Truvox TSA 100 stereo integrated amplifier
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Taken from 'The Gramophone', November 1964

Truvox, for many years a leading British name firmly rooted in the tape recorder field, threw out a couple of new branches in the spring sunshine of the 1964 Audio Fair and put on a demonstration using products entirely of their own manufacture. On that occasion even the tape recordings used were 'home grown' and they were most realistically reproduced by a pair of new Truvox loudspeakers and played via this new transistor amplifier, the TSA 100.

Transistor amplifiers are still far from commonplace and in this event Truvox are to be congratulated both on the boldness of their design policy and on the performance of this example, which, in relation to the price, seems to bring it much nearer than most to parity with existing valve amplifiers. To be fair there is more than a passing resemblance to the recently published Mullard Class B circuit, itself a montage of well-known American practice, and the transistors used in my sample were, with two exceptions, of Mullard manufacture.

A five position input selector switch precedes the two-stage preamplifier; introducing the appropriate equalization by means of negative feedback networks. After the monitor switch and the volume and balance controls there is a further two-stage voltage amplifier, this time incorporating the bass and treble controls in the feedback line. The main power amplifiers use six transistors and a complementary pair driver and 50 dB of negative feedback; no transformers are used. However, as I have pointed out before, there is an immense amount of hard work interposed between the finding of an acceptable circuit and the marketing of a finished product.

From the start the construction and finish of the Truvox TSA 100 inspires confidence. There is a nicely shaped Afrormosia case, which just happily avoids being plain and a two-tone grey panel with clear lettering, attractive knobs and a touch of colour in the illuminated red name badge. In its case or when fitted in a cabinet, easy enough to do, the appearance is business- like, perhaps leaning toward the 'laboratory instrument' style which has recently been popular in America. An interior inspection also produced a favourable reaction; this amplifier is well made of good quality components and the physical layout shows thought and consideration both for those who make it and those who may perhaps later have need to service it.

There has been some lack of compatibility in transistor amplifiers, by which I mean that in some ways they are unsuited for use with existing well-established pickups, tuners and even loudspeakers. It is good to see that in so far as it is reasonable to do so, Truvox engineers have taken care of this and as a result no difficulty has so far been found in connecting to many varied pieces of equipment. For example, the most sensitive magnetic pickup input has a stated input impedance of 50K/Ohms and this will be so at high frequencies where the self inductance of the pickup would otherwise cause loss of treble; but I surmise from the circuit configuration, and I am sure the makers will correct me if I am wrong, that at mid and low frequencies it is probably nearer 15K/Ohms, a figure which may cause deterioration in the channel separation of some pickup designs.

Then, again, there is an internal switch provided to permit operation with electrostatic loud-speakers, which are 'difficult' because, amongst other things, their essential internal transformers decide that at very low frequencies of a few cycles per second their impedance is very low, in the region of one Ohm, and this near short circuit upsets the feedback and can seriously embarrass the output transistors. Incidentally, I once found it necessary to use this switch position when using a matching transformer with a moving coil loudspeaker.

Non-technical readers, to whom I apologize for some of the foregoing which is necessary background to newly developed products, will be glad to learn that with the above proviso I could find no audible difference at any reasonable domestic listening level between this amplifier and a very good valve design - with either type of loudspeaker, even after hooking up the necessary simultaneous input and output switching to permit instantaneous comparison. Another extremely revealing, if musically uninteresting, comparison between valve and transistor or indeed between any two amplifiers, is to feed both together with identical signals and then connect a loudspeaker (and instruments) so that one listens to (or measures) the difference between the two. In this case, as in others I have investigated where design has been exemplary, the difference has proved to be so small as to be inaudible and difficult to measure. So the Truvox has not got 'transistor sound', whatever that may be; it just makes good music gracefully as all good amplifiers should. The tone controls and filters, when intelligently used, seemed capable of dealing with most of the imperfections requiring correction in both radio and disc reproduction and were nicely graded.

After the listening tests the usual instrument measurements were carried out without finding any serious departures from the specification, which was in many cases exceeded. There is a warning in the full instruction book about the dire effects of short circuits in the loudspeaker leads or at the output terminals due to stray strands of flex; it is a pity therefore that the type of terminal used is particularly liable to this sort of thing and has to my knowledge been the cause of numerous requests for service in the past, happily less disastrous with valve amplifiers. One feels that there is an over- powering case for a change to a suitable and safer plug and socket, for this is otherwise a good example of current amplifier practice; compact, efficient, reliably made and musically satisfying. Another feather in the Truvox cap.