A Comprehensive Treatment of an Important Subject

Radio Record, Volume III, Issue 39, 11 April 1930, Page 36

A Comprehensive Treatment of an Important Subject

(By

MEGOHM

ECAUSE the audio amplifier is common to every valve receiver, and also assists in obtaining the best type of gramophone reproduction, it has, in company with the loudspeaker, become a subject of supreme importance. It is the constant desire of’ every constructor to alter and improve . with the aim of ultimately possessing a push-pull amplifier working completely ‘off the electric mains supply. Those who have already arrived at this stage know the extreme satisfaction to be derived from an amplifier capable of handling heavy volume with remarkable clarity and an absence of noticeable distortion. Such reproduction, of course, assumes the provisiou of a loudspeaker worthy of the amplifier. Although a power amplifier may be capable of giving enormous volume without distortion, such volume need not necessarily be used, but forms an amount of "reserve" that ensures the very highest quality of reproduction when normal yolume is used. This is a very different condition to that obtaining when a small amplifier is used, and every part of the circuit is "pushed" to give far more volume than the outfit should be expected to give. The result of this latter method is the pro--duction of plenty of noise, accompanied by very little quality. Apart from the bad effects of amplifying the output of an overloaded de tector valve, there may be inefficient transformers or other components, and valves incapable of carrying satisfac tory volume. Of course, as many. are aware, the valve in the last stage is the main fac- _ tor, so long as it is backed up by other necessary improvements in the circuit, and when it comes to placing two valves in "push-pull" in the last stage. some rather drastic changes may be necessary. But these changes are all worth while, and have to be faced sooner or later by the progressive constructor. ; nae) Power Valves. POWER-VALVES may be used in the last stage of any receiver, and may be classed as "small," "medium" and "larger" or "super," the latter class ineluding the 245, 210 and 250 types. A power-valve has a low impedance, which allows a large "B" current: to pass, and has a thick and long filament in order to increase the emission of electrons. The plate is large, and its high applied ("B’) voltage increases

the electron flow, which in turn is varied by the action of the grid becoming alternately positive and negative, The relative positions of the three electrodes with regard to one another, and the spacing of the grid wires determine the amplification factor ‘of a valve, and in a power-valve the electrodes are placed to give a low impedance, and this, with open-spaced grid wires, gives a low amplification factor. The ability to handle greater volume compensates for the lessened amplification. In receivers with a large overall amplification, and consequently capable of passing heavy volume to the first audio stage, it may be necessary to place in that stage a suitable power-valve, usually a smal) one, so that fair amplification is retained, but where two large valves are employed in push-puil in the last stage, the first audio may have two smaller valves in push-pull also. This is the most satisfactory arrangement of all, but complete operation from the mains is practically a necessity. For the battery-operated set with an output valve that does not handle the required volume very efficiently, it may be desired to place a larger valve in the last stage. Probably such a change must be accompanied by several other changes, because the fact of the larger valve passing more "B" current, possibly at a higher voltage, introduces other matters for consideration. In order to obtain the full advantages of the larger valve, the plate voltage should be near the maximum recommended by the manufacturer, and the actual current in milliamperes taken from the "B" battery will be greater than previously. Where a "B" eliminator is in use the required voltage will probably be forthcoming, and probably the milliamps. as well, because if the eliminator maximum is 150 to 180 volts, it will not be used on a valve requiring more than 20 milliamps., such as the UX171. For dry "B" batteries to last a reasonable time, the total drain should not exceed about 15 mills. so that a 112 type valve is economical, and where

dry "A" batteries are used also, the UX120 is about the limit. Grid bias has usually to be increased when a larger valve is installed, but is easily provided by a 224 volt or other suitable dry "B" battery, if not provided by an eliminator. The Loud Speaker. HE next consideration concerns the output to the loudspeaker. In sets with a small output valve the "B" cur-

rent usually traverses the speaker windings, but as it will probably be only about 6 mills., no harm is done to the speaker magnets if correct polarity is observed in connecting the leads. Where the current passed is more than 10 mills. it is advisable to employ a choke-condenser output filter to protect the speaker windings from saturation by the "B" current. Quality is assisted thereby, and the lower impedance of the output valve will not then affect the speaker output. A valve delivers its maximum undistorted volume when working into an external resistance equal to twice that of the plate resistance or impedance. This means that the impedance of the speaker should be about double the impedance of the valve impedance, though actually there is nothing critical about the matter, especially so long

as the speaker resistance is not less than that of the valve. | An output transformer may be ‘used to compensate for the difference, especially if a dynamic speaker is used. In the case of an ordinary speaker, an output choke usually renders further matching of impedances unnecessary. It also prevents serious loss of plate voltage, prevents heating of the . speaker windings, prevents the placing of a mechanical bias on the speaker armature, and prevents feed-back from the last valve. The inductance of the choke should not be less than 20-to 25 henries, and the condenser 2 to 4 micrefarads. The combination can be pur- | chased as a complete unit. The adoption of large valves means the provision of a power-pack to supply up to 450 volts, and comparatively large current. A pair of 210’s in pushpull will require 36 mills., and a pair of 250’s will take 110 mills. at the highest plate voltage of 450. The 245 is a new valve giving output about equal to the 210, but with only 250 volts on the plate. The filaments of such valves are run from low-voltage raw alternating-current supplied by the power-pack, the voltage of the 245 being 2.5, and of the 210 and 250, 74 volts. The average loudspeaker requires about 100 milliwatts (0.1 watt) to give good volume for a small room, but to obtain consistent quality there should be considerable reserve of power beyond this. The less reserve of power, the greater the liability to blasting or distortion on occasional notes. At the highest plate voltage of 150, the UX 112A gives nearly 2 watt, and with 180 volts the UX171A gives .7 watt output. It will, of course, be understood that these notes are chiefly for the benefit of owners of the humerous batteryoperated sets, many of them homebuilt, that will continue in use for a considerable time to come. Many of the hints, however, apply equally well to home-built a.e. Sets, especially as the last stage in any a.e. set invariably contains an ordinary power-valve, the filament being heated with raw a.c. The average factory-built a.c. receiver has a more liberal audio equipment than its battery prototype, owing to the availability of greater power, and for the same reason the inclusion of a dynamie speaker is usual. Audio Transformers. y QNE of the easiest ways of improving an amplifier is by introducing better transformers than those already installed. Indifferent transformers may introduce a considerable amount of both frequency distortion and harmonic distortion. Frequency distortion is the difference in the amount of amplification at various audio frequencies, A poor transformer will give considerable amplification on high notes, and may give objectionable emphasis known as a "peak" around a particular frequency ; whilst at the same time the amplification of low notes is so slight that below, perhaps, say, 150 cycles, they are practically lost. A well-designed modern transformer is provided with a primary winding of high impedance, so that the low notes are sufficiently amplified, whilst high note loss is prevented by sectional wind-

ing which reduces self-capacity _between the primary and secondary qrindings. In this way the amplification is kept even through the part of the musical scale in general use, and with only a very slight diminution at the extremes. Such transformers contain more material than the inefficient types and cost a little more, but the difference in cost is trifling when compared with the tremendous gain in performance, Harmonic distortion is the insertion of harmonics that were not present in the original sounds. These harmonics may be only weak, but they may easily affect quality. They may be produced in the valves, but more probably by the iron in the transformers, The better transformers have cores of very special’ material in order to prevent ,_ this type of distortion. a But the transformer is not always to blame. Such distortion may also be caused by a too seléctive receiver, too much regeneration or regeneration in too many places, poor detection, and poor amplification, or a combination of these, . Introducing Distortion. ])ISTORTION may also be introduced by improper battery connections, or voltages, or too low a filament voltage, Amplifier filaments should be fixed ‘at the correct constant voltage’ by employing "amperites." The lower the ratio of the transformer, the less the overall amplification for a given number of stages, but the better the quality if the transformer is properly designed. A 6 to1 ratio does. not give twice the overall amplification given by a 3 to 1 ratio; and as the ratio is reduced, the relative amplitude of the lower notes increases, with the result that the apparent decrease in amplification is less than it would be if the amplification were reduced in the same proportion over the entire scale of frequencies. The primary of the first audio transformer carries the detector plate cur‘rent, ‘which is very small, but the second transformer carries the plate current of the first audio valve. This is also small as a rule, but when it ex‘ceeds. 6 miles, as would only be the case ina large amplifier, its case becomes ‘similar to that of the speaker windings at the output, and the current may cause distortion by over-mag-netising the core. A choke-condenser filter can be used in this position, and has actually been employed in some instances, allowing only the a.c. component to traverse the primary winding. The capacity of the condenser across the primary of the first audio transformer is usually stated as .001, but a much’ smaller one may be found better on trial, say .0002 to .0005 mfd. This condenser has to-allow r.f. current necessary for reaction to flow through the detector plate circuit, but -must block the highest audio speech frequencies so. that they have to pass -through the transformer windings. A condenser -is not required across. the second primary unless the low notes are lost and the resulting quality is "tinny," ‘in which case a suitable capacity may be used to subdue the excess of high notes. . Only for this same reason is a condenser put across a secondary, though it is seldom recommended. Audio Oscillation or "Motor-boating." THE output quality of many amplifiers is not as good as should be expected from the components employed, simply because distortion is

introduced owing to feed-back through a "B" battery used for all stages of the receiver, or a "B" eliminator with continuous resistor. In both cases there is high-resistance coupling, and in the case of the dry battery the resistance is gradually increasing as the battery ages, thus making conditions. worse. With a wire-wound resistance of 20,000 ohms in each plate lead except the last, and a 2 mfd. condenser across the plate side of the resistance and the earthed filament, a Ferranti engineer claims that motor-boating is impossible. In actual practice it might only be necessary to make this addition to one or two stages in: the case of battery supply. When a "B" eliminator is used, in order to take advantage of this plate feed system, it is necessary to tap each voltage from a higher point than that required, reducing the voltage for each plate by ‘a separate resistee

ance in its particular lead, also adding the condenser on the plate side. Sometimes an audio choke is used in place of the resistance, but there is no guarantee of its effectiveness. A pushpull output stage prevents feed-back from the output valves, but only if the impedances. are properly matched for both valves and transformers. Speech current does not then: traverse the source of "B" current. Shunting a "B" battery with a 2 mfd. condenser is of little assistance in curing oscillation. Sometimes it is recommended that the connections to. primary or secondary winding. of the audio transformer be reversed. This procedure may stop the tendency to howl at low frequency, but reduce signal strength and create distortion at. a higher frequency, probably reducing amplifica-

tion. A stage of resistance-coupling, followed by transformer coupling, is very liable to audio oscillation. Output transformers do not tend to prevent audio oscillation because they allow the speech impulses to traverse the "B" supply. a Perhaps this form of oscillation is most troublesome when it is incipient; a condition in which it is sufficient to mar quality without being audible as oscillation. . _ Another form of distortion may be introduced by radio-frequency finding its way into the amplifier. By-pass. condensers at suitable points as may be found necessary will subdue this trouble. _ The usual values are .001 from detector plate to filament, .25 mfd. from bias end of .transformer secondaries to filament. B+ end of second transformer. primary to filament, 1 mfd. A non-inductive high resistance (grid-leak type) to suit the amplification factor. of the valve may be placed in the grid lead of the first audio valve. For an amplification factor of 5, 250,000 ohms; . 10, 120,000 ohms ;° 15, 80,000 ohms, and 30, 40,000 ohms. A New Amplifier. ;rrom America comes a description of a resistance-coupled audio amplifier which "Radio . News" states gives practically uniform amplification from 0 cycles per second to 20,000, with no feed-back troubles of any kind. Four stages are used, and'a_ special plate supply is required, owing to the unusual voltages and bias. The amplifier will give a power output of 4% watts over a band of 30 to 10,000 cycles per second. . The voltages used vary from 560 positive to 240 negative. ‘The valves employed are 240, 201A, 112A, and 250. The circuit is a "straight" resistance of special type, without coupling condensers. ° The Loudspeaker. "THERE is no more important unit in the outfit than the loudspeaker. It truly has the "last word" all the time, and can presetve or mar the good product of the rest of the circuit. But the speaker is not altogether in an enviable position, for it frequently has to bear the blame for evil wrought by components earlier in the chain of reproduction. . For the present we. are leaving dynamic speakers out of the discussion, as this article is dealing with home construction of a simpler kind. The cone speaker in one form or another has come to be the popular reproducer in place of the horn type. At

— first a cone was actuated by the same kind of unit or glorified head-phone magnet that did duty in the horn speaker of the period, but as time has elapsed, great improvements have béen made in magnetic units to operate cones. The introduction of the balanc-ed-armature was a big step forward, and this type of unit driving homebuilt cones is now giving pleasure to ay

many thousands of listeners in thig country. Condensers Across Speakers. ENQUIRIES have sometimes been made with regard to placing fixed condensers across loudspeaker leads. With a good amplifier these are usually unnecessary, because when amplification is fairly even, both high and low notes are produced in more or less eorrect relation with regard to strength, and there is nothing superfluous. If the amplifier has a peak on the high audio frequencies, it will favour gramophone scratch, and unduly emphasise some high notes, and in such a case reproduction may be rendered more’agreeable by subduing the higher portion of the musical scale by placing a fixed condenser across the speaker. The necessary value must be found by trial by suit each particular ease. The value should be kept as small as possible to give the desired effect-large capacity reduces volume, and may give "woolliness" to speech. From .01 to .05 should be tried. If the capacity is too large, all the higher portion of the musical scale will be either considerably weakened. or ens tirely lost, and the tone will seem to be lower on that account, but all bril-

liance of reproduction will be lost. If the low notes are sufficiently amplified, they will be heard in true relation without suppressing the higher frequencies. It should be noted’ that "Blue Spot" units have a condenser of small value connected across the. windings and built into the unit, so that any addition to this capacity should not be large. Baffle-boards and Cabinets. "VERY often an improved speaker is more necessary than an improve-

ment in the amplifier, and if a more sensitive speaker is obtained, a given volume is obtainable with a smaller output, ‘and thus the quality is improved simply through the normal volume being further’ from distorfion point than previously. This condition gives more "reserve," and so reduces the tendency to "blast" on heavy notes. Having procured a sensitive unit, a cone must be constructed, unless it is purchased ready-made. In either case a baffle-board or cabinet of some kind must be constructed, because if this is

not done, the air waves which are set up simultaneously by both the front and back of the cone would alternately . neutralise and reinforce each dther and seriously affect the volume, Thus in order to obtain sufficient volume to produce the low. notes, we must provide either a plain baffle, box-baffle, or eabinet for the unit and cone. A plain baffle for a unit such, as 66R is a piece of strong 3-ply or heavier material 40in. square, with a hole in the cents 13in. in diameter. To strengthen this, a couple of stiffeners about 3 x 1 should be attached to the back with small brads driven in from the front, glue being also used. Feet shaped somewhat as in the diagram are cut from lin. rimu. Heavier material than 3-ply may ‘be used with advantage-tin. board glued together without air gaps. Instead of the plain circular hole, the "grille" as shown on the cabinet may be used with advyantage. A flat baffle 40in. square, although the most effective, will not suit everybody’s ideas of bulk, and where it is found more convenient to house the speaker under a table, where it will be out of the way, the baffle may be of 8-ply, 2 feet square, with sides of lin. rimu, 8 inches deep. This is known as a "box-baffle," but the breadth of the sides should not be made larger than the dimensions given-it is better to increase the area of the front board. A diagram shows how to measure the effective size of a baffle. Increasing the distance from X to Y improves the low note reproduction. Shellae or stain will give a suitable finish. Hither may be carefully applied with a piece of cotton rag, going over the surface several times until the desired

depth of colour is attained. This method gives more even results than using a brush. The "grille" effect looks much better than a plain circular hole in the baffle, and to many constructors will prove a simpler task. The end of each aperture is first cut with a bit of suitable size, or slightly smaller than the width of slot. Then the corners A and B are cut out with a sharp knife, to give room for the saw. A fine hack-saw with projecting end served well for the original, but a fine keyhole saw would suit. Coarse and then fine glass-paper round a small block of wood takes away any .unevenness from all parts that are to be finished. No fewer than six fine brads along each side will secure the 3-ply to the frame. The smallest cabinet that may be used with the 66R type measures 18 inches each way inside, and is 9 1-8 inside, back to front, as illustrated. The ‘front may be shaped as desired, but a neat effect is secured by allowing it to project at least 1-8in. at the sides. The box may be of % or lin. rimu, and front and back of 3-ply. The back is shown lying down, and must have about one-third its area cut away by. piercing with holes of any shape, round or. square, .one being near

the centre to allow the adjusting knob to protrude at the back. Attach the back with screws, to be easily remy if able.. Spacing for the grille is shown in.a diagram. Mark the centre space first, then measure outwards each way. The small outside slots will be about 1jin. wide. ‘Cone Construction. THOSE who wish to construct the cone should procure from a printer a sheet of heavy "cover paper" nat less than 17}in. the. shortest dimension, and of soft texture. Take a narrow strip of cardboard not less than 9 inches long, and make two small holes Sin. apart. With a pin through one hole, and pencil point through the . other, describe a circle on the paper. From the centre draw the line A, then measure 12 inches across to another position of the edge, and draw line B,/ then draw line © jin. from B, sloping’ off at centre as shown. After cutting out the segment, which is almost a’ quarter of the circle, glue the fin. strip, and bring edge A flush with B, and pin to a strip of wood.to set. Cut out a circle of the paper iin. diameter, cut out a segment nearly a right-angle; similar to the large cone, and stick on back centre of cone to strengthen it. Next, take apiece of wood about din. x din. and saw a narrow cut din. deep in one end. Go all round the edge of cone with this, bending the paper over to form a bend, leaving a flat rim jin. wide. Now prepare a strip or strips totalling about 42 inches and i 1-8 inches wide, of material for the -"surround." Many kinds of material have been used for this. Rubber is good, but must be very thin, and has the disad-

vantage of perishing in time. Thin felt may be used, and so may serge, ete., but any woven material must be cut "on the bias." The strip is glued all round to the back of the Hin. flange, and no holes must appear at joins in the strip. Now a ring of thick cardboard is cut, the hole in centre of a diameter half-an-inch all round larger than the edge of the paper cone. Glue the surround to this, avoiding puckers, but not drawing very tight. The cardboard ring may be lightly tacked, using plenty of tacks, but not letting them show at front of 3-ply. Glue may be used instead of tacks and has the advantage of preventing any chance of "buzzing" caused by crevices between the cardboard and 8-ply. The unit is attached to a wooden cross-piece and’ fixed in correct position.

Short-Wave News

(Concluded from p. 40.) 4pm. The last hour was devoted to dance music from the Hotel St. Regis. W2XAD was R6 when first heard at | 1.30 p.m. A long talk in a foreign | language was on till after 2 p.m. Volume reached R8 by 2 p.m. and remained at that all through the transmission. WS8XAL at 3.30 p.m. was R3, incréasing to R9, with dance music. Reception was very good. On about 30.2 metres duplex was héard at R9 at 4.20 p.m:, but a stftong morse station spoilt reception. Another station on about 25.5 metres at RO was heard at about the. same time; it also had morse interference. KZRM were R4 at. 7.50 p.ni, with records. From 9.30 p.m. their musical programme was recéived at R7-8, being gushy. RA97: Some fine orchestral items were spoilt by static. Strength R9. |