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Old 14th September 2005, 10:01 PM   #1
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Default Building an LM3886 BrianGT Kit -- Questions

Kits arrived today -- Yay!!!

I'm sure I'll have a few more as I go along. First question -- what exactly does the optional feedback capacitor do?

I'm assuming that it trades off sound quality vs. amplifier stability? If so -- I'll drop it in. I will be hooking up to a wide variety of sources and speakers (building it for use with my keyboards); some not exactly audiophile grade.

Wes
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Old 14th September 2005, 10:43 PM   #2
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Default Make that: Questions and log..

Problem 1: Instructions seems to have mixed up C5/C6 and C1/C2 for the snubberized PS board (10,000 uF Panasonic FC caps).

Trick 1: MUR860s are pretty easy if you stuff a line of four at a time, flip the board with the other half supported at the same height as the diode cases, and solder ever-other-pin.
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Old 15th September 2005, 12:36 AM   #3
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...For anyone following -- my suggested order of assembly is a little different from Brian's:

- Bottom Resistors
- MUR860s
- Binding Posts
- Large Electrolytics
- Snubber resistors and caps
- LED and related components

Second PS board went together in about 15 minutes. Not bad.

Wes
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Old 15th September 2005, 01:54 AM   #4
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NOTE:

I don't know if this has been corrected, but the LED diode diagram is backward on my board set. You should install the LED the opposite way that the boards indicate.
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Old 15th September 2005, 02:01 AM   #5
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Thanks, m0tion, but you're about an hour late. I actually spotted the problem while building my second board, but didn't want to post that problem 'till I had tested it.

Problem 2: LED Long lead points AWAY from Rectification diodes. And the pads/through holes are awfully small, makes desoldering a pain.

Two PSUs built and tested with a spare 24V transformer, one half at a time (didn't feel like hookup up the toroid -- the 24V is already set up with quick-connects which fit my lamp lead).

+/- 36.2V out of each. So when I hook up my 23V toroid, I should see close to 35V rails. Excellent.

Wes
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Old 15th September 2005, 02:56 PM   #6
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I'm thinking about buying a BrianGT kit, but I keep reading about instruction sheet errors, lack of documentation on the LM4780 kit, etc.. Scares the heck out of me. As a noob, you really want to see clear, concise, and straightforward instructions. Wonder why Brian doesn't fix these simple issues...

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Old 15th September 2005, 03:13 PM   #7
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Well, the problems I've hit so far are extremely minor. I'm no noob, but I'm not an experienced builder, either... that said, I'm pretty confident I could build this stuff without any instructions at all -- just the parts list and the schematic should be plenty. The BrianGT kits are definately easier to build than starting from scratch!

If you wind up building a 3886 kit instead, keep an eye on this thread, and you will have all the corrections you need.

(And the power supplies, at least, would be the same.. assuming you get the CarlosFM Rev 1 snubberized PS boards)
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Old 16th September 2005, 12:58 AM   #8
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Okay.

Built my two LM3886 rev 1 boards. Not bad, shook open the package about an hour and a half ago. Zero assembly issues, but I assembled in the following order:

- small resistors
- 47uF cap
- 100uF caps
- PP caps
- big resistors
- LM3886


This order makes it easier (IMHO) to stuff the PP caps -- stretch the leads, stuff, flip the board, and use the electrolytics for primary support.

Question:

If the power grounds are joined on the PCB, why are they both taken from star ground? (Or am I about to screw something up?)

Wes
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Old 18th September 2005, 02:24 AM   #9
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*sigh*

Tough day today. You'd think that the amplifier part of building an amp would be the hard part! I guess that's not the case when you're building Brian's kits, they're really easy. So far.

Because I can't have bolts sticking out the bottom of my chassis (not even just a little bit), I decided to re-use the existing studs which were already installed in its previous life (the only other choice was to make holes in the front).

This turned out to be much trickier than thought! Used square tubing to make "adaptors" -- drilled small holes on the bottom, big holes on the top, and passed the studs through the square tubing, securing them with nuts on the inside (top hole is just big enough for a nut driver).

Drilled and tapped some 6-32 UNC holes in the top of the square tubing, and prepared to mount power supply boards.

Note: Boards in this kit are not drilled to accept #6 screws! If you are using motherboard stand offs, either you will have to hunt for the rare ones which use CD-ROM/Floppy-drive screws (perfect fit), or you will have to drill out the holes in the circuit board to accept 6-32 screws. I drilled them out to 11/64", which gave a little bit of "error slop". DO NOT ATTEMPT WITHOUT A DRILL PRESS AND SHARP BITS!

Now I can almost mount my power supply boards, but need about 1/32" more clearance. I'm going to call it a (LONG) day, and start again tommorow. Hopefully by tommorow, the power supply boards and toroid will be installed (the toroid being another story in and of itself!)

Oh -- another tip -- if you're re-using a chassis and want to take off the old paint... Automotive paint stripper is AWESOME! Spray it on, let it sit, come back and wipe the old paint off with a paper towel...

Wes
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Old 29th September 2005, 11:31 AM   #10
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Last post was more than 10 days ago. Haven't picked up a soldering iron since, but have invested huge amounts of time on chassis work.

Who knew that getting the chassis right would be such a huge amount of work?!?!

It's almost done now. Save a couple of wire-passing holes and an insulated plate for mounting the external connectors, it's ready to roll. I think I'll use a piece of fibreglass PCB with the copper etched off for the plate.

Painted the chassis cover last night. It turned out really well. I used Krylon Fusion textured black paint, followed up by Krylon Fusion satin black, for a nice pebbly black look.

Warning to those using this paint -- the textured paint comes out HARD and THIN. This makes it very succeptible to runs. I'd suggest spraying an additional 6" back than you would with normal paint.

Elegantly splitting the transformer secondaries to go to two rectifier boards was a big tricky. I would up using crimp-on ring connectors for 16awg wire and #6 centers. Using the #6 center connectors gave me the biggest surface area; the OD of this type of connector is constant from one screw size to another.

They are bolted together with nylon screws and a nuts. I still have to figure out where to actually PUT them in the case, though.

Wes
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