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Old 27th November 2011, 09:23 PM   #1
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Default My LM3886 build thread

Hey, All.....

I typically hang out in the "Class A" Nelson Pass forums, but I could not resist putting together a Chip Amp based on the LM3886. Unfortunately, you'll see some of my Class A upbringing in my design, but I still hope this thread provides some useful info for new constructors.

I started off (fortunately, or otherwise....) with some left-over heatsinks and a toroid transformer from my lass Class A MOSFET project, and decided to use them in my Chip Amp design. (Yeah, the heatsink was probably overkill, but I had it handy!)

Here goes..... the heatsink was from Machtron, Inc, and purchased via eBay. It's a MASSIVE hunk of aluminum extrusion, and I elected to use one, split down the middle, to make two heatsinks for this project. (Even so, they run at ambient room temp, even after several hours of use....)

The pics attached show the original Machtron heatsink (unmolested), and then the half that was cut (on a metal bandsaw), trued up (on a milling machine) and then tapped for the LM3886.

The next series of pics show the chassis fabrication and integration of the amp components. The chassis baseplate is 3/16" aluminum, while the rear apron is 1/8" aluminum (T06061 alloy was used). I welded the apron to the baseplate, to avoid using angle brackets and additional hardware. Another pic shows the layout of the rear apron. Despite the size of the heatsinks and toroid, I wanted to give the amp a small footprint--about 9" x 9"--so the chassis and apron layout are pretty compact..... this made for some fun wiring challenges. Final assembly was more like building a watch, than assembling an amp. The PCBs and components are from ChipAmp. Stuffing the amp boards and PSU presented no challenges--as always, read the assembly instructions (and the manufacturer's tech sheets!) to help fabrication. (Trust me, on this last point!)

Since I had welded the rear apron to the baseplate, I used an old fabricators trick during wiring. I inverted the chassis, and since the design layout had "left-to-right" symmetry, I built the amp on the bottom side of the baseplace, effectively using the baseplate as an assembly jig (see pic). This helped me in routing wires and dressing lead length. If you notice, I ran most of the PCB interconnections UNDER the PCBs, to further clean up the assembly. Important, too, to consider routing for the AC and audio inputs (and outputs) in a build like this.

Pics also show the bottom of the baseplate (and the fasteners for the heatsinks, PCBs, etc.....and also the top of the coverplate (which is also 1/8" aluminum). The thickness of the chassis plates is probably also a hold-over from my Class A fabrication, but they certainly make a strong chassis!
Attached Images
File Type: jpg Chip1.JPG (88.0 KB, 1219 views)
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File Type: jpg Chip20.JPG (63.1 KB, 392 views)
File Type: jpg Chip21.JPG (58.9 KB, 407 views)

Last edited by CanAm Man; 27th November 2011 at 09:47 PM.
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Old 27th November 2011, 09:32 PM   #2
GloBug is offline GloBug  Canada
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Nice work, I will be following along.

I am also going to build an aluminum chassis. The welding is a good idea.
I wish I had a band saw. Normally I would cut the 1/4" stuff with a skillsaw and clean up the edges with a file, except one of my pieces is around 3/4". lol
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Old 27th November 2011, 09:37 PM   #3
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Default Part Deux....if you aren't bored yet....!

After test assembly I broke everything down, for final fabrication. (I must have had the heatsinks attached/removed from the baseplate at least a dozen times during contruction to check clearances, etc.) I should mention that the heatsinks were tapped in multiple places with 8-32 taps, and that the heatsinks effectively serve as the side structure of the chassis, sandwiched between the top and bottom aluminum plates. Both top and bottom plates were machined to permit maximum convective airflow for the heatsinks (which, again, was probably overkill....!)

The next pics show the chassis after return from the powder-coater. I use a 10% matte black on my chassis, and I think it's worth the cost (about $30 for this chassis). Also shown are pics of the final wiring. The amp modules were designed to fit tightly between the rear chassis apron and heatsinks, and to minimize input audio and output audio lead lengths. I also routed the AC mains circuilts along the centerline of the chassis, and under the PSU PCB, to keep the AC lines as far as possible from the audio routes.

I also decided to add LED VU level meters for each channel. The VU PCBs were attained from an eBay supplier, and modified for use in this amp. The front panel was attained from Front Panel Express, and was machined to accept the left and right channel LEDs. Since my amp PSU rails are +/- 28 VDC, and the LED PCBs require 12-14 VDC, I added a small circuit board with dropping resistors and 12 VDC regulators to provide power for the VU displays. The VU PCBs and regulator board fit tightly in the forward right-hand side of the chassis, and I was able to use the right-hand heatsink as a mounting point for the two12 VDC regulators (although they run only slightly warm to the touch).

Details are contained in the pics.
Attached Images
File Type: jpg Chip32.JPG (37.7 KB, 337 views)
File Type: jpg Chip33.JPG (52.7 KB, 265 views)
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File Type: jpg Chip36.JPG (68.6 KB, 240 views)
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File Type: jpg Chip39.JPG (48.0 KB, 325 views)
File Type: jpg Chip43.JPG (52.9 KB, 306 views)

Last edited by CanAm Man; 27th November 2011 at 09:51 PM.
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Old 27th November 2011, 09:43 PM   #4
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Default And, fini......!

Pics of the final amp, in operation.....

Again, construction was straight-forward. The use of a "tight" chassis design made the fabrication a little challenging, but that was part of the fun.

The wooden side panels on the amp are solid 1/2" maple, finished with a mix of cherry-dark walnut oil stain.

The LED VU indicators have been calibrated, with the first "red" LED indicating the clipping level (determined with my audio signal generator and scope).

The amp sounds great. I'd recommend the LM3886 for an easy build, with good sound and ample volume levels into efficient speakers. This was a fun diversion from building Class A "room heaters"....

I hope my construction tips might help some of you'all out there.....

Ken
Attached Images
File Type: jpg Final2.JPG (24.6 KB, 404 views)
File Type: jpg Final5.JPG (42.2 KB, 490 views)
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Old 27th November 2011, 09:58 PM   #5
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Quote:
Originally Posted by GloBug View Post
Nice work, I will be following along.

I am also going to build an aluminum chassis. The welding is a good idea.
I wish I had a band saw. Normally I would cut the 1/4" stuff with a skillsaw and clean up the edges with a file, except one of my pieces is around 3/4". lol
I've had access to a HUGE metal brake, which has let me do "straight cuts" that are true and accurate. The curved stuff (like the cut-outs for the heatsink vents) were done on a bandsaw--and the curve of those cuts was a little challenge...... LOTs of filing, afterwards.....! That's one of the nice things about a "flat" powder coat finish--it can hide a lot of "minor sins" in metal fabrication.....!!

Good luck with your project--sounds like fun....!
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Old 27th November 2011, 10:42 PM   #6
GloBug is offline GloBug  Canada
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Thanks.

Wow that was quick!

Effin A, that's a pretty slick job.

I don't think you could ever have too much passive cooling.
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Old 28th November 2011, 01:18 AM   #7
bbm3 is offline bbm3  United States
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Absolutely stunning.
As is all your work.
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Old 28th November 2011, 01:45 AM   #8
6L6 is offline 6L6  United States
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Wonderful! Simply wonderful! The chassis is fantastic, and your love affair with VU meters gets yet another fix!

Send the photos to the chipamp.com website, I'm sure they will highlight it on their site!

Now I really want to finish mine... It's next after a phono stage.

(And then it's back to our regularly scheduled class-A room heaters!)
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Old 28th November 2011, 01:55 AM   #9
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BBM3, 6L6..... Hey, "local guys". Thanks for checking out my ChipAmp thread..... now, if I hadn't mistyped LM3886, in the thread title (as LM2886.....duh).......

Yep..... "holiday spirit"--had to add sparkling lights to the amp, for the festive feeling....!

Ken
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Old 28th November 2011, 02:02 AM   #10
6L6 is offline 6L6  United States
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It's truly a pleasure to see a build of that quality!

Anyway, It makes me really want to visit and hold those heatsinks in my hands, my next project might need a bunch of them. (At least 4 per monoblock!)
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