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Old 15th August 2004, 09:41 PM   #1
kinser is offline kinser  Israel
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Default from 56vdc +/- to 12vdc +/-

Hi all
I am planning to build rod elliottws 300 watt subwoofer amp my problom: that amp needs 56vdc +/- and for the filter I need 12vdc +/-.!
i cant use any of the lm78xx/79xx regulators
please help me
p.s. im building it as a plate amp
Thanks
Kinser
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Old 15th August 2004, 10:11 PM   #2
kinser is offline kinser  Israel
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anyone.....?
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Old 15th August 2004, 11:03 PM   #3
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Default I will do that now.... in one our you will have it Kinser.

Better to use one other small transformer.... and to join the ground pins with your bigger one... this is the best way....because the reduction voltage circuit, will hold 44 volts difference.... will be hot even with small current drain passing trougth... if some damage happens.... your 12 volts circuit will explode!

But, i will do some for you, this way, having both conditions, you will decide the best to your needs....one 15 volts transformer with those positive and negative regulators you already know, or the 56 volts going into one poor devil transistor regulating.

Carlos
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Old 15th August 2004, 11:52 PM   #4
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Default Hehe...people like to help...but when good help is done.

Your problem are half way resolved... you already think in LM IC three pin regulators... you already realise that cannot use with 56 Volts, because too big, IC will explode!

This way, our friends, normally stay out of this subject, because they now you are near the "discovering"... and this is great to be done by ourselves!

I will suggest you, when you need fast help... ask a very stupid question.... tell you will put a lamp to reduce voltage, or something alike..... they will run to help you, avoiding you to burn your circuit!

hehe....brazilian trick!!!.....it works!

regards,

Carlos
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Old 15th August 2004, 11:59 PM   #5
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Default KInsen, this circuit is dangerous... if you short circuit the output...

Transistor will be burned, because current will be bigger than the fuse in one microsecond!..... this way, can go to 5 amps in short period of time, while waiting fuse melt and blow...it takes some time.... small time....but will delay sometime.... you can exceed transistor capacity and Boooooom!.

This way 56 Volts will enter the low voltage circuit.... hehe, i think this is funny, but i am sure you will dislike!

Now, you will have more people here to help you...because Destroyer can "destroy", your amplifier with his destructive idea!

Use a small transformer....15 plus 15 AC, or less voltage.... and use small bridge rectifier and 2200uF condenser.... and LM IC regulators..... 7812 to positive and 7912 to negative....if burns...no problem to your circuit....do not forget to join the earth point with the amplifier's earth point.... the called "star" ground!

"Star" ground is a pretty name for a Ugly conection.... every earth wire same place.... aaaaagh!... ugly!

But works great!

regards,

Carlos
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Old 16th August 2004, 12:05 AM   #6
anatech is offline anatech  Canada
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Hi kinser,
There will be some heat to get rid of. Use a simple zener / transistor regulator circuit to get most of the way there and divide the heat dissipation. You can add a dropping resistor before the transistor if needed. Follow this with a capacitor and 7812 / 7912 regulators. You can put a 15V 5W zener to ground from each regulator output for protection if you wish. Don't forget to figure out how much heat you need to get rid of! And don't under estimate the ability of a couple watts to really heat things up.
-Chris
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Old 16th August 2004, 12:38 AM   #7
Eva is offline Eva  Spain
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The usual approach employed in consumer equipment to get op-amp rails from power-amp rails when several op-amps have to be powered is to use some cheap series pass transistors [ie: TIP41/42] or darlingtons [ie: TIP122/127] configured as emitter followers [ie: bias resistor from base to 56V, collector to 56V, 15V zener and 100u capacitor from base to ground, 100u capacitor from emitter to ground and you get about 14V with decent regulation at the emitter]

Current capability is limited by the heat sink choice and by the SOA of the devices. For 56V in to 14V out we get about 42*Iout Watts of dissipation. For a dozen of typical dual op-amps at 5mA/device dissipation would be 42*12*.005 = 2.5W, so a heatsink providing 20K/W or less would do the job [the case and the main heatsink may be useful too]

This makes more sense than using an extra transformer since regulators and [smaller] heatsinks would be also required
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Old 16th August 2004, 02:03 AM   #8
jaycee is offline jaycee  United Kingdom
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A good way is to use an amplified zener, as other posters have recommended, to drop the voltage to say 20V, then use regular 3 pin regulators to regulate to whatever voltage you need. This spreads dissipation between 2 devices while keeping the lower noise output of the 3 pin regulator. Zener diodes create noise on the supply unless you go to a lot of effort to smooth it out.

I belive Rod has a power supply project showing a zener follower preregulator on his page for precisely this purpose.
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Old 16th August 2004, 02:10 AM   #9
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Default hahaha as you see my dear shalom friend

Nothing as one not so good idea to call people.

That's good, when you discover all the place tricks.

Carlos
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Old 16th August 2004, 02:15 AM   #10
palesha is offline palesha  India
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The circuit can be devided into two parts. First use emmiter follower with 24w zener diode. use this as input & use 78XX or 79xx circuit. this will be the safest. Heat will also be devided.
It is not good to only rely upon zener diode emmmiter follower circuit. Hope this works.
Mahendra Palesha
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