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High-fidelity tuner for the push-pull quality amplifier.
The attainment of the highest standard of reproduction of broadcasting involves the reduction of all forms of distortion to a minimum, not only in the purely LF circuits, but also in the HF and detector stages. The matter is greatly complicated by the necessity for avoiding interference, and the receiver, constructional details of which appear in this article, has been designed to approach perfection in local reception while giving an acceptable compromise between selectivity and quality on more distant transmissions.
It is generally accepted that a receiver designed primarily for high-quality reproduction must introduce a minimum of amplitude distortion and have an overall frequency response flat within very few decibels for frequencies between some 30 and 10 kHz. It was shown, however, that questions of interference may often render a poorer response desirable, so that within limits it should be readily adjustable by the operator. It was shown also that if a straight set be used variable capacitors cannot be employed for tuning, since the selectivity would increase so greatly with wavelength that it would seriously affect the quality of reproduction. Because of this, permeability tuning, which leads to approximately constant selectivity, is employed in the receiver which has been designed to do justice to the excellent characteristics of the Push-Pull Quality Amplifier.
Fig. 1. - The complete circuit diagram of the receiver in which permeability tuning is used with a single HF stage embodying two triode valves for stability and linearity.
The circuit diagram of the receiver appears in Fig. 1, and it will be seen-that a single HF stage, embodying two triode valves, is used, a diode detector, and a triode LF valve. Two tuned circuits are employed before the first valve and are coupled together in the manner of a bandpass filter the actual degree of coupling, however, being slightly sub-optimum. The aerial coupling is unusual, the change from normal practice being dictated by the requirements of permeability tuning. It will be seen that the aerial is connected to the junction point of two capacitors, C1 and C2, which are joined in series across the first tuned circuit. The net result is to make the efficiency of the whole input circuit rise as the wavelength falls, and this almost exactly compensates for an inverse effect in the HF amplifier.
The HF Amplifier
The HF stage is of the double-triode type described by Colebrook and it is employed in preference to a screen-grid or pentode valve owing to its inherent stability and a marked absence of non-linear effects. The major portion or the amplification is given by the second of the two valves, and a gain of 30 times is obtained on the medium waveband - the gain being constant with wavelength. Although the coupling between the two HF valves is nominally resistance coupling, it is actually chiefly capacitive, the capacity being the inter-electrode valve capacities and the inevitable stray wiring capacities. Consequently, the load on the first HF valve varies with wavelength, so that the gain of this stage varies from 2 times at 200 metres to about 4 times at 550 metres. The overall HF amplification thus varies from 60 times to 120 times. As already explained, however, the efficiency of the input circuit has been arranged to vary in an inverse manner so that the sensitivity of the receiver is approximately constant. It is, in fact, much more nearly constant than usual and does not vary at all from the 1,100 μV mark between 200 and 300 metres. At longer wavelengths it falls slightly, but never below 1,400 μV.
The third tuned circuit acts as the coupling between the second triode and the detector, and it is this portion of the receiver which differs most from conventional practice. The cathode of the LF valve cannot be at earth potential, since it provides the phase reversal necessary for feeding the push-pull amplifier, and the cathode varies in potential just as much as the anode, but, of course, in opposite phase. As no point on the input circuit to this valve can be earthed, the detector circuits cannot, be earthed either. The difficulty, is usually got over by employing transformer coupling between the HF valve and the detector, but this cannot be done with permeability tuning at the present time owing to the absence of a suitable component. Apart from this question, a step-up transformer or its equivalent is necessary for coupling the HF valve to the tuned circuit, for the AC resistance of the valve is only some 13,000 Ω under working conditions.
It does not seem to be generally known that the equivalent of a transformer can be obtained by feeding the tuned circuit through a capacitor. The circuit, in fact, is identical with that of the tuned grid coupling so widely used, and the difference lies only in the value of the coupling capacitor, which is made much smaller than usual. Referring to Fig. 1, an HF choke Ch1 is included in the anode circuit of the valve, and the coupling to the tuned circuit is provided by C8 and C10 in series. C10 has a value of 100 pF. (0.0001 μF), and in this case optimum coupling is secured when C8 is about 40 pF.
Normally, of course, only one capacitor would be used, but by using two in the manner shown We can feed the tuned circuit with full efficiency, and yet have neither side at earth potential. The detector circuit itself is entirely conventional, and a duo-diode with its two anodes strapped acts as the detector with a load resistance R7 of 100,000 Ω, shunted by a 0.0001 μF by-pass capacitor C11, and the LF potentials appear across this resistance. Now these LF potentials must be applied between the grid and cathode of the following valve, but as regards high-frequency currents both terminals are live with respect to earth. It is necessary, therefore, to include filter circuits in both leads, and two HF chokes, Ch2 and Ch3, are used with two by-pass capacitors of 0.0001 μF each, C12 and C13.
When the switch S1 is closed the LF potentials are applied through the capacitors C15 and C16 of 0.2 μF to the 0.25 MΩ volume control resistance R9, from which the voltages are applied in a conventional manner to the triode LF valve. As the volume control resistance is considerably higher than the diode load resistance, its presence introduces negligible distortion. When S1 is open, however, the parallel resonant circuit L1 C14 is included. This circuit, which is tuned to 9,000 Hz, has a very high dynamic resistance and provides an attenuation of about 30 dB at its resonance frequency for the purpose of suppressing the heterodyne note between adjacent stations. A tone control circuit is also connected in this portion of the set and consists simply of the 0.25 MΩ resistance R8 and the 0.015 μF capacitor C17. This resistance and the switch S1 are linked together and operated by a single panel control; when this is fully rotated in an anti-clockwise direction S1 is closed and the whistle suppressor is in circuit, but otherwise the full high-frequency response is secured. A small clockwise rotation of the control closes the switch and throws the filter out of circuit, so that the upper register is fully reproduced. For the rest of its rotation the control acts as a tone control to give a progressive reduction of the high-frequency response.
The diode detector is seen immediately behind the two valves on the right which comprise the double-triode HF stage.
The triode used as the LF amplifier is an MH4, and it derives its grid bias from the voltage drop across the 2,000 Ω resistance R10 in its cathode circuit; this resistance is shunted by a 50 μF electrolytic capacitor C18. The coupling resistances are the two 50,000 Ω resistances R12 and R11 in the anode and cathode circuits respectively, decoupling being provided by the 50,000 Ω resistance R13 in conjunction with the 8 μF capacitor C19. Now, in spite of the HF filtering included in the diode output circuit, a small HF input may be applied to the triode; in order to prevent this from having an adverse effect upon the performance it is necessary to provide by-pass capacitors in the output circuit. As regards LF potentials, the capacitors C12, C10 and C8 can the regarded as being effectively in parallel with the cathode coupling resistance, so that we must use an anode by-pass capacity C20 approximately equal to their sum in order to retain a balanced output circuit at high audible frequencies. This capacitor, therefore, has a value of 0.0003 μF.
In spite of the moderate selectivity of the tuning system, a certain degree of sideband cutting takes place, and a tone correction circuit is accordingly connected in the output of the LF valve. This comprises the two identical chokes L2 and L3, and the 0.001 μF capacitor C21; the combination resonates at 8,000 Hz, and gives a rising characteristic to the LF circuits, thus maintaining the responsive curve flat within about 3 dB up to 8,000 Hz. Such correction is not needed on gramophone, and the switch S4 is accordingly used to throw it out of circuit and it is linked with the radio/gramophone changeover switch S3 and the switch S2 which removes the HT from the HF valves on gramophone to prevent any possibility of interference from broadcasting stations.
The tone correction chokes are situated between the electrolytic capacitor and the LF valve, while the whistle suppressor is in the extreme right-hand corner.
As already stated, this receiver is designed primarily for very high quality reception of the local stations; it has been felt, however, that most listeners like to be able to receive a few Continental transmissions on occasion. With a good aerial the sensitivity is actually sufficient to permit this, but to give some factor of safety reaction has been included and it is applied to the grid circuit of the first HF, or buffer, valve. The quality naturally deteriorates somewhat when it is used, but in general it is only needed for those stations upon which interference does not permit the retention of the full high-frequency response. The pre-detector volume control takes the form of a variable resistance R1 connected across the first tuned circuit. In many districts this will not be needed, for the HF stage and detector will handle quite a large input and the LF control gives an adequate range. Where the set is used with a good aerial within about 15 miles of the local station, however, its use will usually be essential in order to avoid overloading.
Fig. 2. - The overall fidelity curves of the receiver and amplifier for various signal frequencies. The small amount of variation with tuning is a direct result of the permeability tuner.
The performance of the receiver as regards fidelity is shown in Fig. 2, with the tone control set for full brilliance and the whistle suppressor out of circuit. The three curves for the medium waveband show how well the high-frequency response is maintained and how little it varies with the tuning. At 30 Hz the loss is under 2 dB, and for a signal frequency of 1,000 kHz the response does not fall below -3.0 dB until the modulation frequency exceeds 8,000 Hz. Even at 10,000 Hz the loss is only 8.0 dB. At 1,400 kHz the loss at 8,000 Hz is smaller, and is only 0.8 db. The fourth curve shows the performance at 250 kHz on the long wave-band. The degree of sideband cutting is somewhat greater than on the medium wavelengths owing to the ratio of L/R in the tuner being higher and giving a higher degree of selectivity. This is little disadvantage, for stations are spaced rather more closely on this waveband, and in general do not themselves transmit the high frequencies so fully.
Fig. 3. - The response at 1 MHz. is illustrated here together with curves showing the effect of inserting the whistle suppressor and the tone control.
The action of the tone control and whistle suppressor is shown in Fig. 3 at 1,000 kHz. It will be seen that the introduction of the whistle attenuates a frequency of 9,000 Hz by 30 dB, thus effectively removing the heterodyne note between adjacent stations: a note of 8,000 Hz, however, is reduced by only 7.0 dB, so that the use of the filter causes very little deterioration of quality. The tone control permits the high-frequency response to be reduced to any desired value between the limits shown by the full-line curve and that marked 'Deep'.
Constructing and Adjusting the Tuner Unit
An under base view of the receiver showing the wiring and layout of components.
The apparatus comprising the receiver unit is assembled on a metal chassis which is obtainable with all holes drilled and with valve holders ready mounted. Care should be taken to see that all fixing screws are well tightened; and it should be remembered that the resistance R15 must be mounted before the permeability tuner, otherwise its fixing holes will be inaccessible. Since the chassis is cellulose finished the paint must be removed around all holes at which a connection to the chassis is desired - these .are chiefly the mounting holes of the tuner, the electrolytic capacitor C19, and all earthing points.
A commercial chassis complete with valve holders was offered to constructors.
The two tone correction chokes L2 and L3 are mounted above the chassis on brackets, but the whistle suppressor coil L1 is mounted by its-centre spindle and itself carries a bracket which supports its tuning capacitor C14. The spindle of this capacitor, is live, so that an insulated support is used, as shown in the drawings.
Fig. 4. - The construction of the whistle suppressor is clearly shown in this drawing; the capacitor C14 is carried by the bracket.
As far as possible the wiring should be carried out in the manner,of the original receiver; that is to say, not only must the same points be connected, but as far as possible the various leads should lie in the same relative positions. Any convenient gauge of wire run in insulating sleeving may be employed, but No. 22 tinned copper is particularly suitable since it is small enough to handle easily and yet possesses a fair degree of rigidity. If this gauge be used for the heater wiring, however, it will be necessary to run a separate pair of leads from the terminal block to each valve, since it cannot carry more than about one ampere without an excessive voltage drop. If a single pair of wires be used for the first three valves, as shown in the drawings, these wires gust be of No. 16 gauge.
Fig. 5. - Two identical coils, L2 and L3, are needed for the tone correction circuit, and this illustration gives full winding data.
The various coils used in the tone correction circuit and the whistle suppressor can readily be made, but can doubtless be obtained ready made it desired. The drawings of Figs. 4 and 5 show the construction, and is unnecessary to make an attempt at an even layer winding. It suffices to run on the wire haphazardly, maintaining the general level evenly, and the operation is greatly facilitated if some form of simple winding machine fitted with a revolution counter be available.
The Preliminary Adjustments
Fig. 6. - The coupling capacitor C8 should be set so that its vanes overlap to the degree shown in this drawing. No further adjustment will usually be needed.
When setting up the receiver the voltages and currents should be checked to make sure that all is in order. The wave-range switch should be rotated in an anti-clockwise direction to the medium wave-band position, the pre-detector volume control set at maximum and reaction at minimum. The inter-valve coupling capacitor C8 should be set at the position indicated by the sketch of Fig. 6, and any further adjustment of this component should be unnecessary. The local station should then be tuned in and the preliminary adjustment of the ganging carried out. Each section of the tuner is provided with two concentrically arranged trimmers - one for the medium waveband and the other for the long. The medium wave trimmers are all adjusted by means of screws, and a narrow-bladed screwdriver should be used. In the case of the rear trimmer it is important that the screwdriver have an insulated handle, and it is desirable that the metal part be very small, for this circuit is not earthed. It should be noted that with these trimmers an anti-clockwise rotation of the adjusting screw gives an increase of capacity. The long-wave trimmers are operated by nuts, for. which a 0 BA box spanner must be used; these operate normally and the capacity is increased by a clockwise rotation of the control.
Having tuned in the local station, adjust each of the three medium wave trimmers for maximum signal strength, progressively reducing volume by means of the LF control if necessary. Then set the tuning control so that the dial setting agrees with the wavelength of the station and readjust the trimmers for maximum volume. If the re-adjustment of the main control is to give a lower dial setting, the capacity of each trimmer must be increased, which means an anti-clockwise rotation of the adjusting screws, and vice versa. A weak station on a lower wavelength should next be tuned in and each trimmer very carefully readjusted for optimum signal strength. The ear will have to be used as the indicator, for there is no circuit giving a suitable change of direct current to operate a tuning meter.
When satisfied with the medium-wave ganging the long waveband should receive attention, and here only the three nuts require adjustment; the medium-wave adjusting screws must not be touched. The simplest course is to set the dial to read 1,500 metres and tune in Droitwich by the adjustment of these three trimmers only.
The only other adjustment necessary is to the whistle suppressor capacitor C14, and naturally this can be done only when a station is found suffering from whistle interference. The filter is intended to provide maximum attenuation at 9,000 Hz, and when it is so adjusted it greatly attenuates all frequencies higher than about 8,500 Hz. It will not, and is not intended to, cut out whistles of lower frequency; it should therefore, be adjusted on the medium waveband on a station which is known to be separated by 9 kHz from each of its neighbours, and, which is consequently accompanied by a very high-pitched heterodyne whistle. The medium waveband is preferable to the long for this purpose, since stations on the latter are sometimes separated by less than 9 kHz and the filter cannot be expected to remove, these. The adjustment is simple and consists merely of turning C14 to the point at which the whistle disappears; in general, the setting will be about one third of the way from the maximum position of the control, and is usually fairly critical.
When used with a good outdoor aerial no difficulty should be found in obtaining good reception of the chief Continental stations, and several should give adequate volume without reaction. The chief purpose ot the receiver, however, lies in local reception, and even with a poor aerial full volume should be obtained in most districts from the local medium wave transmitters and Droitwich. The set is much more sensitive than most local station receivers, but it cannot be expected to give good volume from distant transmissions unless the aerial is reasonably good.
Operating the Receiver
Full details of the construction and wiring of the receiver are given in this drawing.
The quality of reproduction obtainable when a suitable loudspeaker combination is employed is of a very high standard indeed. The departure from an even frequency response of the combined receiver and amplifier are undetectable by ear except in the extreme upper register where they are necessary under present broadcasting conditions in order to avoid interference. The presence of the 9 kHz heterodyne note would mar reproduction far more than does the loss of frequencies of this order. As regards amplitude distortion, the equipment is essentially linear provided that no attempt is made to obtain excessive volume.
Before concluding this article it may be as well to make some mention of the various panel controls, for although the purpose of the tuning control, wave range switch, and radio/gramophone switch are obvious, the two volume controls and reaction may cause some confusion. It is not intended that these be employed simultaneously, and for local reception reaction should normally be set at zero and volume controlled by the LF control, the HF resistance R1 being really a type of local/distance switch, and not meant for providing a smooth variation of volume. For everything but local reception, R1 should be at maximum, and it should only be used on local stations if it be found that with the LF control at maximum overloading is severe. In general, R1 should be at as high a setting as possible without severe overloading occurring when the LF control is set for full, volume. The extent to which it is used, therefore, will depend upon the efficiency of the aerial and upon the distance from the local station. It may be remarked at this point that a form of motor-boating may occur when overloading commences, and if this effect occurs it should be taken, not that there is anything wrong with the equipment, but that the input is too strong, and R1 should be accordingly reduced. This effect does not occur until the full 4 Watts output of the amplifier is obtained, and is rather an advantage than otherwise,since it gives a timely reminder that one is attempting to obtain excessive volume.
When receiving stations other than the local, R1 should always be at maximum, and if the strength of the signal be sufficient volume can be adjusted only by R9. It the volume is not enough, reaction must then obviously be employed, but it is sometimes advantageous to use it on stronger signals and (to reduce the amplification by R9 in order to maintain the desired volume, for the selectivity can in this way be increased somewhat.
The use of the tone control and whistle suppressor was dealt with earlier, but it may be as well to remember that when the knob is rotated in an anti-clockwise direction to a position just short of that at which the switch opens, the full high-frequency response is obtained. A further rotation in the same direction throws the whistle suppressor into circuit, while rotation in the other direction reduces the upper register. Owing to the particular taper of the resistance employed, it will be found that the major portion of the control occurs towards the end of the travel.
The LF Unit
The Push-Pull Quality Amplifier which is employed with this receiver is built as a separate unit, making it suitable for use in conjunction with other special circuits, including those tor television purposes.
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