Taken from Hi-Fi News, May 1958

MEASURING only 10½” by 3¼” by 5” the Mozart must be one of the smallest self-contained amplifiers now available. First shown at the Audio Fair last year, where it created something of a sensation, this compact unit uses printed circuits and has but 3 valves and a metal rectifier. Even so, the sensitivity is remarkably high—only 10 millivolts are required to give some 9 watts output! The stage is somewhat unusual these days, as it uses but a single valve, an EL34. Negative feedback is applied by means of a cathode winding on the output transformer, so the output circuit can be considered as a form of distributed load stage, although a screen tapping is not used. Another interesting feature is a type of pickup matching called the Dialamatic, which is very similar to the Decca system described in the 1957 “Hi-Fi Year Book”. Two calibrated controls are provided, one being the load resistor and the other the attenuator, thus any type of pickup can be quickly and accurately matched. Another feature that perhaps will not meet with the same general approval - at least this side of the Atlantic - is the provision of a variable damping control. When used with a reasonably good speaker system it was very difficult to detect any difference in results if the control was rotated from one extreme to the other, but when a small bookshelf speaker was used it was just possible to note an improvement in bass transient response at a critical damping point. It can be argued, therefore, that the inclusion of such a control is justified in an amplifier of this nature. It must be pointed out, however, that by reducing the effective output impedance of, say,1 ohm (which corresponds to a damping factor of 15 with a 15 ohm speaker) to as far as zero, only means a change of one sixteenth, as the speaker impedance must be considered as part of the circuit. In other words, once a reasonably high damping factor of 15 or so has been achieved there is little to be gained by electronic juggling - it is much better to improve the speaker system itself.

Circuit Details
The first valve is a ECC83 double triode and equalisation and gain adjustment is provided by a negative feed-back loop around the first triode. This is followed by a Baxandall tone control stage and a filter network that precedes the volume control and the second valve - another ECC83. This double triode functions as a two stage amplifier and is followed by the EL34 output valve. Negative feed-back loops are connected from the secondary of the output transformer to both cathodes of the ECC83, plus a positive loop to one cathode via the damping control which, by varying the proportions of feed-back effectively changes the amplifier output impedance from negative to positive values. A proportion of the transformer secondary winding is in the EL34 cathode circuit, and the bias for this stage is derived from a separate winding on the mains transformer and rectified by a miniature germanium rectifier. H.T. current is supplied by a bridge connected metal rectifier.

It will be seen that power output is reduced at both ends of the scale. This is not too serious so far as the high frequencies are concerned because, apart from the cymbals, very little actual power is needed above about 5 Kc/s. The fall in bass power response is more serious - especially as no high pass filter is incorporated to prevent low and even subsonic frequencies from prematurely overloading the output stage, and thus causing audible intermodulation distortion.

Stability
The overall feedback loop of 13.5 dB could be increased to 19 dB before causing instability. In view of the multiple loop feedback arrangement used, this margin of 5.5 dB was considered satisfactory. Frequency response was within 2 dB from 30 c/s to 20 Kc/s and rolled off gradually from 25 Kc/s. Hum and noise varied from -46 dB on Gram to -65 dB on radio input (ref 9 watts). A choice of 3 equalisation characteristics is provided, as shown in the diagram, and provision is made for both radio and tape inputs.

The bass control showed a variation of +12.5 dB and - 16 dB at 40 c/s whilst the treble control gave a variation of +12.5 dB and -13.5 dB at 10 Kc/s. The filter system is a simple resistor-capacity network with an insertion loss of 2 dB - which is very good for this type of circuit. The characteristics as shown in the diagram, and the roll-off of some 6 dB octave are too gradual for most purposes. Fortunately the Baxandall treble control tends to reduce the high frequencies progressively, so a judicious use of both controls can give excellent results.

The sensitivity of 10 millivolts means that most magnetic pick-ups can be used successfully, and among those used during the test period were the Goldring “ 600”, the Miratwin and the Ortofon (with transformer). Changing from one pickup to another was greatly facilitated by the matching controls, as can be imagined. For those desiring an amplifier small enough to stand on a table or bookshelf, and particularly for those with very small rooms, the Pye Mozart can be recommended. Because of the omission of a rumble or high pass filter as mentioned above, it is essential that a good quality transcription motor be used, as even a small amount of rumble or “wow” may result in distortion. The presentation is, I think, very good indeed. The panel is made of copper and it contrasts well with the metal case, which is finished in a matt black. I was pleased to see that the knobs were engraved in such a way as to eliminate parallax errors and I particularly liked the push button Perspex on/off switch which is cunningly illuminated. Finally, with the service engineer in mind, I can report that all components on the printed circuit assembly are quite accessible and can easily be replaced if necessary.
G.W.T.

Manufacturers' Comment:
Whilst the tone controls are of the Baxandall type they are modified to give a stage gain of times ten, which is essential with a 3 plus 1 valve amplifier suitable (as this is) for low output pickups. The exclusion of normal hum reducing components in the H.T. supply is a feature that ensures a very low impedance supply and less heat dissipation; the hum is cancelled out by a feedback resistor from the cathode of the rectifier.