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Old 1st August 2010, 01:48 PM   #1
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Default Why voltage dropped in my new transformer?

My phono amp is using 2 Plitron transformers for independant L & R supply. The consumption is 300mA on each rail. The original spec as printed on the transformer is 44V 1.5A on each supply and therefore I have ordered my new transformer (using OCC silver plated wire as secondary winding) at 44V 1A. With loading, the original Plitron is 46.5V but the new transformer is only 45.8V.

The sonic difference between the 2 transformers is very significant. The new transformer has very musical high, more details but the bass is not as punchy as the original Plitron; although not very much.

I can return my new new transformers to the manufacturer and increase the secondary voltage to 46V that I believe it will produce a higher AC voltage with loading. I have the following questions: -

1. Why a loading of 300mA can reduce the voltage between these 2 transformers, bearing in mind the new transformer is already at 1A per rail?
2. The transformer manufacturer can either rewind the entire secondary of the transformer or solder a new wire and add more rounds but the price difference is high. Will the cheaper second method reduce the performance of the transformer?
3. The manufacturer said I can send my original Plitrons to him and he will rewind it with OCC silver plated wires that he said it will even be better. Could this be true?

Last edited by Sunsun22; 1st August 2010 at 01:52 PM.
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Old 1st August 2010, 02:04 PM   #2
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Hi SunSun, the nominal voltage of the transformer is at full load, as the two transformers have different VA ratings (the first is 66VA and the second is 44VA) they almost certainly also have different regulation (the regulation figure will tell you what the max voltage will be above the nominal voltage, assuming no load). Even with regulation put asside, at 300ma on the first transformer you are only using 20% of the available current for the transformer therefore the voltage drop will probably be less than for the second one where you are drawingt 30% of the available current. If you have the regulation figures for each transformer you should be able to calculate it

I will pass on attempting to answer 2 and 3

Tony.
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Old 1st August 2010, 03:25 PM   #3
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Thanks Wintermute. I have another thinking -

The primary of the new transformer is 220V. I have an isolation transformer that is 220V primary but selectable secondary at 220/230/240V. If I choose 230V output; which is about 4.5% more. I think this will give a boost of secondary AC output by 2V. Could this hurt the primary windings? Is this an option?
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Old 1st August 2010, 07:23 PM   #4
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Quote:
Originally Posted by Sunsun22 View Post
Thanks Wintermute. I have another thinking -

The primary of the new transformer is 220V. I have an isolation transformer that is 220V primary but selectable secondary at 220/230/240V. If I choose 230V output; which is about 4.5% more. I think this will give a boost of secondary AC output by 2V. Could this hurt the primary windings? Is this an option?
It will work but I wonder if it is worth the effort and worry and money. A difference of 0.7V at around 45V is 1.5%. That 1.5% translates to a difference in max output level before clipping of your amp of 0.13dB.
I would be very happy the way it is!

jd
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Old 2nd August 2010, 04:19 AM   #5
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I think I have to give a full picture on my application.

The AC output from transformer will go through a LC filter then to a shunt regulator for 24V regulated output. The shunt regulator; as stated in the manual, needs 5V difference to work properly.

In the original setup, the 47.5V AC output produces 28.8V DC after LC filter which is already very marginal for the required voltage drop. It will then go through the shunt regulator for 24V DC output. Using the new transformer only produces 45.8V AC reducing the voltage before shunt to 26.7V - that is only 2.7V voltage difference left behind.

The new OCC silver plated wire transformer produces very good highs and details but the bass is not as good as the original transformer. I "think" it will be better if I can lift the input voltage to shunt regulator to 29V.

1. Is my "think" valid?
2. I have another though - place a resistor in parallel with the choke to reduce the internal resistance therefore giving a higher voltage before regulation. Is this another option?
3. Rewinding the secondary by soldering a new section of wire to the existing wire only costs me a small amount of money but would this reduce the performance of the transformer? Of course, another option is to rewind the secondary totally but the cost is about 2/3 of the original cost.

Thanks
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Old 2nd August 2010, 05:16 AM   #6
infinia is offline infinia  United States
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Can you post your schematic and any measurements. something does not compute.
47.5 V ( 1/2 ?) *1.41 ~ 33.5 Vdc (or 32 Vdc on the new XFMR) ! Consider using a CLC instead of a LC.

Is it possible for you to lower the regulator output voltage a bit to acheive the optimum headroom and keep power loss lower?
You speak about silver wire on a PS transformer as if the music it self travels through this basic component?
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Last edited by infinia; 2nd August 2010 at 05:33 AM.
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Old 2nd August 2010, 05:54 AM   #7
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yes please post the schematic, and also the DCR of your colis! I'd expect somewhere in the vicinity of 63V (no load) rectified from a 45V ac source with nothing other than caps after the rectifier, so to be getting only 28.8V, over 30V is disappearing somewhere, or the setup is not as I am envisaging

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Old 2nd August 2010, 06:33 AM   #8
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I venture you are using a center tapped secondary and the Rdc and L is preventing the peak voltage from devolping across the C. So adding a C before the L ie CLC will see the output voltage increasingly.
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Old 2nd August 2010, 11:28 AM   #9
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Yes, I also use Duncan Amps Tools and do some simulation by changing the filter from LC to CLC and it will give much higher voltage. The original circuit uses LC filter therefore I just follow it. What are the differences between LC and CLC other than the higher DC voltage output?
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Old 2nd August 2010, 11:44 AM   #10
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rC, rCRC, rCLC all operate as a capacitor input filter. On zero load current, the capacitors charge to the peak of the sine waveform less some rectifier diode drop.

An LC or LCLC or LCRC all operate as a choke regulated supply.
The upper and lower limits of current are defined by component value choices.
If the current goes outside the range for choke regulated then it starts operating as a capacitor input filter. In this low current condition the output voltage rises from ~0.9*Vac to ~1.4*Vac.
LC supplies cannot be used for circuits that demand a big change in current.
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