Stereo Amplifier

Review: Sony TA-F770ES Stereo Amplifier

I must begin with something of an apology, since this amplifier has been in my charge for rather too many months, waiting its turn for a possible review. As always we seem to have more products in the wings than we can actually process in our monthly allocation of pages, though that physical limitation does have the useful advantage of making us still more choosy. It has been said before, but can usefully be restated, that by no means all products submitted for consideration enjoy an assessment on the printed page. Indeed, many are called but few are chosen!

Although it has been on the market for a good while now, the TA-F770ES will remain at the head of Sony's integrated amplifier range until next summer, which is one reason why I felt less urgency in writing about it than normal. The fact of its continued presence says much about Sony's faith in their current technology and marks something of a departure from the familiar Japanese 'change it each year, if only cosmetically' philosophy. Clearly, no product is perfect but until a circuit improvement is demonstrable and repeatable on the production line, there is scarcely any reason to make a change other than to satisfy the requirements of the marketing departments. Aesthetic change in itself is largely futile so far as we are concerned.

The TA-F770ES is a powerful and extremely heavy beast rated at 90 watts per channel into 8 ohms (models supplied to Northern Europe, otherwise 100 watts/channel). In common with several of Sony's top electronics units it is based upon a so-called Gibraltar Chassis, a massive, virtually inert platform intended to suppress the local vibration which is thought to add minute degradations to the wanted audio signal. Subtle modification of the waveform due microphonic pickup has been well known for many years and is particularly associated (because it is much more of a problem) with valves. This is one of the reasons, according to some cynics, for the 'warmth' widely considered to be symptomatic of the breed, an argument with which I admit to having a deal of sympathy. Tap almost any sensitive amplifier circuit, even a solid-state design, whilst monitoring the output on an oscilloscope and a tiny (and sometimes not so tiny) glitch in the waveform can be observed. Acoustic feedback from the loudspeakers, vibration due to the magnetic fluctuations in the mains transformer-such things are surely worth suppressing even if their audible effect is minuscule.

Among the special circuit features of the TA-F770ES is a duplicated power supply arrangement which separately feeds the Class A voltage amplifier and the high-current output stage. The ideal power supply will provide absolutely steady voltage rails, unaffected by the current demands of the output stage. Any fluctuation here is likely to amount to a significant proportion of the low-level audio signal. Although double-ended supplies (i.e. the usual arrangement, with positive and negative supply rails symmetrically disposed about the zero volts, or ground, line) are pretty innocuous in this respect, in the worst case modulation of the wanted signal can occur, adding a kind of signal-related distortion which manifests itself in a 'fogging' of the sound. Sony's Spontaneous Twin Drive system provides entirely separate pairs of supply rails to the small-signal and output stages with the specific aim of combating this kind of effect.

One further 'buzz-phrase' is Super Legato Linear, which is a circuit configuration that Sony claim eliminates the crossover distortion which normally limit s the performance of Class B output stages. Any self-respecting amplifier designer has borrowed or devised some method of reducing such distortion to vanishingly small levels of course, such as the use of negative feedback (which many now eschew as not the cure-all it was once thought to be) or biasing the output transistors into Class AB so as to keep them operating in their linear region. Additionally, Sony use MosFet devices at the output, presumably because of their superior inherent linearity.

The TA-F770ES is presented in the seemingly de rigueur all-black livery of today's hi-fi (suitably austere and business-like but also rather dull, I think) but it has the increasingly used fascia layout whereby the less-often used controls are hidden beneath a full-width flap. The only controls normally visible are for Power On/Off, selection of the seven primary inputs, Source Direct (a purist signal path option which bypasses the tone controls, subsonic filter and balance controls), Muting (drops the output by 20dB-useful for when the telephone rings) and Attenuator (alias Volume-a large rotary knob calibrated in decibel s of attenuation). All of these have adjacent LED indicators: a green 'dot' denoting Power On (this shows red at switch-on while the circuits stabilize and also in the event of a short-circuit at the outputs-relays mute the loudspeakers under these conditions); small green bars for the inputs, orange dots for the Source Direct and Muting; and a red dot which rotate s with the large volume control, this last tying-in with the supplied RM-S703 "Audio System" remote control which duplicates the input buttons (these activate a motor-driven switch-bank at the rear of the unit, adjacent to the input sockets-the right way to do it) and has Up/Down buttons which cause the Volume knob to rotate (it is a motorized potentiometer).