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Old 20th May 2011, 01:35 PM   #31
macboy is offline macboy  Canada
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Quote:
Originally Posted by wintermute View Post
3rd pic is one I did last night, it is a comparison of a single 1000uF cap as post rectifier filter (left channel) vs a single 10,000uF cap post rectifier filter.
I love how this one objectively illustrates the high amount of noise (harmonics of 50 Hz) created by using an over-sized post-rectifier filter cap, and that it shows no reduction in noise at other frequencies. Some people think that you can never have too much capacitance in a power supply, but this clearly shows that more is not necessarily better. The effect on the audio will depend on the PSRR of the devices being powered among other things, but the supply is clearly more noisy.

p.s. I am a big fan of CRC filtering.
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Old 20th May 2011, 01:39 PM   #32
SY is offline SY  United States
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Tony, one missing piece here is your layout. This is especially critical with large charging currents running around! Can you do a quick sketch of how the grounding and layout were arranged? I know it's a PITA, but pretty please?
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Old 20th May 2011, 01:43 PM   #33
macboy is offline macboy  Canada
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Originally Posted by Piersma View Post
Dual LM317 configuration
A little explanation is in order:
The configuration of the 1st 317 results in a constant voltage differential for the 2nd 317, rather than a constant voltage input. What this effectively means is that the 2nd 317 does not need to perform any line regulation (the 1st 317 takes care of that), so it only needs to control the load regulation. The result is better overall regulation at the 2nd 317 output.

(a much better explanation is at Using 3-pin regulators off-piste: part 1 on page 4).
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Old 20th May 2011, 01:55 PM   #34
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Scrounge up a modest value Oscon cap, say 150uF/25V and try that as the output filter (if the voltage isn't too high). I think you'll see a marked improvement in noise at the higher frequencies. Big electrolytics aren't as bad as many think, but the low impedance of the Oscons can be a real eye opener.

I've used the LM317 in scientific instruments where noise was far more critical than any audio use, and IMHO if you can see much of anything with a sound card, you haven't yet extracted maximum performance from the circuit. Layout can get you, topology can get you and the specific component types can get you. BTW, it isn't available on-line, but have you seen the short app note about 3-terminal regulator noise that appears in Bob Pease's book and, I think, appeared in EDN or somewhere many years ago? It covered the fact that a regulator has much in common with an inductor, and the output circuit invariably produces a response peak with the component choices most people use?
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Old 20th May 2011, 02:17 PM   #35
SY is offline SY  United States
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Conrad, we discussed that note in another thread- it's an excellent piece by Erroll Dietz.
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Old 20th May 2011, 02:59 PM   #36
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Hi Macboy, I'm not sure I'd trust the 50Hz stuff, it's been bugging me, because it comes and goes. I'm not sure its coming from the reg circuit. I just decided to give my audigy II ZS another try and in most respects the results are better than with the onboard sound card, but the 50Hz is worse... Having said that though, I do believe that the bigger cap did result in more 50Hz. I'll test again to see if I can get repeatability.

The graph shows (left) output of the reg, and (right) scope probe shorted. As you can see, most of the 50Hz is there with just the preamp and the input shorted (and the PS completely disconnected from the mains as well).

SY, the layout is hideous, have a look at post # 10 it is on breadboard and the component leads are "full length". I hope this doesn't make all the measurements worthless Also the first pic in post #7 illustrates what you are talking about perfectly! It shows the difference with just a move of the scope probe earth wire to a different point on the "same" common earth (the measurements up to that point were basically useless). I did make a post asking for feedback on my proposed layout here --> Obsessing about layout but I never got any feedback BTW I'm probably going to change that after these experiments, but haven't got around to it yet. The 10,000uF's will more than likely go, and it will become 4700 3r3 4700 3r3 1000 (though I also think the third cap is largely irrelevant based on some early results I haven't posted yet.

Conrad, no I haven't seen the app note you mention, but I have simmed the effect of the output cap (especially with low esr) on the output impedance, very low esr results in significant resonance (and the bigger the cap the lower the resonance point) due to the cap forming a resonant circuit with the inherent inductance in the output of the reg.

Also although it now only goes to 48Khz (due to the audigy's 96Khz max sampling rate) , I think you will see that the problems in the higher frequencies were inadequacies in the on board sound card, rather than the noise on the circuit. Post #3 Pic #3 shows the comparison of the shorted input to the on board sound card with the reg output, again the noise is present in the sound card (or perhaps the preamp). However next order I do place I'll add in an oscon

anyway it is way past my bedtime so I'm heading off for the night. Perhaps more tomorrow

Sorry if this post is a bit incoherent, It's late, and I've had a couple of glasses of red

Tony.
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Last edited by wintermute; 20th May 2011 at 03:06 PM.
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Old 20th May 2011, 03:11 PM   #37
SY is offline SY  United States
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Most Aussie reds are strong enough to stun a horse, so I can sympathize.
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Old 20th May 2011, 06:22 PM   #38
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FWIW, other than some very basic things I've never been able to give useful comments on other peoples layouts. I just have to get in there and work it out myself, often confirming what I think with measurements. The appearance of the thing matters little, it's what's connected to what and where that matters. Hand wired Vector board is fine to use, but plug-in proto boards often aren't. The connections have too much resistance and it's too easy to use the busses on the sides. I'll often take a piece of plain copper-clad and cut the surface up with a burr in the Mototool to make the main pads and traces, then do sort of dead-bug construction on top. One hint- the minute you find yourself trying larger and larger caps to reduce some unwanted signal, especially low frequencies, that's a sign of a layout problem or an inherent low frequency noise problem. I have an interest in high precision references and the limiting factor in filtration is often the basic low frequency noise of the device. If there's something going on in the few Hz region, it's tough to filter it out. Random HF noise however, is a piece of cake. Another hint- don't be afraid to kill resonances with a short piece of resistance wire in series with high-Q caps. I like high-Q (low dissipation factor) caps for many reasons, but they can interact with trace inductance if you don't have some damping resistance. Last hint- don't be like me and spend lots of time looking for the mysterious HF noise, only to discover it's the CFL above the bench, or the light dimmer on the wall.

You can read the 3-term app note here: Excerpt from Bob Pease's book

If that doesn't work, it's page 191 of Bob Pease's book, Troubleshooting Analog Circuits
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Last edited by Conrad Hoffman; 20th May 2011 at 06:28 PM.
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Old 20th May 2011, 06:43 PM   #39
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Originally Posted by Conrad Hoffman View Post
Last hint- don't be like me and spend lots of time looking for the mysterious HF noise, only to discover it's the CFL above the bench, or the light dimmer on the wall.

O how true it is. Or wireless routers or switching power supplies.
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Old 20th May 2011, 08:36 PM   #40
gpapag is online now gpapag  Greece
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Or the radiation from the dreadful monitor of the PC to which we connect the soundcard by which we do the measurements.
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Last edited by gpapag; 20th May 2011 at 08:39 PM.
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