Balanced and Unbalanced - AGAIN - sorry...

[tINY]

Originally Posted by mole-man

from my phantom power unit (an AKG n66e), is it ok to use xlr to ts cables (very short lengths - under half a meter) with the shield and negative wires of the XLR to the sleeve of the ts?

That would short the 48v from the negative wire to the ground shield...

You really need a transformer to isolate the 48v common mode voltage from the input. IF YOU ARE SURE THAT THE PREAMP HAS AN INPUT TRANSFORMER (and that it's isolated from the case) you can tie + and - to T and S. Leave the shield disconnected from the input of the preamp (though tying shield to the case may be a good idea).




-tINY

[Richard Crowley]

As tINY says, there is a significant risk of shorting the 48V phantom power. There are several other imponderable risks as well because...

1) We don't really know how the AKG N66E is configured. Does it block the 48V from its output? How does it behave if one side of the output signal is just grounded?

2) We have not the slightest clue what the mic input circuit is for your (unidentified) mic preamp. We don't know if it is prepared for a 48V DC signal (even a transient if the AKG gadget blocks the DC with only capacitors(?)). Nor do we know if your mystery mic preamp can even handle the signal level (or impedance) from a modern low-impedance mic.

WAY too many variables here to offer any significant advice. I would NOT recommend connecting these devices together until you know a lot more about them.

It makes me wonder what is the fascination for this (unidentified) mic preamp with 1/4 inch unbalanced input. It hardly seems like the kind of thing made for modern low-impedance mics. The fact that it is old and uses fire-bottles further casts doubt whether it is compatible with modern microphones.

[crosscutred]

If you are coming from the output of the phantom power unit then xlr to ts are fine. There will be no phantom on that signal.
As to which connections to use I don't know those units so I can't say for sure.

[Lotus 7]

Originally Posted by crosscutred

If you are coming from the output of the phantom power unit then xlr to ts are fine. There will be no phantom on that signal.
As to which connections to use I don't know those units so I can't say for sure.

The old N66E is designed for use with transformer isolated inputs. It has (6) isolated and filtered power feeds but does NOT have any DC blocking capacitors or isolation transformers. The XLR output pins (2) and (3) carry +48 volts and if either is grounded will short the phantom power. If used with a DC coupled single-ended input a floating transformer or blocking capacitors must be used.

It probably needs new 47 uF filter capacitors. You need (9) of them to re-cap it.

The cautions expressed by tINY and rcrowley are exactly correct.

[crosscutred]

Cool, I stand corrected.

Thanks

[Richard Crowley]

If you want to add a low-impedance balanced input to a tube/valve preamp, see my response to a similar discussion....
Adding a low Z mic to an amp with a hi Z input

[Lotus 7]

Originally Posted by crosscutred

Cool, I stand corrected.

Thanks

Although almost any currently produced outboard phantom power adapter uses some form of isolation to keep the DC voltage from appearing on the output, the older vintage stuff often does not. In the "old days" most mic premplifiers and mixer input modules used floating, transformer coupling, so the DC didn't cause any issues. The AKG box in question (from the early 1980's) is really just a well-filtered DC power supply, the required series resistors, and a bunch of XLR connectors.

[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?

Thanks again

[Richard Crowley]

You have THREE issues here, and you seem to be ignoring perhaps the most important one.
1) Providing phantom power to your (unidentified but presumably modern, balanced, low-impedance condenser) microphone.
2) Conversion from 3-pin balanced XLR to 2-terminal (TS) 1/4 inch phone input connector.
3) Conversion from (presumably) low-impedance source to UNKNOWN (quite likely high-impedance) input.

It doesn't matter if you get #1 and #2 solved if you have an input that is incompatible with a modern low-impedance microphone source.

Since you can only offer the details: "old stereo valve mic pre. It has unbalanced 1/4" inputs and outputs." we have no useful information on which to judge whether this will EVER work even if you solve the phantom power issue.

[Lotus 7]

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?

Thanks again

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!!!

[mole-man]

Decided to get some balanced transformers for the inputs to make like easier. now i can use my phantom power unit!

Cheers for the help guys!