Op Amp PCB design question

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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).

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 :)

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.

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.

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.

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.

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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.
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,
Yurk, I recommend you see Multi-Channel Mixer project available in Altium Designer software. That project I've attached below:
Mixer_Routed.pdf - 1.82MB
There is a very useful tutorial for placing electronic components audio mixer. That project uses the THT components, which you could also use.
Secondly, at home is quite difficult to work with SMD, especially when it comes to large projects PCBs.
4+ layers tend to be the norm for a lot of commercial stuff.

Not for baseband analog stuff, even in surface mount. 2 layers is plenty if the layout engineer is not inexperienced or lazy. If you have a load of digital stuff racketing away in there in the same box that is subsequently discovered to cause problems, then the usual resort is an internal compartment.

a 20 dollar difference between 2 and 4 layer boards from that company.

Commercial = cost sensitive.

To put it in perspective, that's $20,000 dollars on a production run of 1000.

I've rarely encountered an audio board I couldn't route on ONE layer with a few wire links or 0 ohm resistors, and the ones that really require two have multi-pin power devices whose positioning is constrained by the need to bolt them to large heatsinks, or as in the case of the Altium mixer, multiple channels and space constraints.

EMC is generally only a problem with switch-mode PSUs and digital boards with a fast clock or short rise-times. Radios generally have the critical bits compartmentalised with feed-through capacitors anyway.

When you design a DAC or other mixed-signal device is the time to reach for a 4-layer board.

i'll agree for simple designs, and cost sensititive. I think using the term commercial is to broadband as it covers a wide span. I have somewhere the Printed Circuit Design & Fab yearly surveys that show how PCB design is evolving, they show some interesting trends, and if I am correct I belive that this years had multi layers just piping double and single sided, by only a couple of percent. That said there are still lots of designs done on double sided, I've just done a Class D amp for Savu, on two layers which was a fun challenge.
Still for anything digital these days I'd prefer multi-layer, as I belive that EMC is a wider problem than just SMPS's etc. Also for SMD it depends on the size of the devices, as they shrink its getting harder to get everything on even 4 layers. Again on a comercial basis having four layers can be cheeper than having to place and solder a few zero ohm links, it depend on the design.
We have found that our layer count for more complex boards is up from an average of 4 - 6 to 8 - 12, due device size and BGA's with multiple voltages, quite often the extra layers are for getting the various supply voltages to the required place, not just for extra routes. These ar mainly digital boards or digital with a bit of analogue, I still prefer to have different boards for digital and analogue circuitry where ever possible.
Well, as far as cost goes, I'm doing this is a home project. I can see saving money in a commercial environment being paramount, not that I want to spend a mint here. I'm a long way from a professional pcb designer, too!

I'm using all old technology, PTH, not SMT. I actually work in a plant that builds both all day long, so if I did SMT I have access to a reflow oven. I just like the PTH for playing with at home.

Looking at that commercial mixer, I'm going to say I think my board looks neater than that, so far (as 2 layer). I may use some jumpers on top for some of my traces that cross planes, but I'm not sure about the traces that go straight to a pin. I'm probably going to have to leave those in. Most of the ones I have on the top layer look like jumpers anyway, only being .5" long or so.

I'll be feeding my analog board with a filtered/regulated DC supply, so no SMPS to cause noises. I guess the worst looking aspect of this build is going to be wiring up all those slide pots! Nothing to be done about it though, I'm using every bit of available space (under 21sq" per board, 4 boards) to keep my cost down some. (over $100 difference).

Lot's of good ideas to think about.
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