Converting old Sennheiser Microphones to XLR (K30AV to K3W)

Table of Contents

How I got here…
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I recently aquired a set of Sennheiser K30AV and Sennheiser ME80. Both components form a microphone as the ME80 is just the microphone capsule and the K30AV is the supply and signal conversion piece in the form of a handgrip. Both parts feature shells from high quality aluminium and are screwed together for usage. The K30AV was a signal conversion unit intended to be paired with prosumer style camcorders in the 80s. The ME80 capsule features a combination of a supercardioid polar pattern with a cardioid polar pattern at frequencies above \(2000\,Hz\). There is a version that has a more pronounced cardioid pattern (ME88), a more supercardioid (ME40) and even a polar pattern (ME20) compatible within the same modular system called TELEMIKE.

Next to some obscure pieces in this system like a lavalier microphone or a telescoping arm, that could be screwed inbetween the signal conversion piece and the capsule, there were several versions of the handgrip. The most relevant next to the version i got is the K3U (which is only different to the K3N by the plug.). The K30AV has a big problem as it cannot be powered by conventional phantom power but needs either a (nowadays) unobtainable battery or a DC power supply on a pin (\(4\,V\) to \(15\,V\)) of its plug. The original battery contained mercury (23PX) and is therefore banned by now. There is apossibility to put a 3D-printed adapter with an equivalent voltage amount of coin cells into the battery slot, however there is another problem with the K30AV, that is not present in the K3U: it cannot do symmetrical audio! While both types have three populated pins, the K30AV features one ground, one signal and a power supply pin and the K3U uses the typical XLR setup, where the two symmetrical signal lines are also used for phantom power.

Here should be an image of a 'Disassembled, yet unmodified K30AV' — Disassembled, yet unmodified Sennheiser K30AV

So I wanted to look into converting one microphone handgrip type into the other. Especially as the K3N/U handgrips are quite expensive and the K30AV ones seem to lie around unused more. I will try an electrical conversion and the handgrip will keep its DIN 45326 connector anf the XLR conversion will be handled by a small cable. This feels a bit illegal, as i tobviously does not conform to DIN 45326 but i will be the only user of that device and the small adapter cables will be living on the microphone. As it will physically not be \(100,\%\) identical to the K3U, i will call my version K3W (W for Werti, if thats not obvious).

Reversing the difference between the microphone handgrips
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To get started I need to determine the common parts and those that differ between the two models. Luckily there is a good scan of the K3N/U service manual online on ElektroTanya or in a much worse quality here.

The document contains everything important. There is a BOM, a schematic and a view of the PCB. I could not find the equivalent document for my K30AV, but that was also not really neccessary as i can just determine the resistor values from the colored rings and printed on values.

Here should be an image of a 'PCB with the original Tantalum Capacitors of the K30AV' — PCB with the original Tantalum Capacitors of the K30AV

The parts values for both devices can bee seen in the following table. Parts, that differ significantly are marked in the “Part” column.

Part	K3U	K30AV
C1	TA-ELKO 22uF/16V	TA-ELKO 22uF/16V
C2	TA-ELKO 1uF/35V	TA-ELKO 10uF/10V
C3	KERKO 6.8uF/50V	TA-ELKO 15uF/35V
C4	KERKO 18uF/50V	TA-ELKO 0.47uF/35V
D1	Zener ZTE 1.5V	Zener ZTE 1.5V
D2	Zener ZPD 8.2V	Zener ZPD 8.2V
D3	1N4148	1N4148
D4	1N4148	1N4148
D5	1N4148	1N4148
R1	\( 220\,\Omega\)	\( 221\,\Omega\)
R2	\( 390\,\Omega\)	\( 392\,\Omega\)
R3	\( 8.2\,k\Omega\)	\( 8.25\,k\Omega\)
R4	\( 8.2\,k\Omega\)	\( 8.25\,k\Omega\)
R5	\( 15\,k\Omega\)	\( 8.25\,k\Omega\)
R6	\( 220\,k\Omega\)	\( 825\,\Omega\)
R7	\( 150\,\Omega\)	\( 150\,\Omega\)
R8	not present	\( 562\,\Omega\)

Luckily there are really only some differences in resistor values and all but one capacitors. I am unsure, how that has happened, but my 16 year old self salvaged the exactly perfect tantalum capacitors from some old hifi equipment i got from a realtive to look inside of. The resistors were part of a THT resistor kit from the local hackspace.

Having shared parts alone wont help, if the circuits design is completely different. There is a Service manual for the Sennheiser K3N/K3U (N and U variant are basically the same microphone with different plugs). The service manual contains a print of the PCB. I have used GIMP to draw along the traces (seperate for each side). Then I did the same for a photo of the K3AVs PCB. These two versions can now be diffed by GIMP on top of the K3s PCB. This shows, that there is really only a very smal difference (yay).

Here should be an image of a 'The difference in PCB layout' — The difference in PCB layout is around *R6 *and has to do with the fact, that there is a difference on how the microphones are powered.

Oops
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While sitting down and starting to convert the microphones, i looked at the schematic again. I have completely ignored the isolation fransformer for the balanced output and was now wondering if I could ever finish this conversion.

I then did some research and found out that the transformer seems to be somewhat standardized¹. I went on a hunt to find the cheapest and smallest XLR isolation transformer I could. Then I stumbled at this convenient set of two transformers² (for half the price in April) and looked at the measurements. If these fit, it wlll be incredibly tight and there will be not a lot of space for wires. Unluckily, the transformer needs to go between the PCB and the plug, so all wires need to pass the transformer as well.

Here should be an image of a 'AliExpress Listing for Generic XLR Transformers' — AliExpress Listing for Generic XLR Transformers

Final Assembly
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Upon arrival of the transformators I was stoked to find out if they fit into the tight aluminium tube of the microphone body (they barely do! wohoo!).