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Old 28th August 2010, 03:58 AM   #1
Yurk is offline Yurk  United States
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Default Op Amp PCB design question

Hey all,

I'm working on an equalizer that will use op amps for the active filters -- along with caps/resistors/slide pots, etc.

Anyway, I've got a layout with 2 layers right now. Bottom layer signal, top layer ground plane. Only things have gotten so complex that I have traces on my ground plane. About 24, to be exact. Two are about 2 inches, one is just over 5 inches. And it's the positive supply rail.

I was thinking of going with 4 layers, just to have an unbroken ground plane, and to get a power plane. Only problem is I'm using a split supply, so I have to decide whether to use the power plane as positive or negative, and put the other supply rail on the topside.

Has anyone done something like this? (Split supply with planes). Should I make a top plane for my other power rail, or run traces for that rail? Should I remove parts of the planes under the opamps (input/output pins)?

Just trying to get a better idea on good practices for keeping noise down and using power planes in a split power design. (+15/-15).

Yurk
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Old 28th August 2010, 06:18 AM   #2
Nisbeth is offline Nisbeth  Denmark
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Good layout practice is good layout practice, but I think you should consider the application requirements as well. If this is a normal analog circuit for audio signals and you are not using excessively fast or picky op-amps, I wouldn't be too concerned that you do not have a perfect ground plane. I don't think you'd see much commercial gear that would use a four-layer PCB in this application, but if you have the skills and patience to lay out a 4-layer board by all means go ahead

/U.
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Old 28th August 2010, 07:36 AM   #3
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A concrete answer as require the schematic Op Amp or PCB as you want to do. If the Op Amp diagram is not very complex, there is no point translated into a two or four layers PCB.
If the diagram operational amplifier is not secret, post it here and then I was able to give you any more valuable or useful guidance.

Regards.
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Old 28th August 2010, 11:12 PM   #4
Yurk is offline Yurk  United States
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Yeah, this is a MFB design based on ESP project 75, and I'm using bog-standard TL072 chips for filters, with NE5532 for drivers. As you say, it's all analog and I'm using bypass caps on all chips. I wondered if I'd get any performance boost with a 4 layer board with more planes, but it sounds like I'm just trying to over-complicate things. I have that problem from time to time.

This is the first complicated/compact board I've tried to design, so I was just trying to bounce some ideas around. I know enough to be dangerous to myself. heh.

Thanks!
Yurk
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Old 28th August 2010, 11:45 PM   #5
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4 layers is overkill in an audio board.

w
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Old 29th August 2010, 03:21 AM   #6
benb is offline benb  United States
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I have no experience with four-layer boards but I suspect they are now "not that much more expensive" than two layers thesedays. But even so, I wouldn't automatically jump to a four-layer. You can keep your ground planes intact without slicing them up with extra traces by adding an amazing thru-hole technology - zero ohm resistors. These were made for automatic insertion machinery, but you can do just as well by hand using "jumpers" instead of traces where a trace might split a ground plane.

Equalizers and other audio electronics have even been made on single-sided boards with bountiful use of jumpers, though of course good ground planes (and quality parts, bla bla bla) may have been lacking.
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Old 29th August 2010, 08:34 AM   #7
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Yurk, you're referring to this project?
Expandable Graphic Equaliser
But just like there? How much you have changed?

My opinion is to translate these project 75 into a CAD/CAE software just as you thought to do. Then post here that diagram and carry the discussion around them. Is the most correct and useful way.

Such projects are usually done in one or two layers PCB.

Regards.
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Old 29th August 2010, 03:53 PM   #8
Yurk is offline Yurk  United States
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donpetru,

Yes, that's the design I've been working from. I'm using his values directly that he derived for a 23 band device. I talked with him via email awhile back, and even though I created this artwork myself, I will not post it because it's derived from his work, as many know he doesn't post his artwork for any reason, and I'll respect that.

The main paper he got his design from was this: http://rane.com/pdf/constanq.pdf It's a longish paper on a constant Q equalizer design.

It's definitely going to be the most complicated (tedious) construction jobs I've done so far. I'm using ExpressPCB to design/order 4 boards, the entire thing will cover 83.79 sq inches. As benb said, it's only a 20 dollar difference between 2 and 4 layer boards from that company.

From what I gather here a 4 layer board is not going to do anything but make my layout perhaps look 'neater'. There won't be any real performance gains in a 4 layer board, so I think I'm just going to stay with my two layer design.

I still have to order the slide pots (and boards) but I'll post some pics up once I get things past the planning stage into the construction phase, if anyone's interested.

Thanks!
Yurk

Last edited by Yurk; 29th August 2010 at 04:04 PM.
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Old 30th August 2010, 11:59 AM   #9
marce is offline marce  United Kingdom
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If you are using SMD devices a 4 layer PCB would be better as you dont have the ability to jump areas with resistors etc, so can get neater and better results. I dont think it is overkill these days as the other problem is EMC, which in our ever more digital world is becomeing more of a pain. That said PTH audio and simple digital designs can be done on two layers, but unless cost is an absolute determining factor 4+ layers tend to be the norm for a lot of commercial stuff.
That said I've done 2 layer designs that have worked, though are more complex to lay out with 2 layers than 4+, again 95% surface mount devices.
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Old 30th August 2010, 12:06 PM   #10
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I am agree,

With most components on SMD it is not fun designing on a 2 layer board. You must use VIA's all the time everywhere and it doesn't get neater. Some may call it overkill or un-nessercerry for audio, but I think the big benefit with a 4 layer PCB is that you can dedicate one layer completely to a unbroken groundplane, without any interruptions (other then the vias and holes).

With kind regards,
Bas
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