Grundig GDM-121

Behold one of the foundational pillars of trashblitz.com: The Grundig GDM-121, 1950s German vintage gold deluxe, and in all its glory quite probably responsible for this website. It is to this day the only gold coloured microphone we own, and it has such a cute little metal table stand with the Grundig four-leaf clover on it! And, more importantly, it was one of the first “vintage” microphones we acquired that did not simply sound old, funny or broken, and it piqued our interest in a very special way: It sounded fantastic, but there was buzz and hum and all sorts of unpleasant background noises mixed into the signal – and here my quest to open up and convert the GDM-121 to low impedance began. I found out that the GDM-121 is a high-impedance version of the Sennheiser MD21, a dynamic omnidirectional reporter microphone which is still being manufactured with only minor design updates, more than 60 years after its release in 1953.

In terms of usefulness, this means that it is 100% functionally equal to the MD21, which was used as a reporter mic and in broadcasting. It has many advantages for these purposes – almost no handling noise, no proximity effect, incredibly sturdy (there was supposedly a show on the Sennheiser stage on some fair in the 1950s where an MD21 was dragged up and down a flight of stairs for hours, and it was still operational after this treatment). Since it was developed for vocal applications mainly, it has a presence boost above 3kHz and can withstand really high SPL. Much like with its cardioid sibling, the MD421, it’s virtually impossible to get a distorted signal out of this mic.

In the studio, it can be used as a soft, jazzy kick drum mic, on guitar cabinets and on many instruments where you want to capture a mellow and natural ambient sound without very detailed transients or highs. This is because it is a large-diaphraghm dynamic omni, and as such probably unique in its properties even in most serious studio microphone collections.

The Grundig GDM 121 = the high impedance version of the 1950s dynamic reporter mic classic Sennheiser MD21, and it's gold coloured!

The Grundig GDM 121 = the high impedance version of the 1950s dynamic reporter mic classic Sennheiser MD21, and it’s gold coloured!

Anyway, the GDM 121 has a built-in impedance transformer (as has the Sennheiser MD21 HN, by the way) which has to be removed in order to use it with contemporary recording equipment and integrate it in one of said collections. Further research showed it was designed for unbalanced tape recorder input, so to convert it to XLR, I feared I even had to bridge pins. Luckily, all unbalanced GDM-121 models I have opened so far actually used balanced cable which only needed to be connected to the body and wired correctly. Now, opening the GDM-121 or the Sennheiser MD21 is a much harder task than I would have imagined, and I’m very glad I found Norbert’s website with detailed instructions. I’ll give you a rough guide in my own words:


How to open up and convert the Grundig GDM-121 to XLR

  1. First, you’ll have to gently hammer or push the metal pins on both sides of the microphone grille into the body, using a slim steel nail, an awl or something similar. If you have a tiny hammer or a non-metal mallet, use it. Don’t lose those pins, or you won’t be able to close the microphone housing anymore!
  2. Take out the microphone capsule. Inside, you’ll find the high-impedance transformer, snugly lodged inside a plastic compartment in the capsule holder.
  3. Bypass the transformer, or desolder all the wires leading to and from it and simply rip it out, your GDM-121 will not miss it.
  4. Connect the textile-coated wires directly to the capsule. Make a note which colour went to + and which one to -, so you can later connect them to the XLR plug correctly.
  5. Attach the ground to the housing. I’ve had no luck soldering inside the microphone’s body, so maybe try scratching it with a file before soldering, or use glue or other mechanical ways of attaching it if you run into similar problems.
  6. Put the capsule back in place and close the mic. This can be quite hard, either you manage to do it alone with the help of a vice, or you get somebody to press the capsule and grille into the body while you try and hammer in the pins.
  7. Cut off or open up and desolder the DIN plug. I recommend to do it this way, the soldering on these mics is usually very neat and you can directly use the already stripped wires for your male XLR plug. Have a look at the notes you made and complete your wiring. 1 is ground, 2 is your hot (+) cable and 3 your cold (-) cable.
  8. And you’re done!

Norbert also mentions that it’s possible to insert a male XLR jack into the tail end of the mic by opening a screw that sits beneath the connection thread for the stand – but to be honest, I never found that screw. I suppose you’ll have to scratch off some paint to spot it. If you manage to find and open it, you can remove the small aluminium cone that holds the cable in place, remove it and insert an XLR jack – that’s quite some mod! As I said, I never managed or bothered to do that, but it sounds intriguing.

Update June 2017:
Frans has found the screw! It is indeed hidden beneath the layer of paint inside the screw thread. He also provided a good strategy for soldering the ground wire to the body - solder some stranded wire to the capsule's ground terminal, cut two small holes into the piece that holds the capsule in place, thread the wire around it, put solder on it from both sides. This should give you a pretty solid ground connection.

Let’s close this article with a bit of version history: There are many different models of the GDM-121, the oldest being the gold coloured ones with the flat grille, sometimes also labeled Grundig MD 121 or Grundig GDM 12 – obviously, they couldn’t really decide how to call it. The one on the picture is a later model, gold with round grille, and there are white and grey ones, too. Also, if you are very lucky, you can find one with DIN small tuchel connector instead of a fixed cable, which has the additional benefit of no built-in transformer. These are ready to go immediately, much like the Telefunken MD21 and the Labor W MD21 (the direct predecessor – Labor W was Sennheiser’s former company name), which have small or large tuchel connectors – both are even closer to the Sennheiser version than the Grundig.

Specs!

  • Frequency range: 40 – 18,000 Hz
  • Capsule impedance: 200 Ohm
  • Polar pattern: Omnidirectional
Style: (5.0 / 5)
Sound: (5.0 / 5)
Uniqueness: (4.5 / 5)
Usefulness: (4.5 / 5)

Trash:Gold ratio 1:4

2 Comments
  1. Found this microphone days ago in Physics in our School and just finished the repairing.Thank you for the Tips! Works like a charm.

    • Hi Mathias, thanks for your comment, that’s great! Physics departments at school can be real treasure troves, it seems!

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