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Old 10th June 2004, 02:24 PM   #1
benmanf is offline benmanf  Israel
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Cool HEHE...My amplifier is almost ready...

I almost finished building my p3a...

now i need a power supply.

25V 300VA costs about 80$ (380 israeli Shekels)..

so this is too much for me.

i have a 33V transformer but with only to wires coming out...

i changed it with a small circuit with 2 diodes + 2 4700uF caps...

but now i have 45 VDC (+45_0_-45) and i need 35 VDC...

what can i do to change it?
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Old 10th June 2004, 02:39 PM   #2
jaycee is offline jaycee  United Kingdom
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45V should be ok, only slightly over spec.. just dont run it into a load lower than 8 ohms !
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Old 10th June 2004, 02:44 PM   #3
benmanf is offline benmanf  Israel
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well...that is the problem...

im going to run it under 4 ohms...
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Old 10th June 2004, 02:52 PM   #4
benmanf is offline benmanf  Israel
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maybe ill put a resistor?
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Old 10th June 2004, 03:10 PM   #5
jaycee is offline jaycee  United Kingdom
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Add an extra pair of output transistors. If you are using the OnSemi transistors MJL4281/4302 they should cope OK. You may want to change the bias transistor to eg MJE340 though.
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Old 24th June 2004, 11:58 PM   #6
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Default Re: HEHE...My amplifier is almost ready...

Quote:
Originally posted by benmanf
I almost finished building my p3a...

now i need a power supply.

25V 300VA costs about 80$ (380 israeli Shekels)..

so this is too much for me.

i have a 33V transformer but with only to wires coming out...

i changed it with a small circuit with 2 diodes + 2 4700uF caps...

but now i have 45 VDC (+45_0_-45) and i need 35 VDC...

what can i do to change it?

can't really change voltage, just be sure your bias is not affected by the overvoltage. you could also build a powerFET feedback regulator and reference it to 35V for both POS and NEG sides(more complicated). Otherwise your circuit will produce more heat.
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Old 25th June 2004, 08:55 AM   #7
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Default Shalom friend with P3A

Good amplifier.

I have a supply, that will make you free of voltage problems from now to eternity.

Can receive high voltage in the input... around max 75 volts... output regulated, and stabilized....voltage start zener voltage more a couple or volts... this case 17 because of zener.... you can reduce zener.... or increase a little if you want.... once adjusted.... can put fire on the P3A.... and voltage will stay the way you adjusted.

I am using 30 years this schematic.... made first by Ampex, to Video Tape Professional Recorders.... and them Sony used.... and them milions used it.


Expensive.... can be or not.... i like to make short circuits in the output, without fuse or protection... this way i put DC voltage in short and i measure maximum current...... if, lets say, 20 ampéres.... them i put 4 times this capacity in output transistors.

Your case on transistor each side will be enough with fuses to protect supply... if you put, lets see, 6 transistors positive side and six transistor negative size.... you can go kidding melting coins in your house or making small electrical soldering jobs.

If not fused.... if amplifiers damage, and no fuses..... amplifier will melt, speaker also, speaker wire also, and supply output wire will be very very hot!

When finished the destruction..... go to measure supply.... OK!...in order!.... you will spend money once.... it will survive for decades, and will be good to a lot of amplifiers...just turn trimpot and adjust plus and minus voltage you want. Maximum stable voltage is aroud 5 volts less the maximum voltage.... regulation stays around 5 percent.

Carlos
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Old 25th June 2004, 08:59 AM   #8
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Default Correcting!

Voltage will reduce 5 percent related your adjustment.
But this will happen from minimum adjusted voltage to the rectifier voltage minus 5 volts.
This reduction is when amplifier sucks great current.... voltage variations because the load... stable.... but no perfect...loose 5 percent.

Carlos
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Old 25th June 2004, 04:26 PM   #9
azira is offline azira  United States
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Default Re: HEHE...My amplifier is almost ready...

Quote:
Originally posted by benmanf
I almost finished building my p3a...

now i need a power supply.

25V 300VA costs about 80$ (380 israeli Shekels)..

so this is too much for me.

i have a 33V transformer but with only to wires coming out...

i changed it with a small circuit with 2 diodes + 2 4700uF caps...

but now i have 45 VDC (+45_0_-45) and i need 35 VDC...

what can i do to change it?
Sorry if it's a dumb question.
How did you get +45 _ 0 _ -45 from a transformer that only has 2 wires coming out?
--
Danny
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Old 25th June 2004, 04:56 PM   #10
joensd is offline joensd  Germany
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Quote:
Sorry if it's a dumb question.
How did you get +45 _ 0 _ -45 from a transformer that only has 2 wires coming out?
--
Danny
Have a look here.
IMO it should work with a gainclone for example without any noise but as a few people stated it´s not recommended I also stay away from further recommending.


Jens
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