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Old 2nd July 2007, 01:31 PM   #11
Did it Himself
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Yes but the limiting factor will now become the wire gauge, which will be too thin to support the additional current to maintain the original VA rating at the new lower voltage.
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Old 2nd July 2007, 03:39 PM   #12
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Quote:
Originally posted by richie00boy
Yes but the limiting factor will now become the wire gauge, which will be too thin to support the additional current to maintain the original VA rating at the new lower voltage.
Reducing the voltage by 15% you will still fall within the current carrying capacity of the #20 wire used for this VA (225) rating.

You can also increase the number of turns in the primary
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Old 2nd July 2007, 04:00 PM   #13
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OK so it requires some research on the wire gauge used, most I would think are run close to the limit though.

As for the primary, by adding turns you will reduce the efficiency/regulation of the transformer. It will usually also require the whole secondary to be removed in order to get at the primary.

Maybe some people have too much time on their hands or are gluttons for punishment
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Old 2nd July 2007, 04:36 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jackinnj
Reducing the voltage by 15% you will still fall within the current carrying capacity of the #20 wire used for this VA (225) rating.
let's take your example.
Reduce the voltage by 15% and increase the current by 17.65% to bring the VA back to exactly where we were before we removed any of the secondary windings.

We all know that Power =I^2 * R.
The internal resistance of the secondary winding has been reduced by 15% due to the shorter winding.
The power loss (=heat) is now 1.176^2 * 0.85 = 1.1765 times higher than it was before we modified this transformer.
The voltage loss at this higher current is also higher. Now, starting from the lowered secondary voltage and with a higher secondary loss, the resulting regulation is worse than the standard transformer. But it gets worse, the higher running temperature increases the resistance of both the primary and secondary windings. This results in more I squared R losses and even poorer regulation and yet higher operating temperature.

In the old days when copper was relatively cheaper and thicker windings were used we could get away with a little removal, but now that the copper content is skimped by the manufacturers, any copper removal will cause deterioration in performance.
Conversely, adding more copper to the windings can improve performance. This can be by increasing the number of turns (more voltage) or increasing the number of windings (multi secondaries), or substituting thicker wire (for lower I squared R losses).
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Old 2nd July 2007, 06:49 PM   #15
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If you have a micrometer you can calculate the gauge or diameter to determine the current carrying capacity of the wire. my experience with Amveco transformers (they use 18ga wire for their 140VA transformers) is that you could easily reduce the Ns by this amount and have sufficient margin of error. The difference in peak currents is about 0.5 amp.

At wholesale, the cost of 18ga vs 20 ga is somewhat less than the differrence in circular area -- but the cost in service of a transformer using 18ga is much lower -- even in a low current application the energy savings is significant...the cost of Cu is a cost factor, but energy efficiency is a selling point.

My suggestion -- measure the guage of the wire -- if it's #20 AWG or lower you're going to be OK.

and one more thing -- don't assume that the impedance of your loudspeakers is actually 8 ohms -- with 6 or 4 ohms the overture chip is going to cook itself to thermal shutoff if you have +/- 40VDC on the rails.
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Old 3rd July 2007, 06:22 AM   #16
Archwn is offline Archwn  United Kingdom
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Hi Andrew.

My PS has a center tapped transformer and it works find with CFM powersupply.

I just connect both AC to the bridge then I get +V and -V out of the bridge. these run pararelly with the center tap in the middle and you could put anything in between these to filter them.

AK
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Old 3rd July 2007, 06:46 AM   #17
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Quote:
Originally posted by AndrewT

let's take your example.
Reduce the voltage by 15% and increase the current by 17.65% to bring the VA back to exactly where we were before we removed any of the secondary windings.

We all know that Power =I^2 * R.
The internal resistance of the secondary winding has been reduced by 15% due to the shorter winding.
The power loss (=heat) is now 1.176^2 * 0.85 = 1.1765 times higher than it was before we modified this transformer.
The voltage loss at this higher current is also higher. Now, starting from the lowered secondary voltage and with a higher secondary loss, the resulting regulation is worse than the standard transformer. But it gets worse, the higher running temperature increases the resistance of both the primary and secondary windings. This results in more I squared R losses and even poorer regulation and yet higher operating temperature.

In the old days when copper was relatively cheaper and thicker windings were used we could get away with a little removal, but now that the copper content is skimped by the manufacturers, any copper removal will cause deterioration in performance.
Conversely, adding more copper to the windings can improve performance. This can be by increasing the number of turns (more voltage) or increasing the number of windings (multi secondaries), or substituting thicker wire (for lower I squared R losses).

Hi Andrew,

I think the VA thing is much less an issue. The transformer may be spec'd at 225VA, but in amp service it will almost never have to supply that power. In normal use the power draw may be just a few 10's of watt average.

One other issue here is the supply capacitor. Extreme capacitances (> 50.000uF for example) will cause extremely sharp and high charging pulses which may heat the wiring of the transformer and cause contraction & expansion noise. But even then, the average power will not raise the average temp.

Jan Didden
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Old 3rd July 2007, 07:44 AM   #18
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Archwn
My PS has a center tapped transformer and it works find with CFM powersupply.

I just connect both AC to the bridge then I get +V and -V out of the bridge. these run pararelly with the center tap in the middle and you could put anything in between these to filter them.
Hi,
what is CFM? From your description it appearrs you are using an unregulated capacitor input PSU.

My comment was in reply to an enquiry about a regulated PSU using twin positive regulators for a dual polarity supply.
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Old 3rd July 2007, 07:51 AM   #19
AndrewT is offline AndrewT  Scotland
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Hi Janneman,
I would be tempted to agree with your overview, but you have overlooked why I replied as I did.
Quote:
Don't forget that you can unwind a few turns of the transformer to attain lower voltage.
Quote:
Isn't the current a function of the BH character of the core
Quote:
Reducing the voltage by 15% you will still fall within the current carrying capacity of the #20 wire used for this VA (225) rating.
and I said and still stand by
Quote:
but that reduces the VA rating in direct proportion to the voltage reduction.
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Old 3rd July 2007, 08:38 AM   #20
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I bought up this issue a while a go. It was quite a heated debate from what I remember!


If you're using just one of these PSUs (e.g a total of 2xLM338) to power both channels of an amp, then you can use a single dual secondary transformer. You may or may not run in to the current limit of the LM338s.


If you want two separate PSUs (e.g a total of 4xLM338), one for each channel, then it "works" with one transformer, in that it didn't blow up and it played music, but it was not working correctly, and you will only see this if you have one of the PSU's negative rails adjusted to put out a lower voltage to the other PSU.

A theoretical example of when you might see this happen is trying to run, say, a 2xLM338 PSU for a 15v preamp, and then another 2xLM338 PSU, running both channels of a power amp at 28v on the same transformer. You will find that the preamp supply ends up being +15v as expected, but the negative rail is -28v.


If you want one of these PSUs per channel then, in my experience - and I am still utterly convinced that I was doing nothing wrong - you will need two separate, dual secondary transformers.
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