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Old 5th October 2005, 06:31 PM   #1
Decker is offline Decker  United States
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Default Help: Need to lose some volts

So I had this great idea to build a chip amp with LM3875s out of the Harmon Kardon Citation 12 chassis that was gathering dust in my garage. The chassis is thick steel and it has separate power transformers for each channel. So far, so good. I added a line filter, a power switch and some new 15k uf caps. But I didn't think ahead far enough. The power supply puts out +/- 42.5 volts. Too much for the LM3875, which has a max PS voltage of +/- 42 volts according to the spec sheet. Can anyone suggest how I can drop the PS voltage a bit, short of regulators? I've never tried a Pi filter in a transistor power amp, or heard of one, so I'm wondering if there is a sonic penalty to this approach. I thought about using a bucking transformer to lower the line voltage in, but there isnt much space on the chassis left after adding the new caps. I could try to squeeze in simple regulators, or capacitance multiplier filters, but if there is a simpler way, I'd love to hear it. Suggestions?
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Old 5th October 2005, 06:55 PM   #2
azira is offline azira  United States
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Do you really need to shave just 1 Volt? How about a diode or two?
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Old 6th October 2005, 02:28 AM   #3
Decker is offline Decker  United States
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No, that would still be too close to the edge for me. Ideally, I would like to get to +/- 30-35 volts.
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Old 6th October 2005, 02:57 AM   #4
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hmm, it would be pretty cool if you built a Citation 12 in that chassis Maybe Nelson's mosfet variant?

Otherwise, you could easily lose a few volts with a CRC and get some sonic benefit IMHO.
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Old 6th October 2005, 04:01 AM   #5
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Default Re: Help: Need to lose some volts

How about an outboard transformer? Companies that sell transformers for using equipment from other countries make all sorts of voltage ratios and they come prewired with power cord and outlet. Specifically, I am thinking of a Japanese/US conversion transformer- 100V on one side 117 V on the other. If you connect the 117 side to the power line and the 100V side to the amp, the PS voltage should drop about 17% (about 7 volts) which puts you right where you want to be. No modifications/chassis space required.

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Old 6th October 2005, 04:54 AM   #6
lgreen is offline lgreen  United States
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Default crc or reg

without adding a 3rd transformer to drop the line voltage to your other 2 transformers, your only options are a CRC power supply or voltage regulators. Remember that the LM3xxx regs just need less than 45 V input to ouput differential so should be well within their operating permaters with your transformers. Or you could do a CRC supply and that would be fine.

As a bonus, both techniques will give you a better amp.
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Old 6th October 2005, 05:31 AM   #7
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You could make a special transistor-capacitor filter.

Use diodes to drop the volts, and a transistor attached to a heatsink to carry the load. The capacitors and resistors provide RC filtering network to reduce ripple.

Now for two rails +/- you would have to make two of these, but one would have opposite polarity and use PNP instead.
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Old 6th October 2005, 06:17 AM   #8
sss is offline sss  Israel
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all those solutions will dissipate power and heat for no purpose .
the best u can do is
1.change the transformer
2.reduse the secondary winding turns of the original transformar ,should be very easy/simple to do ,depending on the type of your transformer.
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Old 6th October 2005, 06:43 AM   #9
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Decker, I wouldn't worry to much if you have +-42.5 V unloaded.
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Old 6th October 2005, 08:55 AM   #10
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Quote:
Originally posted by peranders
Decker, I wouldn't worry to much if you have +-42.5 V unloaded
There are a number of people running 3875s that high or even a bit more, and they still have their smoke inside.

But me, i'd at least do a CRC -- as much to get a better suppl as opposed to dropping V. A CLC or CLCRC would be even better -- if you can figure out where to stuff the parts. It's been over 30 years since i had my Citation 12 so i'm a bit vague on the chassis details.

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