Source for 4 pF polystyrene caps for KM84?

Hi!

I'm looking for a 3.3pF or 4 pF polystrene caps for a Neumann KM84 mic (C2 in the schematic below).
Does anyone know a good source? I'm just hitting ceramics at RS and Farnell.

SCHEMATIC

Cheers!

Stewart

would mica work?

..two pieces of wire in relative proximity should do it...



http://rush-on-line.co.uk/cap_index.php


Jakob E.

Even Surplus sales don't have one that low...
http://www.surplussales.com/Capacitors/Poly-Unelco.html

I'd use a Mica cap myself in place of that, but what do I know...

The smallest polystyrene I ever saw was 15 pF, You can roll your own, but how critical is the value?

JR

Neumann uses ceramics and Andreas Grosser in his mods, too. It would be interesting if there is an audible difference from different materials there. The value is critical, it determines the amount of feedback around the fet.
i just built a km84 circuit (with one of the bv107 transformers from german ebay) into a gefell mv691 body and with the m94 capsule it sounds great.
I used a ceramic for the 4 pF.

I wonder if you could use the inherent capacitance of some devices (like a diode)... While that capacitance might change with voltage which would then introduce distortion.  ???

A NPO ceramic should be OK, but I like poly better if you can find one.. I never saw one that small myself.

JR

4.7pF any good for you?

Thanks for all the comments and suggestions. It looks like Rush have them!

hop.sing said:

Neumann uses ceramics and Andreas Grosser in his mods, too.

Click to expand...

Really? The one I have in front of me is definitely a polystyrene. Maybe they changed over at some point?

Basically I have intermittent noise from something in the high-Z region. The capsule is OK and I've swapped the FET, so I'm looking at the 1Gig resistors and the caps with a suspicious eye.

I'll bung in a ceramic for now so I can at least eliminate that from the equation.

rodabod said:

4.7pF any good for you?

Click to expand...

Thanks Roddy - I'll let you know!

Stewart

Stewart

Carefully lift one side of the cap to find out if it is the cap.  The gain will go up but it will help you find the problem.  I have twisted wirewrap wire measuring with a good meter to make "gimmick caps".  My guess is the caps were custom made, you can find down to 20Pf sometimes.

I've had trouble with polystyrene not being very robust in production... Once had a batch messed up by drying off the PCBs with a hihg pressure air hose. I think it blew water into the caps.

I like the dielectric, but not factory friendly. Fine for DIY handling.

JR

Gus said:

Carefully lift one side of the cap to find out if it is the cap.  The gain will go up but it will help you find the problem.  I have twisted wirewrap wire measuring with a good meter to make "gimmick caps".  My guess is the caps were custom made, you can find down to 20Pf sometimes.

Click to expand...

Thanks Gus.  I have tried this and the intermittent noise seemed to go away, although of course the total noise floor goes up with the gain. I've ordered some bits from Rush.

Really? The one I have in front of me is definitely a polystyrene. Maybe they changed over at some point?

Click to expand...

I was wrong, I opened our km84 and found a polystyrene cap.
But on our Grosser modded u87 it is definitly a ceramic he put in there.
but is there really a difference in sound between the materials with values that small? Or is it more a matter of ohmic isolation between the two capacitor leads, which might be a problem with ceramics?

Something to try is to clean the cap body with distilled water using a clean wipe like a Kimwipe.  Then dry it off. 

Gus said:

Something to try is to clean the cap body with distilled water using a clean wipe like a Kimwipe.  Then dry it off. 

Click to expand...

Well, i used a ceramc cap in my km84 build and the microphone is very quite, so no problems there. But the two leadouts of the cap are pretty close to each other so I cleaned the room in between carefully with alcohol, it did not hurt the ceramic cap, but I do not know if polyprop will survive that, so distilled water is a good idea there.