Yet another satisfied LM3886 builder

[dfdye]

I just hooked up my newly completed LM3886 amp based on Linkwitz's circuit that I cranked out while waiting for the transisitors for my LM4702 to come in. Falling firmly under the "DUH!!!" category for most of the people on this board, I am quite frankly astounded that such nice sound can come from such an amazingly simple package. Right now the parts are flowing all over my workbench, but I will be sure to post some pics when I get it in the purty case I am building. So far it seems perfectly suited for life as my dedicated iPod/computer amp in my office.

Thanks to everybody for help the offered, whether you know you helped or not!

[dgaxeman]

care to share on this ? building my first one. pcb schematics whatever? thanks

[Bearman]

This is a good site with everything you need.

http://hepso.dna.fi/misc/Ukram_Data.html

Hope that works....

[dfdye]

The circuit I used was from

http://www.linkwitzlab.com/images/graphics/3886amp.gif

but the PCB was a custom one that I have attached. There are some things I don't like about the board that I figured out after I sent the boards into be manufactured. For example, CS+ and CS- are in the wrong spot (they should be right next to the chip and after the V+ and V- inputs) and I didn't ground the nonconnected pins (though I fixed that by wiring p2p on the boards when I put them together. Also, though I put a feedback resistor part on the board, I put the resistor directly across the chip pins.

The case isn't finished quite yet, but I will definitely post more pics when I have time to finish it up. It is, however, running strong in my workshop while I do stuff for work, and it sounds great. My Cheap-o Pioneers I bought in high school low these many years ago sound better than they have ever sounded (which is still pretty bad, but at least I have tunes in the shop ) I have hooked them up to my North Creek CM-77's for more testing, and the sound is really nice. I know these aren't the best reference monitors, but my budget (and my wife) hasn't allowed me to get much better than that yet!

Here is the PCB, but I would recommend that a few modifications be made if anyone is interested in using the design. If anyone is interested, let me know and I will post a revision B.

[dfdye]

This is a bad picture from my phone (which is the only digital camera I own ), but it should get the point across. This (obviously) is the "mid/tweeter" version of the linkwitz circuit. The board obviously has places for electrolytic caps for the woofer version, but I haven't built a version of that amp yet with these boards. I have put together a p2p version of that circuit, though, and it has great low end response, but pretty terrible mid/treble. The bass response of the mid/tweet circuit isn't shabby, though, and I would strongly recommend this version if you are going to only build one amp per speaker.

[dfdye]

Here is the revision2 PCB layout if anyone is interested. I took off the feedback resistor assuming that it would be mounted to the chip pins. I also moved the bypass caps, shrank the board a little, and ran a better ground plane that actually connects the loose pins this time!

David

[Greg Erskine]

hi dfdye,

Nice work. We are watching.

Just a couple of points that I have read but not experimented with myself.

1. I don't think you need to worry about the NC pins. They can connect to anything or nothing. Connecting them to a ground plane does seem a nice thing to do though. Have you found it made a difference?

2. I noticed on the Linkwitz schematic that the mute resistor was a reasonably high value, limiting current. I have seem a few posts suggesting a higher current through the mute circuit is better.

regards

[jimbo1968]

dfdye,

What was the price/quantity for your pcbs you had made?

Is single sided cheaper to get made? or is the advantage that you can etch it yourself?

I'd like to design a little 3886 board similar to that for my first pcb. What software did you use? Any chance I could have the files to get me started?

[dfdye]

Originally posted by Greg Erskine
hi dfdye,

Nice work. We are watching.

Thanks! This is definitely a learning experience. I have always relied on our electronics shop to do pcb stuff for me before I found this forum. I am amazed at how quickly you can pick it up! I am sure there is a ton I am doing wrong, but at least I am hafing fun messing things up!

1. I don't think you need to worry about the NC pins.

I have never noticed an issue with NC pins in amps, but in the instrumentation I use in my lab (which is a few orders of magnitude more sensitive! ) there can be issues. Good practice that I learned a long time ago was to tie all NC pins to ground if you can.

2. I noticed on the Linkwitz schematic that the mute resistor was a reasonably high value, limiting current. I have seem a few posts suggesting a higher current through the mute circuit is better.

I haven't played with it, but thanks for the suggestion. So far, I don't have any complaints, but I will have to check that out! Easy swap.

[dfdye]

Originally posted by jimbo1968
dfdye,

What was the price/quantity for your pcbs you had made?

I ordered these boards from Olimex. They have a neat little deal where you can populate a eurocard with as many parts as will fit for one price. I got 2X of my LM4702 boards and 6X of my LM3886 boards for $36.30 shipped (and that included a goofy drill size)

The web page for the boards is: http://www.olimex.com/pcb/index.html

One thing you will have to deal with is Faxing to Bulgaria. That was interesting.

Is single sided cheaper to get made? or is the advantage that you can etch it yourself?

Both. For a board this simple you don't really need to run two sides.

I'd like to design a little 3886 board similar to that for my first pcb. What software did you use? Any chance I could have the files to get me started?

The software I used was Eagle, and you can use the free version and submit the files directily to Olimex. Do a quick google search for Eagle PCB and you should find the software. The tutorial will guide you through how to make the boards, so I would recommend you use this circuit as a test to learn the software.

Good Luck!