kind of OT, how can I best derive +48V phantom power from the HV in an amp's P.S

[Derringer]

Hey there!

Long story short, I turned an old AX84 High Octane build into a tube mic preamp for some DIY recording.
It works great with dynamic mics so I decided to try and add phantom power to use condenser mics as well.

It didn't work so well the way I have initially tried which just using a voltage divider in the power supply.
I am supposing this is an issue because everything "in circuit" including the mircophone when attached, has different resistance too ground, thus creating a different volytage divider.

Below is, basically, what I tried. When I plug my condenser in, the phantom power sags to 15 volts.



Would a voltage regulator cure this? Could I create a voltage divider that gives me ~100volts and install a TL783 circuit to get the requisite 48 volts?

Long story long ...

I don't think it matters much, but here's the preamp circuit I used.
https://tapeop.com/tutorials/30/build-tube-mic-pre/

The roughed out phantom switch+LED is in the article I linked above. I did not make the pad. In the phantom switch circuit I substituted a 270r resistor for R13 and a 100uf 100V cap for C7.

Anyway, my idea was to take that 220K bleed-off resistor in the H.O.'s power supply, R1, and make a voltage divider out of it to derive the 48 volts. 220K and 47K worked perfectly with my voltages. So then I had a switch that turned on an LED when phantom power was engaged. The current limiting resistor for the LED was 10K. When I flipped the switch, all of a sudden my phantom supply dropped below 10 volts. I realized that I was basically creating a new voltage divider since the DC saw that 10K limiting resistor as a path to ground, parallel to the 47K resistor.

OK, I compensated and figured/mathed out a combination of resistors for the bottom of the voltage divider and the current limiting resistor for the LED so thata when the switch was turned on, and the LED lit up, I'd be back to my 220K + 47K divider. I got 48 Volts.

But then when I plugged a condenser mic in, my phantom power voltage sagged back to 15 volts. It worked, but the signal from the condenser was very weak.

So, what's the easiest way to give my build its own, stable for all microphone loads, 48 volts of phantom power?

Thanks!

[antonis]

Sorry buit I can't post pictures from where I am now..

https://epdf.pub/small-signal-audio-designb2a423050b68c4e826f4ec45b9a6637520099.html
pages 538 - 540..

[Derringer]

right on. Thank you. That's a much more detailed regulation circuit than the ones I've been looking at.

That section says that I can get the unregulated supply from a "completely separate transformer's secondary" or from a "voltage doubler", which would imply I have a <48V line to tap somewhere. I'm not keen on tapping the heaters ... unless that's the only option.

What I want to find out though, is can I just do like a 120K + 120K voltage divider instead of R1 in the schem I posted above and use the resulting ~140 volts as the input to a TL783 to produce a stable 48V?

I'd be within the 125v i/o difference range, dropping about 90 volts, but would the regulator be supplied with enough current to do what i need it to do since it's being fed from a 120k resistor? Or am I thinking about this entirely wrong and am asking wrong questions?

[PRR]

Multiplying 6V to 48V is downhill all the way.

P48 must supply from zero to 11mA without drop at the "+48V" point.

It WILL drop at the mike jack because 3.4k (2*6.8k).

Unenlightened figuring: a voltage divider must flow more than 10X the load current for even 10% regulation. So the divider must flow >110mA. Which is a LOT in a tube amp.

[davent]

Morgan Jones has a circuit in his Valve Amplifier book that derives a series of much lower voltages, called the THINGY, to drive references for elevating heaters, no idea whether it could work for what you want.

This link takes you to page a few pages early but you can scroll to page 363 for the schematic, fig. 5.45 and description.

https://books.google.ca/books?id=eddy7svanhcC&pg=PA356&lpg=PA356&dq=morgan+jones+valve+amplifiers+THINGY+schematic&source=bl&ots=_C8MWgQxPE&sig=ACfU3U2iVUq7aWWAq-PA48yz_9yYhG0Dbg&hl=en&sa=X&ved=2ahUKEwj9wuXv_-jpAhXpct8KHeCLCpwQ6AEwCnoECAoQAQ#v=onepage&q=morgan%20jones%20valve%20amplifiers%20THINGY%20schematic&f=false

dave

[Derringer]

Multiplying 6V to 48V is downhill all the way.

P48 must supply from zero to 11mA without drop at the "+48V" point.

It WILL drop at the mike jack because 3.4k (2*6.8k).

Unenlightened figuring: a voltage divider must flow more than 10X the load current for even 10% regulation. So the divider must flow >110mA. Which is a LOT in a tube amp.

and a voltage regulator that is fed by a voltage divider will not be able to supply the necessary current then? I assume?

Is there anyway I can do this with a TL783 or a pair of Tl783s in series from the HT?
Or do I just need to bite the bullet and get a separate transformer?

[Derringer]

Morgan Jones has a circuit in his Valve Amplifier book that derives a series of much lower voltages, called the THINGY, to drive references for elevating heaters, no idea whether it could work for what you want.

This link takes you to page a few pages early but you can scroll to page 363 for the schematic, fig. 5.45 and description.

https://books.google.ca/books?id=eddy7svanhcC&pg=PA356&lpg=PA356&dq=morgan+jones+valve+amplifiers+THINGY+schematic&source=bl&ots=_C8MWgQxPE&sig=ACfU3U2iVUq7aWWAq-PA48yz_9yYhG0Dbg&hl=en&sa=X&ved=2ahUKEwj9wuXv_-jpAhXpct8KHeCLCpwQ6AEwCnoECAoQAQ#v=onepage&q=morgan%20jones%20valve%20amplifiers%20THINGY%20schematic&f=false

dave

Cool circuit. He says it doesn't need to supply current though, only an elevated voltage for the heaters to ride on. I need current.

[antonis]

Voltage is way far easier to handle than Current..
So, grind your teeth and bite the bullet..!!

[Derringer]

yep ... placed a digikey order last night!

There's another one of these preamps in my future, so I'm going to build a phantom box that can power two mics, with room for more if I ever need it.