A Practical Scratch-Filter

Radio Record, Volume IV, Issue 6, 22 August 1930, Page 26

A Practical Scratch-Filter

For the Radio-Gramophone

‘ AMHouGH a new gramophone record of high quality gives very little scratch noise when reproduced by the medium of a pick-up, there are others that are less free from the necessary high frequencies constituting the seratch noises. Much-used records of any kind may be included in this class. If we reproduce all frequencies up to 5500 cycles per second, the recorded music will not be impaired, and by cutting out all frequencies above that figure it is possible to remove entirely the high-pitched "swish" that accompanies gramophone reproduction when a scratch-filter is not incorporated. When a test is carried out. by connecting and disconnecting the filter during the playing of a record, the improvement effected ‘by the filter is easily noted, and those constructors whose aim is to. make reproduction as nearly perfect as possible, will recognise that the addition of a filter is worth while, especially as the task of construction is a comparatively small "one, It must be emphasised that the filter should ‘not have any effect upon the reproduction of good, new records, but the older the record the more will the cleaning-up effect be noticed. . ’ A-honeycomb coil of 1500 turns in combination with a condenser of .008 mf,d. eapacity tunes to 5500 cycles, but a coil of this inductance (200 millihenries) is difficult ‘to make, and at the present time is extremely difficult to procure. By placing.a smali-amount of iron in the centre of the coil the by-passed band is slightly broadened. This is an advantage, so a small audio choke of low inductance with a total of 1500 turns was decided upon. Taps are

provided at 1100 and 1300 turns. For convenience in placing the spool on a winder, a core 5-8in. square was used, and in order to keep the inductance low and. have easily-accessible material for eonstructors the core was constructed of common tin cut into strips 1 3-8 by 5-8 (bare) inches. On the wooden former, which is not less than two inches long, two thicknesses of stout paper 1 7-16in. wide are placed, securing with seccotine. Manilla or simi‘lar hard paper should be used. Spool ends 1jin. square are made of cellu-

loid or other material, to fit over the ends of the manilla, leaving a space of 1 5-16 between. Wind with 36 s.w.g. enamelled wire. The original was layer wound, but the turns may be "run in." Insulate tap joints with tissue-paper and cover the coil with adhesive tape. Half-fill the core space with laminations, filling the remaining space with a slip of wood or strips of cardboard, packing fairly tight. , "THE coil and condenser are connected in series and across the two pick-

up leads, elther at-the gramophone or at the amplifier end.- The taps on the coil may be brought to studs of a. selective switch. Normally, 1500 turns’ are used, but for very noisy records it will be found necessary to switch over to a lower number. Values are not very critical in this piece of apparatus. It will not cut out scratch noises of a frequency below about 5000 cycles without considerably . affecting the quality of the music. Tests were made with numerous condenser values higher and lower than .008, but this figure was found to give best results. If alternating-current is used to run the amplifier, the coil and condenser should be placed in a small tin box which is "earthed," otherwise the coil will probably pick up a small amount of hum. This was the writer's experience, but the tin box cut out. every trace,