Replacing an Electrodynamic Loudspeaker with a Permanent Magnet Loudspeaker
By Charlie DeFir, W7NSQ
First for those who don't know the difference between an electrodynamic speaker and a permanent magnet speaker:
If a coil is placed in a steady magnetic field and then energized with an alternating current (audio frequencies in this case) it will physically move back and forward according to the polarity and amplitude of the applied current. Without a surrounding magnet field it will not move. Further, if the coil is attached to a diaphragm (or cone) which is free to move with the coil (now let's call it a voice coil), it will compress and rarify the surrounding air which results in vibrating the human eardrum resulting in sound.
So much for the physics of a loudspeaker.
In the 1930s permanent magnets with sufficient strength were very large and heavy. Consequently a means was sought to replace the permanent magnets. Voila! A large, many-turn coil was wound around an iron structure at the rear of the loudspeaker. When energized with a substantial current flowing in only one direction (direct current) a steady magnet field was produced. This coil is called the FIELD COIL.
Now the problem was to find a source for this direct current. Each AC operated radio must have a power supply in which the line voltage is stepped up to an appropriate level for operation of the vacuum tubes and since the vacuum tubes require direct voltage (and consequently direct current) this high voltage must be rectified (converted to DC). If we used only a rectifier, the power supplied to the vacuum tubes would be pulsating direct current, meaning the current is always the same polarity but the voltage amplitude would vary from zero to its maximum at 120 Hertz (cycles per second). If this source is applied to the vacuum tubes a very loud hum would be present in the loudspeaker. To change the pulsating DC to smooth constant DC a filter is required. An efficient filter circuit consists of capacitors and one or more filter chokes. A filter choke is a many-turn coil wound on an iron core. Wow! Let's use the loudspeaker FIELD COIL as a filter choke!
This combination of a voice coil and a FIELD COIL resulted in a general loudspeaker design which was used for several years.
Then, the Alnico magnet was developed. The basic magnetic strength of this material was such that a small compact magnet would do the same job as the FIELD COIL. Later the Ceramic magnet was developed and is also a compact replacement for the FIELD COIL.
As you can see, when using a modern Alnico or Ceramic magnet loudspeaker, something must be done to replace the FIELD COIL.
A simple solution is to substitute a filter choke for the FIELD COIL; however most standard filter chokes which will operate at the current levels involved will have a DC resistance considerably lower than that of the original FIELD COIL. Therefore, a resistor should be placed in series with the filter choke sufficient to make the total resistance of the filter choke/resistor combination be approximately the same as the FIELD COIL, otherwise the B+ voltage supplied to the vacuum tubes and other components will be quite a bit higher than the original.
A more common approach is to replace the FIELD COIL with a resistor of approximately the same resistance. This is typically around 1,000 ohms.
The power dissipated in the resistor must be taken into account. Power dissipated equals the square of the current times the resistance. This is expressed as P=I2R. A somewhat typical radio of the era might have a total B+ current flow of 70 milliamperes or 0.070 amperes. 0.07 squared is 0.0049 which multiplied by 1000 is 4.9 watts. Allowing a 100% safety factor, use a 10 watt, 1000 ohm resistor. Since there will be a fair amount of heat given off by the resistor, it should be mounted clear of other parts such as capacitors.