Interesting Chinese field coil 13" driver.

[struth]

That one has bothered me for some time. I assume it is historic, as permanent magnets then were not powerful enough (i.e. could not produce sufficient flux density).

However the power supply has to be very well smoothed, otherwise any residual mains hum would modulate the audio signal. Traditionally they ran off 100V and consumed a fair bit of power.

The only advantage of field coils I can see is that you can vary the energising current, and hence the damping.

How about back emf has less effect on magnetic field integrity and therefore is more accurate under normal operation.?? dont know, just asking