The 88-50—A Low-Distortion 50-Watt Amplifier

Fig. 1. External view of amplifier and preamplifier described by the author. This installment covers only the 50-watt power am p lifie r. The 88-50—a Low-Distortion 50-Watt Amplifier With harmonic distortion of less than 0.5 per cent throughout most of the audio spectrum, this 50-watt amplifier is comparatively simple in construction and requires only ordinary care in wiring. W. I. HEATH" and C. R. WOODVILLE* or audio amplifiers of medium from a pair of KTSS’s is slightly over the biggest gremlins of high-fi apparatus power, the KT66 output tube be 50 watts with a supply voltage of 500 a rumble filler using an attractively F came well known with the Williamson volts. This article describes the design simple circuit is incorporated in the pre amplifier, and its reputation for reliabil and construction of such an amplifier; amplifier. ity has made it much sought after in a second article will give similar details “off-the-shelf” high-fidelity amplifiers, as of a matching preamplifier. They are The Power Amplifier well as in home-built kits. shown together in Fig. 1. From the same stable there now fol The complete amplifier, the “88-50,” The circuit of the power amplifier is lows a new tube, the KTS8, a pentode has been designed to give a high per shown in Fig. 2. A pair of KTSS’s is with a higher platc-plus-screeu dissipa formance and a complete range of input connected in an ultralinear output stage. tion of 40 watts, and a higher mutual and control facilities without compli- They are driven by a push-pull double lows' a new tube," tne' JVi'so, a pentode has been designed to give a high per knwdri TU7'fknr:'ox -j.ixoo‘A is with a higher plate-plus-scrceu dissipa formance and a complete range of input connected in an ultralinear output stage. tion of 40 watts, and a higher mutual and control facilities without compli They are driven by a push-pull double conductance of 11 mA per volt (11,000 cated networks or unusual components. triode (B329/.12AU7) having a low plate microhms. It is therefore reasonably economical to impedance. A high-gain donble triode The KT88 makes it possible to use construct. With its preamplifier it will (B339/12AX7) acts as the first stage familiar circuit techniques to build audio reproduce from any programme source and phase splitter. Over-all feedback of amplifiers giving the higher power out such as radio tuner, magnetic or crystal 22 db gives low distortion and good put needed to handle the “peaks” in phonograph pick-up, microphone, or damping factor. The input sensitivity of high-fidelity reproduction at home, or direct from a magnetic tape replay-head. the power amplifier is about 0.5 volt rms for public address equipment. This A rotary switch selects the required input for 50 watts output. A U52/5U4G recti higher output is obtainable without using circuit and at the same time adjusts fier provides the 500-volt plate supply, a plate voltage higher than that availa sensitivity and frequency correction to and a thermistor1 protects the electro ble from standard components. The the required playback characteristic. The lytic smoothing capacitors against exces KT88 achieves this by virtue of its preamplifier is separate from the power sive voltage during the warming-up lower plate impedance. For example amplifier and is connected to it by a period. The fact that all the plate cir with cathode bias, 30 watts of output flexible cable. Its controls include a loud i A “thermistor” is a resistor having a power is obtainable with a plate supply ness control, a presence control, and a large negative temperature coefficient of of only 375 volts, compared with 425 treble-slope control, all these being con resistance. The type used here is about volts required by the KT66. The maxi tinuously variable with a flat position 3000 ohms when cold at switcli-on, and mum power obtainable with cathode-bias around half-way. A wafer switch pre gradually reduces to about 30 ohms in a selects the frequency at which the treble- minute or two when it has reached its * The General Electric Company Ltd., running temperature under the influence of Wembley, Middx, England. slope control operates. To avoid one of the combined plate and ripple currents. cuits urn in push-pull pairs cuuhles the put stage and prevents unbalanced op 'The balance is improved somewhat by plate supply smoothing to he reduced eration even when grid current flows the use of an mibypassed cathode re to a minimum, with consequent economy during overload. The B329/.I2AU7 was sistor, lilx, in the driver stage. The of components. selected for the driver stage because of power stage uses close-tolerance individ The. ultra-linear connection for output its low plate impedance, about 10,000 ual cathode bias resistors, R 2!. and R is, tetrodes and pentodes has become well ohms. This makes sure that phase shift and this tends to equalize any slight known in recent years for its ability to due to the input capacitance of the out inequalities in the output tube character provide the output power of pentodes put stage is moved to frequencies above istics. at a distortion level as low as, or even 50,000 cps. Combined with the symmetry Tfie over-all push-pull balance achieved less than triodes. As will he seen from of tlie circuit, this greatly assists in en by the above precautions in circuit de Fig. 2, the screen grids are tapped down suring freedom from high-frequency in sign will give a performance which is the primary winding of the output trans stability when feedback is applied over absolutely satisfactory for most pur former so that the audio signal voltage all. poses. However, where an audio genera on each screen is a fraction of the A liigh-gain double triode in the first tor and ’scope are available, adjustment signal voltage at the corresponding plate. stage (B339/12AX7) provides self-bal can be made which will give a minimum The screen-to-plate turns ratio may be ancing in the phase-inverter circuit and distortion figure. For this a preset wire- anything from 20 per cent to slightly adequate over-all sensitivity after feed wound potentiometer, R.J:l, must be in over 40 per cent for satisfactory results back is applied. corporated in the. plate circuit of the to be obtained. However, to avoid in driver stage as shown inset in Fig. 2. stability at very high frequencies when Balancing Circuits The audio generator should be set to a feedback is applied, the output trans The push-pull signal at the plates of frequency between 200 and 2000 cps and former must have tight coupling between the phase inverter stage is balanced to should be reasonably free from second the various sections, and this is easier to about 2 per cent provided that the 1- harmonic distortion. It should be ad achieve with a screen-to-plate turns ratio megohm resistors Rs and R,, are equal. justed to give a signal which drives the. around 40 per cent, that is, each half More perfect balance may be obtained KT88’s up to full power output into a primary is tapped 40 per cent (turns it R ,, is about 2 per cent; higher in value dummy load resistance; this will be in ratio) from the Bt. end. The ultra- than lis, the actual value being unim dicated by a slight flattening of one or linear circuit provides a low output im portant. If a comparison meter is avail both peaks of the output waveform, dun pedance, roughly equal to the load, and able, a good compromise is to use 5 to the onset of grid current. The balance a good damping factor is, therefore, per cent tolerance resistors, making It,, control, RSI„ should then be adjusted so easily obtainable with feedback. the one having the higher value. Stabiliz that both KT88’s reach the onset of grid The push-pull double-triode driver ing capacitors Cs ami Cs should also be current simultaneously as the signal stage gives symmetrical drive to the out of similar tolerance. voltage is increased. It lias been found Fig. 2. Complete schematic of the power amplifier unit. stabilization is that the response of the power amplifier is devoid of peaks, and falls sharply at very low frequencies with the result that there is no tendency for motorboating' to occur when the pre amplifier is connected on the same plate supply. This enables economy to be ex ercised in the smoothing for the pre amplifier supply, to the extent that it is merely required to give adequate reduc tion of ripple. High-Frequency Stabilizing Before feedback is applied, peaks may be detected in the response of most amplifiers at frequencies up to 100 or 200 kc owing to resonances in the out put transformer. With the output trans formers used in designing the proto type 88-50, leakage inductances between the various windings were low and the Fig. 3. Underside of chassis, showing placement of parts and wiring arrangement. first high-frequency peak was detected about 100,000 cps. Such a peak is al that this adjustment gives minimum dis quency response, the reduction in gain ways exaggerated when feedback is ap tortion with a pair of output tubes that required is approximately equal to the plied, and may cause instability under have not been specially matched. feedback that is to be applied.

Load more