Rode NT5 polarization voltage

[k brown]

Anyone know the actual voltage the capsule receives?

 

[Khron]

Might be in the 40-50v range.

A couple years back i had to replace a failed ceramic capacitor on the bias oscillator's output on a buddy's NT5 (symptom was unusually low output / sensitivity). Can't 100000% recall the exact value, but it may well have been 45v or 48v or so.

In the t.bone SC140 (NT55 clone, NT5 with pad & high-pass) it's 48v, that much i remember.

 

[k brown]

So pretty much the full Phantom voltage - thanks.

If that's true, why would it need a bias oscillator in it? Thought they were used to raise the Phantom 48 to something higher.

 

[Khron]

Phantom power is 48v unloaded. Once you draw any amount of current from it, there will be a more or less significant drop across the phantom power resistors inside the preamp.

1mA draw (not unreasonable for a single JFET) will bring the phantom power voltage at the mic down to 44,6v or so.

A Schoeps circuit draws, "officially", about 4mA (including the oscilator), which would drag down the input voltage to about 34v.

 

[mhelin]

Actually the capsule doesn't draw any current so you could get the full 48V if you just used half of the phantom power supply (via either XLR pin 2 or 3, not both) though that leaves a lot less (7 mA short circuited) for the amplifier part, and the output would be single ended, impedance balanced only.

Regarding the NT5 card capsule, has anyone dismantled it? Does it use similar backplate design than MXL capsules (so no exact KK84 copy either)?

 

[kingkorg]

Actually the capsule doesn't draw any current so you could get the full 48V if you just used half of the phantom power supply (via either XLR pin 2 or 3, not both) though that leaves a lot less (7 mA short circuited) for the amplifier part, and the output would be single ended, impedance balanced only.

Regarding the NT5 card capsule, has anyone dismantled it? Does it use similar backplate design than MXL capsules (so no exact KK84 copy either)?

Yes, the one similar to MXL, but better constructed, more consistent, and has appropriate front honeycomb grille.

 

[k brown]

Actually the capsule doesn't draw any current so you could get the full 48V if you just used half of the phantom power supply (via either XLR pin 2 or 3, not both) though that leaves a lot less (7 mA short circuited) for the amplifier part, and the output would be single ended, impedance balanced only.

Regarding the NT5 card capsule, has anyone dismantled it? Does it use similar backplate design than MXL capsules (so no exact KK84 copy either)?

Ah - that explains how the MC-012 gets away without an osc circuit.

 

[Khron]

Ah - that explains how the MC-012 gets away without an osc circuit.

Well, it's true that the Oktava doesn't use an oscillator to derive the capsule bias voltage, but it DOES in fact derive its supply rail from both pins 2 and 3 together, keeping the connection (impedance-)balanced, not single-ended.

 

[ccaudle]

the output would be single ended, impedance balanced only

It would not be impedance balanced if you had unbalanced loads on each leg. For balance considerations you have to look at all devices loading each leg.

 

[k brown]

I've always assumed that's why the '012's output resistors are different values (39 ohm / 51 ohm).

 

[mhelin]

Maybe the balance could be fine tuned using a scope with the amp input shorted if either of the resistors (the 51 ohms is just in the middle) is replaced with a 100 ohms trim pot. In 012 though the phantom power is delivered from both XLR pins, if you removed the R3 (https://medias.audiofanzine.com/files/oktava-mc-mk-012-479633.pdf) the polarization voltage would rise from 40V to 48V (if the voltages on that schematic are real).