±50V DC to 12-15V DC Step Down for PreAmp

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slingy

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Joined 2012
#1
I'm repurposing some gear I've acquired over the years to build/rebuild a subwoofer plate amp.
The main power source is approximately ±37v AC from a Toroidal transformer, which should give me around ±50v DC after rectification (I think).
I need to bring this down to a single ± DC output between 9v and 15v for the pre-amp/LPF.
What decent low cost options do I have, other than a secondary power supply or un/rewinding lower voltage taps into the transformer? I can't find a single DC output buck converter that can handle ±50V input, so I'm hoping someone here might be able to point me in the right direction or suggest an alternative..
 
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#2
If you just need +15V then there is no need to touch the -50VDC supply. Just drop the +50V with a resistor+Zener or a ~7815/317. 50V is a stretch for most regulators but there are HV versions. You need to know the preamp current. If it's 10mA then the Zener current could be 5mA, so a 35V, 15mA resistor is about 2k2 and 1 Watt. A switching regulator is not worth the trouble.
 
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#4
Cheers. I was looking at this thread earlier and wondering if a similar approach would work.

I'm thinking maybe a 2N3055 based regulator with zener to bring the input down to around 20v to feed into a normal 7815.

Hoping to use an existing 4558D based amp or build something around a couple of 5532.
And I've just realised my original question wasn't correct, I would need dual 12v or 15v.
 
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#6
How much current does your preamp need? There are 3 w 1n5930 and 5 w 1n5352 zeners. Peavey tends to use 1.3 w zeners to power 3 or 4 op amps in products like the PV-1.3k but having found them blown I upgraded to the 5 watts when I repaired it. 1 resistor, 1 zener, one capacitor, boom +15 v. Two of each for +-15v. 1n5352 zener test current is 315 ma.
 
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#8
indianajo said:
How much current does your preamp need? There are 3 w 1n5930 and 5 w 1n5352 zeners. Peavey tends to use 1.3 w zeners to power 3 or 4 op amps in products like the PV-1.3k but having found them blown I upgraded to the 5 watts when I repaired it. 1 resistor, 1 zener, one capacitor, boom +15 v. Two of each for +-15v. 1n5352 zener test current is 315 ma.
Click to expand...
You can also use this approach to get a 'mid way' voltage to then drive suitable regulators as mentioned above.
 
#9
All the suggested methods were/are done by many brands and eventually it is the point of failure in many cases. Why not add a small 10VA 2 x 15V toroid? All solved elegantly and no heat issues, no switcher and its terrible noise/EMI, high reliability. Maybe (depends on the load which you haven't specified) you can even use a cheaper and smaller 7, 5 or even tiny 3.2VA type. Adjust the transformer voltage version to the needed regulated +/- DC voltages to avoid useless heat. This will last and have no deterioration problems in time.

Dropping voltage methods are from the tube era, today energy should not be wasted.
 
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#10
Much easier with dedicated transformer.

Shunt regulator resistors get cooked. Easier also to drop only down to a regulator input voltage tolerance with Zener shunt regulator.
Then use 18 +/_ volts for preamp. 3 less volts and current to drop.
18 volt regulator should tolerate reasonable high input voltage, check datasheet

still rather have a separate transformer
 
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#11
What often is forgotten is that the energy to heat conversion requires a larger transformer as the energy converted to heat is not found in the air for free. Tuning the new small toroid to the exact desired DC voltages also may lead to a tiny transformer. Simple.
 
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#14
You do not have a very high voltage to start with, and the range of that voltage is not very wide. I would pursue something similar to this, in keeping with earlier recommendations.
This particular design was built for a very wide range of input, something like 50V up to 200V, and maintain regulated output and cool operation with the heatsinks. So the values look extreme, since it had to run with such a wide range. But the heat loss in your case will be very small, if you are confident of the input. Resistors can be made 0.25W, caps perhaps removed, etc.
Adding iron to run a preamp when DC is already available would be my last choice.
 

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#15
jean-paul said:
All the suggested methods were/are done by many brands and eventually it is the point of failure in many cases. Why not add a small 10VA 2 x 15V toroid?
Dropping voltage methods are from the tube era, today energy should not be wasted.
Click to expand...
The problem with an additional transformer in an amp is that most of my successful amps have a wall of steel between the AC areas with the filters fuses and transformers, and the high gain input areas. Dynaco PAS3, Peavey 1.3k & MMA-875T PV-4c besides wrapping the transformer in iron or copper band, have such a steel bulkhead. CS800s has a whole steel box enclosing the switcher supply with filters at the boundary on all inputs and outputs. Reconfiguring an existing amp chassis while fitting in an additional 10 cubic inches or so requires machine work, if the spare space is even available. Usually not. I did add DC 12 v fans to a ST-120 by installing a wall transformer over on the power strip, and running the DC cord 6' to the amp, with the fans outside the steel enclosure blowing through the mesh. The problem with this is that transformer wall devices have gone the way of the dodo bird. being replaced by the cheaper RF emitting switcher supply. Yes they meet FCC standards. Yes, I can hear the buzz in a phono cartridge.
There is no reason to run a 5 w zener at 5 watts if the load is 1/2 watt. A higher value resistor can run a zener at as low a current as the designer wants, usually double the load current IMHO. The advantage of big zeners is they cost about the same in onesies as the 1.3 watt ones, and never burn up. The freight from the distributor is $10 whether the bag of diodes cost $.20 or $1. Wasting 1/2 to 1 watt is trivial. Much bigger energy loss is running a class AB output for less digital RF howl of a class D or switcher supply. I save 100 watts by not having a sub amplifier or speaker by merely backing my 10 db down at 40 hz speakers up to a hard plaster wall for 6 db bass boost.
 
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#16
Maybe not so for Slingy who is the OP. The electrical elegance of adding a simple small transformer is unbeatable. Have repaired and updated quite some defective devices this way. The other methods are often error by design causing too much heat or other side effects. There are many active loudspeakers/subwoofers that die this way. Costcutting of the destructive kind.
 
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#17
The electrical elegance of adding a simple small transformer is unbeatable. Have repaired and updated quite some defective devices this way. The other methods are often error by design causing too much heat or other side effects.
Click to expand...
Digikey has zero toroid power transformers. They stock one dual winding 15v 115 ac input, 2.4 va . That is 160 ma? To run 4 4580 IC at maybe 8 ma apiece? How much core loss?
I refuse to buy an active speaker or subwoofer. The whole market is doomed to 5 year life. Planned obsolescence. Conanski knows some brands that last longer, and those brands are not the ones in stores in my tiny city of 2000000 population.
Our Indiana electrical cost/kwh is about 1/10 of what the French pay. I'm paying $.38/kwh. Is it that bad in Germany? When we asked the waiter in France for ice tea, he gave us 2 cubes! 8 cc maybe. Maybe Europeans to save electricity should recycle their audio amplifiers and replace them with a wood piano, violin, or clarinet. Of course the A/C or damp chaser to keep a wood piano from molding in a river valley uses 1000 times the electricity of a 2 w line level DC regulator.
 
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#18
It is about elegance and not creating destructive heat. So saving energy, no added useless heat and a device that keeps working. Seems a good choice.

Yeah your country is the best, I know. Founded by Europeans 😉 BTW just checked but toroids in abundance over there.

Did you just assume the load?
 
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