A Chat about Power Amplifiers

Radio Record, Volume II, Issue 53, 19 July 1929, Page 36

A Chat about Power Amplifiers

Worked Completely from Mains

By

PENTODE

O some the words, "power -amplifier" are. slightly misleading. One is inclined to consider that a small battery operated amplifier is capable of giving all the noise wanted, without going to the expense of purchasing or constructing the more elaborate piece of equipment known as the "power amplifier." . Then again a power amplifier does not necessarily operate from the electric power or A.C, mains, although there are reasons which make them mdte easily adaptable to this source of energy, the principal ones being initial cost and running expenses. Of the frequencies that make up the notes which make up the audible range, the lower ones require more energy ‘to reproduce at the same intensity than the higher ones. Bnough has been written upon the desirability of getting equal amplification of all notes for truer reproduction and the public tendency is for the ineyor of notes of the lower register. o this end amateurs change their audio transformers, buy better loudspeakers and even cut out the higher frequencies with by-pass condensers and resistances in an attempt to get more faithful reproduction of the low register. It has been stated already that more power is needed to drive a speaker on the low tones and the amateur is in-

clined to be puzzled Over this point when, as it is, he has to detune and cut down the volume. It is almost a contradictory statement, but when it is understood that power is needed, not actual amplification, the reader will begin to understand. The only power available to the speaker is that which is handled’ by the last valve. This

power is really changes in current accompanied by the voltage fluctuations. In the case of a small valve, changes in anode current are comparatively small and the power available is insufficient to operate the speaker on notes. below a certain frequency. The remedy is, of course, a more powerful valve in the output stage. Unfor‘tunately a larger valve takes more current to operate, higher B and C voltages, and expense is involved. A power amplifier then is one that has ample power at its disposal to fully develop and reproduce low tones, giving more balance to any piece of music. The volume may not be any greater, but there is no comparison between the tone of a correctly designed power amplifier and. that of an amplifier. using general purpose valves and low plate potentials. As in most other things, design plays the greatest part in the final result and the best advice to anyone contemplating the assembly of a power amplifier is to spend plenty of time in designing and arranging. In the case of an amplifier using batteries there are no great difficulties, and given good class: components almost anyone could build an instrument capable of giving results equal to any of the commercial power amphifiers. But when the cost of batteries, ete., have been calculated it will be found to be equal to, if not more than, the price of components of an amplifier capable of being operated from the- A.C. mains. When the running expenses are taken into account a. battery operated amplifier falls far behind in the question of economy

Power Hums. INCE the introduction of the recently perfected A.C. valves, amplifiers can be constructed haying a negligible amount of hum present, yet even when these valves are used the power hum is the biggest hurdle to overcome. Here again, the question of design is

of paramount importance. It will’ be seen shortly how many different factors determine the overall efficiency of any A.C. operated amplifier and that deep consideration has to be given to the suitability of each component used. As three different sources of energy, e.g., the A, B and © supply, have to beavailable, some method of converting the alternating current input into a direct current and its distribution will have to be devised. In: itself the amplifier has to contain a complete A, B and C eliminator, The question of the A power is solved by using the special AC valves for this purpose. The filaments of these valves are made of a fairly heavy gauge resistance wire, usually in the form of a ribbon, which is coated with thorium oxide, and designed to run on low voltage AC. In the case of the 226 type of valve the filament voltage is 1.5 volts, consuming between 1 to 1.5 amperes. By applying raw A.C. to these filaments and taking the grid return to the mid-point, using either a potentiometer or centre-" tapped transformer winding, very little hum is introduced in one audio stage preceding the power valve. In the case of the last valve, raw A.C. can be applied direct to the filament of any power valve. The main transformer

has to be designed to supply A.C. voltages corresponding to the valves used. Current for the B and.O© supply has to be smooth and direct and after being stepped up to the desired potential by the main power transformer it has to be rectified and passed through a succession of chokes and filters, after which it is divided by a resistance’ known as a "potential divider." ‘This part constitutes the B and CO eliminutor. ; 7" In a previous article it has been explained that to obtain a negative bias applied to the grid, either the grid ‘can be made negative with. regard to- {he filament which is at earth potentia} or’ the filament can be made positive with. regard to the grid which is at earth potential. The latter case is usually made use of in an A.C. operated amplifier, so that one single rectifying system gives both B and C potentials at the same time. As the power valve has a certain D.C. resistance of its -own, this valve can be used to calculate the resistance. suitable for the grid poten-. tial. A far easierymethod, however, would be to calculate the resistance necessary to give the required voltage drop, knowing the normal plate current of the valve. Also, as the ‘plate-current has to pass through this resistance, its eurrent-carrying capacity will have to be taken into account. ‘ This’ explains the. fundamental prin ° ciples of the A.C. ‘operated amplifier and Diagram I illustrates the circuit. of a typical two-stage power amplifier. It can be seen that the main power transformer has four distinct secondary windings, delivering voltages suitable for the various filaments and also the high voltage ‘windings, the voltage of which has to be given careful consideration, as will be explained later. Following the B plus lead from the rectifying filament, we are lead to the chokes used in the smoothing circuit. By utilising the field-winding of a moving coil speaker in place of the second choke it not only fully smooths the direct current, but also provides, in an economical way, the -.energy for the M.C. speaker pot. Coe Two or three difficulties have to be overcome here and allowances made in the general design. A speaker field usually operates on the 110-volt direct current supply and passes.as much as: 95 milliamps. Now, if 95 milliamps is passed through the winding theré Wrll’ be a voltage drop of 110 volts across the. speaker field. This voltage will have to be added on before it ean be dropped and the high voltage winding on the power transformer will have to be suitably increased. It was stated that 95 mills would be passed through the speaker when. acting as a choke, No single valve of the 210 type should pass more than 24 mills, so a shunt resistance has to be used to bypass the remaining. 70 mills,

This resistance also acts as a voltage } divider for tapping off the lower plate ) voltage for the first stage, Negative grid bias for this stage is also sup- _ plied by tapping the mid-point of the filament supply at a certain point on this resistance as shown. Two condensers are used to shunt all resistances to offer a lower resistance path to the audio-frequency compon-. ent. The bias resister. for the last stage valve is shown, The valve for which depends entirely upon the current taken by the last. valve. All these important points show the reader the extreme care that has to be taken to cotrectly design the whole piece of ’ apparatus. Not only does it have to be worked out mathametically, but also the arrangement of the eomponents needs careful consideration. The audiofnquedcy. transformers have to be ar ranged so that they do not come in the field of the power transformer, This is

ione by carefyl screening in aluminium boxes. Diagram 2 shows an amplifier using push pull in the last stage, and for a given power output much lower plate voltages are necessary. This not ‘only makes the receiver safer to eon- ' struct, but also provides a great Save ing of condensers. The condensers in the previous arrangement will. of necessity have. ‘to have a high working voltage rating. By using Icwer plate voltages the price of suitable condensers is: greatly reduced. Also a single valve can be used

to rectify. A valve of the 280 type has two plates and filaments, and. rectifies up to 350 volts, , In this diagram an arrangement known as the series filament current is shown. "The bypass resistance described previously. is designed to carry sufficient current to make up the total for the speaker winding. Providing at least 60 mills can be passed through this resistance a convenient form of filament current is available for a 199 type of valve, By including the filament of this valve in series with the main resistance a voltage drop corresnondinz to the voltage rating of the filament, takes places across the two filament leads, and this extra resistancé will have te be calenlated to work in with the main resistance bank.. A precaution necessary with this type of filament supply is to shunt the filament leads with a high valve resistance, so that in the case of the fila-

ment burning out the voltage divider will not be completely disconnected. The value of this resistance can be such that a current of two or three mills is only passing as long as the 199 valve filament is intact, Although this article is not intended to give constructional details, it at least shows that a. great deal of time and careful study has to be given to the design and the characteristics of most of the components depend entirely upon a collection. of the characteristics of the remaining parts. It is certainly not a job for the absolute novice to attempt.