Diode swapping: ordinary diodes Vs. schotky - Page 2 - diyAudio
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Old 7th February 2012, 01:49 AM   #11
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No. Not because of the diodes. You might want to add 0.1uF capacitors in parallel with the electrolytics.
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Old 7th February 2012, 03:46 AM   #12
gootee is offline gootee  United States
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Most of what you're doing here sounds more-or-less OK, as in at least not necessarily harmful and maybe even helpful. But be advised that paralleling an electrolytic with a very-low-loss type of a much smaller cap, such as any film type or an NPO/C0G ceramic, especially with less than say 1% of the electrolytic's capacitance (but could still happen with more), could form a resonance (in combination with the parasitic inductance of the electrolytic), which could excite high-frequency ringing, which is "a bad thing".

You could put 0.1 Ohms or more in series with the small cap, to create a snubber, but that's not really needed unless there's already some ringing going on and would also probably defeat the original intent. Or you might get lucky. Just be aware that it could also cause problems.

If you have the space available, a far better improvement possibility would probably be a large polypropylene film cap, having at least 1% of the capacitance of all the existing paralleled electrolytics combined.

Also, ultra-fast diodes will tend to create some high-frequency ringing, too. I don't see how they could help and would probably prefer going the other way, to soft-recovery types. The big caps are what should be supplying the power to the system, not the rectifiers. You want the rectification effects to be invisible, if possible. The caps are there to smooth their effects so making faster pulses there doesn't make sense. I could be wrong but that's how I see it at first glance.

And note that Schottkys usually have about half as much, or maybe less, for their forward voltage drop. So they're probably mainly just increasing your power supply rail voltages a little bit.

Putting a snubber network across each of your rectifier diodes could be a good improvement. The essential part of a snubber is the resistor. A small cap is put in series just so the resistor doesn't have to dissipate so much power, from the "normal" frequencies that are meant to be there. A snubber is meant to quell any high-frequency ringing. They can also be useful in terminating digital lines, etc.

OK, I'll tell you one secret: As far as the power supply quality is concerned, the main smoothing caps, by the rectifiers, are probably less important, for tweaking, than the decoupling caps that should be at each point of load (i.e. at each active device's power/gnd pins (or sometimes between their + and - rails). Those caps are the ones that supply the fast transient demands for current, and are also the ones more designers probably get wrong.

More decoupling capacitance will usually buy you both more-accurate or faster transient response, and, less disturbance of the power supply rail voltage by the transient current demands (which generate nasty voltages if they have to be pulled through the parasitic inductance and resistance of the power supply rail conductors, which usually can't be done fast-enough anyway).

BUT, note that a capacitor that is physically larger will also have more intrinsic parasitic inductance, which you don't want there. So maybe the best way, especially for electrolytics,would be to parallel two or more smaller caps that add up to the capacitance you want. That lowers both the parasitic inductance (ESL) and the parasitic resistance (ESR). It can be difficult, physically, since you also don't want long leads on them. Adding one or two in parallel but soldered on the bottom side of a board might be easy, in some cases. BUT, if you can use more capacitance but still keep the same case size, or even just the same lead spacing, then I'd go with doing it that way.

You still have to be careful to not add low-loss (e.g. film) types of small capacitances indiscriminately, next to electrolytics, for decoupling, since they could cause unwanted high-frequency resonances. If you have a good oscilloscope, though, you could know pretty quickly if a particular value was OK. But, basically, the physical size determines the speed, not the capacitance (speed is different than resonant frequency). So probably use the most capacitance you can, for a given case size that will fit (but smaller case size is better, too), if you're doing non-electrolytics. Where higher currents are possible, add an electrolytic or two in parallel with (or to replace) whatever electrolytic is already there. In all cases, having the absolute minimum physical connection length is critical, so no axial-lead electrolytics are allowed, for example (and paralleling smaller caps is usually better than using one big one).

One possible problem would be for a fast/digital device where the original designer was very good and took all of the parasitics into account and used exactly the right decoupling caps. Changing them might undo some good design work. On the other hand, if they didn't tweak the decoupling caps properly to begin with, and you can make the LC resonances (as seen by the power pins) match the clock frequency and possibly also the frequency corresponding to the risetime the pin wants the current to have, you might dramatically lower the noise on the power rails, while also letting the chip take perfect-mouthful-sized bites of current. (You'd probably need a good oscilloscope, at least, to do that, though.) You can usually tell if they took the trouble to tweak them by looking at the values. If they just used 0.1 uF or .01 uF, possibly with a 10 uF electrolytic, then they were probably just guessing.

Have fun.

Tom

Last edited by gootee; 7th February 2012 at 03:53 AM.
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Old 7th February 2012, 04:01 AM   #13
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Wow!

Mr. Gootee, thanks for the lesson!
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Old 9th February 2012, 12:42 AM   #14
gootee is offline gootee  United States
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Quote:
Originally Posted by Audio Maniac View Post
Wow!

Mr. Gootee, thanks for the lesson!
No problem. I hope it's mostly correct. <grin>
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Old 9th February 2012, 12:52 AM   #15
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The part about capacitors in parallel is not 100% accurate, more like 80% . More accurate about decoupling - people tend to give too much importance to power supply HF performance, when in reality local decoupling has much more importance.
Related to capacitors on power rails: here.

Last edited by SoNic_real_one; 9th February 2012 at 12:57 AM.
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Old 9th February 2012, 02:42 AM   #16
gootee is offline gootee  United States
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Quote:
Originally Posted by SoNic_real_one View Post
The part about capacitors in parallel is not 100% accurate, more like 80% . More accurate about decoupling - people tend to give too much importance to power supply HF performance, when in reality local decoupling has much more importance.
Related to capacitors on power rails: here.
What parts do you think are inaccurate, exactly? I always like to learn, and always hate to make inaccurate statements.
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Old 9th February 2012, 04:38 AM   #17
mlloyd1 is offline mlloyd1  United States
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there are soft recovery rectifiers. i suggest taking a look at those instead of focusing on high speed.
not to say that schottkys are bad ...
i'm just sayin' ...

mlloyd1
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Old 9th February 2012, 10:50 AM   #18
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Quote:
...could form a resonance (in combination with the parasitic inductance of the electrolytic), which could excite high-frequency ringing...
I think is more about the PCB trace series inductance. The electrolithyc capacitors have little inductance compared to those long PCB (or straight wires). That internal inductance is the ESL (series with ESR) and is given by the leads mainly.
Once you have looong PCB between the capacitor combo and the fast IC (consumer), the series inductance will "isolate" the PS capacitor from the IC (in HF domain). There will be no ringing at the source, because fast transients cannot get from consumer all the way back there. That's why, like you said too, for fast IC's, the local capacitor is important.
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Old 9th February 2012, 01:10 PM   #19
oshifis is offline oshifis  Hungary
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Here is my experience, well in line with that gootee's wrote:
I replaced the PSU capacitors, 3-pin voltage regulator and all the diodes in my Marantz tuner. I used ultra fast recovery diodes. After the modification the sound became somehow distorted, mainly during the first few minutes after turning on. Could it be because of misalignment of the VCO because of the higher voltage, or because of ringing?
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Old 9th February 2012, 01:33 PM   #20
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I doubt that. Probably your electrolytic capacitors didn't "like" that fast ripple current when you started the tuner, but in a few seconds, due to direct voltage applied , their isolation got better. Or they warmed up. You probably needed to add some more solid dielectric ones there.
Aluminum electrolytic capacitors - Cautions: details | ELNA
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