KM-84 BOM Questions

[HellfireStudios]

In the good old days, I think Wima made polystyrenes as well - same housing. Can't remember the series type though - What does the printing on the box say?

The reason for polystyrene here is their very good stability wrt insulation resistance both over time and over temperature/humidity.

Jakob E.

In the pictures of both mics is a WIMA that has 470 100 WIMA written on it. The pictures also depict a different color of this capacitor for each mic. Blue in one mic yellow in the other. The yellow WIMA also has a " - " symbol after the 100. I'm curious as to why Neumann would use an obvious-in-appearance styrene for C2 and C9 and a WIMA at C1. If it were a styrene WIMA, why would they not have used another obvious-in-appearance styrene inside a mic which already had two? Thanks.

-James-

 

[Matador]

If you want to try something different, you can actually replace C1 with the capsule itself!

What is the affect of this?

Well, it gets rid of the effect of the coupling cap in the signal path, and saves a part.  The capsule is after all a capacitor all by itself:  the only reason to bias it the KM84 way was to borrow the front end from the other Neumann designs, where you need to be able to switch the bias voltages between the capsule backplate and front diaphragm to get all the different patterns.

I rewired a KM84-like mike this way and it definitely sounds different, although hard to describe.  It seems to have more high end transient response, was a bit brighter and harder-edged, especially with sharp transient sources like picks on acoustic guitar.  However I found on a really mellow source it helped it stick out in the mix without resorting to shelving EQ's.

It's easy to experiment with however:  since this will be on a breadboard you can simply lift one end of C1 and move the ground connection side of the capsule to that point on the circuit...it's two quick wire changes.

 

[HellfireStudios]

Matador, that sounds interesting, but my intent with a KM-84-ish mic is to get that smoother top it is known for. My M-Audio Pulsar II's are bright/harsh enough Cool tip, though. Thanks.

-James-

 

[Rossi]

You can't connect the MXL603/604 capsule to the FET without a coupling cap. The capsule is grounded via the mic body, that leaves only the center terminal to apply the polarization voltage. As the FET gate is grounded via a high ohms resistor, you need a cap in-between the two.

 

[Matador]

You can't connect the MXL603/604 capsule to the FET without a coupling cap. The capsule is grounded via the mic body, that leaves only the center terminal to apply the polarization voltage. As the FET gate is grounded via a high ohms resistor, you need a cap in-between the two.

There's no way to isolate the backplate connection on this mike?

 

[Rossi]

IIRC the center terminal is the backplate and the only other connection to the capsule (i.e. the diaphragm) is via the chassis. If it's possible at all, you'd have to modify the capsule itself.

 

[HellfireStudios]

I know the original capsule in the MXL 604s/603 is supposedly bass-light, but I thought maybe increasing the transformer coupling capacitor (2.2uF maybe?) might extend the low end to an acceptable degree. Does this seem feasible? Thanks.

-James-

 

[gyraf]

Hi James,

Probably won't help - but try it and let us know.

Jakob E.

 

[HellfireStudios]

What made me consider this cap change was the Royer SDC mod. I know the circuit is not the same, but it uses a 2.2uF instead of the more common 1uF coupling cap. I'm not sure it will work, but I was under the impression that increasing this cap (C4) would extend the low end much like some of the tube mic circuit mods suggest. Is solidstate different in this regard? Thanks.

-James-

 

[gyraf]

It depends on the impedances in question (what is transformer ratio and rated impedances), and on the integrity of the original design. I doubt any designer would go with a 1u, if another octave of operation could be had at 2u2.

The limitation is probably in other areas of the circuit (transformer, capsule)

Jakob E.

 

[HellfireStudios]

The transformer will be a Cinemag CM-2510 at a 8:1 ratio and 9.6k:150 ohm impedances.

-James-

 

[Rossi]

Those MXL603 capsules are not *that* bass shy. The stock circuit limits bass response intentionally by using 100n caps between front end and output stage.

The original KM84 circuit is also a bit reduced in bass response, not by the 1u coupling cap but by the source resistor bypass cap. If you want more bass, you can go higher than 4.7u, but keep in mind that a small transformer can't handle high levels at low frequencies.

Apart from that, the output coupling cap and the transformer interact in the form of a LF resonance. By varying the cap value this resonance goes higher in frequency and becomes more pronounced. A bigger cap makes the resonance lower and less pronounced. Thus, a smaller cap may actually sound bass heavier than a bigger cap. Try different sizes and use the one you like best, but keep in mind that the original designers determined the stock values for a reason.