Electronics > Repair

High Voltage Regulator

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Birstall:
I have a Linn Sizmik 12.45 subwoofer. It is powered by a Linn Class V 500 watt amp.
After rectification 250 - 300 v DC is spit and delivered to two 500uF 200v capacitors and a VB 408 voltage regulator. The Vout on the schematic for the regulator is 12v.
The voltage regulator has blown and no longer available, Mauser have no alternatives. They are still available on Ebay but repeating this failed design seems foolhardy
This is a common problem with this particular amp and usually solved by installing a modern plate amp at a cost of $500.
I am a novice but I am curious to know if there is a workaround to solve this issue. Obviously 500 watt SM amps are still around so how are they designed with lower voltages
Your comments and advice will be most welcome

Pulsepowerguy:
I can think of at least one approach to making a discrete version of the VB 408. The part looks like an integration of some fairly standard items. It would take quite a bit of effort - maybe a fun time for a EE hobbyist. All it really does though is take the rectified line voltage and drop it down (with tons of loss).

A pragmatist would take small 15v 'wall wart' supply and use a linear regulator to take it down to 12V. The wall wart should be a chunky, 60hz type (not a switcher) in order to satisfy the audiophile. This would be highly reliable.

TheMG:
7812 with a suitable resistor in series with the input to bring the voltage down to something it can handle.

If the load is non-constant add a zener diode across the input to clamp the voltage down if the load get very light on the regulator. I would put the zener there regardless as a precaution anyways.

Seen this sort of thing done in plenty of designs where a linear 3-terminal regulator needs to run from a supply voltage greater than it can normally handle. This avoids the use of oddball rare parts like the VB408.

Pulsepowerguy:

--- Quote from: TheMG on July 14, 2020, 12:49:04 am ---7812 with a suitable resistor in series with the input to bring the voltage down to something it can handle.

If the load is non-constant add a zener diode across the input to clamp the voltage down if the load get very light on the regulator. I would put the zener there regardless as a precaution anyways.


--- End quote ---
A resistor in series with a 7812? With 250-300V upstream? For a current on the order of 40mA at 300 volts input the power dissipation of a 7k resistor is on the order of 11 Watts. The big problem though is what happens if the input is 250V. For that case the resistor is too large and the regulator is starved. A 50 Volt variation in input voltage makes this approach fail, to say nothing of line surges or dips. At any rate, I would not go that route.

Birstall: Do you have a schematic of the power supply?

coromonadalix:
I had tl783 regulator in mind,  but they are i think obsolete too,    would an ac-dc adaptor be good ??     an 120-240 vac to 12vdc ???

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