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Getting the best out of the receiver.
No receiver will give of its best unless the associated apparatus is suitable. In this article, hints are given on choosing a loudspeaker for the New Monodial Super and on installing it to the best advantage.
Full details of the construction and the initial adjustments of the New Monodial Super have already appeared and little more need be said about the receiver itself. Since the performance may be greatly affected by such external factors as the aerial system and the loud speaker, however, it is felt that some attention should be devoted to matters of this nature.
The aerial and earth system, although often over looked in these days of sensitive receivers, is still an essential feature. With the receiver in question the earth is perhaps of less importance than the aerial, for the set is essentially stable and hum-free; nevertheless, it is wise to use as good an earth as possible. The best type of aerial, however, is a matter upon which it is impossible to be dogmatic, for it depends greatly upon local conditions. In cases where the set is used in a district remote from any broadcasting station, it is usually best to employ an efficient aerial, even although the sensitivity is sufficient to give distant reception with a poor collector, for the best reception of very weak stations and the full benefit of AVC in counteracting fading will then be secured.
If the set be used in the shadow of a local station, however, it is a wise plan to limit the size of the aerial in order to avoid any possibility of either the HF valve or the first detector being overloaded. Any overloading, of course, would result in the production of whistles. The limitation to the size of the aerial may be either physical or electrical; the latter is often the more convenient, and it can take the form of a small capacitor connected in series with the aerial lead to the set. The optimum value should he found by trial, since it will depend upon the actual size of the aerial and upon its nearness to the local station, but a capacity of some 0.0002 μF will often be suitable. It should be emphasised that such limitation of the aerial size is only rarely necessary, for the receiver will handle an unusually large signal input before overloading occurs.
The loud speaker is the next matter deserving attention, for upon it largely depends the quality of reproduction which is secured. In itself, the receiver gives an overall response which is substantially level up to 7,000 Hz, and if the full benefit of this is to be obtained it is obvious that the speaker should give a similar response. Moreover, since the maximum output is about 6 Watts, the speaker should be designed to handle this input otherwise amplitude distortion will occur.
Dual-matched pairs offer themselves as one type capable of giving a satisfactory performance. In this arrangement, two similar, but not identical, speakers are used, and their natural resonances are arranged to occur at slightly differing frequencies. As a result, the power handling capacity is more than double that of a single similar speaker, and a good dual pair will usually handle 5 to 6 Watts comfortably. Furthermore, by the staggering of resonances, the frequency response curve is smoothed out so that the audible effect of resonances is largely eliminated.
Dual speakers are usually obtainable with field resistances of 2, 500 Ω per speaker, so that a pair of fields connected in parallel is directly applicable for energisation from the power unit, which has, in fact, been designed with this point in view. Many suitable types are available, and the products of firms such as Celestion, Magnavox, and British Rola are all worthy of examination.
Single Speaker Types
The push-pull output stage, together with the mains and smoothing equipment, are built - as a separate unit from the receiver.
It is sometimes difficult to fit dual speakers into the conventional type of cabinet, however, and it is fortunate, therefore, that their use is by no means an essential factor to the attainment of a high standard of quality. Many of the larger type of single moving-coil speakers are designed to handle an input of 6 Watts, and are capable of an entirely satisfactory performance. Speakers of this type usually have a fairly large diameter cone with a very free mounting, so that a large amplitude of vibration is possible, and the field energisation required is often up to 20 Watts.
The BTH RK speaker.
One very well-known speaker which falls into this category is the BTH Senior, and it is rated to handle 5 Watts, its speech coil impedance is 15 Ω, so that the output transformer, about which more later, should have a ratio of 25.8:1, or with sufficient accuracy 25:1. Unfortunately, the field resistance of the standard model is 5,000 Ω, so that it is not directly applicable to energisation from the power unit. It could be used with a separate source of field current, however, or the field could be rewound to a resistance of 1,250 Ω.
A 6.5 Watts permanent-magnet speaker - the Ferranti M1.
Another specimen of this class is the Ferranti M1 model, for this is rated to handle 6.5 Watts; it has a speech coil impedance of 20 Ω, so that here the transformer ratio should be 22.5:1. This particular speaker is a permanent magnet type, so that no energisation difficulties arise; when using it, however, it is necessary to fit a 30 H, 1,250 Ω, 120 mA choke to the power unit.
Guarding Against Resonances
The New Monodial Super housed in a CAC burr-walnut cabinet.
In perhaps the majority of cases the speaker will be fitted into a cabinet instead of being mounted on a baffle board, and if boominess is to be avoided it is necessary to guard against box resonance; Although it is possible so to adjust matters that box resonance occurs at a frequency at which the speaker response is deficient, and so actually improves the final result, this requires a high degree of skill. In general, therefore, it is wise to content oneself with choosing a wide and shallow, rather than a narrow and deep, cabinet, and if any trace of box resonance should be found, it may often be removed by, providing a lining of acoustically dead material. The use of slag wool packing in the well-known manner is also beneficial.
If the receiver be mounted in the same cabinet as the speaker, a further point to guard against is acoustic reaction. This rarely occurs in connection with the valves in a modern set, but usually to the plates of the variable capacitor, and, even if it is not present in sufficient intensity to set up a sustained howl, it may cause a low frequency resonance similar in its audible effect to box resonance. Fortunately, the cure is simple, and consists merely in mounting the whole receiver chassis on blocks of sponge rubber so that it can float freely.
These questions of box resonance and acoustic reaction are of considerable importance, as is also the low-frequency resonance of the speaker, for they are the usual cause of boomy reproduction. Although it is not difficult to reduce them to small proportions, in extreme cases a slight modification to the set might become advisable. Where such resonances are found, therefore, it is useful to note that the bass amplification may be reduced slightly by the simple expedient of fitting a 2 μF second detector anode circuit decoupling capacitor instead of the 1 μF capacitor specified. In the other case, where the bass is deficient through the use of a speaker with a poor low frequency response or a small baffle, the bass can be increased by decreasing the capacity of this capacitor; 0.5 μF to 0.25 μF is then recommended. It should be emphasised that the normal value of 1 μF is right for normal conditions as to speaker and cabinet, and it is actually intended that the receiver be included in the same cabinet as the speaker, so that it is unlikely that any difficulty would be experienced. The remarks in this article are based chiefly upon abnormal circumstances, in order to point out the path which must then be followed for the attainment of normal results.
In conclusion, a word about output transformers is not out of place. A push-pull transformer with a primary inductance of some 60 H, a low DC resistance, a power rating of 6 Watts, and a ratio depending on the speaker, is necessary. In some cases suitable types of transformer are supplied with the speaker, but in others a separate component must be obtained. Unfortunately, there are not many types available with the requisite primary inductance and power rating which have also a good characteristic at high frequencies. It is hoped, therefore, to publish constructional details of a transformer specially designed for use with this receiver in an early issue of The Wireless World. See here.
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