Application of the M1115 100V line, 15W Speaker Transformer to Valve Push-Pull Output Transformer Application.

M 1115 is the part number of a 100V line speaker transformer from Perth/Australia based company Altronics. It is a 15W 100V line speaker transformer with multi-tapped primary and secondary to suit 8 Ohm speakers. Frequency Response: 30Hz - 20kHz ±3dB Secondary Tap: 8 Ohm Power taps: 1.25W, 2.5W, 5W, 10W, 15W Size is about 50mm high, 50mm long (plus mounting flanges) and 27mm wide. Price is A$11.55 retail!

The M1115 rating data is analysed by tabulating the spec for the various terminals as follows. In the left column are listed the original primary termination names. These all connect to a nominal 100V line, in the originally intended application, so this is shown in the next column, with respect to the primary "Common" terminal. The third column shows the rated power each of these terminations is used for into the nominal secondary load, shown in the fourth column. From secondary power and load comes secondary (RMS) voltage by V=(P.Z)^0.5 shown at column five, and then comes the comparison (i.e. the ratio) of primary to secondary voltage, from the preceding figures. The square of this ratio gives the ratio of impedances between primary and secondary, and when multiplied by 8 Ohms, as shown in the last column, we see how an 8 Ohm load would appear, reflected back into the primary. All figures, at this point, continue to use the primary "Common" terminal as the point to which voltages are referenced.

From this table it is observed that the transformer has the characteristic of an 8 kOhm primary with respect to an 8 Ohm secondary. The "Common" and "1.25W" terminations are the two "end" terminations of the primary. It is further observed that the "5W" termination has the characteristic impedance of a centre-tap between these two extremes - from transformer impedance theory the end-to-end impedance is not double, but four (4) times the impedance from centre-tap to the reference end termination; 2 kOhms in this case.

In this second table, the values are shifted to reference the "5W" termination as a centre-tap "0". That it is the centre-tap is easily seen as the symmetry to the "Common" and "1.25W" terminals is made more obvious. That the "15W" and "2.5W" terminals appear to obey a similar symmetry either side of the "0" also now becomes apparent. The "10W" tap evidently has no such counterpart and becomes irrelevant.

In this detail copied from the second table above, it is observed that the voltage ratio at the "2.5W" and "15W" terminals is around 42% of the full half-winding, again with the <"5W" terminal taken as a centre-tap zero reference. This percentage is remarkably close to the oft-quoted ultralinear optimal tap at 43% of the primary half. This all suggested, at least, the possibility of hooking this transformer up as a centre-tapped primary, output transformer; at least it would be worth investigation practically. The connections would be;

The next step was practical experiment. This was done by connecting mains AC via a variac to the secondary, adjusting the secondary voltage to 4.00Vac, (a value large enough to be readily controlled with the variac, but well less than that calculated for maximum power as seen in the first table, above), and measuring the AC voltage appearing across sections of the primary, referenced to the "5W" supposed centre-tap. The results are tabulated as follows;

These results were qualitatively as predicted, confirming the winding symmetry about centre-tap, and the potential application as an output transformer. Even the taps were in near enough to the right place, at 21/49 = 42.9% and 20.2/49.2 = 41.1%. However the voltage ratio to each half was around 49:4, (12.3:1), an impedance ratio of 12.32:1 (150:1), meaning that an 8 Ohm secondary would reflect as 8 x 150 = 1.2 kOhms in each half primary, or four times this, end to end; the transformer measured as a 4K8:8Ohm primary, not 8K:8Ohm as calculated. This result was sufficient to suggest it might at least be worthwhile seeing what happened in an actual amplifier circuit application. But big questions remained. Firstly, only the section of the primary between the common and 15W terminals needed to be capable of carrying high current. In fact the further from the Common terminal a tap was, the less current it originally would have been expected to handle; would this mean that diminishing wire size had been used in its manufacture, to save "copper cost"? If anything, it might be the small size and low cost (i.e. cheapness) of these particular transformers that my count positively; just perhaps the additional labour cost of changing wire size repeatedly during manufacture would mean that it was less costly merely to wind the primary with just one wire only. Maybe . . . Even assuming that the same gauge wire had been used continuously, simply tapped at applicable turn numbers, what would be the overall performance? The transformer cost was itself very low; hardly indicative of much in the way of quality and performance. The public address application intended meant that little more than voice frequencies and elevator musak would be expected of it. How realistic would be the 20Hz to 20kHz -3dB points, in practice? They seemed doubtful at best! As much loop negative feedback as stability would allow, would be called for. This would not be the time to be avoiding global NFB in a minimalist purist design; such a no-compromise attitude should be saved for application with much better transformers than these!

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