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?
Maybe Nelson's mosfet variant?
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.
I_F
I_F
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.
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.
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.
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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peranders said:
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.
dave
Hi Decker
I've been running lm3875s at 39.5 v for months now with no probs.. My ps had no load v of 41 or 42 and I just put a resister on each leg to draw a few ma and bring the voltage down to a safer level. Not the most efficeint method but works.
When I get back to it I'm going to try a CLC cct.
Hugh
I've been running lm3875s at 39.5 v for months now with no probs.. My ps had no load v of 41 or 42 and I just put a resister on each leg to draw a few ma and bring the voltage down to a safer level. Not the most efficeint method but works.
When I get back to it I'm going to try a CLC cct.
Hugh
Running the part at or beyond its rating might work but it will shorten its life.
How about adding, say, two bridge rectifiers? Each one will drop the voltage by about 1.4V. Much cheaper than the other solutions, and doesn't take much space. Adding a significant load (like a big power resistor) will help too.
How about adding, say, two bridge rectifiers? Each one will drop the voltage by about 1.4V. Much cheaper than the other solutions, and doesn't take much space. Adding a significant load (like a big power resistor) will help too.
Indeed printer supplies are real keepers when you do DIY.
I use a HP-printer-supply for my PC-headphone amp ~30VDC at 2A.
How I hate to trade parts I´ve got, rather than just use them...
Usually you lose a buck here and there so...
Will probably go the "diode-route" or try a NTC in series with the primary and see what that does...
greets
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