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[BradL8068]

I am fortunate to work at a recording studio that owns a pair of KM-54's and a pair of KM-56's.
Beautiful sounding microphones. The KM-54's have a slightly higher noise floor.
The noise sounds like RF "hash". We are located in NY Times Square an RF saturated area.

The mikes are quite usable, but if a large amount of gain is required when recording a soft instrument the noise becomes apparent in the KM-54's.

These mics have modern (not original) solid state power supplies and have been fitted with new cables. Since they are pin compatible we have tried swapping the supplies and cables and the noise does not follow. In the KM-54's the electrolytic caps were replaced with no change.

The noise level of the two KM-56's match, and the higher noise of the KM-54's match. The 54's are quite far apart in serial number as well.

So my question is this, does anyone know if the KM-54 has a higher noise floor or is the front firing microphone not shielded as well as the side firing 56? (this is my current assumption)

or should they be sent for repair?

Thank you
Brad Leigh MSR Recording Studios NY

[Kai]

 I have both but no time to check.
In theory KM 56 head should deliver less level, because back and front capsules are connected in parallel, so a signal on the front is loaded with the backsides capsule at the cost of 6dB.
As both mikes have about the same sensitivity (Neumann quotes a 2dB lower one) the amplifier of the KM 56 needs a higher gain, causing more noise, due to overall higher amplification for a given sound level.

If the noise sounds unusual, like RF interference is, the power supply or cable connection might be the cause.
I found the KM 56 more prone to those problems then the KM 54.
There is a test input on the microphone's connector that should be grounded on the microphone side of the connector, but better not connected to a wire.

[klaus]

Two likely sourced for your KM54 noise, and none having to do with the model designation:

1. Bad grounding/shielding scheme between mic and mic pre (yes, I include the XLR line between power supply and mic pre input. Many an RF problem is due to improper shielding connections there, to; see this thread: http://repforums.prosoundweb.com/index.php/topic,1152.0.html

Follow the suggestions there without double guessing They are always right, and you will already be better off, regardless what else may be at work here.

2. Double check that the KM54 tubes are "c" biased (I assume the KM56 are). If not, that is one significant source of stray noise interference and higher noise floor in general.

Let us know what you find.

[usattler]

Check the serial numbers, the improved (lower noise) tube bias scheme was applied to KM54 microphones starting with serial number 5016 and above, also identified with the suffix 'c' to the model designation. Similarly, for the model KM56 the change to the KM56c occurred at serial number 3220.

[BradL8068]

Thank you all!
My mics are revA, I have schematics for Rev A, I'll contact Neumann and see if  I can get the C schematic, then report back after checking all the above info.
Thank you again
Brad

[usattler]

Unless you are fairly experienced, do not attempt to make the conversion yourself! Several components will need to be rearranged and/or added (see attached schematics), and the risk, particularly to the delicate tube, may outweigh any potential saving or other benefit.

[BradL8068]

Yes usattler I am experienced.
I'll mod a single mic then do an A/B.
Again Thank You
Brad

[klaus]

Let me echo Uwe's caution: for this conversion to be successful, the tube filaments need to be heat sunk, and the filament curls untangled and repositioned.

[BradL8068]

Let me echo Uwe's caution: for this conversion to be successful, the tube filaments need to be heat sunk, and the filament curls untangled and repositioned.

Understood. Again thank you for the guidance.
Brad

[BradL8068]

I am about to mod this microphone from A bias to C bias. Looking at the C bias schematic I need a 25uF cap @ 6v parallel with a 3.9k resistor from cathode to ground.  25uF is hard to find and the caps I can find seem to have a very wide tolerance of -10 +75 %. My question is, would a 22uF be suitable in this location?

A second question, what is the composition of the brown capacitor in this photo?

Thank You
Brad

[klaus]

Question #1: Use a larger value capacitor than 22mfd. The cap determines low frequency cut-off, in this biasing scheme, and, unless you want to increase phase shift, increase the low end and decrease the phase shift. 47mfd @ 25VDC will be just fine. Use a high quality type and brand, like Nichicon, Sanyo or Elna.

Just make sure the new cap conforms to the sizing common for this value and voltage rating: 5mm ø x 11mm max, because space comes at a premium in Neumann KM mics.

Question #2: I believe these are early versions of ceramic capacitors.

[Kai]

...I need a 25uF cap @ 6v parallel with a 3.9k resistor from cathode to ground.  25uF is hard to find and the caps I can find seem to have a very wide tolerance of -10 +75 %. My question is, would a 22uF be suitable in this location?

A second question, what is the composition of the brown capacitor in this photo?

Don't worry about the value, 22uF gives a low cut frequency of 2Hz, way out of the audio range. Go for high quality and low loss.
I suggest going for a higher voltage, like 16V, 6V has a much higher failure rate.

The brown cap is a tube ceramic, no longer common these days.

[klaus]

Don't worry about the value, 22uF gives a low cut frequency of 2Hz, way out of the audio range.

Sometimes theoretical and experiential considerations can clash.
Choosing a specific value for this capacitor may not be an issue just of "cut-off" frequency, which can be precisely calculated), but a question of the slope and degree of attenuation, above the cut-off point. I can easily hear low-end distinctions between a 22mfd, a 47mfd and even a 100mfd bypass capacitor in this circuit.

While a more linear response may not be what you want (some Neumann mics intentionally use the source capacitor as low-end eqin FET mics (an analogous application to the cathode bypass cap in tube mics), it should be mentioned, in case you want a phase-coherent low end from your mic.

[BradL8068]

Again
Thank you
Brad

[usattler]

Don't worry about the difference between 22µF and 25µF! On one hand, that is well within the standard tolerance anyway, on the other hand, the corner frequencies of less than 2Hz due to the resulting high pass with the 3.9kΩ resistor place any effect on response or 'phase coherence' is well outside any humanly possible audible perception! In this case it is physical size, not the exact value that matters. For this reason, and I am aware of contrary opinions about them, good tantalum types are often used for cathode resistor bypass.