It was standard design in early 30s & 40s radios to use what was called a field coil to supply a magnetic field to the speakers. Standard circuitry at the time involved routing around 375v DC from an AC rectifier into the field coil of the speaker- the resistance of which dropped the voltage to about 290vdc which was then supplied to parts of the radio.
The field coil also acted as a choke for AC hum( ripple, hence it played an important part overall in the radio.
Restoring such early radios can be problematic especially if the old speaker is either absent or the field coil is not functioning for some reason…
While it is possible to design a high tech filter that combines a DC voltage reducer circuit with discrete semiconductor devices, it would seem that the MP9488 could be a more simpler way to a) reducing the 375vdc to around 290vdc and b) also assist in the overall filtering of 50/100Hz ripple/hum from the 290vdc supplied to the radio valves.
Looking at the suggested schematic for using the MP9488 for the above purpose, i.e. supplying 290vdc at 90mA , could someone give me a steer as to a suitable value for the inductor and its associated capacitor?
Any help…really appreciated
Frencbruce
Hi Bruce, please refer to MP9488 datasheet on pg. 15 for sizing inductor and output capacitor. Thanks.