Originally Posted by mole-man
Thanks ever so much for all you input.
Lotus 7, or anyone else. Would a more modern design of P48 supply effectively solve my problems then? I could run a XLR to TS without any worries of shorting the P48?
If so, could anyone recommend a cheap 2 channel supply that has DC blocking on the output?
As rcrowley said, we can't give you a proper, correct
answer without knowing a little more about the characteristics of your "...old stereo valve mic pre. It has unbalanced 1/4" inputs and outputs
", BUT we can give you an educated opinion.
With 1/4 inch inputs, it's almost certainly a high impedance input. That may, or may not be usable with the signal levels that come from that typical low impedance, balanced output of a contemporary microphone. Some vacuum tube mic preamplifiers are designed to work with high impedance mics which have signal levels that are quite high as compared with a modern low-Z mic.
Additionally, the signal level from modern low-z mics can vary by 30 dB or more depending on model, so it not only depends on the pre amp, but also on the particular mics you're planning on using.
If your vacuum tube mic pre has enough gain, all that may be needed is a phantom power supply with TRS output termination like the Rolls PRB486
. It's fully isolated, AC-coupled, has balanced XLR inputs and TRS outputs. The TRS outputs can also drive a "single-ended" TS input like what you have. It does not provide any signal gain. There is a very good chance that this will work.
If you don't mind having to use batteries, the Denecke PS-2
is a super nice phantom supply. It will power (2) high current LDC mics for 4.5 hours on one 9volt battery. I have several and and use them often. They are rugged, fully isolated, very low noise, and just work! Output is via XLR, but they are on a split cable, so those can be changed to TR plugs, or XLR/TRS adapters can be used.
If your vacuum tube mic pre does not have enough gain, but does have a high input impedance (50k ohms or more) it's possible to still use a isolated phantom PS like the Rolls and connect its output through a low-Z to high Z mic transformer.
These typically will raise the signal voltage level by 20 to 26 dB and will drive even very low gain inputs.
A possible way to go would be to pick up the isolated phantom PS and try it. If you have enough gain for your application, fine, you're done. If it works, but the gain is low for your need, add a couple of transformers so the low-Z mics better match your (presumably High-Z) input.
Finally, if you happen to have, or can borrow, a plain old low-output dynamic mic like a SM-58 or SM-57 lying around, you can safely plug that directly into the 1/4 inch input using a XLR to TS cable
or a XLR/1/4 inch TS
adapter. If it works with the low output from a SM-57 or 58, it will have enough gain to work with an active, phantom-powered capacitor mic. It's a quick, easy way to get an idea of the gain available on your tube mic pre and to confirm that it's actually working.
Hope this helps.
Do you have any specifications at all on the vacuum tube mic pre?
P.S. there are still a few, cheap non-isolated phantom power supplies being sold that have the full DC voltage on the output pins.
BEWARE of the Sampson S-PHANTOM!!!