The Finer Details of Radio

Radio Record, Volume II, Issue 39, 12 April 1929, Page 30

The Finer Details of Radio

Matching Impedances for Dynamic Cones

By

PENTODE

ECENTLY the writer has been experimenting with moving coil speakers, and his practical expernence, gained after many sad failures, may prove of interest to others. In the case of dynamic cone speakers, impedance of the moving coil has to be matched to suit that of the valve. While in England fully 90 per eent. of speakers of this class are wound with high resistance windings suituble only for use with a 1-1 output transformer or a choke filter system,. Americans adopt only the low resistance windings. Output transformers of various ratios are available from 25-1 to 4500 to 1, and it depends upon this ratio as to how many turns are required for the moving coil. It has been stated that the moving coil and valve impedance must be matched. This is quite correct, whatever the method of output, but in the case of a step-down transformer the ratio of the transformer must be given consideration. A mistaken idea seems to exist in some quarters -that the valve has to be matched to the transformer primary, and the moving coil to the secondary of the transformer. This is quite wrong. So long as the impedance of the transformer primary is well above that of the valve its value is not critical, and as far as a

cfliciency is concerned, the higher the impedance the better. It is the load on the transformer that matters, and in this case it is the moving coil. Assuming that a reliable make of transformer is used, it is the ratio that counts. The number of turns on the

moving coil ‘multiplied by the transformer ratio give the impedance of the load in the anode of the last valve. In. the case of a single valve in the last stage, the output transformer must

be of reliable make and liberal in both iron and wire. D.C.. current to the order of 20 to 25 milliampres is flowing through the primary, thereby lowering its impedance, and lowering the efficiency as a whole, and if insufficient iron is used, saturation occurs, and insufficient wire lowers the impedance whatever the current flowing. English and American Methods. OMPARISONS were given of the English and American methods. One must not get into difficulties through working to specifications of the construction of an Bnglish moving coil speaker, and attempting to run it on the.average American valve. Let me explain more fully. In Bngland the standard valve upon which most of the speakers are designed to be used is the LS5A, with an impedance of 2750 ohms. The equivalent American | valve is the UX 210, having an im-

pedance of 5000 to 6000 ohms, and all commercial speaker manufacturers in both countries base their speaker design uuon the characteristics of their more or less national valves. It will now follow that it is inadvisable to use English valves upon an American speaker or visa versa. There are, however, ways out of the trouble, and the following list of turns will not only be of use to any who are designing moving coil speakers, but will help in the selection of a more suituble output valve for either an English or American speaker. Data For Moving Coils.

These figures show that a moving coil of 105 turns in conjunction with a 25-1 transformer would not give anything like efficiency if used after a valve with an impedance of 2500 ohms. In actual fact a marked predominance _of the low tones would occur with an absence of the many harmonics that are the timbre of most string instruments. Valves in Parallel. THIS data concerns only a single power valve. When two valves are worked in parallel the impedance

of the output of the two is a half of that of one valve. Similarly, when two valves are used in push-pull the output impedance is double that of either valve singly. This, then, adds more complications, especially if oue operates a speaker designed for a single LS5SA type valve from an amplifier employing two 210 type valves in pushpull. Surely something would need correction, and in this case a 4-1 stepdown transformer would be indicated. An advantage is gained if a speaker, designed for a 210 valve can be worked from a push-pull amplifier using two low impedance valves, and this brings us to the best methods to adopt in a country whose radio gear comprises half English and half American manufacture. Certainly most of the M.C. speakers in New Zealand are of American make, designed for an output valve of 210 characteristics, while a large propor:

tion of the valves used by the average constructor are of similar types and characteristics as prevail inEurope. Power valves having low impedances are sold, more so than for those for which available speakers are designed. Perhaps it has already been realised that an ideal arrangement, for anyone desirous of getting the best from his outfit, is obtained by the use of a push-pull amplifier using two low impedance valves fed to the speaker through a centre tapped choke to the transformer usually incorporated in the _ speaker itself. No by-pass condensers are necessary, and if a choke is not ayailable the primary windings of a pushpull output transformer will meet the ease, leaving the secondary terminals free. The diagram given indicates the method.

Several correspondents have written to the R.R. asking for a few articles of a more technical nature, now and then. Judging by the text of their letters, they are keen amateurs, and, having passed their ABC stage in radio, want to keep fully abreast with the times. In an attempt to interest this ever-increasing number of real. enthusiasts, a small article of technical value and interest will be given regularly.

Valve, or Turns for Turns for 25-1 impedance. H.R. coil. step-down Ohms. transformer. 2500 1000 50 2750 1300 55 3500 1650 65 5000 2100 105