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Warning: Never operate AC/DC equipment without an isolating transformer.
A view of the amplifier with the protective cover removed, showing the rectifier and Barretter in the foreground.
A considerable improvement in quality of reproduction can be secured by the adoption of the principle of negative feed-back. The advantages of the system are most evident when pentodes are used, for the well-known disabilities of such valves are largely removed. In this article constructional details are given of an amplifier which embodies this principle, and which is capable of giving exceptionally fine reproduction. It is for AC/DC operation.
In the design of equipment for operation from DC mains one is always hampered by the fact that the HT voltage is always less than the mains voltage owing to the loss in the necessary smoothing chokes. It is consequently important that as little as possible of the HT voltage should be wasted. To this end, smoothing chokes and the output transformer must be of low DC resistance, and the output valves themselves must be of types requiring a minimum of grid bias. This last is necessary because the grid bias voltage is subtracted from the HT voltage, unless battery bias is adopted, and consequently reduces the anode potential and hence the power output.
This question of grid bias normally precludes the use of triode output valves in DC mains apparatus, and one is almost forced into employing pentodes, the more so as their efficiency is higher. Pentodes, however, suffer from many defects which make it difficult to secure high-quality reproduction when the conventional circuits are employed. Their AC resistance is so high that the loud speaker is virtually un-damped, thus accentuating bass resonances, and their characteristics are so shaped that a considerable degree of amplitude distortion occurs, and the optimum load impedance is quite critical.
Instead of being slightly curved throughout, as is the case with a triode, the dynamic characteristic of a pentode has a, double curve rather like a much flattened letter S. As a result the valve generates all harmonics in appreciable measure. The harmonic content in the output of a triode consists almost entirely of even harmonics with the second predominating. The distortion of a pentode, however, consists of both even and odd harmonics with the second and third predominating. Either the second or the third may be the greater, according to the value of the load impedance.
When the optimum load, is used, both harmonics are present in about equal degree. If the load is increased, the second harmonic falls rapidly at first to a minimum and then rises again, but the third harmonic continually increases. If the load is reduced, however, the second harmonic rises, while the third falls.
Now when we are using triodes a very considerable reduction in distortion can be made by connecting two valves in push-pull. The even harmonics then balance out and leave only the very small, amount of odd harmonic distortion which occurs with triodes. With pentodes, however, the advantages of push-pull are not so great, for there is no balancing action on the third harmonic.
The Advantages of Push-Pull
This does not mean, however, that push-pull cannot be made beneficial even with pentodes. It is certainly of no advantage if the valves are operated with such a load that the second harmonic distortion is in any case small. Normally, however, the load is chosen for the best compromise between second and third harmonic distortion, It is clear, therefore, that if we use push-pull and so obtain a condition which we can ignore the second harmonics, we can reduce the load impedance to the value giving a minimum of third harmonic distortion.
When using triodes it is usually assumed that the optimum load for a push-pull stage is twice that for a single valve. This is certainly untrue with pentodes, however, and the optimum for a pair of well-matched valves is of the same order as that for a single valve. It is unwise to proceed to extremes, for valves, are not always well matched, and in practice a load of about 1.5 times that of one valve offers a good compromise.
Apart from permitting some increase in output, push-pull is of advantage in regard to the output transformer. With a DC set two output valves are in any case necessary for good volume, and each may consume 40 mA or so. If the valves are in parallel the current through the output transformer is 80 mA, and a large core is essential if saturation of the iron is to be avoided. The transformer thus becomes costly and bulky. With push-pull, however, the steady anode currents balance out in their effect on the core, and the design of the output transformer is much easier.
It can be seen, therefore, that the output stage of a DC mains amplifier designed for good volume at high quality would contain two pentodes in push-pull operated with a load impedance of the order of 1.5 times the optimum for one valve. If the performance is to be comparable with that of an AC amplifier embodying triodes, however, something more is needed, for the third harmonic distortion is still rather high, and the output impedance of the amplifiers is much too high to damp the loud speaker properly.
This something more is to be found in the application of the principle of negative feedback, due to H S Black, of the Bell Telephone Laboratories. For some considerable time his system has been used in communication engineering, and its application to broadcast receivers was discussed rather fully in last week's issue of The Wireless World. It is sufficient to say here, therefore, that it operates by feeding a portion of the output voltage back to the input in such phase that it opposes the input voltage. The gain is reduced, but amplitude distortion is also reduced in about the same proportion, the effective output impedance is greatly lowered, the frequency response characteristic is improved, and the operation is much less affected by small changes in valves, component values, and operating voltages.
It is found that when the amount of feed-back is such that a pentode has an apparent output resistance similar to that of a triode, it requires about the same input voltage. In many respects, therefore, the application of negative feed-back has the effect of giving triode characteristics to a pentode, but the stage retains the low grid bias and comparatively high efficiency of the pentode. The advantages are considerable, and the disadvantage is merely the loss of the high sensitivity of the pentode.
Fig. 1. - The complete circuit diagram of the amplifier. Negative feed-back is secured. through the action of R7, C7, R10, R12, C8, & R11.
The complete circuit diagram of an amplifier embodying this principle is shown in Fig. 1, and it will be seen that two Pen3520 valves are used in push-pull in the output stage and operated with an anode-to-anode load impedance of some 6,000 Ω. Resistances R13 and R14 of 100 Ω each are included in the anode circuit to prevent parasitic oscillation, and grid bias is derived from the 80 Ω cathode resistance R15.
The input is derived from a transformer having a split secondary; this is important, and the fed-back voltages are injected between the negative HT line and the transformer centre points. Referring to the upper valve of the pair, it will be seen that R7 and R10 really form a potentiometer across the output of this valve, for C7 is included only to prevent the HT from being short-circuited. The total value of R7 and R10 is 35,000 Ω, and R10 is 5,000 Ω, so that one-seventh of the output is fed back to the input. In the second output valve the action is the same, but the feed-back network is R12, C8, and R11.
The details of the underside of the amplifier are clearly shown in this illustration.
The frequency discrimination in the feed-back network itself is negligible over the audible range of frequencies. The fed-back voltages, however, must pass through the transformer secondaries to reach the grids of the valves, and it is found that this introduces a phase-change at high frequencies. This causes an increased high-frequency response to, be secured, and in one case at 10,000 Hz the response rose to about +14 dB! In order to avoid this effect, therefore, each half-secondary is shunted by a 70,000 Ω resistance; this artifice enables a very good characteristic to be secured.
The preceding stages follow standard practice. The penultimate valve is a triode decoupled by the 10,000 Ω resistance R5 and 8μF capacitor C6, and biased by the 1,000 *Omega; resistance R6, which is shunted by a 50 μF capacitor C5. A 0.25 MΩ volume control R4 is included in the grid circuit; and when the switch S1 is in the lower position (on the circuit) one pair of PU terminals is joined directly to it.
A fairly large input is then needed for full output, and it is recommended that this pair of terminals be used only for a sensitive pick-up, such as a piezo-electric type, or for the output from a radio set. For less sensitive pick-ups, including the needle-armature type, and fairly sensitive microphones, an additional stage of amplification is included. S1 is then in the upper position, and the first and second valves are resistance-coupled in a conventional manner.
Some surprise may be felt that the bias resistance of the first valve has a value twice that of the second, while the valves are of the same type, and the first has actually a lower anode voltage than the second. The reason is that, because of the lower anode voltage, the current is lower, and a higher resistance is needed to develop the necessary bias voltage. No volume control is connected before the first valve, since overloading will not occur in normal use. A large input should not be applied to this valve, however. The second pair of input terminals should always be used if adequate output can be secured.
It may be remarked at this point that if no external circuit is connected to the first pair of terminals, they should be short-circuited to prevent the valve from running with an open grid circuit. In cases where apparatus is being continually connected and disconnected, however, it would be a wise plan to wire a 1 MΩ resistance across these terminals.
The Mains Equipment
In this view of the amplifier the sheet of copper foil upon which the electrolytic capacitors are mounted can clearly be seen.
Two smoothing chokes are employed in conjunction with 8 μF electrolytic capacitors, and a half-wave rectifier is used to permit operation on AC supplies. The heaters are all wired in series, and a Barretter is used to regulate the current.
In AC/DC apparatus it must never be forgotten that the circuits are live to the mains, and that precautions must be taken to prevent the possibility of shock. The components are accordingly mounted on a Paxolin chassis, and the whole of the upper deck is covered by a cage of perforated zinc which is earthed.
In one corner there is a little clearance between this cage and the smoothing chokes, so to prevent any risk of a short-circuit a piece of fibre, or even thick cardboard, should be slipped between.
The amplifier with the protective cover.
The external pick-up connections are also important. Screened leads must be used to avoid hum pick-up, but they cannot be earthed. If they are to be effective in preventing hum, they must be connected to negative HT. It is, therefore, necessary to use insulated screened cable. Such cable, having a single internal wire for the high potential connection and metal braided screening, can be obtained with an overall insulation from J Dyson & Co. (Works), Ltd. The screen is used for the low potential pick-up lead, and is naturally connected to the input terminal, which is joined to negative HT.
A pick-up having a metal frame will also have to be maintained at negative HT potential if hum is to be avoided. It would be unsafe to connect it directly to this point, however. Fortunately, it will suffice to connect it directly to the screen of the connecting cable through a 0.001 μF. capacitor rated for 2,500 Volts AC working.
On test, the apparatus fully justified all expectations. Mains hum proved inaudible on all the DC and AC supplies upon which it was tested, the amplification proved adequate, and the frequency response almost perfect. Amplitude distortion was also found to be abnormally low, and in every respect the reproduction was of the kind hitherto only associated with high-quality AC equipment. The undistorted output obtainable naturally varies with the mains voltage, being greatest with 250 Volts AC and least with 200 Volts DC. On 230 Volts DC an output of about 3 Watts can be secured, and appreciably more on peaks without noticeable distortion.
For AC/DC use a permanent-magnet type loud speaker is recommended, for if an energised model be used it will be necessary to provide it with a rectifier and smoothing equipment. If the gear is to be operated from DC only, however, an energised type is quite suitable.
Complete details of the construction and wiring of the amplifier are given in these drawings.
List of Parts
Pen3520, HL1320, UR1C & C2.
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