Radford SC 22 stereo control unit (1965 - 1971)
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Taken from 'The Gramophone', January 1966

Any control unit intended to partner the Radford power amplifier reviewed last month must obviously be an outstanding performer. For it to possess similar standards of frequency response, absence of distortion, freedom from self-generated noise and yet not be impossibly expensive calls for extreme skill in design; and to meet the latter requirement this skill must start in a negative way, by making a clear cut decision on what can be left out! In this case out goes provision for 78 rpm equalization, for the use of pickups other than high quality magnetic types, and with them the necessity for elaborate low pass filters. In comes an input stage with a degree of sensitivity which looks past the current pickup cartridge designs to future types with lower outputs (we have noted forerunners such as the Grado) and yet is proof against overload by the high peak output of today's more exuberant Ortofons and others. With considerably more difficulty, in must also come lower levels of hiss and hum, so that the minute signals produced during pianissimo passages are not overlaid by avoidable interference in reproduction, assuming they have managed to survive tape hiss and other deficiencies in the recording process.

As well as the input stage, all the remaining tone control, filter and output stages must be designed for very low distortion and the choice of circuit detail, the type of component used in any situation and the precise arrangement of the negative feedback which will be required, must also be reflected in the signal-to-noise figure of merit. Lastly comes stability; not only electrical stability so that transient response remains unmarred by oscillatory tendencies, but also component stability so that acceptable tolerances are achieved in production and, what is more important, they remain acceptable during the life of the product. The Radford team seem to have safely negotiated all the hurdles with the SC 22 and at the same time produced an article of attractive appearance, as well as one which handles well and is simple to use.

External Appearance
Two versions of the SC 22 are available; one which draws its power from any of the Radford amplifiers (and from most others too if the necessary connections are made) and another which incorporates a small power supply unit energized from the AC mains input. My tests were made using the first type, but the space left for the power supply components helps to make this one of the larger control units on the market, a fact to be borne in mind when considering cabinetry. However the panel area is quite modest, only the depth is increased so that one feels that if mounted and clamped through the usual cut out, a support at the rear would be advisable and probably essential for the heavier mains powered version.

All connections to inputs and outputs are made via phono sockets at the rear; an octal socket takes the supply from the power amplifier and a pair of AC outlets are provided, one switched and the other always alive. Also at the rear is a pair of pre-set gain controls, one for each channel. These are provided to enable this control unit to be used with other makes of amplifier. In fact any amplifier can be fed which requires less than two Volts rms to drive it and this must include the majority. There is also a hum balance control and an Earth terminal and every single connecting point is clearly and unmistakeably labelled.

The front panel is of natural finish anodised aluminium with the various legends exceptionally well printed in black. This panel is set in a polished, chromium finished brass frame. There are five black push buttons set in a row at the lower left centre and they control a. Rumble Filter, a Mono switch, Quiet Listening (a reduction of 15 dB in gain with bass correction according to the latest NPL tests for this level), Tape Monitor, and Mains on/off. Five small knobs of solid aluminium select Input (from microphone, disc, tuner and two auxiliaries), set Bass, Treble and Balance and bring in the switched low pass Filter which has 10, 7 and 4 Kc/s positions. A larger knob with a clear plastic finger guard controls Volume. The four continuously variable controls have that smooth as velvet feel, for the potentiometers used are of the expensive moulded track variety; the switched controls have a light index spring and slip easily and silently (both electrically and mechanically) into the desired position; this is also true of the push buttons with the exception of the power on/off, which has, perhaps deliberately, a heavier action. A tiny Perspex rod sunk into the panel adjacent to the on/off button is internally illuminated to show when power is applied.

Internal Layout
A perforated steel cover finished in light grey enamel is held in place by two screws; soft plastic feet are provided for using the unit 'free standing' if desired. The internal construction is extremely open and accessible, a result of the large size. Most of the components are assembled on three stout, printed circuit boards, one for the pair of input stages, one for the tone control stages and one for the output cathode followers. In the case of the input selector, tape monitor and power switches, long leads have been avoided by extending the operating shafts to the rear of the assembly and there is an absolute minimum of internal screened cabling which might result in loss of treble. All components are of the highest quality, all bear at least 5% tolerance markings, a few 2% and some of the resistors in the input stages are of the low noise, metal oxide variety.

Circuit. Design
As soon as one learns that this control unit is intended to work with a sensitivity in the Disk position of as little as 2½ mV and still provide .a noise figure better than 60 dB, curiosity about the input stage is aroused. Down to about 5mV the single low-noise pentode stage has served its admirably; in recent years the double triode feedback pair has become the most common arrangement, even though it has been necessary to find a source of DC for the heaters to avoid hum transfer. Transistor feedback pairs have also been used but, as we have seen, they can have awkward input impedances, no better noise figures and very low overload 'ceilings'. The recent availability at reasonable cost of low-noise silicon planar epitaxial transistors can remove the first two difficulties and the original (as far as I know) Radford idea of using a triode valve as the second stage gives a handsomely adequate overload figure.

This then is the configuration used for the hybrid input feedback pair to give the exceptional result quoted in the specification. RIAA equalization is carried out by frequency selective components in the feedback path and a further feedback loop includes the rumble filter. When used in the Microphone input position, the equalization is not included and the rumble filter characteristics change to become the bass taper control often required with ribbon or cardioid microphones when used for close talking.

The Tuner and Auxiliary inputs do not pass through the input pair but are fed to the second feedback pair, this time both triode valves, via the Tape Monitor switches and the two pre-set potentiometers which decide the overall gain. Tone and Filter control components are included in both the forward and return paths of the pair and at the output is the main Volume control and the Quiet switch network. A cathode follower using a final triode completes the line up and at the output is the Balance control which is of the type which can completely mute each channel.

By Ear and Meter
Considerable listening was carried out with the SC 22 and its companion power amplifiers before any tests at all were made. Some of the listening was done with a very low output moving coil cartridge (not available in this country) which required the maximum sensitivity. No matter what demands were made on it, the unit passed with flying colours. I was pleasantly surprised by the subjective adequacy of the simple low pass filter arrangements and found the rumble filter effective on some records which have substantial amount of low frequency noise (fortunately my motor has very little). The tone controls are quite mild, but the shapes and slopes available are right for the job; the bass moves in point of origin as well as degree, while the treble acts more as a movable 'step' of fixed frequency.

After the listening tests I was not unprepared, when measurements were made, to find complete agreement with the specification, which is itself quite a tight one. Mr. Radford is obviously a man to whom a single dB of error would be an affront and so, for example, the plotted RIAA curve taken on my sample would cover up the textbook one and the measured frequency response with the controls set at level is level and reaches the -1 dB point at 50 Kc/s! Transient response is also excellent and the sight of a 10 Kc/s square wave after passing through both control unit and power amplifier would satisfy the most critical observer . . . Just as this combination satisfies me and would do the same for anyone, for it sets a standard which is unlikely to be considered in any way inadequate for many years to come.