Mimco Loudhailer

[David Goodall]

Does anybody have information on how these things work ? They take four 4.5 volt batteries. I suspect that there is no amplifier. I looked it up on Google, and found no image or reference. I thought it was from around the 1960's, but having seen some images from that time this one looks to be a lot earlier.
Many possible uses. Suggestions welcome !

[brenellic2000]

MIMCO - Marconi International Marine Company? These would be 1950s for ship-borne work I'd imagine. Echo sounders perhaps - just guessing!? Ping!

Barry

[David Goodall]

Marconi International Marine Company. That would account for the build quality, which is rather impressive. It is handheld by the way. Are you suggesting that somebody in the Norman Wisdom mold would have been seen shouting "Ping" into this thing ?
"Pitkin, what the hell are you doing man?"
By the way, I should have said that I fitted four batteries, and it didn't work.

[John M0GLN]

I don't know if yours has the facility, but some of these loudhailers could be used in reverse, you would loudhail someone with it, then switch to receive and listen through the mic' using the speaker to pick up any faint sounds.

John

[David Goodall]

I don't think it does. It has one simple push button.

[Herald1360]

Didn't some of these things simply have a beefed up carbon microphone driving the horn speaker directly / via a matching transformer? I should think the output power available would be a few watts which makes a LOT of noise fed to a nice efficient horn.

[David Goodall]

The microphone unit does look rather beefy. I shall have to take the hailer to bits, and show you some more pictures. I suppose the first check should be continuity on the microphone, and trasnformer. What should I expect in ohms from a carbon mike?

[David Goodall]

Having got round to looking inside, I've found the push button is not working. It even looks rusty. I have checked the speaker coil (11.4 ohms) and the microphone. This is around 400 ohms, going up to about 3000 when I shout the word "Hooooorn" into it. Less so for tinny words ! I recorded the waveform, and noted that it seems to be only half wave. Is this normal ? Also, how on earth does this thing amplify without an amplifier ?

[Herald1360]

A carbon microphone in itself has considerable power gain from sound power in to electrical power out. The sound in modulates the resistance of the carbon granules in the capsule using a piston coupled to the diaphragm to vary the pressure on the granules. With a reasonable amount of microphone current, suprisingly large amounts of signal power can be generated. Coupled properly to a horn loaded speaker, itself probably better than 10% efficient (compare with typical hifi speaker at maybe 0.1 - 1 %) it works quite well.

For a simple demonstration of carbon mike gain, simply connect an old telephone mike, earpice and a 3V battery in series. If you bring the earpiece up close to the mike, the system will squawk- oscillation resulting from a loop gain of > unity.

There's some information out there in the www about high power carbon mikes, some were ferocious watercooled beasts capable of handling 10s of watts!

Some interesting stuff here:

http://www.douglas-self.com/MUSEUM/C...mp/mechamp.htm

[jamesperrett]

Originally Posted by brenellic2000

MIMCO - Marconi International Marine Company? These would be 1950s for ship-borne work I'd imagine. Echo sounders perhaps - just guessing!? Ping!

The word MIMCO brings back memories of sailing on RRS Discovery in the 80's. Just about every deck, lab and cabin had a grey metal MIMCO intercom box and it was the main means of communication between the bridge and people working on deck. In the early 90's they were all replaced by a telephone system.

James.

[David Goodall]

That's very interesting. I had no idea about carbon microphones. Unless there is a transformer in the microphone housing, I don't think there is one.

[mrmagnetophon]

No these things simply usually had a microphone being fed by a few batteries connected right up to a horn speaker. I know a guy on youtube who had one. Carbon mics output quite alot of voltage compared to other mics and don't require a transformer, when used with high impedance inputs.Also carbon mics didn't need shielded wires as the quality wasn't that great to begin with.

-Chris

[Lucien Nunes]

I have one of these, and it works passably well. The microphone is probably not in great condition so it's rather noisy, might improve if I were to dry the carbon out. I've never taken it apart though, just lashed up a battery pack and it worked. Does it take 4x 1289? I didn't have any to hand and just strapped a load of alkaline AAs together.

You can't get a much simpler audio device!

Lucien

[Herald1360]

Originally Posted by mrmagnetophon

Carbon mics output quite alot of voltage compared to other mics and don't require a transformer, when used with high impedance inputs.Also carbon mics didn't need shielded wires as the quality wasn't that great to begin with.

-Chris

I would expect a carbon mike to transmit most power into an impedance similar to the average resistance of the mike itself- probably a few hundred ohms or less. To drive a high impedance (kohms or more) a step up transformer will provide a higher voltage to the load than just the mike itself. A typical telephone circuit from the carbon mike era has just such a transformer to get best signal levels at the transmission impedance of the telephone network, though it maybe called an "induction coil" or even "anti sidetone induction coil" depending on the design of the phone.

Quality of the sound from the mike has nothing to do with shielding requirements. The relatively high level output does, though, since it means a reasonable signal to noise ratio is possible even with some unwanted pickup in the leads. Balanced circuits also help considerably with noise pickup rejection.

[David Goodall]

I'm going to hook it up tonight, as I've just picked up the batteries (1289 x4) What's the correct way to work out the wattage ? Can it be worked out just by knowing : 11.4 ohms coil. 400 ohms microphone, and 18 volts supply.

[Herald1360]

Tricky....

You can measure the power from the battery easily enough, but that will probably bear little resemblance to the signal power actually delivered to the speaker.

You need to measure the signal ac voltage across the speaker, and the signal ac current through it plus the phase difference between, all whilst whistling into the mike at a reasonable constant frequency and level.

Twin channel 'scope territory, preferably with storage facility

Some idea maybe from just signal rms volts squared / 11.4, but that may only be sensible at frequencies below where the horn is useful.

Maybe just feed the speaker from a tranny with about 250mW output and see subjectively how loud that is compared with someone shouting into the mike?

250mW into a horn should sound about as loud as 2-3 watts into a typical cone loudspeaker.

[Station X]

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