Neumann KM56 w/ NN48a PSU - How much hum?

I have a vintage Neumann KM56 on the bench, I've replaced the faulty polar pattern switch with a replacement from Soundlock Engineering. Unit is working again and the polar patterns switch nicely now.

It seems to have a bit of 120hz hum, the supply has been recapped with modern caps and measures 130V DC on the B+ and 4.3V DC on the heaters.

Does anyone know if this mic has a fairly high hum? The mic is still usable, just not the quietest. With 20db of gain from my mic pre the 120hz hum is coming in around -70dbfs

Is there anything in particular I can check for regarding the hum aside from typical culprits?

Be sure the ground betwwen mic body and head and power supply are all good.

Are you using a Neumann NKM type power supply?

Get that heater voltage for the AC701's back down to 3.8 to 4.0 volts. No more.  Good way to ruin an expensive tube.  Check this with the mic connected, or a 40 ohm resistor "dummy load" for initial setup.  If you have a NKM supply, there are "stabilytes" connected in series.  If these go bad, you will have hum.  These acted like high value electrolytic capacitors in a small volume.  (I changed these out to NiCAD's in one of my NKM supplies.)  There are also some nice PCB regulators out there.  There is a small tapped wire wound resistor in series with the heater output that can be tapped up (higher resistance) to drop the heater voltage.  Use a good DMM known to be good for these measurements.

B plus (HT) is a little high.  That should run around 120 volts  (No more).  Rectifiers OK?  (Old selenium rectifier can become leaky.. you will know if it shorts out.)  The 150B2 tube should be lit in the NKM supply.  If not, replace it.  This can be a reason for excessive HT voltage.  (A 0A2 tube can be used with socket rewiring.)  The 150B2 tube is a nice feature of the NKM supply; it regulates the HT voltage to the filter capacitors at 150 volts.

One real quick check (after getting the heater voltage correct):  Connect the mic, turn on the supply, measure with power, wait a few minutes, turn OFF the power,  immediately listen and measure the hum (mic powered by residual charge in capacitors only).  If you still have hum, check the cables.  Hopefully, the AC701 isn't damaged.

The KM56 is a nice mic.  Especially for classical music recording.

Thanks for the tips.

This supply says NN48A on the side, it appears that it had be recapped and "modernized" before it came to me for repair. The tube rectifier is gone and replaced with a silicon one.

I will check on that heater voltage as well as all the ground connections.

outoftune said:

The tube rectifier is gone and replaced with a silicon one.

Click to expand...

Something wrong here. NN48a never had a tube-rectifier!