Bob Cordell Interview: Power Supplies - Page 12 - diyAudio
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Old 26th November 2006, 09:18 AM   #111
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Quote:
Originally posted by MikeBettinger

So in reality there is current flowing into the center tap whenever there is current through the speaker.
No, only when the caps are charging. This is the point I have been trying to make all the time. Whenever the caps are not charging, the diodes are reverse biased so the current loops through the windings, and hence through the center tap, are effectively broken. However, I am sure you know this too, so we must be seriously misunderstanding each other somehow.

Quote:

In my amps I dig into the transformers and bring the coil wires out separately and join them at the central ground, not allowing the positive or negative loops to mix. There is no possibility of unequal currents causing problems because the +/- loops are completely separate. (within the transformers limitations)
This sounds like something we can agree on. Besides, I have never understood why people use transformers with center taps anyway. I don't even think I have ever seen one, but maybe they are common in some countries, for whatever reason.
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Old 26th November 2006, 10:51 AM   #112
moamps is offline moamps  Croatia
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Gentlemen, the discussion on Upupa's beautiful amplifiers deserves a thread on its own so it has been moved to "Pavel Dudek's (Upupa Epops) amplifiers", which is to be found here:
Pavel Dudek's (Upupa Epops) amplifiers

Enjoy!

Regards,
Milan

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Old 26th November 2006, 01:07 PM   #113
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Quote:
Originally posted by Christer


No, only when the caps are charging. This is the point I have been trying to make all the time. Whenever the caps are not charging, the diodes are reverse biased so the current loops through the windings, and hence through the center tap, are effectively broken. However, I am sure you know this too, so we must be seriously misunderstanding each other somehow.



This sounds like something we can agree on. Besides, I have never understood why people use transformers with center taps anyway. I don't even think I have ever seen one, but maybe they are common in some countries, for whatever reason.
We have a fundamental disagreement here in that I have found that in my amps (as described) that the flow of power is a loop that starts at the rectifier output, flows through the output transistors, through the speaker and returns back into the secondary to complete the loop. The filters are there to smooth out the power and act as a source during dead time and to help absorb reactive energy reflected from the load. The capacitors are not actually in the signal path as far as the power output stage is concerned. This is where the difference between what the ground is doing (reference or current source) comes into play.

You can create a virtual ground without a center tap and, I know it works when powering low level circuitry because, as I stated, the ground is only used as a reference, and I'm assuming that you could build a power amp this way and force the return currents backwards through the filters, although I have not tried this, having always used the center tap, Iím sure it would work. but why use it in this application?

It would place the sound of the amp squarely on the quality of the caps and it would inseparably inter mix the charging currents with the load currents.

Implementation wise, by running the return currents back to the caps rather than the centertap a voltage is developed across whatever wire runs from the filters to the centertap. This is the source of the unintended feedback that I mentioned in paragraph 4.

If you think about a tube power amp with a positive-only supply to the output stage; the power flows from the supply through the transformer load the output tubes and is returned to the ground side of the supply completing the loop. A push pull amp that uses a +/- supply also needs to close this loop, only it has two separate loops.

Regards, Mike.
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Old 26th November 2006, 04:19 PM   #114
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One other thought. Think of the supply as two +/- DC sources without charging currents, rectifiers or filter caps, only a +/- DC potential with a center tap. Where does the current flow?

Regards, Mike.
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Old 26th November 2006, 05:24 PM   #115
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Quote:
Originally posted by MikeBettinger
One other thought. Think of the supply as two +/- DC sources without charging currents, rectifiers or filter caps, only a +/- DC potential with a center tap. Where does the current flow?

Regards, Mike.

That is a totally different situation. I am really starting to wonder if you actually even disagree with the figures I posted earlier? If so, we really have a fundamental disagreement of a most puzzling type.

Anyhow, since you seem to prefer not to have a center tap at all if there is a choice, we probably agree on that case at least.
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Old 26th November 2006, 05:43 PM   #116
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Quote:
Originally posted by Christer



That is a totally different situation. I am really starting to wonder if you actually even disagree with the figures I posted earlier? If so, we really have a fundamental disagreement of a most puzzling type.

Anyhow, since you seem to prefer not to have a center tap at all if there is a choice, we probably agree on that case at least.

I must not be being very clear
What I'm talking about is needing a center tap!!! I don't know where the disconnect is.

I don't have time now to sort it out since I'm getting ready to leave for the week. Going for LabView training in NC. If I can manage a internet connection while I'm away I can try. Did I lose everyone?

Regards, Mike.
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Old 26th November 2006, 06:01 PM   #117
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Mike,

Let's just conclude that either we have some fundamental disagreement or, more probably, we misunderstand each other severely. So let's end the discussion here. It is not really important to me to sort this out and nobody else seems interested in the discussion either.
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Old 26th November 2006, 08:40 PM   #118
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Quote:
Originally posted by john_ellis
Hi all

Rather than use additional transformers to add the extra volts to the power rails, why not use the voltage-doubling rectifier method?

I've used this on several amps now. As the load needed by the VAS etc is only a few tens of mA relatively small caps only are needed. A linear regulator then can drop the excess volts and provide a highly stable supply to the critical input stages. Only needs four smallish caps and a pair of HV diodes (1N4007's) per side, plus decent regulator tranny per side.

I agree with the comment that separate compartment regular transformers give better isolation than the toroids.

cheers

John

I agree, john, using voltage doublers should work perfectly fine, as the currents drawn by the VAS etc are rather small. Doubling gives you quite a bit more voltage than you need, though, so you'll burn a little power in the down regulators. On the other hand, a 10-20 VA transformer with a pair of isolated secondaries is only a few dollars.

Bob
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Old 26th November 2006, 10:18 PM   #119
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Bob Cordell
.....so you'll burn a little power in the down regulators.......

Bob
The power dissipated may not be insignificant if the first stage of drivers i included...
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Old 27th November 2006, 01:25 PM   #120
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Quote:
Originally posted by Bob Cordell

On the other hand, a 10-20 VA transformer with a pair of isolated secondaries is only a few dollars.
EXACTLY !
I found transformer solution cheaper compared to electrolytic caps for doubling! These should be rather high value.
Adam

P.S.
A question:
Have anyone had any problems with double suplly rails at startup?
I assume small power regulated rails are charging slower at startup, so I've added some diodes so that early stages don't have lower voltage.
I am not sure if this is mandatory. Anyone?
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