Bass Amplification Described for the Serviceman
Sensitivity of Human Ear Varies With Volume and Frequency
Excerpted from Philco Serviceman, February, 1937
The function of a radio set is to convert a faint electrical signal originating somewhere in a microphone or phonograph into sound waves for the benefit of some listener. The ideal set is one which enables this listener to forget that there is an electrical link between him and the initial sound source. This ideal criterion implies that the set will reproduce every audible frequency. It does not, however, require that the amplitude of the several sound waves of different frequencies correspond to the amplitudes of the original sound waves. Rather, this correspondence is dependent upon both frequency and volume.
It is a characteristic of the human ear that its sensitivity varies widely with frequency and volume. At moderate volume a sound wave of a given energy level will have the same loudness regardless of its frequency. At a lower volume level a sound wave of a given energy level will sound louder at medium-high frequencies than at low frequencies or very high frequencies. The energy level at which a radio set is operated is generally lower than that at which the sound is originated. It is, therefore, necessary to amplify the low audio frequencies more than the middle tones to compensate for this physiological effect. However, as the volume level at the set is increased, the amount by which the bass tones are augmented should decrease.
In the practical radio set the range of audible sounds which can be reproduced is limited by the capability of the set, the amount of the radio frequency spectrum which can be devoted to the transmission of the electrical signal, the amount of noise signal, etc. The lower audio frequencies should be amplified more than the higher frequencies by an amount dependent upon the volume level of the reproduced sound to compensate for the physiological effect. In addition, the set should make up for the deficiencies incurred in the wire and space transmission to the set.
Philco 690 Audio System
The PHILCO 37-690 provides for this additional amplification of the lower audio frequencies by use of a separate channel in the audio amplifier. To obtain the necessary variation in amplification of the lower frequencies in accordance with the total volume level, the degree of amplification in this channel is controlled by a special gain-control system. The two separately amplified signals are then combined and amplified in a power output system and again divided and supplied to appropriate loud speakers.
The functional diagram given below shows the entire 690 audio amplifier and sound-reproducing system. For simplicity, many power-supply filters, bypass condensers, voltage dividers, etc., have been omitted, only the essential elements being included. The main signal paths have been indicated by the heavy lines. The amplifiers and circuits have further been segregated in units to indicate their particular function.
Signals are supplied by the detector through suitable filters to the volume-control potentiometer. The filters are so chosen that the signal across this unit is substantially a pure audio signal devoid of any intermediate frequency components. The a.v.c. system is so arranged as to maintain this audio signal at a constant level independently of the strength of the received signal, but directly proportional to the degree of modulation of the carrier wave. The user selects the volume level at which the sound is reproduced by manipulation of the volume control. Signals thus adjusted for strength and amplified in the first audio amplifier are supplied to the main amplifier and bass amplifier channels.
Main Amplifier Channel
In the main amplifier channel the whole audio signal is transmitted to an adjustable fidelity control, which in turn passes it on to the 6R7G amplifier stage.
The fidelity control consists of an adjustable resistor and condenser and operates as a low pass filter. At the cut-off frequency the impedances of the condenser and resistor are equal, and if the contact is at the junction of the resistor and condenser, signals above the cut-off frequency will be attenuated. Signals of lower frequency will not be attenuated as much, however, for with lower frequencies the condenser impedance is greater. If, on the other hand, the contact arm is at the other end of the potentiometer, that is, the end connected to the proceeding amplifier, then the amount of signal transferred will be substantially independent of frequency, since the amount of signal across the whole unit is substantially independent of the impedance of the unit. For intermediate positions of the contact, the amount of signal attenuation will vary accordingly. At low frequencies the signal will be transmitted without attenuation regardless of the position of the contact arm. In the 37-690 this control is mechanically gauged to the intermediate frequency band-expander so that the band widths of both the audio and I.F. stages are jointly controlled.
From the fidelity control the signal is transferred to a second amplifier tube (6R7G), which also contains the diode for the bass amplifier gain control. The load circuit of this tube contains a 10 k.c. filter to eliminate any adjacent channel whistle. This filter is a tuned circuit having a high impedance at its resonant frequency (10 k.c.), but low impedance for frequencies off resonance. At resonance its impedance is much greater than that of the load resistor and high-frequency bypass condenser, but for audio signals up to about 8 k.c. the reverse is true. Thus audio signals up to about 8 k.c. will build up across the resistor and be transferred to the driver stage, whereas a 10 k.c. whistle builds up across the tuned circuit and forms practically no signal across the resistance and condenser. Only signals formed across the resistor are transferred to the driver stage.
Bass Amplifier Channel
Signals similar to those supplied to the main amplifier are likewise supplied to the bass amplifier channel and first amplified in the 6R7G tube, whose gain is controlled by a special control system. The transfer impedance for this tube has the characteristic that the amplitude of the signal transformed decreases with frequency in such a way as to compensate for the normal deficiencies of the ear. The net result is that the bass frequencies are amplified by an amount which decreases with increasing frequency, while the high frequencies are not transmitted. The amplified signals are thus transferred to a bass volume control which enables the listener to select the degree of bass amplification which he desires.
The output signals of the main and bass amplifier channels are combined in the grid circuit of the driver stage and then amplified in a push-pull, high-power output stage. A bridge circuit is used to combine the output signals of the two channels and at the same time prevent any interaction between the two output tubes which might cause distortion. Throughout the system the amplifiers are operated at power and voltage levels well below their rated values to keep any possible distortion at an absolute minimum and to insure a tremendous overload capacity.
The cathedral speaker is directly coupled to the output stage and is energized by the whole audio signal. Two additional speakers responsive only to high-frequency signals are used to improve the high-frequency response. A condenser in series with the output transformer forms, in combination with the leakage reactance of the latter, a high pass filter which transmits the higher frequency signals to the additional speakers, but prevents the transmission of the lower frequency signals, which, having a larger amplitude, would tend to overload these speakers.
The output signal from the driver stage is also used to control the gain of the bass amplifier channel. This is accomplished by rectifying a portion of this signal in the diode unit of the 6R7G and using the rectified signal, appropriately filtered, to bias the input grid of the control tube in the bass amplifier channel. As will be apparent, the input grid will be made more negative as the output signal increases in amplitude. In this fashion the amount by which the lower frequencies are augmented is caused to decrease with increasing output level. A delay voltage is used to prevent the control from functioning until the output signal exceeds a certain threshold value.
It is not sufficient that a sound-reproducing system reproduce sustained signals which appear to the listener to be the same as the originating signals. It is further necessary that the reproduced signals have no distortion. In other words, when a kettle drum is thumped, that initial thump should be clearly and distinctly heard, and when the drum is stopped by the player damping it with his hand the reproduced sound of the drum should stop instantly and not drag on and slowly die out. This requires that the cabinet, which is acoustically resonant at some low frequency, be sufficiently damped to prevent the formation of acoustic reverberations. The acoustic clarifiers serve this purpose. In the functional diagram they are indicated as diaphragms connected to a base through a resistance. The resistance corresponds to the damping material between the apex of the cone of the clarifier and the frame. The diaphragms are tuned to the resonant frequency of the cabinet and at these frequencies absorb sound energy from it which is dissipated in the damping material. Thus the acoustic clarifiers damp the cabinet and prevent it from resonating in an analogous way to that in which a resistance will damp a resonant electrical circuit and prevent it from oscillating. Transient distortion is thus prevented.
The sound-reproducing system of the 37-690 thus serves three purposes. It brings to the listener sounds of every audible frequency which it is practical to transmit. It modifies the amplitude of the sounds of the several different frequencies in such a way as to compensate for the physiological difference in hearing at different energy levels. It enables sounds of all frequencies, and especially low frequencies, to build up and die out in such a manner as to reproduce faithfully the variations in amplitudes of these signals at the originating source.