680W LM3886 Chipamp

[krips]

This is meant as a flexible, good sounding amp for my home theater. I've almost completed building the 10 mono kits, and have my 800W 20V toroid already. I plan on making a MDF enclosure, and have the top vented using perforated aluminum sheet:

For cooling, I was planning on using an extruded aluminum square bar, measuring 14.5"x1.5"x1.5", then have aluminum plate on top of that allowing me to add heatsinks that I already have.
Here are the pictures of my progress thus far.




Comments/tips/questions would be appreciated. Thanks .

[Hdale85]

Very intersting Chip amps aren't my favorite in several ways. They sound pretty good but I've heard better for sure. My biggest gripe with them is their sensitivity to noise! But you can work it out eventually lol. Get ready for a lot of tweaking to get rid of it though :B

But it looks pretty neat so far! That's a lot of amps that's for sure.

[krips]

Can I wire my transformer like this? I have dual primaries and quad secondaries.

[Johnloudb]

If you got 120VAC line, which I suspect you do, then yes you should wire the primaries in parallel. Just make sure to get the right wires together.

No, you shouldn't wire the secondaries like that. If you have quad 20VAC(?) secondaries you should wire 2 of those in parallel. Also making sure you get the correct wires together.

You want +/- 20VAC, so you need to wire the two paralleled secondaries in series. And the center wires are power ground. Get your wires straight. Dots mark the negative phase if I remember correctly. Yes, I just checked a transformer.

It's hard to say exactly without seeing the markings on your transformer. Are you sure there are quad secondaries (8 wires). Usually you have dual secondaries (4 wires).

[krips]

This is my exact transformer: http://www.antekinc.com/AN-8420.pdf

[fjhuerta]

Very intersting Chip amps aren't my favorite in several ways. They sound pretty good but I've heard better for sure. My biggest gripe with them is their sensitivity to noise! But you can work it out eventually lol. Get ready for a lot of tweaking to get rid of it though :B

But it looks pretty neat so far! That's a lot of amps that's for sure.

That's weird. I haven't had any noise problems yet with my 41Hz kit (even with a tube pre), and I like its sound a lot.

My 100W Tripath kit, on the other hand, is like a huge noise-picking antenna. It sounds great... when it's not humming or beeping.

[Johnloudb]

Ignore my previous description about the secondaries, as you don't want to connect any in series.
Here's how the tranny wires should be connected. You also need to be careful to make the correct connections to bridge rectifiers. You should have 2 bridges with 4 pins each, or you should have eight rectifier diodes.

[cjd]

Bad grounding can make them sensitive to noise. Otherwise I just find that they're not good at hiding noise in the system, no matter where it exists.

[Hdale85]

I have a pretty good ground setup in my house. I just find you have to wire the ground in a particular way to get the noise out. If you have the slightest bit of noise somewhere though it certainly is going to bring it out

[Amphiprion]

My 100W Tripath kit, on the other hand, is like a huge noise-picking antenna. It sounds great... when it's not humming or beeping.

What type of power supply and does the chip use any feedback? Is the amp SE or BTL?

[Hdale85]

Aren't the 41hz kits different though?

[cjd]

Uh. Not ground in the house. Ground layout in your amp. They're sensitive to this. Picky, even.

C

[Amphiprion]

Is it anything that a 4-layer signal/ground/V+/V- stackup wouldn't fix?

[Hdale85]

Ah yes CJD, we are talking of the same thing then

The way I found that works best is to use a fat ground cable between a common star ground spot and your chassis. Then use medium size wire to your amp boards. Use the thinnest wire for the non ground items. The reason I have gathered to do it this way is it leaves the ground wire as the least resistive for ground loops and what not.

[cjd]

Is it anything that a 4-layer signal/ground/V+/V- stackup wouldn't fix?

Yes. Maybe. Depends on the amp layout overall, really.

C

[Amphiprion]

Ok. That totally answers my question

[cjd]

It's just really easy to add a ground loop, and having a ground plane doesn't specifically solve it. It *might* but it depends how you pull it all together. I think too many people forget to include the internal circuit in the op-amp when considering ground paths.

In this case, the likely problem will be merging the grounds of each of those separate amp boards! If you had 10 on one circuit board and were planning a ground plane properly you'd probably already have that issue solved.

[krips]

cjd, what is this ground plane that you mention?

[Johnloudb]

A ground plane is a layer in a PCB that is connected to signal ground. You don't need to worry about it, as you already have your PCBs. But, just star ground all the grounds from the different PCBs to one point at the power supply ground. And you don't want multiple ground paths.

[krips]

So my transformer and my power supply boards are going in one unit and my amp chips in another. I'm not quite sure how I'm going to make up one of those 'umbilacal cords' though. I was thinking maybe RCAs but someone told me that wasn't the best idea. Would I be better to go with binding posts? XLRs?

[Kevin Haskins]

I did a quick and dirty PCB based upon the LM4780 application notes and it is dead silent. You can get noise with any amplifier design but that doesn't necessarily have anything to do with the chip design. It has to do with layout and implementation which is up to the DIYer.

If you follow the application notes you get really good results.

[cjd]

I did a quick and dirty PCB based upon the LM4780 application notes and it is dead silent. You can get noise with any amplifier design but that doesn't necessarily have anything to do with the chip design. It has to do with layout and implementation which is up to the DIYer.

If you follow the application notes you get really good results.

Yup.

Though if you're trying to glom lots of these together it's SO easy to build yourself ground loops that are nearly invisible without a solid understanding of how the chips are wired internally and including that in your schematics...