo.

To hook up caps on veroboard, just hold your wire firmly against the pin/leg of the capacitor. Bend the pin/leg of the capacitor over the wire (it holds the wire). Apply flux, and solder into place.
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