Field Coils -- how to find proper operating point?

[farleybob]

Hi,

I just bought a field coil coaxial speaker and was wondering how to determine the proper operating voltage, if not printed on the speaker. It's a Stephens E52A, I think from the mid-1940's. I've not seen one before. I'd love to hear, but I'd hate to destroy it!

Thanks and Best Regards,
Steve

[Hoerninger]

Hi Steve,

some details of the powersupply will probably give some hint or information.
For comparison, I've got an Klangfilm fieldcoil loudspeaker from the 1940s, it works with 250 Volt DC.

Greetings
Peter

[Ken Pachkowsky]

Steve Schell could probably tell you everything there is to know about field coil drivers. Send him a message.

Ken

Hi,

I just bought a field coil coaxial speaker and was wondering how to determine the proper operating voltage, if not printed on the speaker. It's a Stephens E52A, I think from the mid-1940's. I've not seen one before. I'd love to hear, but I'd hate to destroy it!

Thanks and Best Regards,
Steve

[Steve Schell]

Hi Steve,

The proper DC supply voltage for these old field coil speakers has to be determined by measuring the DC resistance of the field coil and making an educated guess of the intended dissipation of the unit. I don't have any literature at hand that describes the E-52, and field coils were often wound for various supply voltages anyway, depending on the application.

The Lansing woofers of the same period were rated for 30W minimum and 40W maximum dissipation. I think it would be safe to calculate your Stephens for 30W, perhaps trying a slightly lower supply voltage first. These structures will become warm after being on for awhile, but should not become more than moderately warm to the touch. They take a couple of hours to reach full operating temperature.

Using the Ohm's Law equation of E = the square root of P x R, you can calculate the supply voltage. For instance, if the DCR measured 500 ohms, then the correct voltage would be the square root of 15,000 (30 x 500), or 122 volts. I would recommend applying a voltage perhaps twenty percent less than this to the field coil and monitoring its temperature for a couple of hours before applying the full voltage.

I'll also add my stock recommendation to those new to field coil speakers. Do not apply an audio signal to the voice coils without the field coil being energized first. Although there will be a small output from residual magnetism in the pole pieces, virtually all input is converted to heat in the voice coils and they can burn out easily.

Your E-52 is a rarity, and probably a very good driver. Stephens claimed, falsely I think, to have built the first multicellular coaxial speaker unit. I believe that honor goes to the Altec Lansing 601 field coil Duplex, but Stephens followed with the E-52 within a year or so. They were replaced by the P-52 permanent magnet unit by the time that Stephens moved to Culver City in 1947, so the E-52 was made for only a short time. I've tried to score one for my collection, but haven't succeeded yet. Please let us know how it sounds when you get it going!

[farleybob]

Thanks for the info everyone. Thanks Steve -- I'll check out the field coil resistance and calculate an estimate from there.

By the way, great pics of the Altec 601! Wow.

STeve