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Old 4th December 2012, 04:56 PM   #11
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
to be able to stack (add) the voltages you will need to get inside, as Gmp described
There is an alternative.
Use the 15.9+15.9 as one winding. = >32Vac when open circuit.
Use the 12+12+7as the other winding. =>32Vac when open circuit.
But your output current is limited to 750mAac by the 7Vac winding rating.

A third alternative is to add a 2A rated winding of ~7Vac to make a 31.8Vac + 31.8Vac @ 2A output. That is a total VA of 63.
A lot of work for 63VA !
In addition, the lack of accurate resistance/voltage balance will generate significant amounts of 50/60Hz ripple
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Old 4th December 2012, 07:39 PM   #12
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Quote:
Originally Posted by AndrewT View Post
to be able to stack (add) the voltages you will need to get inside, as Gmp described
There is an alternative.
Use the 15.9+15.9 as one winding. = >32Vac when open circuit.
Use the 12+12+7as the other winding. =>32Vac when open circuit.
But your output current is limited to 750mAac by the 7Vac winding rating.

A third alternative is to add a 2A rated winding of ~7Vac to make a 31.8Vac + 31.8Vac @ 2A output. That is a total VA of 63.
A lot of work for 63VA !
I was trying to recycle this transformer for use in a LM4780 amplifier. It's my first build and I'm on a limited budget. I guess this is not gonna work for me. I'll need a higher VA than this will provide. From what I've read they like to see about 300, right?
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Old 5th December 2012, 03:41 PM   #13
AndrewT is online now AndrewT  Scotland
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I have suggested many times my preference for a minimum of 160VA for a power amplifier transformer.

The general guidance spread around this Forum and elsewhere is to use a VA rating between one times and two times the total maximum power of the amplifier/s.

eg.
2 channels of 50W amplifier is 50+50 = 100W of total maximum output power.
Use a transformer with a rating from 100VA to 200VA.

There is a third recommendation to buy and fit the biggest that you can afford.
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Old 29th January 2013, 04:11 PM   #14
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Rather than start a new thread about this transformer, I'll update this one.
I finally got a transformer for my project that fits my specs.

http://www.antekinc.com/details.php?p=80

This transformer has dual 32v secondaries
http://www.antekinc.com/pdf/AN-4232.pdf

The PCB that I used for my amp has only 3 connection points for the inputs to the rectifiers

Click the image to open in full size.
I have measured the voltage across the blue and green wires and found that either blue to either green gives me around 32V. Should I cap one of the green wires and use the 2 blue and the remaining green for this connection, or should the 2 green be used together? Or probably there's a better way that I haven't thought of. I plan on using the connections on ESP for wiring the mains(120) minus his bridge rectifier and caps of course.

Power Supply Wiring Guidelines

Here is the schematic for the board
Click the image to open in full size.
Thanks for your help.
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Old 30th January 2013, 03:48 AM   #15
DUG is offline DUG  Canada
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With dual secondaries you should hook them up as in the bottom of the jpg.

The red box shows where the three connections are on the pcb.

The two windings should be connected to give twice each winding when measured.

Example:

A-B 32VAC
B-C 32VAC
A-C 64VAC

Something I noticed:

Looking at the data sheet for the LM4780 suggests a maximum operating supply of 84V or +/- 42V.

A 2 x 32VAC used in this configuration will provide more than 88V or +/- 44V.

( the 94V or +/- 47V maximum rating is for no signal conditions)

None of the graphs in the data sheet go above +/- 40V

(The circuit board is labeled for 28VAC)

Perhaps someone more experienced with these Overture series amps can tell you if this is OK to do.

Hope that helps.
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Last edited by DUG; 30th January 2013 at 03:50 AM.
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Old 30th January 2013, 09:00 AM   #16
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by DUG View Post
None of the graphs in the data sheet go above +/- 40V

(The circuit board is labeled for 28VAC)
The Circuit board is marked 28V, but that should read 28Vdc.
IT IS NOT for 28Vac.

Check the traces and determine which is -28Vdc and which is +28Vdc.
The capacitors have the -ve marked with the white bar.
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Last edited by AndrewT; 30th January 2013 at 09:02 AM.
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Old 30th January 2013, 10:50 AM   #17
DUG is offline DUG  Canada
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Quote:
Originally Posted by AndrewT View Post
The Circuit board is marked 28V, but that should read 28Vdc.
IT IS NOT for 28Vac.

Check the traces and determine which is -28Vdc and which is +28Vdc.
The capacitors have the -ve marked with the white bar.
You are correct, my error.

It is marked 28V.

But it could be 28VAC and still operate within spec limits.

It does have a bridge rectifier on board.

If you applied +/- 28VDC it would not matter which of the (non-ground) terminals the + and - were connected to...the diodes would direct the supplies to the required polarity.

I just think 32VAC is a bit too much.

Or is that just me?

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Old 30th January 2013, 08:17 PM   #18
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Can I use this transformer if I run it through 2 FWB rectifiers and voltage regulators? Maybe something similar to this?

LD1585CV - STMICROELECTRONICS - IC, REG LDO, 1.25 TO 28V, 5A, | Farnell United Kingdom

I originally thought that since there were diodes and filter caps on the board that this was the rectifier portion of the circuit. Is that not the case?

Thanks again
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Old 31st January 2013, 02:22 AM   #19
DUG is offline DUG  Canada
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Quote:
Originally Posted by hondatech739 View Post
Can I use this transformer if I run it through 2 FWB rectifiers and voltage regulators? Maybe something similar to this?

LD1585CV - STMICROELECTRONICS - IC, REG LDO, 1.25 TO 28V, 5A, | Farnell United Kingdom

I originally thought that since there were diodes and filter caps on the board that this was the rectifier portion of the circuit. Is that not the case?

Thanks again
The rectifiers and filter caps are on the board in post #14.

You could build two separate (isolated) regulated supplies as in the jpg.
(don't use LM78xx regulators as the current rating is not high enough.)
I would recommend at least a 10A cct

or

You could rectify and filter before the board and drop the voltage with a couple of power MOSFET's...That way you could still use the transformer you have.

Select high current N-ch with standard gate threshold.

Or wind some extra turns on the transformer and reduce the secondary voltage.

This was covered in another thread.

Or use another transformer.

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File Type: jpg FULL-WAVE-D.JPG (28.5 KB, 21 views)
File Type: jpg PS-DUAL--SPLIT-b-reg.JPG (27.9 KB, 21 views)
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Old 31st January 2013, 03:12 AM   #20
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So, I could run the transformer through a couple of rectifier/filters and regulators, and still run it through the rectifier/filter set that is already on the board? Isn't that kind of redundant? I mean, I haven't done this before, so I don't know. Would I lose anything running the rails through 2 rectifiers?

My board is completely assembled already, I just used that pic for reference since it was easier to see everything.

I had actually toyed with the idea of just using external rectifiers/regulators and connecting to the points that the filter caps connect to.

As I say you guys know way more than I.
Thanks for keeping in touch.
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