+48 volt phantom supply

This will only be practical in 120V lands.

Why not change the 2K+4K7 to 3K3+3K3? Better distribution of heat, and you get the two-fer price on resistors.

C1 has to be rated 170V (use 200V). C2 could be 110V, but might as well use 200V here also.

A dandy source of few-hundred uFd 200V caps is old PC power supplies. Cheap ones yield two 100uFd, good ones can run as fat as four 470uFd.

Using just one of the two primaries cuts the total transformer rating, perhaps to 3/4 of what it was sold as. While you can use the nominal secondaries, allow for Phantom draw (about 8VA) and overall derating.

Great idea. I'll put it in the PS META.

Also please consider a couple of safety issues:

First, isolation class between windings. Most power transformers has strong isolation between primary and secondary, but NOT between two individual primaries!

Second thing is that if you loose a Zener diode in this circuit, you will probably loose your condenser microphone as well!

Jakob E.

[quote author="gyraf"]Also please consider a couple of safety issues:

First, isolation class between windings. Most power transformers has strong isolation between primary and secondary, but NOT between two individual primaries!
[/quote]

Been there...
Once wanted to squize an Avel torroid into 1U for tube pre and use second primary with a voltage doubler. As always after completing project, wanted to leave the thing for 48 hours. The first 8 hours was checking every once in awhile--everything nice and transformer stayed cool. Left on overnight just to realize the next morning the fuse was blown--primaries shorted. Never ever again!

Uhhhh ok..... So, it's in the META so folks will know what not to do. :wink:

[quote author="buttachunk"]Marik, I've used a split primary tap in the PRR VariMu for over 6 months now and haven't had a problem yet... maybe something else was wrong ?[/quote]

I have actually no any idea what happened there (definitely--I was sleeping :grin: ). Everything else looked fine. After that I put a Hammond and it works for 2 years with no problems, so far.
It also might be that Avel had particularly bad isolation...
As my mom used to say--never step into the same shit twice and I just can't take any chances going to a session, when the customer pays about 2 grands a day for a hall rent alone.

Oh, whenever I need to use a phantom, I made a battery one--5 nine volters.

I once asked my transformer manufacturer about this, and they strongly recommended to avoid this! The main problem is not the casual burn-out, but the poor isolation from mains voltage. Simply dangerous.

Jakob E.

[quote author="buttachunk"]then maybe the *safest* way to do the phantom tap is to use 2 transformers like many of Jakob's designs, but in reverse-- for approx 60vAC ? Then drop down with one 1k 2watt resistor into the zeners. 60vDC shouldn't blow most mics...[/quote]

If there is a choice of a second transformer, why would not you just get a dedicated one, with no hassle.