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Old 19th August 2012, 04:32 PM   #701
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Kindharman, yes you are right things are getting a little lost in the thread and I honestly hope that our highly knowledgeable friends do not lose interest because of the trend of late.

If I can summarize so far is that the capacitance is 3000uF/Amp still to be confirmed the transformer VA rating is > 150% of the rms power rating (I take it into any load specified).

We are waiting for the final simulations to confirm what capacitor value would become irrelevant. It would appear that the capacitor value is as dependent on both the low frequency signal and the line frequency.

We have had members arguing that one could design for a higher than necessary rail voltage since more energy can be stored at the elevated voltage and that rail sagging would not affect the signal. The argument makes sense but has not been confirmed.

We have concluded that there must be distibuted capacitance on the PCBs as close to the power devices as practical, also the cable runs to and from the PCBs need be twisted tightly together and kept as short as practical.

It was also mentioned that multilayer PCBs where there is a positive, negative and earth plane could be very beneficial because of reducing the inductance.

I am a believer in regulating the front end of the amp, but no-one has actually confirmed whether this is an alternative for large reservoir capacitors - probably not.

Have I missed anything?
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Last edited by Nico Ras; 19th August 2012 at 04:37 PM.
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Old 19th August 2012, 04:37 PM   #702
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Originally Posted by magnoman View Post
However a good engineer doesn't assume this, but rather analyzes the issue and tries to understand all aspects (wasn't really thinking about hum). Eventually a compromise is reached which results in what he or she believes is the optimal solution to accomplish their goal, accounting for the given constraints (including time and money).

Hope this helps
-Antonio
I didnt assume, I already mention to look for the Y parameter to see that the transfer from colector to emiter is very low
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Old 19th August 2012, 04:39 PM   #703
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Originally Posted by liching1952 View Post
Yes am still waiting for an answer of my question in post 487 mr Dadod. Kan jij een antwoord geven meneer Dadod?
I don't think they speak Dutch in Croatia.
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Last edited by Nico Ras; 19th August 2012 at 04:54 PM.
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Old 19th August 2012, 04:41 PM   #704
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Originally Posted by DF96 View Post
I'm not certain that is true. If you increase the quiescent supply rail voltage enough, it doesn't matter how much it droops (assuming PSRR is not an issue) so you can have a small cap. Your stored energy will increase because of the higher voltage. What you can do is calculate the smallest cap required, given a stated Vdc (exceeding Vpk by whatever margin you deem helpful). You can also calculate the minimum stored energy (see my earlier post) but that merely lets you then choose C (given Vdc) or choose Vdc (given C).

I suspect you are all looking for the gold at the end of the rainbow: there is not an optimum value for C or Vdc, although there may be an optimum combination. Total 'AC in' to 'sound out' efficiency may be a useful parameter, as raising Vdc unnecessarily reduces efficiency.
DF,

Not looking for optimum C, yet. Just simply bounding C, and really only the lower bound.

You say it might not be true (that there must be a "smallest viable C") but then you proceed to the same place I was!

But calculating the C value is NOT straightforward, unless you assume ripple is small and the load is a constant current! THAT is the whole problem.

And non-approximate (non-"small ripple") mathematical solutions are extremely complex. We COULD go that way. I have found the papers with the math done, but not with a sinusoidal load current, although Basso has a nice solution for an SMPS load. I would probably enjoy the mathematical approach more but it seemed quicker and easier, for the purposes of this thread, to use simulations. I am almost done with the first full round of sims, during which I am automating as much as possible (and making it simple so others can use it), doing a single transformer voltage with different VA characteristics. but other transformers will then be much quicker and easier. I will post the semi-automated simulation setup soon. My only worry is that my one available measured transformer model is not great for more than 120 VA, although I have paralleled those to get 240 and 360 cases as well.

We could just make the transformer voltage higher, too. But we want to know the limits for real transformers of different sizes and voltages, don't we?

Cheers,

Tom
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Old 19th August 2012, 04:47 PM   #705
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Quote:
Originally Posted by Nico Ras View Post
Kindharman, yes you are right things are getting a little lost in the thread and I honestly hope that our highly knowledgeable friends do not lose interest because of the trend of late.

If I can summarize so far is that the capacitance is 3000uF/Amp still to be confirmed the transformer VA rating is > 150% of the rms power rating (I take it into any load specified).

We are waiting for the final simulations to confirm what capacitor value would become irrelevant. It would appear that the capacitor value is as dependent on both the low frequency signal and the line frequency.

We have had members arguing that one could design for a higher than necessary rail voltage since more energy can be stored at the elevated voltage and that rail sagging would not affect the signal. The argument makes sense but has not been confirmed.

We have concluded that there must be distibuted capacitance on the PCBs as close to the power devices as practical, also the cable runs to and from the PCBs need be twisted tightly together and kept as short as practical.

It was also mentioned that multilayer PCBs where there is a positive, negative and earth plane could be very beneficial because of reducing the inductance.

I am a believer in regulating the front end of the amp, but no-one has actually confirmed whether this is an alternative for large reservoir capacitors - probably not.

Have I missed anything?
Yes its important to stabilize the front end. Its even better to use a separate PSU for the front end.

150% of the amp rms value for the transformer is far not enough.
The efficiency of an Class AB amp is around 35%. That means that you need 3 times more power then the rms value. For a 2x 100Watt amp you need at least a 600Watt transformer.
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Old 19th August 2012, 04:53 PM   #706
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Originally Posted by liching1952 View Post
......The efficiency of an Class AB amp is around 35%.
You sure about this?
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Old 19th August 2012, 04:54 PM   #707
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Quote:
Originally Posted by liching1952 View Post
....The efficiency of an Class AB amp is around 35%. That means that you need 3 times more power then the rms value. For a 2x 100Watt amp you need at least a 600Watt transformer.
This is completely different from any previous values I have seen that I can rely on for reasonable accuracy.
The efficiency of a ClassAB amplifier is around 65% when delivering full power without clipping of the sinewave output.
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Old 19th August 2012, 04:58 PM   #708
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Originally Posted by liching1952 View Post
Power supply sag cant be a problem in a good design. One designes an amp with a certain power supply voltage. Then the power supply must be designed to deliver that voltage at mayimum current. Thats all, the voltage will be higher when the current draw is lower but thats not a problem.
liching,

That is what this thread is about.

If you know how to design the power supply, please post the mathematical steps that arrive at the component values, starting from the variables for the specifications, and allowing for variable transformer and rectifier diode characteristics.

Tom
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Old 19th August 2012, 05:20 PM   #709
gootee is offline gootee  United States
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Originally Posted by liching1952 View Post
Better then simulation is reading a good book about this matter. Every basic electronic book describe this with advantage that there is a change that you understand what really happens, simulation doesnt.
liching,

Give me a break, man. You insult us.

Many here are electrical engineers, with vast experience and education.

You still totally miss the point of the discussion.

The "basic electronics" books do not even attempt to show the real solution to the "simple" linear PSU.

Tom
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Old 19th August 2012, 05:23 PM   #710
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Originally Posted by liching1952 View Post
I didnt assume, I already mention to look for the Y parameter to see that the transfer from colector to emiter is very low
Already done that as posted earlier, it appears not "low" enough for all.

Hope this helps
-Antonio
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