An unusual loudspeaker

[Karen O]

I was invited to a private bonfire party last night and in the host's garage was a very interesting object. A loudspeaker which I gues was once part of a radio set (hence posting in this section of the forum).

It resembled an electric heater more than a speaker. The owner says he found it in a loft and that it is made out of bakelite, though I couldn't get close enough to confirm that.

It was round, perhaps twelve inches in diameter and had a central lattice of nested squares to protect the diaphragm, which had no features revealing the position of the voice coil (as would be the case for a modern speaker). The diaphragm had no flexible region at the edges either - it was just a continuous curved surface stretching from one side of the aperture to the other.

Any ideas what era this speaker likely comes from? Should I urge the owner to look after it better (it's currently in his garage). Should I suggest he donate it to a museum?

I'm always concerned that antiques such as these survive into the future.

[lesmw0sec]

In the absence of a voice coil, it is just possible that you have an inductor-dynamic type (circa 1920's). If this is the case, at the rear of the cone should be a coil or two wound on a permanent magnet (rather like on old headphone unit), but with the diaphragm being replaced by a pole piece coupled to the cone with a steel spindle.

These devices were quite efficient as regards sound output, but the power handling and frequency response was rather poor.

(Total guess here, without seeing it!)

Les.

[peter_scott]

Perhaps a Philips circa 1927?

http://4.bp.blogspot.com/_qUJoz3Oi6Z...te+speaker.jpg

See: http://www.qsl.net/vk2dym/radio/philips.htm

Peter

[boxdoctor]

If you can get a look at the back of this speaker, see if there is a small knurled knob projecting from or inset in the back. If so this will confirm that it is a moving-iron type drive unit. The knob is used to control the mechanical bias on the armature to offset the D.C. current flowing in the anode circuit of the output valve.
Sounds like it should be saved from the bin anyway. Tony.

[PJL]

Or a BTH but that is not a lattice. http://www.radiomuseum.org/r/bth_type_e_form_b.html

[Boom]

Almost certainly 1920s moving magnet. I have a couple here. I wouldn't be too happy leaving it in a garage especially not a damp one.

[val33vo]

I dont know what its worth (Just as much as anyone will pay for it ) but try shoving it on e-bay I do seem to remember however that some old valve sets had a elctromagnet rather than a permanent magnet as is used nowadays it also had a second function as it was wired in opposite phase to the ripple on the HT line and reduced the mains hum emanating from the set

such a type would not be easy to connect to any modern equipment

regards

val33vo

[Maestro Maker]

We used to call them "Balanced Armature" speakers.
They were very sensitive if the adjuster knob is carefully set. More than 70 years ago I worked one off a crystal set in the north London area getting a very strong signal by using a long outdoor aerial. They were better quality than the horn type popular in the 1920s but when I graduated to a 2 valve receiver in 1938 and changed to a moving coil speaker I was in the realms of HiFi - but that description hadn't appeared then!
I have come across 5 inch round versions that at first glance looked like a modern moving coil variety. They were used as a "tweeter" in larger multi-speaker American "tombstone" radios of the late 1930s early 40s.
Cheers MM

[Maestro Maker]

Further thoughts!

Does anyone remember a moving coil speaker produced in the 1960s and made as a flat expanded polystyrene moulding resembling a ceiling tile or packing piece. About A4 paper size in area and having a "picture frame" around the outer rim formed by a moulded groove about 25mm in from the edge. This groove was very deep and approx. 10mm wide so as to leave only a thin section to act as a compliance allowing the inner flat portion to move as a diaphagm. It had a voice coil positioned in the centre of this diaphragm and a decent permanent magnet to complete the drive unit. I think the whole tile was about 2 inches thick. They were meant to be very cheap and damproof and could be hung on a wall like a picture. We evaluated a variety of them but although the sound was passable they were not thought very practical.

MM

[roffe]

Poly-Planar P20 was one of the models.
http://www.faktiskt.se/modules.php?n...wtopic&t=40879

[Maestro Maker]

Wonderful - many thanks Roffe!
I didn't imagine it after all.

MM

[Maestro Maker]

Originally Posted by val33vo

. . . I do seem to remember however that some old valve sets had a elctromagnet rather than a permanent magnet as is used nowadays it also had a second function as it was wired in opposite phase to the ripple on the HT line and reduced the mains hum emanating from the set

such a type would not be easy to connect to any modern equipment

regards

val33vo

You remind me that the moving coil speaker I refer to in my post above No:8 was the electromagnet type (refered to in my day as mains energised) and it was a big disappointment when I first used it - so I took it back to the shop! They explained the magnet was very weak without a current to energise it and the winding was high impedance and made to be placed across the HT supply and not in series as a smoothing choke.
Connecting the energising coil across the 120v DC supply out of my "Battery Eliminator" (q.v.) brought forth full volume.

MAGIC !
Happy schoolboy

MM