Phantom separate transformer

Hi,

I am gathering information to build a tube microphone preamp, I've built guitar amplifiers so far.

Why is it so important to have a separate transformer for the phantom circuit? Can't I use just a different secondary winding (ie: HT - heaters - phantom)?

Thanks!

Of course you can, if you have that winding available.

Thank you! I think I'll custom order a toroidal transformer then.

What's the recommended amperage for that phantom secondary winding to power just one microphone?

HOLD ON!

Tube mics don't need phantom power. Unless I'm missing something or read the question wrong. What you need is high voltage (around 160V, depending on tube choice) B+, filiment, and capsule polarizing power (if its multi-pattern.) You only need phantom to power a solid state condensor mic if it is designed to run on +48 volts. That's why all tube mics have their own dedicated supply.

Tod

Mr. Snoid said:

Tube mics don't need phantom power.

Click to expand...

No, but it would appear that he's trying to build a microphone preamp where the active elements are tubes (such as the G9 or the NYDave one-bottle), rather than a preamp for tube mics. In that case (mic pre built out of tubes) you DO need phantom if you want to be able to supply condenser mics.

To the original poster: The max current isn't hard to figure out. P48 is implemented as +48V fed through a pair of 6.81k resistors to both of the mic lines. If for some reason both lines are shorted to ground, how much current would flow? (This is more than the P48 spec says, which is 10mA I believe, but especially for DIY it never hurts to be safe in case of ground shorts).

More, much more information on this can be found in the Microphone Preamplifiers META. Well worth a read.

JDB.

Wikipedia says:

The IEC standard gives 10 mA as the maximum allowed current per microphone.

fluzo18 said:

Thank you! I think I'll custom order a toroidal transformer then.

What's the recommended amperage for that phantom secondary winding to power just one microphone?

Click to expand...

Lots of different routes you can take here. You don't necessarily need to have a mid voltage winding (35-50VAC) on the transformer. You could go with a transformer that has a high voltage winding and say a 15V winding. You can take the 15V winding and use it for regulated 12VDC for your filaments and also use a voltage tripler circuit to get your 48V. (I have a rough schematic for this, PM if you'd like to see it).

You might not even need a custom transformer. A 12.6VAC tap can get you *close* to 48VDC with a voltage tripler and LM317 regulator... most condensers will run on ~ 44V just fine. In fact, some of the Behringer and ART stuff actually has ~ 30V for their phantom power IIRC.

I had Edcor wind me a 200-0-200 (50 mA) / 12.6VAC (1A) transformer - $32.46. The part number is XPWR115 if it's not on their website yet. Save you the $40 design fee.

Mason

OOPs... I guess I didn't read it right. My bad.... duh!

Nevermind...