Neumann KM84 Circuit Question

I have a pair of MXL 993's that I modded a while back with the 'Gus mod' and they sound pretty good, but lately I've been craving to gut them and build a whole new circuit while still using the 993 capsule. I want to build the circuit based around the Neumann KM84 but I came across two different schematics for it. I'm still a beginner at this stuff, but I'm hoping that this is a full schematic and not just a single stage in the circuit because it looks like something I can do:

http://www.xaudia.com/xaudia/Schematics/Pages/Neumann_files/Media/Neumann_KM83-84-85/Neumann_KM83-84-85.jpg?disposition=download

The other, more complicated schematic is this one:

http://www.xaudia.com/xaudia/Schematics/Pages/Neumann_files/Media/NeumannKm8345sch/NeumannKm8345sch.jpg?disposition=download

Help me out here! Are these both full schematics of the KM84, the first one just being an earlier version? Would you recommend this build to someone of beginner level skill?

I've also been looking into transformer-less SDC circuits such as the 'Alice Circuit' to save on costs, but I'm not sure if I can use that particular circuit with the 993 capsules. If anyone has any answers to my questions or ideas of any small diaphragm condenser circuits I could build around these 993's, help and suggestions would be greatly appreciated! 

Hi Bart,

These are both schematics that I 'found' on the web. The second one is the full schematic, and the first just shows the amplifier, without the details of how those voltages are generated from the phantom supply.

Hope that helps a bit. In fact if you take a bit of time the full schematic is not so much more complicated. Most of the extra parts are just supplying the power from phantom, and then filtering.

Cheers

Stewart

As far as the km84 circuit goes... I tried doing the same with a 991, but I couldn't find a transformer small enough to fit in the mic.

As far as the alice mic goes, well I don't know how it sounds. I do know that the mxl circuit isn't too bad as far as topology goes, and switching out some caps did help the sound a lot. After that, I think the capsule is your real limiting factor. I recall a mod for the backplate, but I can't remember where. It involved changing the air space behind the capsule, which meant machining a new backplate. (tiny plastic one). If you're crafty you could diy something.

Thanks for the replies. Are there not any transformers at all that can fit into the 993/991/603 size body?
The 993 circuit is the same as the 991/603 with the addition of a -10dB pad and a low cut. I swapped some caps out on one of them and that made significant improvement in the sound. It sounds good and I'm pleased with the mic. I'm looking to do this as a learning experience in understanding how mics work rather than trying to make the mic something it's not.

Why not put the circuit inside one of the wider-bodied MXL mics like the 990 or 440? These look like LDC mics but are in fact using the same capsule as the 441, 603,993, et al. They are not expensive on the used market, and the only disadvantage is that they are side-addressed. There should be enough space for a small transformer in there.
Save your 993s if you think they sound good now. I think they work really well for acoustic guitar.
Or you could just buy some of the capsules off of e(vil)-bay for not much at all and build the whole thing from scratch.
Lots of folks on this forum have done some pretty cool stuff using metal tubing, and even tuna-fish tins.

I remember this topic cropping up before, and it was determined that the important discrepancy between the two drawings is that the drain resistor should be 47k, as shown, slightly fuzzily, in the full schematic - and not 4.7k, as shown, erroneously, in the simplified drawing.

I've got a pair of the alice mics... another variation of the schoeps circuit.... the capsule used is an electret and is extremely noisy.... but trading one schoeps ciruit for
another seems like a waste of time
I just bought a couple of BV107 xformers for a pair of km84's, so you can find them around... or try marik.... his transformers are excellent quality and may be small
enough to fit in this application (although I'm not sure as the only transformer I've purchased from Mark is a ribbon)

I built a 5840 tube mic with a Cinemag 2480 into a 993 mic. It just fit into the body. Tab makes a 7:1 with the T14 sized bobbin that will fit. Marik's transformers sound very nice.

The 993 has the larger body that will take a transformer. You will have to make your own PCB.

The Schoeps circuit can sound nice in these mics.

Jim Jacobsen
JJ Audio

> different schematics.... I'm still a beginner...

The simple form looks like a magazine story, simplified by cutting-away the ugly yet "obvious" details. Look at it and ask how/where the +22V and +44V come from.

> I'm hoping that this is a full schematic

The +22V and +44V power appears "by magic".

The full plan shows how these power nodes really happen.

Does anyone know what R1 is supposed to be in the schematic? It's too blurry for me to read.

That's a 1 gigohm resistor

cheers

A better readable schematic...

Thanks for the clearer schematic! Definitely helps. What I mean though is that R3 is labeled select. What value of resistor is usually recommended here?

Select so that the drain of the FET sits at 10V, which means a current of 0,25mA. Something around 3k-4k will probably get you in the ballpark. Measure the voltage and adjust the resistor accordingly.