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Old 18th June 2003, 04:20 AM   #21
jcarr is offline jcarr  United States
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EC8010:

>Even better, switch the auto-router off and take the time to work out a layout that doesn't need more than one layer. One layer per power supply (and 0V) is nice, but shouldn't really be necessary for an analogue power amplifier.<

Although I agree that in many cases it is better to avoid the auto-router like the plague, designing for one layer only isn't a worthwhile priority, IMO. Although my circuits tend to be fairly complex affairs as far as audio designs go, usually there will be circuit nodes that benefit from the impedance control to be gained by intimate proximity to a ground or power plane. If parasitic capacitances are detrimental to a given circuit node, open the ground or power plane in that locale only.

Compared to designing 2-dimensionally with a single-layer PCB, designing quasi 3dimensionally with a multi-layer PCB is far more likely to get you a compact circuit that has smaller antenna area and therefore less sensitivity to RF pickup. Besides, 4 or more-layer PCBs allow you to do various tricks that would be impossible with a double-sided or single-sided PCB, unless you don't mind _lots_ of jumpers (been there, done that).

That circuit may not _need_ more than one layer, but it will probably measure and sound better with a more sophisticated board design.

SY:

>What do you think the tradeoffs are between ground planes and putting some bandwidth limiting at the input to keep the RF out in the first place?<

I do both.

hth, jonathan carr
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Old 18th June 2003, 05:17 AM   #22
alvaius is offline alvaius  Canada
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I have had a chance to review a lot of designs at the PCB level. There are many many cases where a GROUND PLANE IS NOT a good idea, especially for audio circuity. When you have a ground plane, you have very little control over where the current flows, hence you can get current flow from your high current outputs into your low level inputs. In almost all cases for analog design, a very careful layout of traces (of appropriate widths), will result in a better layout than just pouring a plane.

Digital of course is a whole nother matter where the primary goal is power supply stability at the IC pins, controlling impedances, etc.

Of note, improper use of power and ground planes can even contribute to radiated noise.

Alvaius
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Old 18th June 2003, 05:42 AM   #23
usekgb is offline usekgb  United States
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One thing I have noticed in a lot of commercial designs is poor power supply design. They can almost always use improvements. All too often, the supply caps are too small, the bridges can't handle enough current, and mosfets aren't able to dissapate enough power. 90% of the amplifier and electronic repairs I do are power supply related. I have seen more power supply caps that have exploded than I can count.
One thing we can learn from this is to try and engineer everything to handle any power load we may throw at it. I like to over-engineer my power supplies if I can. For example......Use caps that can handle higher voltages than the circuit needs. This will help if you have any power surges or unusually high current draws from your supply. This goes for the rectifiers as well. Go ahead and use rectifiers that are over rated for your aplication.
These are just my thoughts on the subject. I hope this helps.

Cheers,
Zach
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Old 18th June 2003, 05:57 AM   #24
SY is offline SY  United States
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jonathan, thanks. So would you say that there are advantages, other than consistency unit-to-unit, to using a PC board as opposed to point-to-point?
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Old 18th June 2003, 07:25 AM   #25
Shaun is offline Shaun  South Africa
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Quote:
When you have a ground plane, you have very little control over where the current flows, hence you can get current flow from your high current outputs into your low level inputs. In almost all cases for analog design, a very careful layout of traces (of appropriate widths), will result in a better layout than just pouring a plane.
The thing to note here is that there are two grounds: signal ground and power ground. Don't mix them up, and you'll avoid this problem. Inevitably they need to get connected electrically, but this should be done at a single point on the board, away from heavy current paths. It is still possible to have a star point with copper floods on the board (and you aren't limited to just one).

On another point, one of the constraints (money) often makes it impossible to have multiple development iterations of boards, so we try to do it best first time around. An inexperienced builder, could be forced into overcomplicating the design because of trying to get all the best goodies in at once. Rule here is to keep it simple, and grow slowly, steadily gaining confidence (OK, I know I'm just repeating here what someone said above...)
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Old 18th June 2003, 08:11 AM   #26
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Jonathan,

thanks for the links. I had already read nr. 2, that's a good one.
I just has a brief look at the first one, and it seems quite
useful too.

I am afraid your japanese texts wouldn't be of much help to me.
Although I did take a short course in Japanese a few years ago,
just out of curiosity about the language, I didn't get very far
and I don't remember more than a few useful phrases like
"watashi no atarashi jidoosha akai" (which I probably
don't remember correctly anyway and besides, my jidoosha
is shiro not akai). It's an interesting language,
though, which I wouldn't mind learning more about if I only
had the time.
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Old 18th June 2003, 08:52 AM   #27
jcarr is offline jcarr  United States
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Alvaius:

>When you have a ground plane, you have very little control over where the current flows<

On the contrary, it is feasible to analyze the grounding and locate the circuit nodes that tie into the grounds in a manner that allows for rather well-controlled ground currents.

>hence you can get current flow from your high current outputs into your low level inputs.<

The most basic precaution that you can take is to keep the power and signal grounds separate and tie them together only at one point. With multilayer circuit boards, it is just as easy to star planes as it is traces.

>In almost all cases for analog design, a very careful layout of traces (of appropriate widths), will result in a better layout than just pouring a plane.<

Naturally, the designer should study, measure and think carefully before he does anything. "Just do it" isn't good enough. For low frequency applications, starring traces is usually sufficient, but for wider bandwidth designs, I would recommend starred planes, or combining starred traces and planes.

My designs also incorporate active grounding (discrete circuits which accomplish similar functions as the TLE2426), which allows me to take care of many grounding requirements while simplifying the currents flowing in the grounds.

>Of note, improper use of power and ground planes can even contribute to radiated noise.<

No doubt. Incompetent use of anything can cause problems, but more often than not, the problem lies with the designer, not his tools.

hth, jonathan carr
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Old 18th June 2003, 09:15 AM   #28
jcarr is offline jcarr  United States
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Sy:

>Would you say that there are advantages, other than consistency unit-to-unit, to using a PC board as opposed to point-to-point?<

Oh yes. Planes and striplines in P2P are an iffy proposition at best . And multilayer PCBs allow you to deal with relatively complex circuitry that would drive you batty if you ever attempted them in P2P.

But PCBs also have drawbacks, including problems with high-impedance nodes and leakage currents, and only two surfaces for installing parts. This is why in my own designs, multilayer PCB layouts with comprehensive ground and power planing serve as the foundations, but these are then augmented with component and board stacking, localized P2P, guard rings, floating isolater pads, teflon-insulated standoffs, clover-leaf terminals, and various other structural go-faster tricks.

regards, jonathan carr
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Old 18th June 2003, 09:44 AM   #29
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Most DIY'ers (and Pro's) mostly SS I think fall short in the PSU department. Using SS diodes with huge caps right behind them.

http://www.nutshellhifi.com/library/...est_Talk2.html (bottom part)

I am assuming that Lynn Olson is right here. And my gut says he is.

Cheers,
Bas
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Old 18th June 2003, 10:35 AM   #30
EC8010 is offline EC8010  United Kingdom
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Default PCBs

jcarr, what I was getting at, in espousing single-layer boards, is that many PCBs have longer, more convoluted tracks, and links than necessary, but single-layer design forces you to think hard. Of course, from a point of view of design efficiency, the time required to achieve an ideal design is not worthwhile - there's a parallel here with computer code.

SY, aren't input filtering and ground planes two different issues? Ground planes help us to avoid local noise, and input filtering assumes that we picked up noise on the incoming cable.

In case anyone here thinks I'm in the business of teaching grandparents to suck eggs, I only stuck my oar in because I've had disappointments caused by "professional" PCB designers. The best example was when I allowed my boss to persuade me to let a PCB designer take my circuit and implement it. The (digital) circuit compared the timing of two video signals in three stages of severity. It worked on breadboard (even the 4.43MHz phase comparator), and it worked on wirewrap. The first PCB couldn't even manage the middle stage test. (The electrical connections were fine, and it matched the circuit diagram, but the poor layout stopped it.)

Everyone else has now said all I want to say about PCBs. Good layout is an art, and is driven by many (and conflicting) rules.
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