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Old 24th February 2009, 07:12 PM   #1
tojoko is offline tojoko  Netherlands
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Hi there

I have a question, I have a transformer that supplies 2 x 48 volts after rectifying, I need max 2 x 35 volts, is there a simple way to be able to use this transformer (500 VA) for the chipamp, with simple I mean cheap, the transformer are the most expensive part for a chipamp based amp.

thanks for any reply....

tojoko.
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Old 24th February 2009, 10:13 PM   #2
mjf is offline mjf  Austria
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hello.
what power do you need for your chipamp?
perhaps you can use a voltage regulator (with big heatsinks)......
greetings..........
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Old 25th February 2009, 07:37 AM   #3
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Is it a dual secondary or a center-tapped transformer?

With a dual secondary you could connect the secondaries in parallel and create a single supply. Then use the single-supply application from the datasheet.

With a center-tapped transformer you create a single supply by using only one half of the secondary, and waste the other half.

mjf's idea with voltage regulators is viable, but may not be cheap, because you need big heatsinks, regulators with high current rating, additional capacitors and space in the enclosure. A chipamp LM3886 fed off 35 V would push out ~2,5 A per channel into an 8 Ohm load. Your voltage difference would be 13 V. 2,5 A * 13 V = 32,5 W of power dissipation per channel. About as much as the chipamp dissipates, so the heatsink for the regulators would have to be as big as the one for the chipamp.

If you add it all up, you might be better off, buying a new transformer and either selling the other one or keeping it for another project.
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Old 25th February 2009, 09:26 AM   #4
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If your line voltage is 115v cheapest thing to do is add a 2A or
larger 24v "bucking" transformer.
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Old 25th February 2009, 10:26 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
you could add a capacitance multiplier between the (lower value) smoothing caps and the amplifier.

The cap multiplier can be set up to lose any preset voltage across the main pass transistor. The heatsink can be much smaller and if you use a metal cased To3 device you can attach it directly to the sink and run it very hot.

The amp will draw a low average power/current and this is what will determine the average power to be dissipated by the regulator/multiplier.
A 60W amplifier should be operating at an average power no higher than 1W and preferably between 100mW and 500mW average power.

Look at the dissipation graphs and see that the chip needs to get rid of <10W of heat when very low average powers are being generated.
A 0.5C/W sink with two chips on it will probably rise by <15Cdegrees above ambient when running flat out @ <1W of average output power.

This high voltage transformer can only be used with high impedance speakers. Do not try 6ohm, nor 4ohm nor 4 to 8ohm speakers.
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Old 25th February 2009, 04:16 PM   #6
tojoko is offline tojoko  Netherlands
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Hi

Thanks for your reply, I just wanted to mention ,that the thansformer is a 230 volt type(primary) and 2 times 36 volts secundairy, I have big heatsinks and enough (big) capacitors, so I do not have to spend money on that, just need a schematic and a partslist, I am not an electronics-technician, so be gentle with me.......

thanks for any (more detailed) help!

tojoko.
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Old 25th February 2009, 04:43 PM   #7
AndrewT is offline AndrewT  Scotland
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+-48Vdc from a 230:36Vac transformer is very low.
What was your mains voltage when you measured the DC voltage.
What was the load on the DC supply when you took that measurement?

35Vac normally gives over 50Vdc and can rise significantly above that even when supplying bias current to the connected amplifier.
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Old 25th February 2009, 04:46 PM   #8
AndrewT is offline AndrewT  Scotland
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the maximum AC voltage is usually accepted as 25Vac for 8ohm speakers.
If you design carefully and use gentle speakers you can get away with <=28Vac.

36Vac is way above what a reasonable multiplier would normally have to cope with.

Similarly, a fixed voltage regulator will have to dissipate a lot of heat to get back down to +-<=42Vdc.
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Old 25th February 2009, 05:09 PM   #9
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you are right AndrewT. I think some people do not realize that secondaries voltages increases after bridge rectifier.my amp has 22vac secondaries and after bridge rectifier the voltage gets to 32vdc.Good enough for 8ohm speakers.
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Old 25th February 2009, 06:16 PM   #10
tojoko is offline tojoko  Netherlands
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Hi

Thanks again for your reply, I measured 36 volts, normally(i was told) the transformer will give about 5 % more without load, so I guess it will give about 34 volts under load, than 48 volts would be a reasonable guess, I will measure when I get the parts for the rectifier.
I gave in the data in a few supplier-sites for buck down IC's, but I got a bit confused by the results, the input Vmax was not a problem, the 35 volt Vout with 3 ampere was....
Maybe you guys know more and give me a partname/number I can use.

thanks again, tojoko.
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