Post
by hrschlosser » Mon Mar 09, 2009 2:44 pm
I've converted several Gefell mics, also I built some external converters, so I want to make some comments to this discussion.
First, the MV 691 and 692 do NOT use T-Power. This is very important, because if you use the wrong adapter it may do harm to the mic. Both mics (as well as other Gefell types like PM 750 and PM 860) use Phantom Power, but with 12V, known as P12. In addition to that the mics can be used with a proprietary powering, where 12V is suplied via Pin4. In that case you can wire the outputs to unsymmetrical. If you use Phantom Power, the output can only be used unsymmetrical (which should be fine for all recording purposes).
Second, I'd never recommend using any types of resistors in the output cables, to get the Power down from P48 to P12. This is unreliable (what happens, if the power from the mixer (or preamp) is not precisely 48V but 52V? Right, the mic is getting too much power in this case. Also, as pointed out before, the mic output impedance is changed (for worse) in that case. This can cause problems depending on cable lenght and input impedance of the mic preamp.
If you want to use an external adaptor, you need decoupling caps, which do not let phantom power pass. You take P48 from the input, regulate it with a zener, a transistor, some resistors and caps down to P12 and pass it back to the cable after the decoupling caps. But here again you can see the limitation of most available converters. They are built into the smallest possible housings, leaving only space for electrolytic decoupling caps, which are not the first choice if you want the best possible sound. Due to the low mic impedance you need coupling caps of at least 22?F, assuming the mic preamp has "normal" input impedance to not get a bass roll-off. MKT caps are available in this size, but they are big and expensive.
Third, the only good solution is to convert the mics internally. Good means: inexpensive (less than 5$ for the parts), the sound goes unaltered. You'd need the same regulator design as mentioned above (for external adaptors), but without any decoupling caps. The Power (P48) is taken from the center tap of the transformer and regulated down to 12V. Here again, you need a zener, a transistor, two resistors and a cap. The problem is, the space inside of the mic only allows for a smd (at least partially smd) design.
Using a resistor to drop the voltage from 48 to 12 V is not reliable. You can never be sure that the preamp or mixer delivers 48V precisely. If it is more, the mic gets too much power. If it is less, the mic get's too little power. Both is undesirable. In the case of too little power the mic won't get hurt, but not work to specs. The output will be lower. Note, that the capsule also changes sound, when it doesn't get the right power. So, having the mic powered correctly is slightly critical for it's sound.
