Patent GB-489178-A (GB)
Title: Improvements in or relating to sound recording apparatus
IPC: G11B7/00
Filed: 1937-01-22
Abstract: 489,178. Gramophone &c. recording circuits; light-valves ; valve amplifying circuits. WESTERN ELECTRIC CO., Ltd. (Electrical Research Products, Inc.) Jan. 22, 1937, Nos. 2005 and 24102. [Class 40 (iv)] [Also in Group XL] In a system in which audio-frequencies are supplied to moving conductor instruments such as a light valve or galvanometer for electro optical recording, distortion may arise due to voltages generated in the conductor by its motion in the magnetic field and which have a maximum value at mechanical resonance, and also due to transient currents or clashing of the ribbons of a light valve. This is avoided by feeding back to the input of the instrument or of a stage of the amplifier, compensating voltages derived from the voltages set up by the motion of the conductor. The arrangements described comprise bridge circuits which feed back such compensating voltages but prevent feed back of the audiofrequency input. Any desired frequency response may be obtained by frequency discrimination in the feed back path. Specifications 272,622, [Class 40 (iv)], 367,479, 382,663, 390,157, [all in Group XXXVI], and 479,083, [Group XXXVIII], are referred to. In the arrangement shown in Fig. 1, the source, such as a microphone, pick-up or wireless receiver, is connected to the input 10, 11 of amplifier 16, 17, the output 21, 22 being connected to a light valve which is biassed. in accordance with the envelope of the input by a circuit 23-26 connected to a centre tap on the output transformer. The secondary of the transformer is shunted by two resistances 27, 28 forming a bridge circuit with the light valve and a variable resistance 20, and a feed back supply circuit 29, 32 is connected across diagonal 19, 22, The bridge is balanced as regards the output of the amplifier but voltages set up across 21, 22 by the light valve give rise to voltages across potentiometer resistance 32 which are amplified at 33 and fed to the input or other suitable point of the recording amplifier, or to the output transformer. Condensers 29, 34 discriminate against feed back of the lower frequencies to prevent feed back of bias-voltages from circuit 23-26 and to increase the low frequency response of the recording system, whereas circuit 30, 31 introduces a phase-shift compensating the phase-shift due to the output transformer which has a tendency to cause positive feed back. In the modified arrangement shown in Fig. 2 for use with various different types of light valve, the output of the recording amplifier is connected at 49, 50. In the case of ordinary single ribbon valves, the valve is connected to terminals 43, 45 with 44 unconnected, and the biassing circuit (23, 26, Fig. 1) is connected to terminals 40, 42, terminals 47, 48 being strapped together. The inductance 51 provides additional smoothing for bias and tends to prevent audio-frequency currents flowing back in the bias circuit. The feed-back controlling bridge comprises a resistance and potentiometer shunt 55-57, adjustable impedance 53, and the light valve. Voltages due to motion of the light valve are set up across potentiometer 58 and applied over transformer 60 to an amplifier 63, 64 and fed back into the recording circuit by a low impedance transformer 54. Condenser 59 discriminates against low frequency feed-back and circuit 61, 62 modifies the phase relation of high-frequency feed back to increase the effective high frequency response of the valve. Feed-back amplifying valve 64 has a local negative feedback path 65, 66 to reduce its effective impedance. In the case of the light-valve shown in Fig. 3 with a connection to the mid-point of the loop, terminals 67-69 are respectively connected to terminals 43-45 and the bias circuit is connected to terminals 41, 42, terminals 46, 47 being strapped together. The recording circuit may also be used with similar connections to the double valve shown in Fig. 4 for producing a pair of push-pull records. The bridge comprising resistances 53, 70 and the two limbs of the valve or valves, is balanced to the audio-frequency input but voltages set up by valve motion cause voltages across diagonal comprising resistances 71, 72 the junction of which is connected over potentiometer 58 and the lower wire of the recording circuit to the mid-point of the ribbon loop or to the junction of the two ribbons. The voltages across potentiometer 58 are fed back as before. The two limbs, Fig. 3, or two valves, Fig. 4, are in series as regards bias voltages and in parallel as regards audiofrequency input and the bridge is conjugate as regards audio-frequency input and bias voltage. In using Fig. 2 with the modified double valve shown in Fig. 5, the ribbons A, B replace the resistances 53, 70 of Fig. 2, so that the four ribbons form the bridge, bias being applied at 41 and terminal 69 being connected to terminal 45. The direction of signal current flow is indicated by the arrows on the ribbons and of bias current by the arrows alongside the ribbons. The first Provisional Specification describes arrangements for accurately balancing the bridge with the aid of a meter in the feed back path and the second describes an auxiliary conductor moving with the main conductor for providing feed-back voltages.