Another LM3886 PCB!!

[troystg]

Hi sek-

And please don't take my last post the wrong way.

I am more than willing to pay you for your time and if not at LEAST I will send you some of the boards that you designed and I have made.

Your kindness has really helped me out and I promise to return the favor.

[sek]

Let me 'investigate this further' over the weekend.
I'm a little sick (cold) at the moment... Can't respond today as quickly as you're used to...

Sebastian.

[cjd]

For gits and shiggles...



I have an inverted chip-amp layout that'll sit in place of the NI layout with all the inputs/outputs in exactly the same spots.

1 inch wide, 2.9 inches long.

I have not yet verified 100% that this is actually entirely compliant with the schematic, but I'm 99% sure at this point.

C

[troystg]

Hey cjd-

Nice board and layout.

Good size demensions but in my case I need a board that has the chip on the narrow side.

What SW did you use to draw it?

[sek]

Hi,

Originally posted by rabstg
First off I really want to thank you for your time. The PCB you drew was exactly what I was trying to do.

That sounds good, glad I could really help (and not only try).

I've read in another thread that you're away over the weekend. I guess you're reading this later, so I'll sort out a couple of things first.

I have a couple of questions on how to modify the slime-line PCB you drew for me.

No problem, but look how you drive me into doing it for you again (in the step between question 1 and 2):

1. How do I move ...
2. How do you move ...

Funny

I don't state this because I'm lazy or don't care. I just don't know your situation as good as you do. See, sorting everything out, making sure you know every measure, every constraint, every part of the plan, and such; this is all much more work (and fun) than doing a small layout. Thus, while you have the plan, I can only quess what you want exactly. However, I won't let you alone on this!

Every little lack of information on my side could lead to a serious amount of work in order to correct for 'design changes' that have to be done after a correction.

That said, all we have to do is clear all the important issues to get this first sketch into a working building block. Let's see what we (both) know and what we (me in terms of information, you in terms of solution) need:

The PCB can use top and bottom layer since it will be made at a factory.

This is good news, as you (and Steve, for that matter, whoever he is) don't have to fiddle around with processing/etching something from someone else's work (which is always difficult, as every DIYer has own preferences on how to prepare photo films and such).

OTOH, commercial board manufacturers usually specify:
- what data they need (supposedly either gerber files, a foil image or project files),
- what limitations their manufacturing process has (i.e. minimum distances, wire widths, drill sizes, etc.),
- what they recommend in order to help reduce production cost (like wether you're allowed to align multiple boards for later cutting apart; and if so, how...).

It would be great if you found this out first; either look at the web site of the board house or mail them for e.g. a CadSoft Eagle 'design rule' file.

Also the components (except the supply +- V caps) can be placed on top AND bottom.

Yes, that would be my next question: How much space is around each board?
You probably have a sketch or a drawing?

As there are only caps, resistors and the chip, it's clear which components can also be placed on the bottom layer: the resistors.

1. How do I move pad 3 to the left a little more to straighten out the trace from C2?

By using the command 'move'? No, kidding. I'll do that once everything else is pointed out.

Such as: which components do you actually intend to use?
This mainly has to do with grid (pin distance), diameter and distance/spacing.
Most important to know are the supply capacitors and the 'power' resistors.

For the capacitors, I assumed the smallest 1000uF/35V type that is available to me, for the resistors I don't know.
But please keep in mind: for bridging/paralleling, you'd need high precision resistors that probably don't come in standard 0207 size.

2. How do you move pad 5 (and the one next to it) and Rin to the right? I also will move the trace to the top layer.

Same answer as above: once every component is determined, this will be no problem for me. I'm looking at your picture to see the direction you want everything to go.

3. Can you add 2 output resistors to parallel multiple boards?

Now this is really something new: I'm not sure wether I understand your question right, because I have no schematic to start from. I used the one you posted, but it was neither correct, nor complete...

I guess we're talking about BR100, PA100, BPA200 as of National's AN-1192. The thing I clicked together doesn't really match their core building block yet.

Even the simplified circuit as of AN1192's Figure 17 requires the capacitors Ci, I would therefore recommend to reserve space for them.

Also, the recommended supply in Figure 16 doesn't require large supply bypass capacitors for each chip. You already said that you'll have a large capacitor 'block' in the power supply, so we can probably minimize the supply capacitors on the board. The regulated supply you probably want to implement in a later stage won't require significant capacitance near the chip amps at all...

The LM3886's datasheet requires (or highly recommends) a capacitor at the mute pin in order to prevent turn-on pops; have you thought about this yet?

4. Separate traces as much as possible but alternating top and bottom layers.
5. Widen traces as much as possible?

This will become easier now that I have two layers available.
The first attempt had too slim and too narrow traces; after all, it was just a first attempt...

6. The board can be lengthened to give you more room in front of the chip.

Good. I assume the available length will be shown in your drawing.

What about the regulators? Will they have to be designed, too? (Easy...!? )
In this regard, it would help to know your actual application, e.g. what purpose(s) the finished amp will be used for.

Also, we forgot mounting the board itself so far. It needs holes and the screws through them have to hold somewhere in your enclosure!

Anoter point: think about the positions of every connector, you'll have to be able to reach them on all the boards inside the enclosure, and I guess it will be very tight in there once everything is mounted.

Okay, that's the list of 'unclarities' at the moment.

Cheers,
Sebastian.

[cjd]

Originally posted by rabstg
Hey cjd-

Nice board and layout.

Good size demensions but in my case I need a board that has the chip on the narrow side.

What SW did you use to draw it?

ExpressPCB. Gotta try to switch to Eagle.



1 inch wide, 2.65 inches long.

better?

The only thing I don't like about it is that the low level input is right near the high level output. And the power traces are a bit longer.

[sek]

Hi cjd,

better?

The only thing I don't like about it is that the low level input is right near the high level output. And the power traces are a bit longer.

For rabstg's purpose the inverting topology doesn't seem too useful.

Also, I'd consider the supply and output traces too narrow and, as you mention, too close to the input circuitry. They might introduce noise/hum or cause oscillation without proper bypassing and input- and output-filtering.

Hope this helps,
Sebastian.

[cjd]

That's not inverted. . .

And, how wide would you recommend the traces be, based on what thickness copper?

C

[sek]

Yep, you're right. Sorry, I misunderstood you in a former post when you said the following:

I have an inverted chip-amp layout that'll sit in place of the NI layout with all the inputs/outputs in exactly the same spots.

As for the trace widths, I'm not so much talking about absolute thickness. But they seem too close to each other at some points, and - as you already mentioned yourself - seem too close to the small signal circuitry. My point basically is: changing this will certainly lead to too small traces. Also, how would the regulators be cooled in your arrangement (e.g. they could be bolted to a heatsink underneath the board)?

I'm not telling you that it won't work the way it is, though!

Sebastian.

PS: As one can see in my project I pointed to in a former post, I would highly recommend SMT components at this point. The integration that is required in this thread requires a very large portion of the board area for copper traces (and thus very small components), as the reduction in board space doesn't lift the physical requirements on the current that flows through the copper traces.

[cjd]

Ahh - I understand your confusion on the inverted bit. Somewhere a while ago I put together IGC and NIGC boards to go with a regulated power supply that were an inch by an inch and a little (don't recall exactly how big). The inputs, power, and outputs all lined up between the two, which means tossing the regulator portion to an inverted setup would be simple. Just was an option, and if it ever goes bridge-parallel it may be a worthwhile and needed option.

I'm not convinced that a board with the chip on the narrow end makes any sense at all, for a lot of reasons. As you noted, the regs in this case still need some heat-sink, and if you put a HS on the side, why not put the chip on the side then?! You're probably right about SMT as well - this was a quick "can I do this" try more than anything. I'll only ever make the previous layout I posted, which puts the chip to one side and keeps the traces further apart. And that one hasn't been given all that much consideration yet, as far as making sure it won't cause any problems (such as those you mentioned regarding this recent one).

I suspect there may be some trouble on the regulator portion with respect to traces being too close together. Which may mean I just need to make it a bit wider, or find a different solution.

By the time I get around to making more amps, I may just go P2P for a rats nest of beauty, and one heck of a miserable mess to root through if I ever need to troubleshoot. Not that I'd need to do that.

C