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Old 14th May 2005, 12:04 AM   #1
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Default +/- 15VDC from +/-75Vdc Is there a better way?

Is there a better, quieter, more elegant way of developing +/- 15VDC from +/-75VDC rails??

An amp i am working on uses this schematic to power the front end from the +/- rails. It works, but they used two 5 watt resistors piggybacked on top of each other without any sort of support to the PC board, or each other. and because this amp see's regular road use, the piggy backed resistors ended up breaking the solder connections causing the amp to fail.

this just seems like a brute force way to do this. and Most V-Regs like a 7815 etc cant handle that much on the input side.

If i calculated the power dissapation correctly, then there is about 5.5 watts going up in heat which was enough to discolor the resistor legs and board around them.

I thought of changing the resistors and zeners to develop +/- 30VDC then using regulators, but i thought there must be a better way to do this using a transistor as the pass device etc.

There is not enough room in the chassis to mount another transformer, but i can put a small PC board vertically inside next to the cap bank board.

Can anyone show me a schematic that i can adapt. and would there really be any benifit to going through the trouble??


Zc
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Old 14th May 2005, 01:06 AM   #2
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if you use a regulator you would need to mount it on a heatsink and as youve said you are limited on room as a small remedy in regard to heat try increasing the power rating of the pass resistors
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Old 14th May 2005, 01:11 AM   #3
anatech is offline anatech  Canada
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Hi Zero Cool,
I hate, crappy cheap supplies. You may gain a lot from using a switching supply off the rails. A 555 timer set for a 30% duty cycle ( or there abouts), followed by some filtering and normal regulators would get you there. Use the 555 to drive transistors on the rails through optocouplers. You can use FETs or BJTs since this is low power.
I'm sure there are more elegant ways to do this, this is just to get you thinking.

-Chris
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Old 14th May 2005, 01:34 AM   #4
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Quote:
Originally posted by mastertech
if you use a regulator you would need to mount it on a heatsink and as youve said you are limited on room as a small remedy in regard to heat try increasing the power rating of the pass resistors
If i remove the resistors and mount a PC board vertically, i could mount a pair of TO-220 devices on heat sinks. there is enough room for that but not for a transformer.


I dont know that i could build a switcher supply and make it quiet enough. i have no experiance building those.

I am going to dig through my old Audio Amatures and see if there is a V-Reg sam in there i could modify or something. if nothign else i will be changing the resistor setup to be more solid and road worthy.


Zc
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Old 14th May 2005, 01:43 AM   #5
jleaman is offline jleaman  Belgium
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What about using the Nelson Pass Zen regulator ?
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Old 14th May 2005, 03:14 AM   #6
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Quote:
Originally posted by jleaman
What about using the Nelson Pass Zen regulator ?

DUH! you know i never thought to look there! But look what i found, a simple easy circuit that looks like it will do the trick!

Now according to the text, the Zeners need to total about 4V higher then the desired output voltage. so, i should be able to just remove one of the 10V zeners per side for a total of 19.1V remaining minus the VGS of the mosfets should yield 15.1V thereabouts.

Now, will i need to change R1 & R2??? and i wonder what wattage those should be. I just so happen to have some of those mosfets on hand, but i think my front end is drawing only 100ma or so, so i may try and find some smaller mosfets to conserve space.

Zc
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Old 14th May 2005, 03:16 AM   #7
jleaman is offline jleaman  Belgium
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I'm thinking about building a few of these regulators for some stuff at home i already have a board layout. : O )
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Old 14th May 2005, 03:26 AM   #8
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Oh yeah, forgot the disclaimer...

The schematic show above is of course the property of Nelson Pass and is shown for reference and Education only etc etc. DONT STEAL THE DAMN THING AND CALL IT YOUR OWN OK!!! hahahahha


Zc
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Old 14th May 2005, 03:33 AM   #9
jleaman is offline jleaman  Belgium
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Quote:
Originally posted by Zero Cool
Oh yeah, forgot the disclaimer...

The schematic show above is of course the property of Nelson Pass and is shown for reference and Education only etc etc. DONT STEAL THE DAMN THING AND CALL IT YOUR OWN OK!!! hahahahha


Zc

Yup im building mine for personal use not To sell : O ) He has great design's..
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Old 14th May 2005, 05:18 AM   #10
Enzo is offline Enzo  United States
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It may be crude but it works. If the trouble is the resistors cracking free, they need support. I often have to resolder them in Crate and Fender products for this reason. A line of silicone will hold them in place and seriously reduce the motion that causes the solder to fail. Or mount larger resistor to the chassis wall in clamps and run a couple wires. More complex circuitry is not always the best approach.
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