Phantom power, why I'm getting 82V???

Hello dear Diy Family, I'm building power supply fro mic pre. I'm using schematic from JLM Power Plunt.  On Phantom power LM317 input I'm getting 81 V, why? I have beautiful +24V rail good  and on phantom power regulator i have 80V on input, I'm pretty sure i did all the components right,  is there a mistake somewhere in schematics? Cause on diodes D1 and D3 it's 82V how come?

here is a schematics:
http://www.jlmaudio.com/Power%20Plant/POWER%20PLANT%20Schematic.pdf

I'm using 2*12V transformer

80V at the input to the phantom power regulator is not too bad with no load but it should drop considerably if the regulator is drawing any current (its quiescent should be about 10mA which should bring it down). What is the voltage at the regulator output?

Cheers

Ian

D1 and D3 are part of a voltage tripler, the high voltage is expected.

[silent:arts] said:

D1 and D3 are part of a voltage tripler, the high voltage is expected.

Click to expand...

Not from what I see .........  To me they look like part of a standard bridge rectifier that creates another 24v supply that  is sat on top of the 24v supply for the  mic amps.    The caps that feed the D1-4 bridge are just to isolate it from the 24v supply so you can sit it on top & create the 48v for Phantom..

Hey Amigos, Thank you for a help (just like always). I've tried to setup trimmers for a 48V and it burned some 120 resistor, but after I plugged dummy load, voltage dropped down and I was able to setup 48V with no prob. Thanks Ruffrecords i forgot that  it's better to setup trimmer on power supplies with a load.

Rob Flinn said:

Not from what I see .........  To me they look like part of a standard bridge rectifier that creates another 24v supply that  is sat on top of the 24v supply for the  mic amps.    The caps that feed the D1-4 bridge are just to isolate it from the 24v supply so you can sit it on top & create the 48v for Phantom..

Click to expand...

From the JLM ACDC schematic:

[silent:arts] said:

From the JLM ACDC schematic:

Click to expand...

That's a different diagram to the one that the link goes to in the first post .......

Rob Flinn said:

That's a different diagram to the one that the link goes to in the first post .......

Click to expand...

The tripler section is the same (but a fixed tripler in the first schematic, switchable doubler / tripler in the ACDC)

Both schematics have a BIG FLAW. I would not ever put either into a production product as it would be returned for repair. The LM317 has a max differential voltage limit of 40 volts. The LM317HV has a 60 volt max limit. The phantom voltage design is “48 Volts” this means that the input must be several volts above the 48 volt level. When the supply is first turned on the OUTPUT is 0 Volts (must charge the caps) and the input is >> above the 40 volt max limit. The LM317 or LM317HV is passed its design limit. Turning on phantom power or plugging in a mic of shorted cable can cause a FAILURE.

The circuit should be changed so the used parts do not exceed the rated limits. The circuit should also have some sort of current and or power limiting to insure against failure.
Duke

The TL783 could be a good solution :

http://www.ti.com/product/TL783

Bri

Best pratice having separate phantom winding from power transformer at ~40-45Vac, it has a few advantages compared to doublers/tripplers and similar. Since your PT is already in use, i would look at Ruffrecord's TL783 circuit values as attached.
For 1ch mic amp first cap can be 470uF/100V (4700uF is there for powering more channels), other values should do as on schematic without heatsink. Guess you know the filter which is typicaly mounted on phantom switch, Jensen's "mic input schematic" shows values of cap and resistor.

Do you have the junper in the JLM PSU in "tripler" or "Doubler" ?