T+A – P10 MKII


The P 10 2 valve pre-amplifi er is the most sophisticated unit in our series of high-end two-channel analogue equipment. Its no-compromise construction and the equally uncompromising sophistication of its circuit design elevate this pre-amplifi er to the very highest level. Even from the outside, the arrangement of the valves clearly indicates the amplifi er’s totally symmetrical double mono layout. The signal paths are absolutely identical, as is the sound quality. The interior of the unit (see right page) confi rms this beyond all doubt.

Circuit concept: the P 10 2’s fi ve valves per channel represent an extraordinary level of complexity. Why such complication? The answer is simple: we were not prepared to accept the quality of sound which was available in the 1950’s. Modern highresolution source devices, with their unprecedented bandwidth, dynamic characteristics and minimal harmonic distortion, are far more demanding than their predecessors. If we insist on maintaining the very highest standards in terms of sound quality and musicality, and if the pre-amplifi er is not to become the limiting factor in the reproduction chain as far as sound quality is concerned, then simplistic electronic concepts are no longer acceptable. All the amplifying stages of the P 10 2 take the form of differential cascode amplifi ers. This circuit design is responsible for the machine’s extremely broad frequency bandwidth and excellent linearity. The amplifi er valves are the “LPS” version (Long Plate / Spiral Filament) of the 12AX7, whose outstanding feature is particularly low distortion. The potential sound quality of these valves has already been proved in the D 10 2 and the front end of the V 10 2. In fact, the linearity and quality of the P 10 2’s amplifi cation stages are so high that we decided against including any form of negative feedback. Even though the classic negative feedback is entirely absent, the P 10 2’s total harmonic distortion and frequency linearity values are excellent. The output stage is of fully symmetrical construction, and features both unbalanced (Cinch / RCA) and balanced (XLR) signals. The excellent ECC99 high-current triode valve has proved its worth in the D 10 2, and we decided to use the same component for the P 10 2 as it enables us to achieve outstandingly low output impedances. The result is that it can drive active loudspeakers or output stages without any problem, even where long cables are required.

All-pole input section: when the inputs are switched on the P 10 2, “all poles” are switched – i.e. the earth connection of the source device is switched in addition to the signal conductor. This is very complex to implement, but ensures that all source devices are galvanically isolated. At any one time only a single device – the one currently selected for listening – is connected to the amplifi er. In conventional Hi-Fi systems earth currents are free to wander around between the individual components, and these can have a signifi cant adverse effect on sound quality. The problem is solved completely by the allpole input selection system adopted for the T+A P 10 2. As with the V 10 2, the input switch is based on an extremely solid mechanical angular rotation device featuring a zero-wear opto-electronic sensor. This control, with its slop-free operation and accurate magnetic detents, gives a feeling of incredible solidity and precision.

Floating ground concept: balancing and interference currents, some of them signifi cant, fl ow between the earth connections of Hi-Fi devices. These currents can result in unwanted dips in voltage at the connecting leads, and interference potential at the amplifi er’s input, either of which can have a perceptible adverse effect on the sound. We were determined to eliminate these unwanted effects in the P 10 2, and have adopted a solution which is entirely new in the field of audio equipment. The low-impedance earth connection can be isolated at the amplifi er’s inputs, and we employ differential amplifi ers in the P 10 2’s input instead of a conventional earth-related amplifi cation circuit. This technology is used in ultra-sensitive laboratory measuring equipment, where it is known as a “fl oating ground” circuit. The important factor with this type of circuit is that the input stage must be able to cope well with high common mode voltages. However, this is precisely the point where, by their very nature, valves have an advantage over semi-conductors. Valves operate at high voltages – more than ten times higher than those used with transistors – and this makes them ideally suited to this purpose. Since this advantage applies to any source device, i.e. including those which do not feature XLR outputs, we consider this concept to be more fl exible than the use of XLR inputs.

Mains power supplies, voltage supply: valve amplifi ers place much more exacting demands on the mains power supply than conventional transistor units. The anodes of valves require high operating voltages – in our case more than 350 V – as well as high-current heating voltages. These requirements are very divergent, and for this reason the P 10 2 is fi tted with separate, specialised mains sections. A shared basic feature of both mains power supplies is their layout as high-frequency fl yback converters. This type of circuit effectively eliminates the low-frequency hum interference which is unavoidable with conventional mains transformers. For the heating circuit we use a D.C. voltage which is stabilised extremely accurately, and also features a “soft-start”. This feature also prevents feedback hum, and avoids premature valve failure, as they are heated gently and always operate at the optimum heating voltage, regardless of fl uctuations in the mains voltage. The overall result is an extended effective life of the delicate amplifi er valves. A micro-processor constantly monitors the entire system. The anode voltage is also stabilised electronically with great accuracy. The slightest residual interference to the anode voltages is eliminated by the use of separate passive fi lter circuits for each channel, and reservoir capacitors of extreme dimensions (2000μF/450 V).

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