Western Electric Field Coil Speaker
|I can't believe this
speaker sat atop a speaker cabinet here for 5 years; I bought it in 2012
for $21 but it cost about $55 to have it shipped from Chicago.
Part of that delay was due to not being able to figure out how to remove
the transformer from the top. Another reason is that time flies
and the older one gets, the faster it seems to pass by.
Anyway, the first 4 photos show the speaker in its original condition; it supposedly dates back to 1923 and I was informed that it's a Western Electric, despite there not being any markings. At some time in the past, it appears to have been modified; note the plastic plate at the rear.
The transformer on top is to match the voice coil impedance to that of the output stage of an amplifier. Unbeknownst to me until the speaker was disassembled, what I saw as a voice coil surprised me. Since in its original configuration it would be useless unless an amplifier similar to the original was obtained, serious modifications were made. Therefore, this can't be called a restoration but rather a re-fabrication. More on that weird voice coil later but here are some other parameters.
The impedance matching transformer has 3 wires to the primary and two from the secondary, which is the two turns of heavy gauge copper wound around the outside of the transformer. The dc resistance of the primary is about 400 ohms and is center tapped.
This transformer was not used in the re-fabrication but was left on the speaker. The original voice coil has been stored in a box.
The field coil measures 3660 ohms and is also center tapped. There is no hum demodulating coil, (hum bucking coil) as the speaker was probably unable to produce an audible output at 120hz due to the stiffness of the moving system. There may have also been a high pass filter in the output stage. The field coil most likely was used as a filter for the amplifier's dc supply. High voltage electrolytics were large and probably expensive back then due to the lack of efficient dielectrics.
|The secondary windings of the impedance matching transformer. Note the two tapered heavy gauge copper connectors from the secondary to the voice coil.|
|The voice coil. This thing measures 0.1875"
(3/16") wide, is 0.050" thick and is 2.0" inside
diameter. There's enough copper in that coil to make a lid for a peanut butter
jar. The total dc resistance of the whole secondary of the
transformer and the coil is less that 0.01 ohm, essentially a short circuit.
The Fluke meter oscillated between 0.01 ohm and 0.005 ohm. However,
the following SWAG may be true in whole or in part. With the coil surrounding the pole piece and with the field coil
energized and at audio frequencies, the actual impedance will be considerably
higher. Unfortunately, I didn't make that measurement.
SWAG = Scientific Wild Ass Guess
|The field coil in it's original location (left) and re-centered (right)|
The following series of photos should be self explanatory.
The voice coil has two layers of 22 turns each of #36AWG copper wire.
The winding height is the same as the thickness of the top plate, 0.1875" (3/16")
The coil form is 0.002" brass.
The cone had to be shimmed about 0.1 inch as trimming the apex would have enlarged the hole into which the voice coil fits.
The original bolts were installed for appearance only.
The 6 steel sectors originally used to hold the cone covered the outer corrugation of the annulus.
|The wires leading
into the field coil were covered with shrink tubing and extended as the
original insulation had dried to the point of being useless and also
hazardous, especially with 234 vdc.
The holes for the voice coil binding posts were difficult to drill as the metal used for the frame seemed to be as hard as the drill bit. They had to be drilled initially at 1/16" and successfully drilled larger to 7/32".
The field coil power supply (lower right) will easily apply 234vdc into the 3660V of the field coil for a power of 15 watts.
The dc is filtered with a 680uF/400vdc capacitor which gives straight line dc (<7mv ripple) under load, 0.063 amp
Impedance curve with 15W to the field coil
Fs = 85.7 hz Z(fs) = 18.4 V
Nominal impedance, Z(nom) = 9.1 V at 232 hz
Considering its age, 94 years, T/S parameters were deemed pointless and therefore not determined.
However, I couldn't resist running a near field response, just fer schitts 'n giggles.
The red curve is the black one smoothed at 1/4 octave.
Manufacturers are notorious for using this but at 1/3rd octave as it makes the curve look nicer by removing all those nasty peaks and dips.
It is somewhat of a deceptive practice, unless it's specified.
Undoubtedly, the original may have had more serious peaks and dips due to the extremely thin paper used for the diaphragm,
but that may have also increased the high frequency response beyond 5700hz.
The original cone measures 0.016" thick and the replacement measures 0.027"
The near field response here above about 1000hz is probably useless as cone break-up at higher frequencies introduces errors. However, that principle is usually adhered to when running a sweep at 1w1m. This sweep was run at considerably lower power levels due to the proximity of the mic to the cone, about 0.125 inch. The response is quite smooth up to 1000hz whereas at 1 watt, it usually becomes very erratic around 400hz. Many manufacturers run near field below 400hz and 1w1m above.
Running a 1w1m sweep meant mounting the 45 pound speaker in a cabinet and raising the cabinet up from the floor so the duct cap would be midway between the floor and ceiling. This also meant moving the DC power supply.
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