321 – my favourite microphone type number of all times! There’s the AKG 321, a fierce contender of the Shure SM58 back in the 1980s, there’s a Sennheiser probe mic called MD-321 which was used to acoustically identify faulty parts in all sorts of industrial devices, and then there’s this little feller, the Grundig GDM-321. It’s a slim and rather elegant looking omnidirectional microphone in a sturdy black metal housing and has an identically-looking cardioid twin, the GDM-322. In fact, if you get your hands on one of these, it’s quite likely that the flimsy metal foil label has long come off and you won’t know which one you got – until you see the side vents on the GDM-322 which make it cardiod, of course.
There’s one more catch, though, namely its connector (oh no, not again). If you look closely, you’ll see that it has two pins, one a little thinner than the other, and a screw thread to fasten the cable. Yes, two pins only. This normally suggests that this is an unbalanced high-impedance mic, but in that case I suspect that the sleeve is the ground. It works either way. This microphone jack is actually so rare, I’ve never seen it anywhere else but on this particular mic (even though there are similar jacks on some unbalanced Japanese mics from the 70s and 80s, but they’re not compatible).
Which means that, if you find one of these roaming free, chances are it has lost its connection cable and is basically useless, unless you want to steal its capsule. I’ve read that there are 3-pole-DIN connector versions as well, but this seems to be a misunderstanding. I have yet to see one to confirm that – from what I could gather, all those mics have the strange 2-pin-connection and rely on their custom adapter which also has a high-impedance transformer built into the DIN plug. If you want to convert the mic to XLR, you won’t have any use for it anyway and can simply cut it off.
For use with old tape machines, this is quite useful – they didn’t have to make any changes to the mic itself, yet it is still compatible to both tube and transistor devices as required by DIN 45594.
If you are lucky, though, and find one in its original box, you’ll probably get a metal stand with a clamp (which you don’t need, you can use any mic holder you’d normally use for a small diaphragm condenser, the GDM-321 is, as mentioned before, quite slim), the connection cable, and there might even be a small silver plastic ring included. It serves as a sort of presence booster to increase voice intellegibility; I’ve seen similar constructions on old small diaphragm mics made by AKG.
According to a reliable source, namely a former Sennheiser employee, this microphone is also one to add to the long and mysterious list of Sennheiser OEM mics. It sure sounds the part, although it’s not too full-bodied and needs a lot of gain, but it has a pleasant clarity to its sound. So far, we’ve used it as drum overhead, which sounds very jazzy and vintage with lots of air, but the studio recording purposes for dynamic omnis are, of course, rather limited. If you want to go sound hunting out in the field and don’t have access to 48V phantom power, then the GDM-321 might just be what you need, though.
- Impedance (of the HL version): 200 Ohms on pins 3 + 2, 70 kOhm on pins 1 + 2
- Polar pattern: omnidirectional
- Frequency range: 40 – 18,000 Hz
- Length: ~21 cm
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Trash:Gold ratio 1:2