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[Bjur]

Greetings!

Our studio has a Neumann UM 57 with a UN 61 power supply. The power supply is wired for 220 volts input. Can this be modified to accept 110 volts? If not, would you recommend a voltage step-up transformer?

Thanks in advance for your help.

[klaus]

Unfortunately, the Neumann, Gefell UN61 power supply was equipped with a power transformer fixed for 220VAC. You will indeed need to purchase a step-up transformer with a minimum rating of 50 Watt to make that mic work in the U.S.

But you are not done with the mere acquisition of a transformer: nominal AC household voltages have changed over the decades, both in Europe (now 230VAC) and the United Stats (now 120VAC).

The other variable to consider is the often imprecise 1:2 voltage conversion of consumer step-up transformers.

Because transformer output voltages vary so widely, and because of the changed AC environment, I strongly recommend that you calibrate both, B+ and heater voltages inside the power supply to your mic, once you have purchased the transformer.

[Bjur]

Thank you, Klaus. Do you have a recommended source or manufacturer for a good transformer?

Also, I am new to this, so will need some instructions for calibrating the B+ and heater voltages. I've read elsewhere that I should do this for my U 67s as well. Is the process similar?

Incidentally, our building AC voltage seems to hover around 124-125 volts. Is this cause for concern?

[klaus]

Do you have a recommended source or manufacturer for a good transformer?

I used to buy Franzus converters, but for your mic's relatively high current draw they will not be quite sufficient and will overheat. I am sure someone else who is frequenting this forum may have a recommendation for a 50W+ converter.

Also, I am new to this, so will need some instructions for calibrating the B+ and heater voltages.

Thanks for telling us. Please DO NOT undertake this task by yourself or on your own. Dangerously high voltages are involved in the calibration and parts modification of microphone power supplies, that can expose you to lethal amounts of current. Engage a seasoned specialist and ask to watch him/her in the process.

I've read elsewhere that I should do this for my U 67s as well. Is the process similar?

The process for calibrating your U67 supply is the same as for all other tube mic power supplies, with a few notable exceptions (U47). For tube mics to operate noise free, and for capsule and tube retaining maximal life, both, high voltage (B+) and heater voltage need to stay fairly close to the voltages prescribed by the microphone manufacturers.

Incidentally, our building AC voltage seems to hover around 124-125 volts. Is this cause for concern?

If 124/125VAC is your average house AC voltage during the hours you would be operating the mic (voltages may fluctuate during the day, depending on the electric company's ability to keep its AC steady with heavy usage load periods), you would need to take that into account when calibrating the mic power supplies. Some power supplies do not have the circuitry to keep output voltages stable with fluctuating input voltages.