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Old 4th December 2012, 04:54 PM   #1
mvorn44 is offline mvorn44  United States
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Default What should I do with my R-Core transformer

I'm a noob so I don't know anything and your help is greatly appreciated. History: I bought a new 55" LCD TV. The sound sucks and it only has one digital output. I have some nice 2.1 PC speakers so I bought a DAC to get analog out to my speakers. It sounds great. The problem with this setup is I have to get up every time I want to change the volume. So I ordered a pre-amp board kit with remote and r-core transformer from china. Here is where I need help. The input power for the board is 10-14 vac or 11-15 vdc. The transformer I received has 9vac and 15vac outputs. Do I... 1) Sell this transformer and get a new r-core to match board. 2) Add a full wave rectifier circuit that will change it to DC and in turn will drop the voltage by 1 to 2 volts. 3) Sell the r-core and get external 12 vdc power supply. 4) Any other suggestions you have. Thanks.
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Old 4th December 2012, 05:18 PM   #2
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See what kind of voltage regulator ( probably it has one) it mounts, if it is a 7809 or 7812
or something else. They need +3V than the output to work properly.
And they can accept till 25-35 V DC at their input ( they have three-pins : input-ground-output)
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Old 4th December 2012, 06:40 PM   #3
mvorn44 is offline mvorn44  United States
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I have a 7805 & 7809. I assume the 05 is to power the lcd and the 09 is for the Sanyo LC7542 pre-amp chip. From the little I've learned, I don't think you can send AC voltage to the voltage regulators or the pre-amp chip so there has to be a rectifier on the board. I will try to locate it and get the part number. Then try to find the data sheet. What are the consequences if i run 15vac to the board instead of the recommended 10-14 vac. Will it just cause the regulators to produce more heat? Here's the board I ordered. 4 Way Remote Preamp Kit Treble Bass Balance Controller | eBay

Last edited by mvorn44; 4th December 2012 at 06:45 PM.
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Old 4th December 2012, 07:05 PM   #4
Mooly is offline Mooly  United Kingdom
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The regulators will produce a bit more heat with a higher voltage. One concern might be if any parts of the circuit are fed from the unregulated supply. 14 to 15 volts shouldn't make a difference but that said if the transformer produces more than 15 when lightly loaded and your mains is a bit high then that 15 could easily become 16 or 17. Best to measure it and be sure without it connected to the unit.

Without seeing the circuit or close up detailed pictures its hard to advise with certainty.
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Old 4th December 2012, 07:06 PM   #5
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You should be OK with the 15V AC attached to the PCB where it says 14V AC.

If the regulators get a little warm you might need to attach small heatsinks to them.
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Old 4th December 2012, 08:05 PM   #6
mvorn44 is offline mvorn44  United States
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@Mooly - I don't think there is any unregulated supply but I'll look closely. I will test the output from the R-Core too.

@KateandDad - I think I'll add heatsinks just to be safe. They don't cost much.

Thanks for your help. Once I test the supply from the R-core and it's close to 15v I'll feel comfortable connecting it to the board. But before I do that, I'm going to check the specs on the rectifier. My understanding is the rectifier will drop the voltage a little and the large cap following it smooths the supply. Once I have power to the board I will test the voltage just before the two regulators. I know I ran across a formula to calculate the amount of heat the regulators will produce. Then I can spec the size of the heatsinks.
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Old 7th December 2012, 03:23 PM   #7
mvorn44 is offline mvorn44  United States
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Tested the transformer and it's 14.9 volts. Got the specs for the rectifier and it has a 1 v drop. I hope to assemble and test the preamp this weekend. I didn't get mating connectors with board though. My lab at work didn't any that would fit and we have a lot of different styles. The connectors are a 2 way and 4 way with a 2.54mm pitch. I might have to buy new ones at Radio Shack.
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Old 7th December 2012, 05:06 PM   #8
Mooly is offline Mooly  United Kingdom
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If you can hold and keep a finger on the metal tab of the regulators then the temperatures fine. I would be happy running these at around 65 C continuously. Thats like a hot radiator.
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Old 8th December 2012, 02:21 PM   #9
AndrewT is offline AndrewT  Scotland
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check both the secondary output voltages with no load on the windings.

The 9Vac and the 15Vac should be different.
14.9Vac is suspiciously low for a no load measurement.
What was the mains voltage when you tested the transformer?
What is the rated input voltage of the transformer?
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Old 16th December 2012, 07:25 PM   #10
mvorn44 is offline mvorn44  United States
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Quote:
14.9Vac is suspiciously low for a no load measurement.
What was the mains voltage when you tested the transformer?
What is the rated input voltage of the transformer?
I had a lab tech at work test it with 110vac.
The transformer has an input rating of 115v and 230v.

I re-tested the transformer at home. My main voltage at my home is 118.5vac.
The 15vac wires output 18.9vac and the 9vac wires output 11.1vac. Does that make more sense?
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