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Grundig GCM3 microphone (aprox. date of manufacture1956)

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Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby forumuser722082 » Sun Aug 07, 2005 6:45 pm

Hi, i have got a Grundig GCM3 microphone and am having problems using it. The plug on the end of the wire conected to it is a DIN plug with 3 pins. I was wondering if anyone knew of a converter to make it into an XLR/jack etc. or if they are easy to re-wire with limeted skills in that area, cheers for any help
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby rodabod » Mon Aug 08, 2005 1:15 am

Yes, either re-solder an XLR plug on, or make yourself an adapter cable by buying parts from Maplin.

Reply if you are not sure what you are doing.
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby rfornal » Sat Aug 20, 2011 10:07 am

I know this post is old, but I've got one of these microphones coming in and would be interested in learning how to build the adapter; what parts and all ...
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby Mike Stranks » Sat Aug 20, 2011 11:02 am

Suggest you Google this... seems like it's a condenser which needs 100v 'phantom'so it's not just a case of changing plugs or making up adapters...

Oo-err!
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby Hugh Robjohns » Sat Aug 20, 2011 11:06 am

First thing to say: If you are unsure about how the mic works and what the wires are doing, DON'T RE-WIRE IT. There is a risk that you will damage or destroy the microphone -- or whatever you connect it to! Seek professional help instead -- someone who can dismantle the mic, understand how it works, and wire it accordingly.

And secondly, DO NOT APPLY NORMAL PHANTOM POWER TO THIS MIC UNDER ANY CIRCUMSTANCES!

A quick bit of googling suggests that this mic is something of an odity -- it is a high output DC-biased Capacitor mic, but without an impedance converter in the mic itself!

I've seen posts elsewhere to suggest that it has an unbalanced output (no surprise there!), and requires a 100V DC bias voltage to activate the capsule -- which would normally be supplied by the (valve-based) tape recorder.

Normally, an unbalanced DIN connection has the ground on pin 2 and the signal input to the recorder on pin 3. That being the case, I'd assume pin 1 would be the DC bias voltage... but you'd have to check that before any attempt at re-wiring.

Since valve recorders of the day operate with very high input impedances anyway, I presume the mic relies on that to avoid leakage currents from the capsule... and the very high bias voltage would also help bestow the alleged high output.

Either way, this mic won't be at all happy being connected to a modern (relatively low impedance) preamp input -- or via a balancing transformer for that matter, and it won't work at all without the appropriate DC bias voltage correctly applied. Essentially, it will require a bespoke high-impedance input buffer amp stage and a suitable high voltage, high impednace biasing power supply -- the precise facilities provided by the partnering Grundig valve tape recorder, in fact.

It would, theoretically, be possible to power the mic using a phantom supply in a vaguely similar way to how many electret mics are powered via phantom, and to build a high-impednace buffer with a cascode FET stage or similar... but this is not something I'd like to advise about with a sight-unseen vintage mic on a forum like this.

I've also read reports that the very largre diaphragm is prone to cracking... so to be honest it's really probably not woirth your while trying to get this thing going. Stick it on a shelf as a pretty antique instead!

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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby Mike Stranks » Sat Aug 20, 2011 11:41 am

Hugh Robjohns wrote:Normally, an unbalanced DIN connection has the ground on pin 2 and the signal input to the recorder on pin 3...

I never thought I'd do this... and thus do so with a great deal of trepidation, but I think I disagree with m'learned friend!

Back in the day when I had Phillips tape recorders and a Uher mixer I always made up unbalanced low-impedance DIN cables as Pin 1 signal and Pin 2 ground... or have I misunderstood you Hugh?

As for the rest of Hugh's response... can only agree in spades. I typed-up a reply about DIN-XLR adapter leads with caveats about high-impedance mics and then "Googled" to see if I could find out more about the mic... I ditched my proto-response and came to the same conclusion as Hugh... mic looks nice, but really not practical to use.
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby Ian Fairclough » Thu Oct 06, 2011 2:35 pm

I got one of these. A GCM3. Here is the circuit diagram of a solution to the power problem as designed by a tech I know...

http://www.flickr.com/photos/58610460@N05/6216997257/

We used 10 10V watch batteries to power it. If you cant access the link let me know and I'll figure out another way of getting it here.

I tested it the other day and although the pre works fine (no hiss or hum)and the mic looks in great condition I think the diaphragm has perished with time as it sounded pretty toppy with no body to the sound as you would expect with a large diaphragm mic.
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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby Hugh Robjohns » Thu Oct 06, 2011 2:49 pm

Mike Stranks wrote:I never thought I'd do this... and thus do so with a great deal of trepidation, but I think I disagree with m'learned friend!

How very dare you!

As I recall, different manufacturers used different wiring conventions -- although pin 2 did always seem to be earth and pins 1 and 3 were always left and 4/5 always right. But whether 3/5 were inputs and 1/4 outputs seemed to vary with each item of equipment. I seem to remember Quad preamps being wired in the opposite way to most Japanese hifi, for example.

Perhaps it was a Europe/Far East disparity?

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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby James Perrett » Thu Oct 06, 2011 3:19 pm

To confuse things even more, Philips tape recorders used the same pins (3 and 5) for both input and output, depending upon whether the record button was engaged or not. Pins 1 and 4 were always a low level, low impedance input which was in accordance with the DIN specs.

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Re: Grundig GCM3 microphone (aprox. date of manufacture1956)

Postby fenderchris2 » Tue Nov 27, 2012 5:40 pm

Ian Fairclough wrote:I got one of these. A GCM3. Here is the circuit diagram of a solution to the power problem as designed by a tech I know...

http://www.flickr.com/photos/58610460@N05/6216997257/

We used 10 10V watch batteries to power it. If you cant access the link let me know and I'll figure out another way of getting it here.

I tested it the other day and although the pre works fine (no hiss or hum)and the mic looks in great condition I think the diaphragm has perished with time as it sounded pretty toppy with no body to the sound as you would expect with a large diaphragm mic.


Could anyone make me one of these?
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