Quad II Pre (Control) Amplifier
RECORD EQUALISING
The control unit provides the correct equalisation for all of the records likely to be played on the equipment. Recording engineers are by no means agreed upon an optimum characteristic and indeed agreement is unlikely since the choice relies on a subjective assessment of a number of conflicting factors. The result is that most manufacturers of records use one or other of the seven or eight different characteristics generally recognised, of which four are in very popular use.
Push buttons are used for the selection of characteristics for reasons which will now be apparent. The four red push buttons on the control unit select the four main characteristics in a completely straightforward manner and cover all normal listening requirements. Engineers and experimenters, however, will be delighted to find that other characteristics are provided by combinations of buttons. For example the NAB characteristic is obtained by pressing AES and ffrr 78 together; RCA Orthophonic by using AES ffrr 78 and STD78; NAB bass with AES top by using AES and STD 78. All buttons relaxed still leaves the unit switched to gram but with completely uncompensated response - a useful feature for the calibration of pickups.
PICKUP MATCHING
For the finest possible gramophone record reproduction, it is desirable that the amplifier input be correctly designed for the pickup with which it will be used.
Different types of pickups vary in respect of their source impedance, their required load impedance, amplitude or velocity characteristic and their output level. The circuitry for any pickup should meet all these requirements and at the same time provide optimum conditions for signal to noise ratio. In the QC II there is a small plug-in unit at the back. By inserting the unit appropriate to the pickup selected the input circuit will be automatically correct in every respect for that pickup.
The advantages of this feature are rather more than the obvious one of correct matching, particularly where very low output pickups are concerned. Moving coil pickups, for example, can be matched directly without the use of any transformer, thus eliminating attendant problems of hum, lead capacity, etc.
RADIO MIC INPUTS
Three other inputs are provided on the control unit and they are switched by two white push buttons on the control panel. Two of these inputs may be used for radio or alternatively for microphone and radio respectively. The choice of these alternatives is available when selecting the pickup adapter unit. The third input is selected by pressing both white buttons simultaneously and is used for tape playback.
STANDARD INPUTS
Each control unit is supplied with a standard pickup plug-in unit designed to suit most velocity pickups and to provide two alternative radio inputs. The gramophone input as standard therefore, is in line with contemporary practice in other amplifiers so that an alternative plug will only be required for special pick up arrangements.
TAPE RECORDINGS
Two sockets are provided for connection to a tape recorder, one for recording, and one for playback. The former is unaffected by the volume, balance and filter controls which should be used in the normal way when playing back. Gramophone compensation is effective.
EXTERNAL POWER SUPPLIES
Two sockets providing HT and LT supplies for tuner units are available on the control unit, the HT supply to each being switched when the appropriate radio push button is actuated. Where a tuner unit has its own power supply, this switched HT may be used to operate a relay within the receiver. A spare pin on each socket carries permanent HT for use where an unswitched supply is required.
CONTROLS
There are five controls on the front panel.
The large knob provides adjustment of volume level and combines the on/off switch for the complete equipment. The Acoustical nameplate is illuminated when the equipment is switched on.
The treble and bass controls are most subtly designed to obtain correct musical balance to suit the environment in which the equipment may be used. The remaining two controls adjust the equipment to suit the useful range of the particular program material available and thus to reproduce the highest quality inherent in that programme.
A cancel position is fitted on the filter switch control. In this position, bass, treble, and filter controls are automatically bypassed to give a level response. Thus a switched reference standard is provided for useful comparison with the setting of controls found by aural assessment.
In use the controls require the minimum of adjustment. The listening environment is usually constant so that the bass and treble controls, once set, will rarely require alteration. The filter switch should normally be set to 7k and the filter slope rotated from “level” as may be required by imperfections in the recording or radio transmission. With older recordings or with poor radio reception the filter switch should be in the 5k position. The 10k position is for very fine adjustment on the very best possible program material.
CIRCUIT DESCRIPTION
The control unit employees three stages. The first stage provides pre-amplification for microphone and is also used in a versatile feedback loop for recording compensation. The second and third stages are enclosed in a further loop forming part of the treble and bass control, together with further amplification up to the filtering circuits. The output of the unit is 1.4 Vrms to which level all figures refer.
THE INPUT STAGE
The circuit of the first stage is completely versatile and is associated with the nine pin socket on the back of the unit.
Also wired to this socket is an impedance equivalent to the replay characteristic appropriate to the selected push button. The range of impedance characteristics has a common value of two 270,000 ohms at 1000 c/s. The insertion of the plugin unit completes the circuit to suit the pickup selected.
When the replay impedance is used directly in a parallel feedback circuit the equivalent impedance at the valve input is less than 2 kΩ resulting in very low thermal noise. By suitable choice of series value, very high sensitivities can be obtained provided the pickup impedance itself is low.
By using stepped parallel feedback it is possible, effectively, to multiply the playback impedance by a constant so that the arrangement can be made to provide a high signal/noise ratio for the pickup impedance to be used. The variable elements are within the plug-in unit so that optimum conditions can be provided for any pickup in terms of source impedance, load requirements and sensitivity, as well as providing for constant velocity or constant amplitude signal. The maximum sensitivities which can usefully be obtained are 3 mV at low impedance and 6 mV at 100 kΩ impedance, for full output at 1000 c/s. Ample gain is therefore available for moving coil pickups without an intervening transformer and without noise.
Permissible input overload is approximately 40 dB for plug-in units requiring 15 mV or more for full output, reducing to 20 dB for plug-in units providing maximum possible sensitivity.
The microphone import utilises the same stage with a lower load and a level response, fully loading with 1.5 mV at any impedance up to 100 kΩ.
The volume control follows the first stage and this is also the entry for radio inputs.
CONTROL STAGES
The continuously variable bass and treble controls are so arranged that the central position of their travel will always give a level response with virtually no error. The bass rise operates by varying an impedance in the feedback circuit of the two final stages and bass fall is obtained by varying an impedance in the top arm of the feed to the filters. These two impedances are of like value so that a linear bass control is used. This is advantageous because a linear control does not appreciably change its scaling and any change in overall value will be symmetrical. Further, the two fixed capacitors are small and of like value, thus easy to match and extremely unlikely to drift.
In the maximum rise and fall positions, the maximum slope is 5 dB/octave and an intermediate settings the slope is gradually reduced to zero and at the same time the turnover point reduces in frequency. This is preferred to the step obtained by variation of amplitude or the constant slope obtained by variations of capacity alone.
The treble control operates both rise and fall in the feedback circuit. The slope is always asymptotic to zero so that a boost in musical brilliance in the treble musical register is not accompanied by boosted distortion at very high frequencies, nor is the reduction in the treble musical register accompanied by a complete loss of harmonics. This feature of musical balance control is, of course, only made practical because of the independent filtering controls provided.
AUDIO FILTERING
The filtering is accomplished at the output of the control unit where the signal level is high and the impedance relatively low. This enables optimum circuitry to be used and does not preclude the use of inductors by hum problems.
For the best possible performance, the filter must produce the required curve with the minimum of deleterious effects. The effect on transients due to ringing is a direct function of the response curve together with any mis-matching in the circuit. The first is a natural law and is, of course, common to all methods of filtering, the advantage of an LC configuration lying solely in the more favourable conditions for correct matching. With the added advantages of greater range and complete freedom from distortion a properly matched LC network becomes the obvious choice when other circuit arrangements do not preclude its use.
The controller slope operates from frequencies of 5 kc/s, 7 kc/s and 10 kc/s, these being equal musical intervals. The slope is continuously variable from level to 50 dB/octave.
It will be seen that the slopes can be made to follow the mirror image of distortion present in practical programmes. In this way, wide range with low distortion becomes possible. It should be emphasised that intensity of treble is entirely independently controlled since the filters modify the higher harmonics only and do not operate in the treble musical register.
ULTRASONIC FILTERING.
In the level position one of the inductors is coupled in a secondary circuit to provide a 20 kc/s low pass filter. In this way the response above 20 kc/s is always attenuated.
The output of wide range pickups frequently contains components beyond the audio range, the amplitude peaks of which exceed the peak amplitude within the audio range. Unless these are reduced, they can cause unsuspected distortion or unnecessary limiting of available output power. For similar reasons all circuits prior to tone controls and filters will handle signal levels 15 dB above that required for full output. In the “cancel” position bass, treble and filters, including the 20 kc/s filter, are out of circuit giving a level response (other than recording compensation if in use) from 20 c/s to 60 kc/s. The coupling cable forms part of the circuit and should not be altered without consultation with the manufacturers.
Quad II Power (Main) Amplifier
CIRCUIT DESCRIPTION
The Quad II main amplifier differs from contemporary practice both in the output stage and in the penultimate stage contributing materially both to performance and efficiency in a matter that would delight the engineer.
THE OUTPUT STAGE
It has been shown that by proportioning the influence of screen and anode currents on the load, a series of operating conditions may be obtained offering low distortion and high efficiency. Since cathode current is common to screen and anode, it is convenient to use cathode windings for reasons which have also been explained.
The output stage has used in the Quad II appears before feedback is applied as the equivalent push-pull triode circuit as far as amplification and effective output impedance are concerned but with less than half the distortion and an increase of 40% in efficiency over the equivalent triode circuit. The screen and anode circuits are more favourably arranged for efficient smoothing, a factor which reflects itself in the overall size of the equipment.
THE OUTPUT TRANSFORMER
The output transformer employees five windings subdivided into fourteen sections ingeniously coupled so that the output stage phase shift is extremely small within the range where the overall feedback loop gain exceeds unity. The small size of the output transformer resulting from optimum choice of flux and material should be noted. The distortion at 25 c/s at 12 W output shows no significant increase over that at middle frequencies.
THE PENULTIMATE STAGE
Full benefit can only be obtained from this output circuit if the feed is accurately balanced throughout the frequency range, since it is some three times more sensitive to unbalance than conventional operation. For reasons of phase, high stage gain is desirable and some form of see-saw circuit is called for. Such circuits however are grossly unsymmetrical in relation to the HT supply. In the Quad II two EF 86 valves are used, each feeding one output valve. The second EF 86 is fed from the first but with the signal 6 dB lower than would be required for balance. The EF 86’s are then coupled through their screens and cathodes so that they are always approaching balance. This arrangement is inherently stable, satisfies symmetry requirements and the balance error is automatically maintained through see-saw action. The output valve grid returns are applied to a fixed signal point in phase with one and out of phase with the other, the small signal so applied being equal to the small resultant balance error previously mentioned. In this way the complete phase change centres around perfect balance and provided the anode load resistors are of equal value, the error with random valve selection is considerably better than 1%.
FEEDBACK AND LINEARITY.
The overall feedback is an integral part of the amplifier design and cannot be considered separately. The standard of pass figures for distortion is, of course, well below audibility no matter how refined the test.
PERFORMANCE ADVANTAGES.
The complete circuit shows a number of performance advantages over contemporary equipment. Stability is complete and entirely independent of the load. Low distortion is not dependent on matched valves and the figures given cover the whole tolerance of commercial valve manufacturers tests. The frequency response is completely free from “ears”. The input is not prone to embarrassment by the presence of frequencies outside the audio range. The importance of these features in terms of performance and the holding of that performance through service is obvious. The efficiency of the circuit provides a high tolerance of reliability due to the fact that all valves are operating well below maximum dissipation and the HT supply does not exceed 340 V.
Quad FM Tuner
DRIFT
As drift is quite important at the frequencies used for FM, temperature compensation and AFC have been incorporated to ensure that the QUAD FM Tuner really stays on tune.
TUNING INDICATOR
A unique display has been developed giving a positive indication of accurate tuning, with the advantage over conventional types that it is not necessary to detune the receiver to check whether or not it was on tune! This indicator shows a tuning error of 1 part in 10,000.
TUNING
A precision mechanism prevents tuning error due to backlash and provides a smooth and accurate control.
DIAL
The dial shows the frequency range of the receiver (87.5 to 108 mc/s) covering both British and American bands, and adjustable Station Indicators. set to the scale settings corresponding to local stations, enables programmes to be selected without difficulty.
MOUNTING
The mounting arrangements are of the same design as other units in the QUAD series, combining simplicity and rigidity with a “clean” finish.
AERIAL
The strength of signal to be expected in any area will depend on the distance from the transmitter and the nature or the terrain between transmitter and receiver. The QUAD FM Tuner is suitable for use in fringe areas, but the type of aerial will depend mainly on the locality, and the local dealer will be in the best position to advise on this matter.
