|Rogers RD Junior Amplifier, Control Unit MkII and switched P.M. Unit|
RD Junior Amplifier, RD Junior Control Unit, Mk. II
& RD Junior Switched P.M. Unit.
Visual examination of the RD high fidelity unit received for review gave instant pleasure at the beautifully wired chassis and superb workmanship throughout. This goes a long way towards guaranteeing the reliability of the equipment.
The three units are designed to be used together as a matched equipment, although each item may be used in conjunction with any appropriate high-fidelity system. When used together, the units are conveniently inter-connected by means of plugs and sockets, the control and tuner units deriving their H.T. and L.T. supplies from the power pack on the main amplifier chassis. The amplifier and tuner unit are in chassis form, intended for mounting in a cabinet, whereas the control unit is totally enclosed in a steel case, and may be stood on a shelf, etc., if desired. A suitable cabinet is obtainable from the manufacturers.
RD Junior Amplifier.—Two EL84 valves are used in an ultra-linear circuit giving a nominal output of 10 watts. One half of an ECC83 acts as a phase splitter, and this in turn is driven by the other half, used as the input A.F. amplifier. An EZ81 rectifier supplies H.T. from a double wound mains transformer, the smoothing components being inside a magnetic screen. H.T. and L.T. sockets are fitted to supply the control and tuner units. The output transformer has adequate dimensions and contains additional negative feedback windings, feeding back to each cathode of the ECC83.
The degree of feedback measured on the amplifier received for test was 18 dB, measured at 1,000 c/s.
Provision is made for matching loudspeakers hawing impedances of 2-3, 6-8, or 12-16 ohms, changeover from one output impedance to another being carried out by inserting the appropriate octal plug into a socket at the rear of the chassis. Negative feedback is also adjusted to suit automatically by the connections within the plug.
The output valves are run under normal loading conditions, and so the power output may be measured by injecting a sine- wave. The maximum power output without visible distortion on the oscilloscope was 9.5 watts (12 volts r.m.s. across a 15 ohm load), and to achieve this an input signal of 0.55 volts was required. A wide frequency response extending from 30 c/s to over 20 kc/s, ± 1 dB, makes this amplifier well suited for high fidelity reproduction. Distortion is very low, whilst the hum level is negligible. A thoughtful addition is a hum-dinger in the heater circuit to the control unit and tuner unit supplies, but, although mounted on the main amplifier chassis, will only control the hum level when these two units are in use. The circuit diagram issued with the amplifier may be of a provisional nature, but service technicians may note that the diagram received differed from the chassis in so far as the connections to the two halves of V (ECC83) are reversed. Furthermore, a grid stopper is included in the first triode section.
RD Junior Control Unit, Mk. II.—This incorporates a pre-amplifier as well as all the controls necessary for an audio amplifier, and has five input sockets. These are: Radio, microphone, tape replay, P.U.I, and P.U.2. Each may be selected at will by means of a switch on the front panel, and an excellent feature is the inclusion of an independent pre-set volume control for four of the five inputs at the rear of the chassis, the microphone being excluded from this feature. These are in addition to the master volume control which is on the front panel.
Three more controls are also on the front panel. These being: Treble control, bass control, and filter switch, a flexible control of frequency response being obtained by suitable adjustment. At 50 c/s, the bass control had a range of 30 dB, whilst at 10 kc/s the treble control was 27 dB. This means that it is possible to compensate the amplifier to any of the existing disc or tape recording characteristics.
The filter control is a low-pass filter, having cut-off frequencies of 20, 9, 7. and 4.5 kc/s marked on the four position switch. On the "20" position, the control unit has a flat response from 30 c/s to 20 kc/s, above which frequency the response falls rapidly. If a high pitched needle scratch is present on certain recordings, the 9, 7, or 4.5 kc/s position may be selected, the response falling rapidly at frequencies higher than each one selected. The measured fall-off in response above each of these frequencies was about 12 dB per octave.
As a qualitative example of the effectiveness of the filter and tone controls, a 9 kc/s whistle was superimposed upon a radio programme, the amplitude of the whistle being approximately equal to the peaks of the radio signal. With the tone controls set to give a flat response, the interference was unbearable. Adjustment of the treble and filter controls almost entirely eliminated the whistle, however, making the programme of entertainment value.
Provision is made for adjusting the response to RIAA standards as well as any other disc characteristic, whilst for the replay of tape a CCIR characteristic is obtainable by removing a shorting link. A tape record output is available, but it should be clearly understood that a separate bias oscillator is required before recordings can be made. Again, although tape replay facilities are provided, a tape pre-amplifier must be obtained, as the gain at the tape input socket of the Mk. II Control Unit is insufficient for direct connection to the magnetic heads.
When used in conjunction with the RD Junior Amplifier, the following input voltages were required to produce 10 watts output, at 1,000 c/s.
RD Junior Switched F.M. Unit.—This unit contained a cascode R.F. amplifier followed by a triode pentode frequency changer, one I.F. amplifier, limiter, Foster-Seeley discriminator, and cathode follower output. The oscillator frequency is automatically stabilised, and all of the above functions are carried out by three triode-pentodes (ECF80), and one double triode (ECC84).
A four position switch is the only control provided on the front panel, allowing instant selection of either 'Home, Third or Light programmes. Incremental inductive tuning is used. In the fourth position the unit is off, although the valve heaters remain alight ready for immediate use. The master switch for the mains is on the Control Unit. A pre-set volume control is a useful addition, and is fitted on the chassis.
The workmanship and component layout is excellent,
and the performance leaves nothing to be desired. The quality when used
in conjunction with the RD Junior Amplifier and Control Unit and a good
loudspeaker system was very pleasing. Sensitivity was such that at the
location in which the tests took place (in the service area of the local
transmitter), the only aerial required was a piece of wire two feet long,
plugged into the aerial socket. It must be emphasised, however, that in
many cases a good aerial must be used for satisfactory results.