KM-84 BOM Questions

[HellfireStudios]

I am modding two MXL 604's to KM-84 type mics. I've researched as much as I can
take in at the moment, and I have not been able to find answers to some specific
questions about part values (see schematic below). Here is my BOM:

R1 - 1Gohm
R2 - 1Gohm
R3 - 25kohm pot
R4 - 47kohm
R5 - 10Mohm
R6 - 56kohm
R7 - 10kohm
R8 - 10Mohm
R9 - 2.2kohm
R10 - 2.2kohm
R11 - 47ohm
R12 - 47ohm
R13 - 10 ohm
All Resistors Are Dale/Vishay @ 1/4 watt

C1 - 470pF WIMA 63V
C2 - 3.3-4pF Styrene 50V
C3 - 1.5uF Tantalum 15V
C4 - 1uF Tantalum 25-35V
C5 - 4.7-5uF Tantalum 25-35V
C6 - 0.1uF WIMA/Panasonic 100V
C7 - n/a?
C8 - 4.7-5uF Tantalum 25-35V
C9 - 15pF Styrene 50V
C10 - 0.1uF WIMA/Panasonic 100V

T1 - 2N3819

U1 - Cinemag CM-2510 (8:1)

Gr1 - 1N4749a

S1 - DPDT slide switch (only wire one side)

My questions are:

What is the value of R13? Why is there no C7? Are my cap choices correct for the
circuit I'm cloning? Should any of the tantalums be electrolytic? Any
suggestions for a slide switch that works in the mic? Any other concerns before
I get my parts list in order? Thanks.

-James-

 

[Sredna]

R13 indeed seems to be 10 Ohms.
Unusual as Neumann didn't use one in the KM88.

Are you shure about the inductors (R11, R12)?

 

[HellfireStudios]

Two 47uH inductors were used on the vintage U-87. I figured it was the same in the KM-84. If someone could confirm one way or the other, I would appreciate it.

-James-

 

[Sredna]

Well, the schematic shows R11&12 as 47Ohm resistors and on this picture it looks like such.

 

[Sredna]

...and on the other side you can see the ground connection (black wire) going to a 10Ohm resistor.

 

[MatthisD]

Yes 47Ohm resistors, leave them out for 200Ohm output. You might want more than 10Kohms adjustable resistance for R3 to find a suitable FET. There are several threads for this circuit which talk about that process. I've used 4pf in one mic and 15pf in another for C2(with no C9) and 10pf/33pf C2/C9 for pad switch version.

 

[HellfireStudios]

Thanks. Picture evidence is hard to dispute. I'm really just making guesses as to what types of components should be used. I see the U-87 schematic I have indicates those two inductors as 47E. In the KM-84 schematic they are just marked with just 47. I suppose the difference should have been obvious. Anyone know why the U-87 would use inductors and the KM-84 used resistors?

I need to revise my BOM to reflect what I found in the internal pics of the KM-84. Thanks.

-James-

 

[HellfireStudios]

Yes 47Ohm resistors, leave them out for 200Ohm output. You might want more than 10Kohms adjustable resistance for R3 to find a suitable FET. There are several threads for this circuit which talk about that process. I've used 4pf in one mic and 15pf in another for C2(with no C9) and 10pf/33pf C2/C9 for pad switch version.

From the Neumann schematic it would seem that output impedance would be determined by the transformer wiring. Would it not? Also, why did you double C2's and C9's value in comparison to the original circuit? What affect does that have on the frequency response? Which cap setup was the most pleasing to your ears?

BTW, My 2N3819's are all rated at an IDss of 3.8mA and would probably need a 25k pot to bias properly. Thanks for the heads up.

-James-

 

[HellfireStudios]

BOM was just edited. For C6 and C10, should I choose WIMA pulse film or Panasonic poly caps? It would seem the Panasonics would be more accurate, but I hear WIMA makes a great cap. Does it even matter at this part of the circuit? What voltage rating is recommended for these caps? Thanks.

-James-

 

[Sredna]

Looks like Wima to me, voltage maybe 100V (have to take 46V).

 

[Sredna]

BTW, are you making a PCB layout?

Im planning to do the KM84 circuit in a Gefell MV691 body,
would be an idea to make a PCB fitting both (MXL604 & MV691)

 

[HellfireStudios]

Looks like Wima to me, voltage maybe 100V (have to take 46V).

I can't tell from the pic, are the caps bright red with sharp squared edges, or are they more brick red in color with rounded edges?

BTW, are you making a PCB layout?

Im planning to do the KM84 circuit in a Gefell MV691 body,
would be an idea to make a PCB fitting both (MXL604 & MV691)

I plan on cutting perfboard into the same dimensions as the original MXL PCB, carefully making room for a transformer, and wiring point-to-point. Unfortunately, the PCB is an integral part of the structure in the MXL. The internal design/construction may be too different for a single one-size-fits-all PCB. Use google images to search internal pics of each mic to understand what I am saying.

-James-

 

[Sredna]

Looks like Wima to me, voltage maybe 100V (have to take 46V).

I can't tell from the pic, are the caps bright red with sharp squared edges, or are they more brick red in color with rounded edges?

I found the pictures on the web but they look like darker red with rounded corners typical for older wimas. See also the blue 47pF (470) Wima in the front.

 

[MatthisD]

From the Neumann schematic it would seem that output impedance would be determined by the transformer wiring. Would it not? Also, why did you double C2's and C9's value in comparison to the original circuit? What affect does that have on the frequency response? Which cap setup was the most pleasing to your ears?

BTW, My 2N3819's are all rated at an IDss of 3.8mA and would probably need a 25k pot to bias properly. Thanks for the heads up.

-James-

Yes for series or parallel secondaries, from what I remember reading those 47ohm resistors were there as optional for preamps which expect 100ohm sources.

I built one recently in an LDC body and just used what caps I had at the time, but also to aim for more than the 10dB pad for recording loud instruments (which I haven't done yet). I have a 10pf polystyrene to use for C2 now but I don't expect to be able to tell the difference, maybe in a good studio I could!

I have also used 330n and 470n polypropylene for C4. Personally I like the polyprop there but in the SDCs I use tantalum to save space.

 

[HellfireStudios]

From the Neumann schematic it would seem that output impedance would be determined by the transformer wiring. Would it not? Also, why did you double C2's and C9's value in comparison to the original circuit? What affect does that have on the frequency response? Which cap setup was the most pleasing to your ears?

BTW, My 2N3819's are all rated at an IDss of 3.8mA and would probably need a 25k pot to bias properly. Thanks for the heads up.

-James-

Yes for series or parallel secondaries, from what I remember reading those 47ohm resistors were there as optional for preamps which expect 100ohm sources.

I know, if you remove the bridge from the 47 ohm resistors, it will convert it into the KM-84ip. I assume what you are getting at is the bridge makes the 47ohm resistors inactive. Which makes the circuit see them as "not there" and, subsequently, unecessary. Can I just leave them out? Any room I can make in these mics woukd be great, as long as the train of thought I am on is the right one. Thanks.

-James-

 

[Matador]

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

 

[HellfireStudios]

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

What is the affect of this?

 

[gyraf]

..and C1 probably want to be polystyrene..

Jakob E.

 

[HellfireStudios]

..and C1 probably want to be polystyrene..

Jakob E.

I'm not against the idea, but internal pictures of two different KM-84's both show WIMA's in that position. That is why I picked that particular capacitor type. What would be the benefit of a styrene at C1? Thanks.

-James-

 

[gyraf]

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.