Power Supply Resevoir Size

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Increase the 0.22R resistor to 1 ohm and use a wire-wound 5W or larger ceramic/sandstone power resistor. When used in conjunction with the 10,000uF cap, this gives you the 15Hz corner frequency I mentioned.

Have you tried a suitably sized inductor in place of the resistor? I can't help thinking that the resistor is a bit of a waste of power and we can get better HF filtering with a choke. Recently I've played a bit with CLCLC filtering using iron dust toroidal chokes (values of a few hundred uH) and my rudimentary measurements show they do get rid of ripple nicely above a kHz or so. However I've yet to discover what happens when a real amp is used as a load, rather than my test resistor. :D
 
Actually, almost always, the power supply capacitor current is exactly the signal that you hear. For very-convincing "proof", see the image from an LT-Spice simulation that I attached to post # 372 of this thread:

http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-38.html#post3117390

Cheers,

Tom

Tom,

From what I can tell, you show in post 372 is that the PS reservoir capacitors are delivering current to the load via the output devices, which are following the input signal. That's pretty normal stuff, exactly what should be expected.

I believe that what you are inferring by the post above is that voltage fluctuations that are on the rails will show up at the load in the same way. That is at a minimum a very misleading statement (if that is indeed what you were saying... :scratch1: ). I am sure that you have heard of power supply ripple rejection ratio... it's the ability of the amplifier to reject voltage fluctuations on the rails. For my favorite chip amp, the LM3886, this starts out around 100 dB, falling to 75 dB at 1k Hz (worst rail). So the 1V ripple at 120 Hz becomes an 0.00001 V ripple on the output (assuming 100 dB PSRR). Hardly of any concern and entirely negligible.

I see how what you are trying to say above, but the reversed logic is faulty.

-Charlie
 
For my favorite chip amp, the LM3886, this starts out around 100 dB, falling to 75 dB at 1k Hz (worst rail). So the 1V ripple at 120 Hz becomes an 0.00001 V ripple on the output (assuming 100 dB PSRR). Hardly of any concern and entirely negligible.

Speaking of faulty, this has an error in the calculation :) The PSRR is referred to the input, not the output. So with a gain of X20 its 26dB worse than your figure. The PSRR at 20kHz for the LM3886's negative rail falls to a fairly dismal 50dB - what's on that supply rail therefore just gets 24dB attenuation on the output in a typical implementation.:eek:
 
Have you tried a suitably sized inductor in place of the resistor? I can't help thinking that the resistor is a bit of a waste of power and we can get better HF filtering with a choke. Recently I've played a bit with CLCLC filtering using iron dust toroidal chokes (values of a few hundred uH) and my rudimentary measurements show they do get rid of ripple nicely above a kHz or so. However I've yet to discover what happens when a real amp is used as a load, rather than my test resistor. :D

LC filtering can be done, however, you may inadvertently create a resonance above the audio band (or so I read) because of interaction with other components (I forgot what exactly, sorry!). I do use wirewound power resistors, which have a small inductive component, but most of the attenuation is purely RC.

Let's run some numbers to see what the power loss might be:
I suggested 1R + 10,000 uF to give a corner frequency of 15 Hz. I mentioned that the cap size was appropriate for about 20W+20W of power. At full power this would be about 4A RMS current, so 4V loss. But this would only really happen if the amp was operated long enough at "full sine wave power" to draw that much current long enough to drain the caps down 4V. Mostly there will be brief high power peaks with the average being much less (like 10-20dB less, e.g. my crest factor value typical for music), so the average rail voltage loss would only be around 0.1 - 0.3 V, which really is not much of a loss considering that with this one component (the 1R resistor) you clean up the rails significantly.

I just happened to be working on a chip amp last night with almost this exact same PS. After the bridge rectifier I have one pair of 10k uF caps, then a 1R resistor on each rail, then another pair of 10k uF caps. Attached are two full power (about 25W+25W) measurements that I took, one with the 1R resistor and one with it replaced by a wire short (0R). The mains and ripple harmonics have all but disappeared - the 120Hz ripple is reduced by 20dB. With the 0R connection between pairs of caps, the ripple amplitude was almost as much as the second order distortion peak!

-Charlie

Edit: Also of note is that the IMD is reduced/eliminated. You can see the small side bands (e.g. 1k Hz plus 120Hz, and 1k Hz minus 120 Hz) which are present for the 0R case disappear in the 1R case.
 

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Speaking of faulty, this has an error in the calculation :) The PSRR is referred to the input, not the output. So with a gain of X20 its 26dB worse than your figure. The PSRR at 20kHz for the LM3886's negative rail falls to a fairly dismal 50dB - what's on that supply rail therefore just gets 24dB attenuation on the output in a typical implementation.:eek:

Hmm, interesting, thanks for pointing that out. But I think most junk on the rails is ripple harmonics (at least below 20k Hz) and those die out by 1k or 2k Hz even in the worst case. At least that is what I see in my measurements...

-Charlie
 
LC filtering can be done, however, you may inadvertently create a resonance above the audio band (or so I read) because of interaction with other components (I forgot what exactly, sorry!). I do use wirewound power resistors, which have a small inductive component, but most of the attenuation is purely RC.

I agree its something to watch out for and not something that will show up accurately in LTSpice (being as it knows nothing about frequency dependent loss in inductors). The chokes I'm starting to play with are by deliberate choice lossy into the 100's of kHz so I doubt whether there's really a problem in practice. I'll have to suck it and see though.

Let's run some numbers to see what the power loss might be:
I suggested 1R + 10,000 uF to give a corner frequency of 15 Hz. I mentioned that the cap size was appropriate for about 20W+20W of power. At full power this would be about 4A RMS current, so 4V loss. But this would only really happen if the amp was operated long enough at "full sine wave power" to draw that much current long enough to drain the caps down 4V. Mostly there will be brief high power peaks with the average being much less (like 10-20dB less, e.g. my crest factor value typical for music), so the average rail voltage loss would only be around 0.1 - 0.3 V, which really is not much of a loss considering that with this one component (the 1R resistor) you clean up the rails significantly.

I agree that for such a low BOM cost the series resistor is great bang for the buck in terms of cleaning the rails. However I think your estimation of the voltage drop is too optimistic for a couple of reasons. Firstly the crest factor of the resistor current in the schematic you've drawn will be less favourable than a sine because of the lowish capacitance you have prior (2.2mF). And secondly the amp gets much less efficient at lower power levels.

A quick way of estimating this is that the load itself is 8R, there are two of them effectively in parallel, so 4R. With 14dB crest factor the average power for a 20+20W stereo amp in the two speaker loads (combined) is 1.6W. I'd expect an average 0.4W in each resistor so roughly 0.6V drop. This ignores the crest factor issue I mentioned and ignores RMS/average conversion so in practice it'd be somewhat worse by a few 10s of percent points but it gets in the ballpark.
 
Regarding the value of the series resistor, you are correct (abraxalito) in being a little concerned about the 1R value. IMHO it is about the most I would put there, but in return you get a great deal of attenuation/rejection. As you showed, the voltage drop that will result is pretty minor.

Here are my calcs:

Code:
[B]10,000uF + 1R[/B]

Frequency     Attenuation   Harmonic
60            -11.8 dB      
120           -17.6 dB      2
180           -21.1 dB      3
240           -23.6 dB      4
300           -25.5 dB      5
360           -27.1 dB      6
420           -28.4 dB      7
480           -29.6 dB      8
540           -30.6 dB      9
600           -31.5 dB      10


If the capacitance goes up, the resistance can decrease for the same amount of attenuation. For instance using 45,500 uF and 0R22 (0.22 ohms) give exactly the same attenuation in dB at each frequency that is shown above, however, there will be about 4.5 times less voltage drop in the series resistor (if that is a concern).

Also, as the amplifier power increases and larger caps are required to give the same ripple voltage on the rails, the resistance can be reduced while attenuation remains constant. The higher power amp will draw more current, so the voltage drop will probably remain about the same all things being equal. Using larger than average capacitors will allow the resistor value to be decreased even further... this counters the "minimum capacitance" concept that I have seen mentioned in this forum. More is indeed better in this application.

-Charlie
 
Originally Posted by gootee
Actually, almost always, the power supply capacitor current is exactly the signal that you hear. For very-convincing "proof", see the image from an LT-Spice simulation that I attached to post # 372 of this thread:

http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-38.html#post3117390

Cheers,

Tom


Tom,

From what I can tell, you show in post 372 is that the PS reservoir capacitors are delivering current to the load via the output devices, which are following the input signal. That's pretty normal stuff, exactly what should be expected.

I believe that what you are inferring by the post above is that voltage fluctuations that are on the rails will show up at the load in the same way. That is at a minimum a very misleading statement (if that is indeed what you were saying... :scratch1: ). I am sure that you have heard of power supply ripple rejection ratio... it's the ability of the amplifier to reject voltage fluctuations on the rails. For my favorite chip amp, the LM3886, this starts out around 100 dB, falling to 75 dB at 1k Hz (worst rail). So the 1V ripple at 120 Hz becomes an 0.00001 V ripple on the output (assuming 100 dB PSRR). Hardly of any concern and entirely negligible.

I see how what you are trying to say above, but the reversed logic is faulty.

-Charlie


What?!! Think again! That was MUSIC, NOT NOISE.

You originally said:

Originally Posted by CharlieLaub
<snipped>
Remember, this is just the power supply. There is a tenuous connection to what you would actually "hear" from the amplifier, so do not do listening tests to "proof" your tweaks!!!
<snipped>
-Charlie

and I was merely pointing out that the connection is not tenuous at all.

The power supply current IS what you hear.


There was no "inferring by", no "reversed logic", no "faulty", and no "misleading", at least not on my part!

Whiskey Tango Foxtrot?!

How could you have thought that I was implying, as you said, "voltage fluctuations that are on the rails will show up at the load in the same way"?

I said NOTHING that could possibly be construed to have implied anything at all like that.
 
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@Charlie : In terms of the higher harmonic rejection/filtering of the series R, ISTM there's going to be a limit imposed by the main cap (10mF in your case) ESR. With 10mohm ESR and 1R series R the best attenuation you can hope for is 40dB for example. Splitting those components in half does better - a two stage filter gives better numbers than a single, where the numbers are higher harmonic rejection. With a series L though the attenuation even with a single stage keeps on going, at least until you reach the SRF of the inductor.

The series L is probably costlier, definitely bulkier at lower powers - at higher it won't need heatsinking though so probably wins out in the kinds of applications Nelson Pass designs. In the link you removed from your original post, I see he's using 67mohms between two banks of 40mF. My concern with that is that while this looks good looking in from the rectifier, looking back from the power amp the first set of caps all of a sudden has +67mohm of ESR which seems to me to be something of a waste of decent caps...:p
 
gootee said:
Actually, almost always, the power supply capacitor current is exactly the signal that you hear.
Be careful of causality here. The current through the cap may be the same (apart from charging pulses) as that through the speaker, but neither is set by the cap; both are set by the feedback loop (if there is one, which is usually the case for SS amps). Now if you go 'non-feedback' things get harder, but that is only for people who take pleasure in making life harder for themselves.

If you really can hear the PSU caps in a conventional SS amp then something is wrong: grounding, PSU caps too small, NFB take-off point or ground reference incorrect etc.
 
If you really can hear the PSU caps in a conventional SS amp then something is wrong: grounding, PSU caps too small, NFB take-off point or ground reference incorrect etc.
Actually, it's pretty easy to "hear" the PSU caps in a "conventional" amp, that classic loss of dynamics, or compression of sound that starts at a certain volume on most systems is the obvious symptom of the caps starting to fail to do their job properly. Which is why amps with monstrously oversized power supplies always impress people, because finally the power supply's "fidgeting" is at a low enough level to not disturb the amplifying circuit's functioning ...

It becomes easy to recognise this behaviour once you've modified a normal sized power supply to work properly: all of a sudden the amplifier sounds 10 times as powerful, even though nothing in the amplifying circuit has altered ...

Frank
 
fas42 said:
compression of sound that starts at a certain volume on most systems is the obvious symptom of the caps starting to fail to do their job properly.
Sounds to me like an obvious symptom of the caps being too small. Note that the simulations and algebra both show that as audio power rises to the maximum possible for a particular transformer and power amp then the capacitor value becomes more and more critical and in fact rises without limit. Note that too high ESR, by dropping some voltage, also means a larger cap is needed.

No magic is happening, it is simply that the cap is not doing its job properly, as you say. Bigger cap or bigger transformer is the answer. Circuits really do obey circuit theory! You will note that I included 'PSU caps too small' in my list of things which might be wrong.

'Loss of dynamics' presumably means that the gain of the amp is varying with signal level. That is mainly an amp design issue, but it can be eased by bigger caps so the supply rail droops less.

I think the main problem is that people don't realise what actually sets the required cap size. They do average DC calculations, not realising that this can automatically put them a factor of 2.8 too low in current draw. Then the marketing men get involved so an amp/PSU which can do 100W on an isolated HF peak is then advertised as a 100W amp even though it can only do, say, 60W on sustained low bass. People treat it as a bad 100W amp when in fact it might be a very good 60W amp. Doubling the PSU cap size might make it an 80W amp. Doubling again makes it a 90W amp (roughly) but then people turn up the volume and start complaining that it can't do 150W on HF peaks (they are still limited to 100W). This also might be described as poor dynamics.
 
What?!! Think again! That was MUSIC, NOT NOISE.

You originally said:



and I was merely pointing out that the connection is not tenuous at all.

The power supply current IS what you hear.


There was no "inferring by", no "reversed logic", no "faulty", and no "misleading", at least not on my part!

Whiskey Tango Foxtrot?!

How could you have thought that I was implying, as you said, "voltage fluctuations that are on the rails will show up at the load in the same way"?

I said NOTHING that could possibly be construed to have implied anything at all like that.

Ah, then it looks like I completely misread what you meant in your post. Mea Culpa. You are correct in that the current from the power supply to the amplifier is resulting in the music emanating from the speaker.

Here's what transpired from my point of view:
I wrote: "Remember, this is just the power supply. There is a tenuous connection to what you would actually "hear" from the amplifier, so do not do listening tests to "proof" your tweaks!!!" to Danielwritesbac in an EMail where I went on to talk about his power supply design. What I had in mind was him using a light blinker to test his supply, and his unorthodox cap in parallel with diode add-ons. I had in mind the amp PSRR at the time when I made the statement about a "tenuous connection" and I was thinking of ripple, or other non-DC component on the rails. I wanted to go back and change it later to be more clear, but you only get a few minutes to change your posts after you make them, and that time had passed.

I had in mind that ripple and other rail VOLTAGE fluctuations will not be discernible at the amplifier output because of PSRR, and that making a tweak in your power supply and then listening for the change in the sound coming out from the loudspeaker is not a good way to go about it IMHO.

I thought that you were invoking your current measurements from post 372 and saying that everything on the rails will come out at the speaker. This would not be true, unless it was a signal at the input signal of the amplifier, which is precisely what you were measuring/showing in post 372 - a (music) input signal resulting in current delivered to the load that mirrors that input signal (if I am understand your post correctly).

So again, my apologies for misconstruing your post as something that it was not.

-Charlie
 
Actually, it's pretty easy to "hear" the PSU caps in a "conventional" amp, that classic loss of dynamics, or compression of sound that starts at a certain volume on most systems is the obvious symptom of the caps starting to fail to do their job properly. Which is why amps with monstrously oversized power supplies always impress people, because finally the power supply's "fidgeting" is at a low enough level to not disturb the amplifying circuit's functioning ...

It becomes easy to recognize this behaviour once you've modified a normal sized power supply to work properly: all of a sudden the amplifier sounds 10 times as powerful, even though nothing in the amplifying circuit has altered ...
Frank

Hi Frank! That a great comment.
I'm looking at it trying to name one thing or the most likely thing that could cause what is described. Bass dynamics? Undersize transformer isn't good for bass dynamics. Compression? Well, that confuses me a bit. Compression sounds like: "YAAAAAA!!!!" and something is having trouble driving something else, most usually a computer without a buffer. There seems to be several fixes, but I do not know the cause.
 
'Loss of dynamics' presumably means that the gain of the amp is varying with signal level. That is mainly an amp design issue, but it can be eased by bigger caps so the supply rail droops less.

Hard to see how this could be, short of the amp clipping. Under clipping of course the amp's gain is reduced, but exclude that and for a typical NFB amp the feedback resistors' values would have to be altering.

I rather suspect 'loss of dynamics' is associated with the perceived noise levels rising in a program-modulated fashion. The rails have more noise (increased ripple) on them and this gets through to the output slightly delayed due to the reservoir caps. Without the delaying factor probably the extra noise would get subjectively masked - perhaps explaining the 'small res cap sounds better' observation. The mechanisms can be non-infinite PSRR and sub-optimal grounding and decoupling techniques.
 
Compression, as I hear it, and think of it, is the inability of the amp to resolve high frequency information correctly once past a certain volume. Driving rock records with lots of splashy cymbal work is an easy way to pick it; the shimmer is maintained up to certain SPLs, and then that sound just goes flat, the drummer might as well be banging on the bottom of a saucepan.

My first decent amp, 25 years ago, a Perreaux 2150B, theoretically had plenty of grunt, but you could hear the power supply start to collapse in this fashion even at moderate sound levels. So the initial big tweak was to rip out the usual caps of the day, two big, fat screwcaps, and replace with myriads of parallelled, small valued radials, with about 10 times the capacitance overall. This made a huge difference of course ...

Frank
 
Be careful of causality here. The current through the cap may be the same (apart from charging pulses) as that through the speaker, but neither is set by the cap; both are set by the feedback loop (if there is one, which is usually the case for SS amps). Now if you go 'non-feedback' things get harder, but that is only for people who take pleasure in making life harder for themselves.

If you really can hear the PSU caps in a conventional SS amp then something is wrong: grounding, PSU caps too small, NFB take-off point or ground reference incorrect etc.

Absolutely. The capacitors are passive and should be configured to be transparent and their currents are used to produce the sound and we hear the results of the work done by the currents and hope we don't hear anything that is not due to accurate reproduction of the source signal, as orchestrated by the amplifier that is using the power supply.

But unless the reproduction is ideal and perfect, we actually are always hearing "system effects", which most of us wish to minimize. We hear each component of a non-ideal system, to some degree, which we hope is negligible in every case.

If Daniel could hear an improvement in reproduction accuracy due to a power supply change, then he was first hearing the effect of something being wrong and after the change he was hearing the effect of it being less wrong (assuming his perceptions were accurate and correct).
 
But unless the reproduction is ideal and perfect, we actually are always hearing "system effects", which most of us wish to minimize. We hear each component of a non-ideal system, to some degree, which we hope is negligible in every case.
Exactly, Tom. As a system gets closer and closer to being a "good" system (hah!) I hear 2 characteristics changing: firstly, more and more detail is apparent, the sound picture becomes richer and fuller in all the subtle aspects; and secondly, that fine detail is fully "integrated" into the whole, nothing jars, sounds harsh or irritates - it's always musical, for want of a better word ... these 2 aspects are rarely in alignment in hifi!!

Quite often alterations can bring the first element to the fore, but not the second. The hardest trick is to get both happening together, which means all audible "system effects" are sufficiently minimised ...

Frank
 
Frank I would agree with the concept of detail or the ability to resolve small a signal in the presence of larger signals.

However, as soon as the power supply modulation increases to similar levels than that of the small signals it not only masks the small signals, but also mixes with them creating new spurious inter-modulation products which could be dissonance and of course fatiguing.
 
Your implementation of the diodes does NOT seem to be a practical approach, and I believe that the diode drop is reducing the PS rail voltage.
Not as much loss as regulators. However, I do apologize for not showing the "output diodes" as Schottky like MBR1045, or even a fast silicon, like an MR. Credit to diyaudio.com member "TheProf" for that idea. This is not intended for every power supply in every application. Let's not lose sight of the target. Remember, the application had a near zero length umbilical cable and possibly borderline size transformer.
However, if the transformer is about watts*3=va and the umbilical cable is long enough to even the odds, then the "power board output diodes" would be useless except for minor stereo separation enhancement.
 
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