Methods of Volume Control

Radio Record, Volume II, Issue 7, 31 August 1928, Page 27

Methods of Volume Control

ee rr ee we ISTENERS who are situated only 2 compartively short distance from. a broadcast station are particularly interested in the subject of volume conitrol, but it is also of interest to any possessor of a good receiver. R.F. Stages. (THERE are several ways of controlling volume, a good method being the placing of a variable high resistance either in series with the aerial lead, or across the aerial and earth terminals. In either case the resistance must vary from zero to a very high figure, not less than 200,000 ohms in the first case, and several megohms in the second. . a ~+> a a i

If the energy 1s not controled abl tiv entrance to the receiver as above, then a variable high resistance with a maximum of about 400 ohms may be placed in series between secondary coil and R.F. transformer and filament. This method, however, decreases selectivity. Another method is to place a 200,000 ohm maximum variable resistance il the B positive lead to the R.E. valves. There should always be provided a bypass condenser of not less than .005 capacity, placed on the side of the resistance nearest the primary coil, the other end of the condenser being connected to the earthed filament lead. This method is very satisfactory with most receivers, and is really a way of reducing amplification, or oscillation control. The filament rheostat control is very popular, and very satisfactory when wisely used. This method functions by limiting the emission current, which increases the impedance of the yalve, and if the current is unduly cut down, distortion is possible, so that care is required to see that the reduc tion is kept within reasonable limits

The Detector. (SONTROLLING volume in the early stages is a good scheme, but in a set employing regeneration it is understood that reaction should be suitably reduced before other controls are used, and very often will be sufficient, leaying other controls set at the point of maximum efficiency. Sometimes a variable high resistance of the noninductive type is shunted across the tickler, and though this system is used in many circuits, especially those for short-wave reception, is not always the most satisfactorv. and moving coil or condenser control are more popular. If control is left until after the detector stage, it is not so satisfactory as control in R.F. stages, because the detector may then be overloaded with strong signals, causin’ distortion that cannot be eliminated by any subsequent system of volume control or reduction. Audio Control. UDI1O control may consist of a variable high resistance across primary or secondary of transformers, maximum 100,000 and 500,000 ohms respectively, or across the output, that is, across the speaker leads, in which case a resistance with a maximum between 25,000 and 50,000 ohms will suit. Volume control by detuning the nerial circuit is bad when the detun-

ing only amounts to a few kilocycles, and can cause bad distortion in a selective receiver. In an unselective receiver detuning will usually have no ill effects. In a selective receiver detuning is only permissible if through i. good many degrees on the dial, which is an amount that would cut reception out altogether. If the receiver is unshielded there will always be a certain amount of piek-up by the coils.