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Old 4th September 2012, 06:24 AM   #971
gootee is offline gootee  United States
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The table in my last post had the theoretical maximum output power for a single sine tone, i.e. it took the average ripple minus half the p-p ripple voltage and assumed that was a sine's maximum-possible peak voltage. So it divided by sqrt(2) to get the RMS voltage then used V-squared over R to get power. Consequently, those values appeared to be very-conservatively small.

The attached similar tables give the theoretical maximum output power assuming a square wave instead of a sine wave. For a square wave, the peak value IS the RMS value. So the theoretical maximum power shown in these tables really is around the maximum usable power (for cases where the transformer could support it), but an amp could be well into clipping at the levels shown, since there often needs to be four or five volts headroom between signal and bottom of ripple.

The scalable spice transformer model is great, because I can now automatically sweep parameters such as VA rating, and Volts RMS rating.

NOTE: 7.5 Ohms and 3.5 Ohms are the average impedances seen across the transformer's output terminals, during charging pulses, with 8 and 4 Ohm loads. Those impedances include the whole load, including the amplifier, not just the output resistor. Since the measurement setup for these tables doesn't include an amplifier, I used 7.5 and 3.5 Ohms to get the voltages but used 8 Ohms and 4 Ohms to calculate the power delivered to the load.

Oh, the reason I started making these tables was initially just to test the new scalable transformer model. Then I also thought they would be nice to have, to see if a particular power/load/VA combination was "reasonable" to try to simulate, to use as a data point for the required capacitance per amp of output current.

I was going to post all of the spice files for everything so far, but it's getting late so it will have to be tomorrow.

Cheers,

Tom
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Last edited by gootee; 4th September 2012 at 06:28 AM.
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Old 4th September 2012, 12:01 PM   #972
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Quote:
Originally Posted by Kindhornman View Post
danielwritesbac, I have been trying to keep up with this thread and I must admit that much of the technical discussion goes over my head. But at the same time when you questions whether any other type of filtering would reduce the requirements of the power capacitors I just don't follow that. There does obviously seem to be a balance between the allowable maximum voltage for a particular circuit with particular devices staying within the SOA and the required capacitor reservoir size to limit to some agreed upon % of ripple currents. What am I missing here that changes that? If you have a transformer with sufficient voltage output, high enough VA rating, and a minimum capacitance, how is that affected by any additional non-capacitance type of filtering? Steven
For an example of huge capacitance easily defeated:
Let's grab an old style half wave car battery charger (he say "Narrrr") hook it up to the car battery and measure the AC voltage present. It is about 6v added peak noise (total output ~18v of garbage). Any audio equipment attached to that gets "Narrr" noise. That's a great example of what capacitance doesn't do. And, using a bigger transformer won't help either. And the bass is insufficiently supported while the charger is on, thus it seems that even the biggest capacitance is insufficient whilst charging. Near identical conditions occur in audio amplifier power supplies--except milder than this big example. I totally guess that the dramatic bass differences between different model caps of same value is a closely related question.
Result: Some critical bit of design is missing. Since there's some things capacitance doesn't do, I guess it is a parallel filter of some sort that might help. But I haven't found out yet.
And:
I'd like to know whether this is my imagination or real, so I put the question to Tom and Terry and I'm willing to accept any answer they might give.
Else:
If they say "no" then I'm perfectly willing to assume that some caps are simply harder to charge than others and I had attempted to do it with a "borderline" too small transformer, thus going definitely too small when pushing the more stubborn models of caps with the totally expected consequences that the bass went missing. Although that's the most plausible answer, its only useful if its the real answer.

Which is the most likely case? I don't know, so I'm asking.
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Old 4th September 2012, 02:15 PM   #973
DF96 is offline DF96  England
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Quote:
Originally Posted by danielwritesbac
Let's grab an old style half wave car battery charger (he say "Narrrr") hook it up to the car battery and measure the AC voltage present. It is about 6v added peak noise (total output ~18v of garbage). Any audio equipment attached to that gets "Narrr" noise. That's a great example of what capacitance doesn't do.
Charger plus battery has no capacitance. How can that be "a great example of what capacitance doesn't do"? It is an example of what 'no capacitance' does do!
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Old 4th September 2012, 02:39 PM   #974
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Originally Posted by DF96 View Post
Charger plus battery has no capacitance. How can that be "a great example of what capacitance doesn't do"? It is an example of what 'no capacitance' does do!
So, paralleling a capacitor fixes that situation?
In this case, the parallel filter I was looking for is simply adding more capacitance?
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Old 4th September 2012, 03:00 PM   #975
DF96 is offline DF96  England
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Not 'more capacitance', but 'some capacitance'. However, it will need a lot because a lead-acid battery has quite a low impedance. Ideally you would need a battery with 4 terminals so the voltage drop across the terminal contact resistance caused by charging pulses does not go to the audio circuit.

Why bring batteries into the discussion? A lead-acid battery and a big electrolytic are not the same thing, even though they both use electrolysis. They use electrolysis for different reasons: one to store charge by migrating ions and depositing metal on plates, the other to build an insulating dielectric layer.
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Old 4th September 2012, 03:07 PM   #976
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Nice Job Tom. The truth is that we need the transformer to provide much more than is normally perceived. Your data may very well have brought out the balance that is needed between VA and Farads. Could you direct me to the decoupling data

Last edited by OnAudio; 4th September 2012 at 03:10 PM.
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Old 4th September 2012, 03:43 PM   #977
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Quote:
Originally Posted by DF96 View Post
Not 'more capacitance', but 'some capacitance'. However, it will need a lot because a lead-acid battery has quite a low impedance. Ideally you would need a battery with 4 terminals so the voltage drop across the terminal contact resistance caused by charging pulses does not go to the audio circuit.
Why bring batteries into the discussion? A lead-acid battery and a big electrolytic are not the same thing, even though they both use electrolysis. They use electrolysis for different reasons: one to store charge by migrating ions and depositing metal on plates, the other to build an insulating dielectric layer.
Well, I did notice that some electrolytics work better for smoothing than others, and some are practically useless for smoothing, such as the car battery, which was an overly extreme example.

However, there's also some caps that can transport a signal extremely well and perhaps intact, yet that same cap doesn't work well on the power board. I'd sure like some "grip" on that problem so I can more reliably select capacitors. It is rather disheartening to have done an excellent job on the power board assembly and with excellent quality parts, only to discover that the job didn't get done. So far, adding more capacitance is not the effective fix; but changing the model of capacitor does it no matter if you increased the capacitance or not. I sure can't explain it, but I wish I could predict it a bit better.
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Old 4th September 2012, 04:05 PM   #978
AndrewT is offline AndrewT  Scotland
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Originally Posted by danielwritesbac View Post
Well, I did notice that some electrolytics work better for smoothing than others, ....................... I sure can't explain it, but I wish I could predict it a bit better.
are you sure you know what you are talking about?

Creating a DC supply from AC is easy.
Transformer, if the AC voltage is not right, + rectifier and smoothing capacitor. That is all that is needed. Miss any of these out and you won't get the correct DC voltage as the input to any charger.

You need to wise up by doing the research.
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Old 4th September 2012, 04:58 PM   #979
gootee is offline gootee  United States
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Quote:
Originally Posted by danielwritesbac View Post
Well, I did notice that some electrolytics work better for smoothing than others, and some are practically useless for smoothing, such as the car battery, which was an overly extreme example.

However, there's also some caps that can transport a signal extremely well and perhaps intact, yet that same cap doesn't work well on the power board. I'd sure like some "grip" on that problem so I can more reliably select capacitors. It is rather disheartening to have done an excellent job on the power board assembly and with excellent quality parts, only to discover that the job didn't get done. So far, adding more capacitance is not the effective fix; but changing the model of capacitor does it no matter if you increased the capacitance or not. I sure can't explain it, but I wish I could predict it a bit better.
There are only a few variables. Unfortunately, each one multiplies the chances of us not knowing whether or not you have comparable or analytical setups by infinity, or in some cases infinity squared.

As Terry and others have mentioned, it could be challenging, even under rigorously-controlled conditions, to know the true causes of some effects. (But here's a tidbit for your own mental toolbox: "Current does NOT follow the path of least resistance (or impedance). It follows ALL paths, in inverse proportion to their resistances (impedances)." Filtering is different than transmitting input or output, because with filtering the signal has two paths to take, rather than just one.)

However, in case it's something that is easily explainable and measurable, such as the capacitors' ESRs or inductances, you could download the spice model I posted and do things like: automatically plot a series of frequency-responses, one for each ESR in a sequence you can define. You could plot the frequency response for any range of steps of variation of _ANY_ parameter, anywhere in the circuit! If there are correlations with certain frequency ranges, you will immediately SEE them, as the multiple plots (of ratio of output vs input amplitudes, over frequency) all take form on the same axes, right before your eyes.

Interested in distortion? It's almost as easy. You can tell anything at all in the whole schematic, including any parasitics you choose to insert, to vary over whatever range of steps you desire, and use a sine wave as the input, and you can receive a THD number for each step of the variation that you commanded. The only difference from above is that the THD numbers get placed into a text file that you then have to open, after the automatic sequence of stepped variations has finished running.

If you could do something like that, what would be the first thing you would try looking at?
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Old 4th September 2012, 05:22 PM   #980
DF96 is offline DF96  England
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Quote:
Originally Posted by danielwritesbac
Well, I did notice that some electrolytics work better for smoothing than others, and some are practically useless for smoothing, such as the car battery, which was an overly extreme example.
Category error. A car battery is not an example of an electrolytic, extreme or otherwise.

Capacitor ESR, and possibly inductance, will play a role. Capacitor nonlinearity might play a role as it could cause hum IM.
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