Field Coil as Filter Choke

The thrifty 1930s receiver put the loudspeaker field in series with the HT line. Its inductance smooths rectifier ripple; the DC that must flow anyway energises the magnet. This plate analyses that capacitor-input CLC filter.

Parameters

Voltage on reservoir C1

V_{C1}

Load current

I

Field inductance

L

Field resistance

R_f

Reservoir capacitor

C_1

µF

Smoothing capacitor

C_2

µF

Mains frequency

Rectifier

Fig. 11Capacitor-input π filter. The speaker field doubles as the series choke.

Filter performance

Output ripple

V_{r2}

865 mV p-p

Attenuation @ ripple-34.1 dB

Ripple on C1

V_{r1}

43.8 V p-p

Resonance

f_0

14.1 Hz

Ripple frequency

f_r

100 Hz

DC drop in field

IR_f

70 V

B+ output260 V

Field power

P_f

4.9 W

☞ Field dissipates 4.9 W, energising the magnet for free.

Transfer function |V_out / V_C1|: frequency response

How it works

Theory · assumptions · formulas

Ripple on the reservoir

Between charging peaks the reservoir capacitor $C_1$ supplies the full load current alone, so its voltage sags in a sawtooth of peak-to-peak amplitude

V_{r1} \approx \frac{I}{f_r\,C_1}, \qquad f_r = \begin{cases} 2f_{mains} & \text{full-wave} \\ f_{mains} & \text{half-wave}\end{cases}

The LC section

The field coil (inductance $L$ , resistance $R_f$ ) and $C_2$ paralleled by the load form a voltage divider:

H(j\omega) = \frac{Z_p}{R_f + j\omega L + Z_p}, \qquad Z_p = R_{load} \parallel \frac{1}{j\omega C_2}

Above the resonance $f_0 = 1/(2\pi\sqrt{LC_2})$ the response falls at 40 dB per decade, so ripple well above $f_0$ is attenuated by roughly $(f_0/f_r)^2$ . The output ripple is $V_{r2} = V_{r1}\,|H(j2\pi f_r)|$ . The sawtooth's harmonics are attenuated even more, so this is slightly pessimistic.

The price: DC drop

The field resistance drops $IR_f$ volts, which the transformer must supply in addition. Designers chose the field resistance so that this drop, multiplied by the current, gave the magnet its rated power (typically 5–10 W for a domestic set):

P_f = I^2 R_f

Resonance warningIf

f_0

approaches the ripple frequency, the filter amplifies rather than attenuates, and load transients from a class-AB output stage can excite ringing. Keep

f_0

below about a third of

f_r