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| Composite amplifier: LM3886 + LME49720 |
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| JeanLeMotan:
Thanks a lot for the info Tim. You've given me quite a lot to process so it's a busy weekend for me. Now I'm trying out alternative PCB layouts based on your advice to see what I can come up with - but it's clear that I need to understand better PCB design for amps, especially in the presence of multiple separate grounds (signal, low and high power). --- Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pm ---Thanks for the reminder. ;D Schematic and simulation -- can't find a fault. Those are damn good opamps, and as long as it's stable, you're set. I was about to complain that DC coupling allows the bias servo to saturate (if left biased long enough), but I realized it is AC coupled, the cap is just shorted over for simulation purposes. :) Layout -- flipping a THT component is a really awful hack though. I'd flip it back and clean up the routing, yes it'll be a bit fiddly but the result is easier to build, and more compact. --- End quote --- I was actually proud that I found a clever way to keep the feedback loop short by flipping the 3886 ;D... I was completely unaware that this is a bad practice. Not sure what will happen if I remove the flip - but the feedback loop will definitely be longer. Are there any reasons except aesthetics and volume? --- Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pm ---Seems like it's missing an inner layer ground, or something? I see ratsnests on the layout, GND isn't fully connected. --- End quote --- Yes, the low-power ground was missing a connection to the high power one. I intended to make this connection on the PSU-PCB but since I don't think there are big currents on the ground traces (as it's a bridged amp and the speaker doesn't return to ground) I think I will just connect them on the main amp PCB. --- Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pm ---That's my other complaint: no ground pours or planes. Inner layer would clean that up nicely. 4-layer boards are cheap and plentiful, hardly worth arguing about, especially considering the amount of time invested in making a PCB layout in the first place -- you've more than paid for that luxury already. :D That also gives another routing layer, which may help relieve some congestion. In bipolar supplied designs, it's not really feasible to pour both supplies and ground; but you could pour the two important ones (VCC, VEE), and route ground on outer layers as just another signal. Which really, in a design like this, that's what it is! --- End quote --- I played around with ground planes but I wasn't sure how to go around it. From what I've read, the best way seems to be to use a ground plane for the signal ground and route the power ground by hand. The new PCB does this (still WIP) on both the top and bottom layer. I still have to do some serious via stitching. Fixed another issue while was at it: the output zobel inductors were parallel and they could couple magnetically. Now they are 90 degrees apart. 4-layer boards are super tempting and I did one in the past: way more fun than 2-layer ones. It does increase cost a bit and I think I will have to do at least 2-3 iterations of this board but compared to the final amp cost (PSU, components, etc) the PCBs will probably be peanuts. The thing is - I know it's doable in 2 layers so I take it more like a puzzle. If I fail completely and the whole thing oscillates and I don't have any clue why, then 4 layers it is. But I want to give 2 layers a try at least. --- Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pm ---Mechanically speaking: don't be afraid to extend the board a bit further behind the power amp footprints. I don't know if you'll be using offset pinned components, but I've seen those before (the leads are formed so the pin rows extend in front of the component body, rather than being straight vertical overhead as the silkscreen suggests). Even if not, it's quite feasible to add a heat spreader to space the components off the enclosure wall / heatsink face. Just grease both sides, bolt it down and you're good. I'd go with bigger mounting holes, and more meat around 'em, myself -- ~0.125" fits an M3 or #4 well enough, but I default to #6 for this kind of thing, which is comfortable in a 0.15" hole. Leave at least 0.2" of board width ("meat") around the hole. This may be a personal preference thing as much as any; #4 screws aren't exactly weak or anything. The holes near the heatsink may not even be needed -- the component leads will take up that stress without much problem, and they'll take up the strain of mounting errors and thermal cycling as well. (That is kind of an iffy instruction to make: counting on component leads to take up flex is a risk factor for fatigued ("cold") solder joints. 2+ sided PTH boards help greatly with this, holding the pin through the entire board thickness. Compare to the old days of 1-sided punched phenolic board, solder joints only on the bottom surface: any lead stress at all causes failure. The motivation here is, if those nearby screws are used, they have much more leverage against the component leads, making them a risk factor for mounting errors or fatigue as well.) If you do use them, try to fit everything in place, then solder the power amp chips as the very final step, so there's as little lead strain as possible. --- End quote --- Good point about the holes. I will make them bigger and remove the ones on the amp side. The LM3886 footprint seems to match the datasheet spacing between the pins and the heatsink - so no extra space there. For now I don't need that extra routing space but good tip with the heat spreader. I'll think about routing power through there, see where that leads. Maybe it solves some of the ground place issue. Thanks a lot for all the advice & info! |
| T3sl4co1l:
I wouldn't worry about feedback path length, at these frequencies it could be meters before you'll notice. Assuming noise or other feedback doesn't couple in along the way, of course (hence why planes are so great), and assuming the path length isn't on the feedback node itself (which shouldn't be loaded by much capacitance, but this also can be compensated with more "speed-up" cap). --- Quote from: JeanLeMotan on March 03, 2019, 12:37:08 am ---I was actually proud that I found a clever way to keep the feedback loop short by flipping the 3886 ;D... I was completely unaware that this is a bad practice. Not sure what will happen if I remove the flip - but the feedback loop will definitely be longer. Are there any reasons except aesthetics and volume? --- End quote --- The heart of the argument I think is more about manufacturing and space. If those are unimportant (as a low quantity hand-built is), it's meaningless. :) I feel a kneejerk about it because it's very dissonant against what you see out there, a style issue -- this is however the weakest possible argument (and being honest enough to notice when this is the case, is a challenge unto oneself). But as for useful justification, it's a design-for-manufacture (DFM) issue. In production: - You'd have to hand-solder at least one pair of amps. No chance of running the board over a wave solder machine twice. - Heatsink screws on both sides is fiddly, and makes order of assembly that much more critical. (Probably can't mount the board before finishing those screws?) - Requiring access means whatever mounting for the board (standoffs/bosses) has to be very tall. On the other hand, all the same... if you're going for an industrial chic look, it might be a feature-not-a-bug. Say you tilt the whole board so the LM3886s look like they're in rows or something. Idunno. In the latter case, fiddliness of assembly is probably a feature-not-a-bug, too. :P --- Quote ---Yes, the low-power ground was missing a connection to the high power one. I intended to make this connection on the PSU-PCB but since I don't think there are big currents on the ground traces (as it's a bridged amp and the speaker doesn't return to ground) I think I will just connect them on the main amp PCB. --- End quote --- Yikes! Don't run a critical signal through a wiring harness! Incidentally, be very weary of advice you see about star grounding. There are some very beautiful (in the industrial-chic sense) implementations of this out there... and they stink to high EMI heaven! Fortunately, audio amplifiers are extremely lenient, so that most any beginner can put together a schematic (with pretty much any layout) and get acceptable results, at least assuming they aren't in an RF warzone. The better way is simply to route current paths, and signal loops, away from each other. With high frequency currents returning on the ground plane underneath the trace, loops are where you put them. In this case, it should be easy to locate the speaker terminals (if this were an unbalanced amp) near the power amp, and have the signal amp on the other side of that connection. Also, it's just clicking that this is intended for bridged operation... so you could have some opportunity to measure the differential voltage, and feedback with that. A differential sense amp, that's easy, but maintaining balance is actually a bit harder. A fully differential amp could deliver +/- output (feedback) signals, but, I doubt one exists that has nearly as good specs as the '49710. You could feedback to one phase (the "slave" or "follower"?), and leave the other (the "master") alone (similar to what they did back in the day: the paraphase phase splitter). That is probably a bit better than the split path method, but it feels inelegant. Point is, you're approximately doubling the errors in the system -- the errors from both outputs add (well, subtract, but assuming random variables y'know). This is not wrong to begin with -- you've doubled the output power too, after all! Using a single error amp, would reduce it to a single error, not a double error, so it's worth about 3dB SNR. (And, again, not that this is really needed, but given the spec of the '49710, I assume you're going for the best possible.) So, it's merely an incremental improvement, versus, there may not be any good way to implement it with off-the-shelf parts. I don't think I would do anything different. This is more to illustrate design analysis, the kind of contemplation and insight that any good design needs! --- Quote ---I played around with ground planes but I wasn't sure how to go around it. From what I've read, the best way seems to be to use a ground plane for the signal ground and route the power ground by hand. The new PCB does this (still WIP) on both the top and bottom layer. I still have to do some serious via stitching. Fixed another issue while was at it: the output zobel inductors were parallel and they could couple magnetically. Now they are 90 degrees apart. --- End quote --- Ah, cool. FWIW, nearby (air core) inductors typically have k < 0.1, which isn't much, and I doubt you'd notice anything here. Actually, the ones on opposite phases of a given channel, are getting the same current anyway, so they act in series and the coupling is equivalent to a slight increase or decrease in total inductance. Should be no problem there. The total value would be slightly different from what's intended, but these are low-Q parts, errors over a factor of 2 would be cause for concern. :D Crosstalk between channels, via inductor, isn't going to be noticeable until near cutoff, either; keeping them isolated, really just feels better, and that's fine. --- Quote ---Good point about the holes. I will make them bigger and remove the ones on the amp side. The LM3886 footprint seems to match the datasheet spacing between the pins and the heatsink - so no extra space there. For now I don't need that extra routing space but good tip with the heat spreader. I'll think about routing power through there, see where that leads. Maybe it solves some of the ground place issue. --- End quote --- Actually, y'know, leave the holes in, in case they're useful -- see how it goes in the real build. Don't HAVE to put standoffs and screws in them. If nothing else, maybe they'll prove useful for wire ties? Who knows. :) That's another nice lesson -- prepare for options. Not many options to worry about on an amplifier, but it can be nice to have a number of footprints for testing different components and combinations when you expect you have a design that's going to need some fiddling. Or... conversely... if you're not averse to defacing a PCB, there are many things that can be changed on an already-fabbed board, for varying degrees of difficulty. Cutting traces and running wires is easy. Drilling holes -- if you have free space to do so without cutting or shorting anything out -- is pretty easy, too. Patching traces in a like-new manner, is more difficult, but can be done. Rebuilding board material entirely, well... So, even if you don't put holes in the design, they can be added (cough... subtracted?) later, if the space is still reserved for them. Tim |
| JeanLeMotan:
--- Quote from: T3sl4co1l on March 03, 2019, 03:51:25 am ---I wouldn't worry about feedback path length, at these frequencies it could be meters before you'll notice. Assuming noise or other feedback doesn't couple in along the way, of course (hence why planes are so great), and assuming the path length isn't on the feedback node itself (which shouldn't be loaded by much capacitance, but this also can be compensated with more "speed-up" cap). --- End quote --- I did some simulation of what effects the feedback path length has for phase margin. Attachment 1 shows no parasitic inductance: The Phase Margin is 64 degrees. Attachment 2 shows a 10nH parasitic inductance (which corresponds to 1 cm of trace length): PM = 56 deg Attachment 3 shows a 30nH parasitic inductance (3 cm of trace length): PM = 43 deg Attachment 3 shows a 100nH parasitic inductance (10 cm of trace length): PM = 29 deg The phase margin degrades somewhat although it's not clear for me what this degradation means: is it bad? is it acceptable? irrelevant? Anyway, according to this page, feedback track length is critical for performance: https://www.neurochrome.com/taming-the-lm3886-chip-amplifier/stability/ --- Quote from: T3sl4co1l on March 03, 2019, 03:51:25 am ---The heart of the argument I think is more about manufacturing and space. If those are unimportant (as a low quantity hand-built is), it's meaningless. :) I feel a kneejerk about it because it's very dissonant against what you see out there, a style issue -- this is however the weakest possible argument (and being honest enough to notice when this is the case, is a challenge unto oneself). But as for useful justification, it's a design-for-manufacture (DFM) issue. ... --- End quote --- This is very interesting, I never thought about these issues. I guess it takes a lot of training and experience to design something for mass production. Do they actually teach this in school? --- Quote from: T3sl4co1l on March 03, 2019, 03:51:25 am ---Also, it's just clicking that this is intended for bridged operation... so you could have some opportunity to measure the differential voltage, and feedback with that. A differential sense amp, that's easy, but maintaining balance is actually a bit harder. A fully differential amp could deliver +/- output (feedback) signals, but, I doubt one exists that has nearly as good specs as the '49710. You could feedback to one phase (the "slave" or "follower"?), and leave the other (the "master") alone (similar to what they did back in the day: the paraphase phase splitter). That is probably a bit better than the split path method, but it feels inelegant. Point is, you're approximately doubling the errors in the system -- the errors from both outputs add (well, subtract, but assuming random variables y'know). This is not wrong to begin with -- you've doubled the output power too, after all! Using a single error amp, would reduce it to a single error, not a double error, so it's worth about 3dB SNR. (And, again, not that this is really needed, but given the spec of the '49710, I assume you're going for the best possible.) --- End quote --- That's actually a great idea! I didn't think about that but it makes a lot of sense. The 49710 would correct a bridged-paralleled amp made with 3886 and their servos. Like this it will correct all issues coming from both the paralleling and bridging. This will also reduce the BOM and routing density/constraints. I wonder what this will do to the outer loop stability. Hmm, choices.... [Later edit] Here's a new schematic where I use one LME49710 per paralleled group: https://easyeda.com/jeanleflambeur/amp-lm3886-composite2 Simulations in progress. |
| T3sl4co1l:
--- Quote from: JeanLeMotan on March 03, 2019, 11:24:59 am ---I did some simulation of what effects the feedback path length has for phase margin. --- End quote --- With inductance added as shown in the article? No, that's output path length -- that fits under the rule of keeping current loops and paths tight and separate. In other words, you don't run load current through your feedback trace! Same net, but the branch carrying output voltage (not current) from '3886 output pin to the '49720. Modest L and C in that path should be very tolerable, of course you'll want to add some damping if it's incredibly long. I can't imagine you could have any problem even for a very circuitous trace on a board of these dimensions. --- Quote ---Attachment 1 shows no parasitic inductance: The Phase Margin is 64 degrees. Attachment 2 shows a 10nH parasitic inductance (which corresponds to 1 cm of trace length): PM = 56 deg Attachment 3 shows a 30nH parasitic inductance (3 cm of trace length): PM = 43 deg Attachment 3 shows a 100nH parasitic inductance (10 cm of trace length): PM = 29 deg The phase margin degrades somewhat although it's not clear for me what this degradation means: is it bad? is it acceptable? irrelevant? --- End quote --- A modest amount certainly looks to be acceptable, which tells you that a corresponding length of loop-sharing is acceptable. So, about an inch, give or take. :) --- Quote ---This is very interesting, I never thought about these issues. I guess it takes a lot of training and experience to design something for mass production. Do they actually teach this in school? --- End quote --- :-DD They teach very little practical engineering in school. School is about two things, only one of which they tell you about. At school, you learn theory, so that you have a toolbox, a framework to build hypotheses on. You also make connections with other students, faculty and professionals, which benefit your career track. Practical things, you learn on the job, and from observing what others have done in practice. More importantly however, understanding what reasons there might be driving those decisions. It is very rare indeed to find any kind of design documentation; even when picking up existing designs in full, you're largely left to figure it out yourself. I've almost never been asked to produce design documentation myself -- and I'm in the business of making designs for others! I think in some cases, they don't really need it (if they continue to engage with us for design updates, they won't need to know). Others, it would be beneficial, but maybe they forget to ask, or they figure the cost of producing that documentation would be more expensive than having their guys look it over. Well, interpretation is up to the reader. That process can vary in terms of accuracy. They may miss subtle points; or they may discover catches that the original designer never anticipated, or intended. Well, analysis is up to the writer, too, so that can vary in terms of accuracy. On a recent job, I got to read one of these rare documents, actually. I wrote several pages of notes and critiques on it in the process; it was, uh, sad to say, rather lacking in many points. Such is engineering. There is no simple process to determine capability and fitness. At best, you can get more eyes on it for review, but you'll get about as many opinions as eyes*. Good engineering is ultimately tested in the lab, and then the marketplace; but that's a very expensive test indeed, and you usually only get one or two tries of that before heads start to roll (slipped deadlines, exploded budgets, rolling revisions; fired managers, or teams..). *Heh, nice. This implies approximately two opinions for every person. Which is probably true enough. If it passes testing (EMC, shock and vibe, environmental..) first try, that's encouraging (I pride myself on having done this more than a few times 8) ), but you may ask if the product is more expensive or bulky than it could otherwise be. You could bring in an expert to review the design, but how do you verify someone's credentials? They're only going to tell you about their successes, of course. Well... in any case, there is good reason why product development is so costly... Tim |
| Wallyboy:
--- Quote from: T3sl4co1l=topic=172005.msg2236821#msg2236821 date=1551573428 ---A fully differential amp could deliver +/- output (feedback) signals, but, I doubt one exists that has nearly as good specs as the '49710. --- End quote --- Actually the LME49724 is a very good candidate for the fully differential solution. Using this, both bridged halfs could use the same inverting arrangement thus lowering the large common mode signal that is present with the non-inverting arrangement (lower distortion). Walt |
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