+48 phantom power

[johnheath]

I have an idea of a preamp with rectified 6,3vac for the heaters of a pair of 6SN7GT tubes... and... 48volt phantom power .

That is nothing spectaular but I was thinking of using  a power transformer like this Hammond 369JX and now I wonder if it would be doable to use the 50v tap and rectify it and use it for the phantom power.

I have searched the internet for  ideas and hopefully a schematic but with no luck.

Can anybody tell  me if that would work? I guess i would if I filter carefully.

http://www.hammondmfg.com/pdf/EDB369JX.pdf

Thanks

/John

 

[ruffrecords]

That 50V tap is intended for creating a negative bias for tube power amps. There is no reason it could not be used to create a +ve supply. It would need to be half wave rectified and smoothed. This would produce about 70V dc. Then you could pass it to a suitable regulator to create the required 48V.

Cheers

Ian

 

[Landins]

1. Use this as inspiration.

2. Simulate with Duncan Amps Psu design tool.

 

[johnheath]

Thanks both of you…

Landins… you schematics is an inspiration but it rises a question  and that is…If I use the 50v tap to the rectifier bridge there are no other tap (unless the 0V red/yellow) to connect to the other end?

 

[Landins]

Ah. Didnt see the schemo of the trafo attached. I figured you had a complete 50 v winding. A bridge will not work. Follow ruffrecords tip with halfwave rect and cap to ground.

 

[johnheath]

Yep… just what I thought.

Thanks

 

[ruffrecords]

I would recommend the TL783 circuit above. I use something similar myself. Be aware that the winding you are using appears to be rated at 69mA. This needs to be shared between the HT AND the phantom supply. The TL783 needs  about 10mA to regulate properly and if you allow 10mA for the attached mic you need around 20mA total for the phantom supply.  The transformer needs to be rated to provide 1.6 times this current or 32mA. That still leaves about the same for the HT which means you can probably draw another 20mA dc from the HT which should be plenty for a mic pre.

Cheers

ian

 

[johnheath]

Thanks for the input Ian

The preamp I am planning…or thinking about, to build draw around 22 - 24mA for two channels.

The suggested transformer is not decided yet… I can always buy a "bigger" =)

Thanks

John

 

[ruffrecords]

OK, as it is two channels then you also need to double your budget for phantom power. There are lots of mains transformers with suitable HT and heater windings but many do not have a 50V tap which limits your choice. It might be worth considering a separate small  transformer for the phantom power. It only needs to be about 5VA. Something like this might do:

http://uk.farnell.com/multicomp/mcfm50-22/transformer-toroidal-2-x-22v-5va/dp/9531564?categoryId=700000005694

Cheers

Ian

 

[johnheath]

Yes I have thought of a seperate transformer, but many of the hammonds have that 50v bias tap so it will not be a problem to find a "large" enough.

Cheers

John