Gefell MK221 and MK202

[Rumi]

I had a phone call with Andreas Grosser, and as always he gave me a truckload of interesting data. I hope I recall it correctly.

He told me that the old MVs are unbalanced, which means that cable lenght is a big issue. His PSU uses transformers to balance the signal, and he has to shield the trannies twice, once with MU metal and once with copper. The trannies are pretty expensive, which makes his PSU quite expensive, too. He told me that Neumann often balanced the output of their mics just by connecting minus to ground (or even leaving it open, I am not sure here), which results in a "pseudo balanced" output that has half of a wave on one side and nothing on the other. Of course that leads to more THD etc. If the M221 is balanced in the same way, that might not be ideal.

I have just now bought a pair of MV201 with 6 capsules on ebay (MK202, MK102 and MK301), and will have Andreas make me a PSU for them. The total price will not be that much below the new M221 with second hand capsules, but anyway, I trust Andreas a lot, and I also feel better with a dedicated PSU for the 200 Volt the capsules require than to rely on phantom power.

Edit: I just realized that I obviously confused some prices - my set will be around half of the costs for a pair of M221. Now do I feel lucky!

[Me2]

Originally Posted by Rumi

I had a phone call with Andreas Grosser, and as always he gave me a truckload of interesting data. I hope I recall it correctly.

He told me that the old MVs are unbalanced, which means that cable lenght is a big issue. His PSU uses transformers to balance the signal, and he has to shield the trannies twice, once with MU metal and once with copper. The trannies are pretty expensive, which makes his PSU quite expensive, too.

I have 3 MK221 capsules and MV201 preamps for use in a Decca tree. I know the mv pres are unbalanced and have the circuit diagram. Building my own 90V/200V polarization PSU is no problem for me. I've also found the correct physical connectors. The original Gefell SPL measuring meter had a very high impedance unbalanced input using a capacitor coupled discrete transistor buffer amp of ±1Mohm input impedance via a stepped attenuator. Output is anyway pretty hot at 50mV/Pa.

I'm still undecided about whether to electrically balance the output with a 600 Ohm driver, or to use a transformer. Transformer may be more colored, but a driver requires yet more PSU voltages and still leaves the possibility of ground loops.

Do you have any data on what Andreas changes e.g. which transformer he uses? Especially transformer impedances? I was looking at some Beyerdynamic or Sowter transformers e.g. mu metal core mic input transformer wired backwards, but it's not clear to me what's the best option.

Or does anyone have the schematic of the m221 / mv225?