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♦ PDF Cover Page ♦ ♦ Verso Filler Page ♦ Ultra-Linear Operation of the Williamson A m p lifier
DAVID HAFLER':: and HERBERT I. KEROES:::
The Famous "Williamson" can be improved simply by replacing the output transformer and making a few minor changes in other components. The results are well worth the effort and expense.
RO:\I Tllr. TIME an article I appeared descrihing the principle of Ultra F Linear operation of power output tuhes, considerable interest has been evidenced in rhe application of this new circuit improvement to the famous WiIliamson amplifier. The William son circuit ha$ hcen publicized in several arrangenlents including at least one commercial one, and the configur atiou is undoubtedly the most popular The �uthors' Ultr�· high-quality audio circuit ever devel Line" �mplifier com oped. For Illany people there is little bined with the power necessity to attempt to improve this supply on a single basic amplifier circuit. Its listening chassis. quality is excellent; it is easy to COil strtlct; and it provi(les top quality at a cost comparahle wirh units which cannot measure up to its capahili ties. The one category in which the \Vil liamson amplifier is significantly de .ficient is with regard to efficiency and power-output capabilities. Peak power output is less than 15 watts, and it takes a 450-voIt supply at approxi mately 130 ma to achieve this power screen grids of tetrodes are energized TO-300, which was designed for use output. It this limitation can he over from a tap on the primary of the out with tubes of the 6L6 type. Its 6600 come without deterioration of quality, put transiormer. This connection, on ohms primary impedance therefore, is a change in the original design is justi which patents are pending, modifies the also correct for 51:\81's and 807's in the fied. If simultaneonsly it is possible to operating characteristics of t.he tube. Ultra-Linear hook-up. In alhlition, KT- improve the amplifier hoth in measure Proper location of the tap results ill 66's can be Ilsed without deterioration able aspects and in listening quality, optimum input-output linearity simul of quality as the slight mismatch is in then a change is not only justifiable, it taneously with ellicient operation, a favorahle direction with respect to is mandatory. power capahilities approximatdy douhle distortion characteristics. Thereiore, It is difficnlt to impr"ove on some those of a triode connection, alld low this transiormer can be used with the thing which is really good. There .re impedance output such as is offered by tube types normally used in \\'illiam some audio enthusiasts who will scoff triodes. In short, it �>crmits better per son ampl ifiers w i thou t compromise of at lhe illea that the \Villiamson circuit. fonuance than either triode or tetrode characteristics. It is of interest to note can be improved. However, it has heen connection oi the tubes, and this is sub that the change in impedance to 6600 fi,'e years since l\[r. Williamson pub stantiated in comparative listening does not violate �[r. Williamson's de lished his circuit; and in the course of tests an(1 distortion measurements. sign considerations. The modi fied tube five years, there is little which can The unique merits of the Ultra-Linear characteristics of the Ultra-Linear con maintain supremacy without change or stage are particularly applicahle to the nection require this impedance if we renovation. \Vhen a basic circuit im Williamson circuit. The mating of the wish to preserve operating conditions provement-the Ultra-Linear output two seems to have been ine,·itable. The similar to those of the original amplifier. stage arrangell1ellt- 26 AUDIO ENGINEERING • JUNE, 1952 -'1----0,. \10.'1---.0 • ICI-----'O 4 c ",..+----:-911 =+---1- acllO To-� ---- '.TO�� __------�------t- -I�� __TQG 'AlII __ to-tO � 500 w" 11711 Fie. 1. Schematic of the Ultra-Lintar Williamson amplifier. The components in dotted circles life those which are changed from the origin,l circuit i" making the conversion of an ellisti�1 amplifiir to Ultra-Linear oper.ltion. performance. One of these is the all taps of a tapped - secondary winding. creasing one pair of coupling capacitors change in value of the feedback resistor The amplifier, as converted, now from .05 Ilf to .25 Ilf gives a five-to-one to 10,000 ohms in order to maintain 20 surpasses the original with respect to ratio of time constants for the two db of feedback. In the Ultra-Linear response, distortion, and transient char pairs of networks and increases the stage the gain of the stage is greater acteristics. In addition, it was con low-frequency stability margin at nom than for a triode stage. In addition, the sidered desirable to make certain other inal increase in cost. change in primary impedance changes slight changes which primarily increase The insertion of a lO,OOO-ohm para the proportion of voltage fed back. the stability under feedback conditions. sitic suppressor- in the input grid and Thus the feedback is increased unless The low-frequency time constants of a 100-1111£ capacitor across the feedback the feedback resistor is changed to the original circuit's interstage coupling resistor adds to the high-frequency compensate. The readjustment of this networks were the same for both such stability margin and eliminates a slight resistor to the desired value then per networks. This is not particularly de ringing in the vicinity of 200 kc. mits the added gain of the Ultra-Linear sirable in a feedback amplifier since a One last optional difference from Mr. output stage to increase the amplifier given frequency loss is accompanied by Williamson's original circuit lies in the sensitivity. It can now be driven with maximum phase shift. Separation of use of a bypass capacitor across the a little over 1 volt as compared to almost the time constants permits less phase cathodes of the output stage. This has 2 volts required for the original amp shift for the_ same frequency loss. In- been found beneficial in both the Ultra lifier. The feedback is taken from the 16- ohm tap regardless of the speaker con nection. This tapped secondary arrange - - ment is extremely convenient when Fig. 3. Squue-w�ve shifting to speakers of different im performance �t 20 a cps lIeft) and at 50 pedance as it does not require change - - in the value of the feedback resistor. It kc (right!. is made possible -by special transformer design (on which patents are pending) which permits equivalent response on I Linear conversion and in the triode ::f.'tt-: �'4I' I Williamson2 at high levels of operation I as distortion at the overload point is Fig. 2. Curn of in diminished. te""odul.tion distor There are no changes required in / tion n. .Htctin the remaining stages nor in the power / sine-wave-power out- supply. Most of the publicized versions � put. � : Kerocs, H. I. "Building the William o :so son amplifier.l" Radio and T elevisioK News, December l �SO. AUDIO ENGINEERING • JUNE, 1952 27 oi the circuit utilize power transformers on a 'scope. By observing the transfer which iurnish �OO volts at 200 ma. characteristic, it is possible to detect hy Since the draio of the circuit does not eye harmonic distortion of less than 2 exce�d 130 to I�O ma, the voltage oh per cent. The power curve of the amp tained out of a capacitor input filter and lifier thus de"iates frol1l tlat by less than range 20 cps to kc. 5 V� recti fier is ahout �50 volts. This is 1 dh over the 30 \s intimated above, the circuit has the correct value for the circuit as cou excellent listening qualities. This is a ,·erted. Lower voltage will limit the confirmation of the measurements. The power output capahil it ies. additional power available shows up in cleaner and better articulated bass. The Performance overall effect is of greater smoothness, Fig"re 2 shows iutennodulation dis more definition of detail in the soune!. tortion versus power OIIlPIlt. I t can he and better transient response. Ultra� seen that the power out)lut oi the cir Linear circuits seem to have a wider cuit is effectivelv dOllhle,1 over that oi tran sient bandwidth-all audible benefit the original circ'uit for a gi\'en distor which is not readily susceptible to tion. At low levels, around I watt, the lIIeasurement. The combined effect of 1:\1 hit� snch phenolllenal \"alues as the Williamson circuit configuration .06 per cent. It is only 0.3 per cent at a wide-band, low-distortion arrange 13 watts. This curve is hased on eClui v ment-plus an output stage of decreased alcnt sine wa ve power in order to make distort ion and higher power capability, it comparahle with all the other pllh a stage which exceeds the original lisht'd and advertised (lata on the Wil specification and operating parameters, liamson circuit.J The \'allles graphed lIIust be heard to bc appreciated. in Fig. 2 can he divided by 1.47 for those wllo wish to have direct cOlllpara hility with the lIIeter readings ohtained on the illlermodlllation test eqllipment. Fig"re 3 shows oscillograms of square-wave traces taken throllgh the COml)lete amplifier with repetition ratcs of 10 cps :11\(1 50 kc. Traces at mter mediate irt'qllencies appro.1ch theoreti· cal periection. and t'\'en such a rigorolls test as the 50-kc wave shows up ex trelllely well. The waveform has not "sined off," ami the extent of ringing- is It'ss than that exhihited hv the SO()()· cps wave of many good 3 Sarser and Sprinkle, "Musician's amplifier," AUDIO E:'lGINEERl:>G, No\"clllhcr 19 .. 9. AUDIO ENGINEERING • JUNE, 1952 43