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Old 16th September 2012, 01:48 AM   #1211
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Terry Given,
So if we use you concept of the double sided board with minimum hole size just for pass through of a lead does the actual shape of the board have any affect? Could you have the capacitors all in a row and make the board narrow, in other words not much wider than the diameter of the capacitors?
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Old 16th September 2012, 01:52 AM   #1212
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Quote:
Originally Posted by Terry Given View Post
here is a parallel-plate transmission line pcb layout.

As you can see the PCB layout is very, very simple. the top layer is +Vdc (+24V in this case), the bottom layer is 0V. and each layer is almost entirely solid Cu, only having holes in it where unconnected component legs are.
This is exactly the technique I used for the key, caps adjacent to the chip amp in my OTT gainclone project. No measurements done, but the sound was good enough for me ...

Frank
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Old 16th September 2012, 02:43 AM   #1213
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Hi Frank! Your timing was perfect.
Quote:
Originally Posted by gootee View Post
I guess you could probably save the unsuspecting folks from messing up the designs with low-loss caps by just using low-loss caps with a small resistance in series.
National Semiconductor typically lists 1R to 2.7R for the resistor in the output zobel of their chip amplifiers. Of course this means that commercial television/home-theater amplifier manufacturers want to use the lowest cost cap--a polyester dip cap with typical ESR of 4 ohms. The resulting RC has 5R~6.7R worth of resistance, and matches most common guidelines.

Next, Audiosector replaces this cap with a more attractive looking high efficiency box cap. And, then lastly they recommend omitting the output RC due to inferior audio quality. Egads. Prettier appearance with wrong values. And, the wrong values flubs the audio quality. Correcting the resistance value could have worked better than omission,
except. . .
Also, I think it weird to recommend arbitrarily 100nF or even 220nF values every time. These values are a little bit high and risk having people make assumptions about output RC and audio quality. Is bill of materials monotony really worth decreased audio quality? Actually about 47nF is lower risk of causing omission, yet still able to work for HF noise reduction.

So, the little polyester dip cap (or tiny value / hv electros) that are internally RC's (little high loss caps easily parallel with big elecros--no fight when lossy little caps have a small enough capacitance value), can be misleading if the RC task isn't mentioned anywhere.

But I have a rule of thumb:
If the task is obviously snubbing and/or if the series resistor is less than 4 ohms, I suspect that the right cap for the job will be a high esr filter-purpose cap. Right tools for the job?
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Last edited by danielwritesbac; 16th September 2012 at 02:47 AM.
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Old 16th September 2012, 06:31 AM   #1214
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Tom @ #1206,
you could achieve this, but what it takes is lots of physical space. I assumed you would be twisting the pairs anyway, so all thats left for magnetic isolation is physical separation. Shielded cable will give excellent electrostatic shielding, but the magnetic shielding wont be very good because of the shield (braid) opacity (how many holes in it) and thickness - shields tend to be very thin, so eddy-current shielding wont do much at all for the frequencies we are interested in here (unlike the electrolytic cap cans, which are nice and thick - and get thicker as the caps get bigger).

plus of course shielded twisted pair takes up more space. wads of bulky, well-spaced cables - urgh.

consider my paralleled 100nF caps - if I move the caps further apart I can decrease the mutual inductance, but this then:
1. takes up more space (in my case thats a no-no)
2. allows each caps ESL to radiate H-field over free space - exactly what I want to prevent for best EMI results.


I've been playing around with a conceptual SYMEF layout, and making the local decoupling on the amp look like this really is difficult - the layouts that Harrison (OnAudio) has done are pretty damn good, given all the constraints - they are much better than all the audio amp layouts I have looked at to date.

but what is dead easy is to make a separate rectifier/capacitor PCB using this approach. There are almost no components (input connectors/wires, rectifier(s), lots of caps, output connectors/wires) so its utterly trivial to make a DS-PCB with one layer a solid 0V plane, and the other side having a solid V+ plane on one half, and a solid V- plane on the other half.

This can be placed nice and close to the transformer, then connected to the amp with tightly twisted (not plaitted even though its prettier, 'cos AndrewT is quite right that twisting has lower inductance).

This will do a brilliant job of killing any AC line noise stone dead as it will be very low Z up to the MHz region, and also keeps the Xfmr 2ndary wires (again twisted) really short, greatly reducing (removing?) magnetic coupling of the cap charging currents into the amp.

placing the fuses in series with the DC bus interconnect gives us a poor-mans CRC filter - and the fuse resistance (if we play our cards right) ought to damp the L_wiring/C_amp circuit too. After all, L_wiring is small and C_amp is moderately big so it wont take much R_fuse to do this.

we can put the fuses on the amp PCB or the rectifier/cap PCB. I doubt it makes much difference, but I'd put them on the rectifier/cap PCB because the fuses do form a reasonably large loop. I'd also place the two fuses at right angles to minimise mutual inductance between them (why not - this pcb isnt exactly complicated).

Then we can do the best job possible with the amplifier local decoupling, and if the various constraints mean its not quite as good as we want - oh well, at least we annihilated the AC line noise and cap charging current problems.
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Old 16th September 2012, 06:45 AM   #1215
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Tom @ #1210:

paragraph 1: yes. this is exactly how one designs high power AC motor controllers (if you dont want them to go bang). its a real PITA though, as it means the entire system must be designed simultaneously (cue a team of mechanical & electrical engineers, draftsmen and technicians). breaking the cap/rectifier board off as described above is IMO an excellent compromise as it avoids this horror, and is easily achievable in DIY form.

hell, there's no reason why one couldn't retrofit this approach to many (most?) amps. it doesnt solve all the problems of the fully integrated method, but it solves a whole bunch of them without needing a mega-dollar team & budget - you can do it with some blank DS-PCB, a sharp knife and a small drill - like the DIY cap bank picture I posted earlier.

I'll keep playing with my SYMEF layout. I think I can do an awesome job with a 4-layer PCB (that dc bus I showed is actually 4-layer 2-Oz Cu, 1mm thick, equi-spaced layers. TL, ML2 = +24V; ML1, BL = 0V I just didnt show the inner layers), but I'm working on a DS-PCB approach, almost exactly what you describe in paragraph 2 (I'm trying to decide if parallel or orthogonal small-signal PCB works best, by the simple expedient of doing both layouts and picking the best one). great minds think alike - or fools seldom differ. Franks done this too, so hopefully its the former.....
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Old 16th September 2012, 06:54 AM   #1216
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Steven @ #1211:

reductio ad absurdum: a ginormous Nx1 cap array facing away from the power devices would not share very well. I'll dig into the maths of current diffusion and see if I cant come up with a sensible answer. bugger actually doing the maths, thats way too hard (Schwarz-Christoffel transforms? I dont think so) - but I'm pretty sure I've got an analysis of a step-change in conductor width kicking around somewhere - and from that I ought to be able to finagle a "spreading depth" (analagous to skin depth) over which the current will spread out to be evenly distributed. If I was any good at using FEA freeware like FEMM I could just run a few sims and rip the answer out of the current density plots, but I'm not.

you can also force the cap array to share. if we make a rectifier/cap PCB for a single supply, with a rectangular (or square, doesnt really matter) form, and place the rectifier in the center of one edge, with the dc output in the center of the opposite edge, then most of the caps will share the current - the caps in the four corners might not though. but if we extend the PCB a bit (how much? dunno, thats what I'll figure out as above) on either side, and leave the cap bank in the center, then even the corner caps will share the current, as the extensions allow the current to spread out from the rectifier (connector) to cover the full width of the PCB
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Old 16th September 2012, 08:27 AM   #1217
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Not quite as effective as diode tricks, but you could try to block the all-in-one amp/power combo-board's dullness caveats by 100u//100u//100u high efficiency caps for amp decoupling.
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Old 16th September 2012, 02:56 PM   #1218
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Tom, @ #1191 and #1200. There are some electrolytics (let's take for example Mundorf MLGO 10000/63) which state low ESR, something like 14 mOhm, so just similar to that of a film cap, and a 20nH ESL.

Any resonation risk around? This might be the reason why AKSA (#33) and others suggest the 0.15R resistor between first and second cap.
Would in that case be interesting (and cheaper) to adopt a two reservoir caps config with decreasing ESR (the nearest to the bridge with higher ESR)?

Wow, during the night, you all worked a lot on the thread! A lot of good stuff to read and understand... (let's hope)
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Old 16th September 2012, 08:15 PM   #1219
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Here is some more data, the 100W case to go with the 25W, 50W, and 75W cases in post # 1175, which is at Power Supply Resevoir Size .

I also attached the spreadsheet with all of this. Just remove the .txt from the filename after (or while) downloading it.

It looks like "2500 uF/Amp", using RMS Amps into the load at 2X the maximum rated output power level, would be more than sufficient, for these four 8-Ohm cases, for which the transformer is well over-sized. That gives about 5% as the maximum drop in rail voltage (the drop is relative to its average level), in each case, and about 0.005% average RMS error, in each case, in the square wave that draws 2X the rated power. But you could go as low as 1600-1800 uF per amp, and even lower when rated output power is less than 100W. But for higher than 100W rated output power with the same transformer ratings, more uF/amp would probably be required. I will try to run some other cases, as I get time.
Attached Images
File Type: jpg 8Ohms_100W_240VA_44VCT_SQ_PLOTS.jpg (159.2 KB, 196 views)
Attached Files
File Type: txt PSU_Square_Load_Data_PU2 - Copy.xlsx.txt (48.0 KB, 26 views)

Last edited by gootee; 16th September 2012 at 08:27 PM.
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Old 16th September 2012, 09:47 PM   #1220
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Quote:
Originally Posted by seppstefano View Post
Tom, @ #1191 and #1200. There are some electrolytics (let's take for example Mundorf MLGO 10000/63) which state low ESR, something like 14 mOhm, so just similar to that of a film cap, and a 20nH ESL.

Any resonation risk around?
No, any electrolytic or solid polymer, OS-CON and the like, will always be safe, even when paralleled with a film cap. Tom doesn't quite agree with me on this, but I suspect the bad behaviours he's come across are because of film caps at a distance from each other, i.e. the lead inductance between the caps comes into play.

Always remember that ESR is not a locked in characteristic, it varies all over the place depending upon just about everything, so don't get hung up on precise values ...

Frank
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