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National Horizon 10 Article Horizon 20.Pdf AUDIO—HIGH FIDELITY The National Hori- zon 10-watt amplifier. By ROBERT F. SCOTT TECHNICAL EDITOR Circuit Features in Power Circuit analysis of the single-ended push- AMPLIFIERS pull amplifier in National's Horizon series N the preceding installment (August) plate-cathode junction. Although the Fig. 4. Battery BA1 is eliminated by we saw how the Mclntosh unity- load is in its cathode circuit, we cannot using the drop across R3 as cathode coupled output circuit solves the consider VI as a cathode follower be- bias for V2. Batteries BA2 and BA3 are I problem of switching transients and cause the signal voltage is applied be- replaced by a single supply (BA4) de- notch distortion caused by high leakage tween the grid and cathode rather than livering a voltage equal to that of BA2 reactance in output transformers in between grid and ground as in a cathode and BA3 in series. The load is connected class-AB and -B service. A different follower. The load also appears in the between the plate-cathode junction and solution is offered by Drs. Donald B. plate circuit of V2 which, like VI, has the junction of two large capacitors (Cl Sinclair and Arnold P. G. Peterson. its grid driving signal applied between and C2) in series across BA4. Theirs is the single-ended push-pull grid and cathode. Fig. 5 illustrates another variation circuit used in the National Company's The voltages at the ends of the sec- of the basic circuit. This is used when Horizon 10 and Horizon 20 amplifiers. ondary of a transformer are 180° out one side of the load must be grounded. In the conventional push-pull circuit of phase, so that connections to sec- Although the optimum load impedance (Fig. 1) the tubes develop a.c. signal ondary No. 2 are transposed to show of these circuits is only one-quarter that voltages that are series-aiding across that the grids of VI and V2 are driven used in conventional push-pull opera- the load while the plates are paralleled in the same manner as in Fig. 1. tion, it is still too high for direct across the d.c. supply. The new single- The basic single-ended push-pull am- coupling to voice coils in the con- ended push-pull circuit is the dual or plifier with signal voltages supplied by ventional impedance range. A single converse of the conventional in that the a phase inverter is shown in Fig. 3. 6A7-G used for VI and V2 has an opti- tubes are in series for d.c. and supply We have retained the dual power supply mum load impedance of around 280 the load in parallel. Thus the optimum of Fig. 2. Grid excitation for VI is ohms and it may be used to drive two load impedance—as in the Mclntosh developed across Rl, the phase inverter 500-ohm speakers (Stephens or equiva- circuit—is only one-quarter of the plate- plate load resistor and the grid return lent) in parallel through a direct con- to-plate load in the usual push-pull con- for VI. By careful selection of operat- nection. However, we must still use nection. ing conditions we can use this direct- an output transformer for matching Fig. 2 shows a basic form of the new coupled circuit with VI being biased speakers with voice coil impedances in circuit. The tubes are in series across by the drop across El. Tube V2 is driven the range of 4 to 16 ohms. a d.c. supply with the load connected by an equal but 180 °-out-of-phase signal Fig. 6 is the equivalent circuit of Fig. between a tap on the B supply and the voltage across E2 in the phase inverter 4 with pentode output tubes trans- cathode return. Battery BA1 supplies former-coupled to the load. The output operating bias for V2 because ground- transformer has a split primary. ing R2 directly would make V2's grid Screen-to-cathode bypass capacitors Cl positive with respect to its cathode. and C2 and filter capacitor C3 effective- Two of the three d.c. supplies can be ly connect the halves of the primary eliminated by using the variation in LOAD in parallel for audio signal voltages. Fig. 1—Conventional push-pull circuit. |BA4 Fig. 2—Single-ended push-pull circuit. Fig. 3—Phase inverter feeds amplifier. Fig. 4—Amplifier uses single battery. SEPTEMBER, 1955 79 AUDIO—HIGH FIDELITY Refer to the simplified circuit in Fig. 7. circuit the signal voltage for VI de- Since the capacitors, at audio frequen- velops across Rl and that for V2 across cies, provide a direct connection, the R2. Battery BA1 supplies a negative designers have in .effect used capacitive bias voltage that opposes the d.c. drop unity coupling and eliminated the need across R2 and prevents V2's grid from for magnetic coupling. (The designers going positive with respect to the reported that they verified the fact that cathode. A large filter capacitor pre- magnetic coupling is not needed by vents an a.c. signal voltage from de- using separate chokes in place of the veloping across BA1 and R3. split primary.) Thus, switching trans- The screen grid of VI is supplied by ients and notch distortion caused by current flowing through L2 from the insufficient magnetic coupling can be B supply line, and V2's screen is sup- eliminated with this circuit. plied from the plate-cathode junction As in Figs. 2 and 3, VI and V2 through winding LI. The connections (Fig. 6) are excited by signal voltages are polarized so the magnetic fields pro- Pig. 5—Variation of single-ended cir- applied between their grids and duced by the screen currents oppose and cuit—one side of the load is grounded. cathodes. The phase inverter tube take cancel each other. its plate supply voltage from the center The circuit in Fig. 6 requires twice of the series-connected output tubes. If the voltage and half the current needed its load resistor (Rl) is connected di- for the same tubes in a conventional circuit so the power input is the same for either type. But this particular variation has the disadvantage of re- quiring power supply and filter com- ponents with twice the normal voltage ratings. .LOAD The Horizon series amplifiers The need for twice-normal plate supply voltage is eliminated by wiring the circuit so direct tube currents flow Fig. 7—The capacitors effectively place through the primary windings as in the primary halves in parallel. National Company's Horizon 10 and Horizon 20 amplifiers. The circuit of rectly to the B supply, Vl's grid excita- the 10-watt Horizon 10 in Fig. 8 is a tion voltage would appear between grid simplified version of ';he experimental Fig. g—Single-ended circuit using pen- and ground thus causing it to operate 25- and 50-watt amplifiers described todes transformer-coupled to the load. as a cathode follower. In the Fig. 6 ;by Peterson and Sinclair in The General I2AX7 -PREAMP SECTION VOLTAGE AMPL PHASE INV "1 6V6-CT (2) v« ±~ Fig. 8—The National Horizon 10 is a 10-watt amplifier featuring a single-ended push-pull output stage. 80 RAD I O-ELECTRONICS AUDIO—HIGH FIDELITY Field-Tested. • • Proven Best! Radio Experimenter, October, 1951, and in the January, 1952, issue of Proceed- ings of the IRE. Here the plate currents for the phase inverter and V2 and the screen current for VI flow through L2. The cathode current (sum of plate and screen cur- rents) of VI flows through LI. This arrangement causes a difference in the direct-current flow in LI and L2 and results in a d.c. drop across both windings that reduces the supply volt- age for Vl's screen and the plate of V2. This circuit and current unbalance is minimized by careful design to avoid flux unbalance in the transformer. Unlike the basic circuits in Figs. 3 to Electronically Analyzes, CORRECTS 7, R-C coupling is used between the Picture Tube Trouble in 2 Minutes! phase inverter and the grids of VI and V2 in the Horizon amplifiers. Fixed Cleans Picture Tube elements, burns off oxygen, welds grid bias is developed by a half-wave open cathodes and filaments . automatically and selenium rectifier and a simple R-C electronically! The TV Serviceman can now RESTORE picture tubes IN THE HOME! Service Charge just filter and applied to the output grids. $9.95 to customer—may be sold with minimum The plate supply for the phase in- guarantee! 7"x9"x4" 6'/2 Ib. steel-cased kit. verter is taken from the plate-cathode junction of the output tubes. The output signal developed at the plate of V2 is in series with the phase inverter d.c. plate supply and the negative feedback voltage thus developed reduces the dis- tortion and gain in this stage. The loss in phase inverter gain is minimized by returning its cathode load resistor to the bias supply to increase the effective plate voltage. The voltage amplifier is conventional and uses fixed bias obtained by return- ing its cathode to a tap on a B plus voltage divider. Distortion in the power amplifier is further reduced by 16 db of negative feedback from the secondary of the output transformer to the voltage MUTUAL CONDUCTANCE amplifier cathode. AND EMISSION CHECKER Preamplifier-equalizer Accurately checks Picture and Circuit Tubes for The preamplifier and equalizer section filament continuity, emission, gas, shorts and has four input channels. One for a leakage under actual load conditions—in 20 radio tuner, one for playing back tape seconds! 7"x9"x4" 7 !b steel-cased kit.
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