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Old 5th September 2012, 04:35 PM   #1
tsiros is offline tsiros  Greece
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Default use symmetrical transformer for single supply amp?

I have a pair of mono amplifiers... kinda silly but oh well i didn't know much back then and i just wanted to assemble a simple circuit.

for now i am feeding them about 40V(dc) from one of the two rails of a 28-0-28(ac) transformer.

I have no idea if it is a good idea or not, but it's been working like this for months. Yeah the transformer gets hot, but you can keep your finger on it. For five seconds

Can i somehow make it use both rails but still output 40Vdc (28Vac)? I still have a leftover rectifier from the assembly of my other amplifier.

(also, why does the LM4780 need a decoupling capacitor at the speaker output? I thought symmetrically powered amps like this don't need it? Since the output sits at ground when idle? It's a 22μF ... dunno if it affects the signal any )
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Old 5th September 2012, 04:55 PM   #2
DF96 is offline DF96  England
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If the transformer is really 28-0-28 then you can't change what you have. If it is really 0-28 plus 0-28 then you can parallel the secondaries instead of having them in series.
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Old 5th September 2012, 05:12 PM   #3
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If it is a centre tap transformer you can full wave rectify each side to be positive and then parallel the rectified DC voltage.

The center tap will remain 0V, and two +V that are paralleled would be capable of twice the current. What you say DF, think it is practical solution?
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Old 5th September 2012, 05:22 PM   #4
DF96 is offline DF96  England
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Assuming that he currently has the usual bridge rectifier, then he is already using both sides of the winding. You can't add two independent bridges to one CT winding. Draw it out and you will see.
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Old 5th September 2012, 05:29 PM   #5
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Quote:
Originally Posted by DF96 View Post
Assuming that he currently has the usual bridge rectifier, then he is already using both sides of the winding. You can't add two independent bridges to one CT winding. Draw it out and you will see.
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Old 5th September 2012, 05:45 PM   #6
tsiros is offline tsiros  Greece
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This is it

Click the image to open in full size.



Click the image to open in full size.

can i measure it somehow? without an oscilloscope. i touched the YG and B and it went FZZ and sparked all over o_O
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Old 5th September 2012, 05:57 PM   #7
AndrewT is offline AndrewT  Scotland
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Old 5th September 2012, 06:02 PM   #8
DF96 is offline DF96  England
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The label appears to say 2x28V, so you may have two secondaries but the wires have been cut short and joined to form the CT. I assume the mains input goes to the bottom tags, and secondary at the top. If so, not a good idea to use mains cable for the secondary as this could be confusing.

You need to separate the two wires going to the YG tag. Use a resistance check to see which connects to which side of the secondary. The one which is the other end of blue should go to brown. The one which is the other end of brown should go to blue. Then the two secondaries are wired in parallel, hopefully the right way round.

If at all possible, use a lamp limiter when you try it. Borrow or make one.
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Old 5th September 2012, 06:11 PM   #9
tsiros is offline tsiros  Greece
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andrew: i've already made a lamp "limiter"

this psu, which is for the lm4780 works as it should. the bulb flashes very briefly (caps charging)
Click the image to open in full size.

(pay no attention to the stupidity that i did, using singlestrand cable with crimps . But it has fusebox, certified switch and all that it should have))

df: yes, I shall try your suggestion, with the lamp limiter, of course. And put better plugs. Think it will work?
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Old 5th September 2012, 06:29 PM   #10
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I would be tempted to separate the two joined wires thus making two separate windings.
Provided the wire insulation is intact.
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