Neumann K87 and K67 capsule capacitance

[bxt403]

Hi there,

I need to know the rated/specified (or even actual/measured) capacitances for the original Neumann K67 and K87 capsules to figure out if I keep the bypass capacitor for a clone at 470pF or increase it to upwards of 1000pF. 

It's my understanding that low frequency roll-off occurs at a capacitance that's less than 10x the capsule capacitance and that the ideal bypass capacitor value should be >=10x the capsule capacitance. 

I've cross-referenced with http://recordinghacks.com/capsules/ to figure out what the exact capacitor value needs to be (as it changes for every different K67/K87 type capsule).  The only problem is that (without knowing the capacitance for the original Neumann capsules): I'm uncertain if the 470pF capacitance is actually part of the U87/U87ai characteristic sound (in that having a bit of low frequency roll-off is actually desired and is part of the design).

 

[Khron]

Which exact "bypass capacitor" do you mean?

 

[bxt403]

Which exact "bypass capacitor" do you mean?

The one that's in series with the capsule that goes to the gate of the MOSFET T1 on the U87 and U87Ai schematics.  It's between two parallel 1G Ohm resistors on the U87 schematic.  On the respective schematic: it's labelled C103 for the U87Ai and C4 for the U87.  In both cases: it's 470pF.

 

[bxt403]

Actually: I didn't even notice that the information's on the Neumann U87 and U87Ai schematics.  The KK87 (so K87) is shown to be  2 x 50pF.  This makes me suspect (using the Recording Hacks capsule page as reference): I can consider the KK87 as 50pF. 

For the U89: the KK89 is shown as 2 x 80pF and is using 1000pF (with 1 G Ohm at either side).

The KK67 is also shown to be 2 x 50pF on the U67 schematic but I've yet to find a schematic that shows what capacitor's used (I can't see where it is on the U67 schematic).

 

[ln76d]

U77 and U67 doesn't use this capacitor at all. Try also without it

 

[Biula]

Hi there,

I need to know the rated/specified (or even actual/measured) capacitances for the original Neumann K67 and K87 capsules to figure out if I keep the bypass capacitor for a clone at 470pF or increase it to upwards of 1000pF.

It's my understanding that low frequency roll-off occurs at a capacitance that's less than 10x the capsule capacitance and that the ideal bypass capacitor value should be >=10x the capsule capacitance.

I've cross-referenced with Large-Diaphragm Microphone Capsules to figure out what the exact capacitor value needs to be (as it changes for every different K67/K87 type capsule). The only problem is that (without knowing the capacitance for the original Neumann capsules): I'm uncertain if the 470pF capacitance is actually part of the U87/U87ai characteristic sound (in that having a bit of low frequency roll-off is actually desired and is part of the design).

"Bypass capacitor"? No at all, this is the capsule coupling cap! It should be roughly 10 times the nominal capacitance of the capsule (50pF), given the 1 gigaohm gate resistor value it permits 0.33 Hz -3dB rolloff frequency. More is ok, but 1000pF is just overkill...

 

[soliloqueen]

the actual capacitance of the capsule is about 70pf, but you're really not going to run into any issues at 470pf either way.

 

[Biula]

the actual capacitance of the capsule is about 70pf, but you're really not going to run into any issues at 470pf either way.

No sir, KK87 and K67/KK67 are 50pF per side... but, as said, do not really matter.

 

[soliloqueen]

No sir, KK87 and K67/KK67 are 50pF per side... but, as said, do not really matter.

It says that in the schematic but that's not true in real life. It seems like either they started out with that capacitance during r&d but they decided to decrease the distance between the backplate and the diaphragm later on and didn't update the schematic, or maybe the schematics just never had accurate capacitance. Either way, I have a very early capsule here with the wire mount and even it has a capacitance of 67. My later k87s have capacitances between 65 and 75.

 

[Biula]

Maybe this applies to recent models: the vintage ones are around 50pF as stated in the schematics...

 

[soliloqueen]

Maybe this applies to recent models: the vintage ones are around 50pF as stated in the schematics...

The schematics are simply wrong. That's the long and short of it. I don't know why they're wrong, nobody knows why they're wrong, but it gets brought up from time to time. There have been a couple theories, like that the 50pF is supposed to be charged capacitance, but the drop in capacitance from charging tended to be only about 5pf so that ended up not being the case when someone tested it. It's been a real head scratcher for a very small group of people for a while...

 

[soliloqueen]

My wiremount k67 from the '60s and my '70s and '80s k87s all have 70pf capacitance ish. The wiremount one is slightly lower then the newer ones but still well above 50pf.

 

[Biula]

Actually: I didn't even notice that the information's on the Neumann U87 and U87Ai schematics. The KK87 (so K87) is shown to be 2 x 50pF. This makes me suspect (using the Recording Hacks capsule page as reference): I can consider the KK87 as 50pF.

For the U89: the KK89 is shown as 2 x 80pF and is using 1000pF (with 1 G Ohm at either side).

The KK67 is also shown to be 2 x 50pF on the U67 schematic but I've yet to find a schematic that shows what capacitor's used (I can't see where it is on the U67 schematic).

The U67 doesn't need this capacitor: the capsule is coupled directly to the grid of the EF86 tube, which is biased at -1.6v via a 400Mohm resistor (resulting in a 8Hz -3dB roll-off system).

 

[Biula]

"The schematics are simply wrong"... Maybe at Neumann nobody can write down a schematic right (or maybe German picoFarads are in metric units, not imperial )!

 

[soliloqueen]

"The schematics are simply wrong"... Maybe at Neumann nobody can write down a schematic right (or maybe German picoFarads are in metric units, not imperial )!

No clue! It's been discussed a fair amount, and nobody I've seen has figured out why they denote 50pF for the capsule capacitance. Maybe that's the cap replacement value if you're injecting test signal to get the same results they have in their handbooks for technicians?

 

[Biula]

Well, jokes apart I have the feeling the clue resides in that "C.A." note that Neumann schemes print near the nominal capsule capacitance: probably denoting the standard conditions of measurement they use... I'll ask Siegfried Thiersch next time I'll contact him, maybe he knows.

 

[rock soderstrom]

Well, jokes apart I have the feeling the clue resides in that "C.A." note that Neumann schemes print near the nominal capsule capacitance: probably denoting the standard conditions of measurement they use... I'll ask Siegfried Thiersch next time I'll contact him, maybe he knows.

I think CA here means circa, a Latin word for "around" which is used at least in the German language in the more common spelling "ca." with the meaning approximately.

I am quite sure because elsewhere in the same schematic it is translated as approx. in English.

 

[Lampie519]

Yes, "ca." translates to "approximate"

 

[Horizoneer]

In English we also use "Circa" in more technical conversations, the Romans got us too.

 

[rock soderstrom]

It seems like either they started out with that capacitance during r&d but they decided to decrease the distance between the backplate and the diaphragm later on and didn't update the schematic,

That sounds plausible to me. When the circuit diagram was first drawn up, the exact final capacity was probably not yet known, hence the estimate.

More than 30% tolerance is a bit much for a real German "Ingenieur"...

This was then retained in all subsequent circuit diagrams, such as here in this US version. Note the capacitance specification, which supports my previous statement.