Radio today (Jan-Mar 1939)

Dynamic testing — Part XII ELIMINATING HUM IN AUDIO AMPLIFIERS Practical step-by-step method for tracing source of hum in radio sets and amplifiers By Vinton K. Uleich, Managing Editor When there is too much hum in the output of a radio set, the dynamic method of tracing it to its source is most effective. Actual hum voltage measurements in various parts of the circuit are often misleading since the serviceman seldom, if ever, knows just how much hum voltage is normal. Hum is introduced into a receiver in several manners. The most common is insufficient filtering of the B power supply. Next on the list is a hummy tube — generally with a leak from cathode to filament. A third source is the induction of hum voltages into the signal circuits of the set. This latter type often occurs when leads in low level circuits are in close proximity to the AC leads. While the actual induced voltages may be low, after amplification through several stages the hum reaches tremendous proportions. In amplifiers having audio stage coupling transformers, it sometimes happens that AC hum voltages are induced into the windings of the transformers— this is particularly true with high-gain P. A. amplifiers using an input transformer. How to reduce or eliminate the voltages naturally depends upon the way in which the hum is introduced into the receiver. If there is insufficient filtering of the power supply, the obvious answer is to improve the filtering, generally by increasing the capacity of the condensers. Other times a nominal amount of filtering is used in the power supply, and hum bucking between the stages is employed to cancel the small amount of remaining hum. Hummy tubes generally are discovered on the tube checker, although in some instances it might be necessary to substitute new ones. Principle of hum bucking Fig. 1 illustrates the principle of hum bucking in the stages. A small amount of hum from the filter (after the field or choke) remains and is fed into the plate circuit of the first audio stage. This hum is passed on to the grid of the output stage as shown in 1-A. As an example, the induced voltage via the plate circuit of the first audio stage is one volt. The voltage amplification of the output stage being five, means that an output hum voltage of 5 volts is produced by the tube. However, the plate supply for the output tube is not filtered so well as that fed to the first audio tube — in this instance 5 volts hum is introduced into the plate circuit of the output tube. Both the 1 volt and 5 volt hums are in phase when they leave the power supply. But when the 1 volt hum is amplified to 5 volts as shown in Fig. 1-B, the phase of the voltage is changed by 180° because of vacuum tube action. In Fig. 1-C the two 5 volt hums are shown 180° out of phase. Note that when they are added, the resultant hum is zero. This method of hum canceling is used quite often — although actually, the cancellation is seldom complete. By employing such circuits, the cost of the filter is reduced since a less effective one is needed. Hum bucking coils as used in dynamic speakers operate on a similar principle. The hum introduced by the field cancels that introduced to the voice coil via the output stage. Stage-by-stage analysis The first step in the stage-by-stage hum chasing is to short the input of the output stage. Push-pull stages are quickly shorted by connecting the grids together, if transformer coupled. Or both grids may be grounded directly if the grid is at ground potential for DC. If the grid is at a negative DC potential, the grounding for AC voltages may be effected by connecting a large condenser across the grid input circuit. Single-ended stages are likewise grounded as shown in Fig. 2-A and 2-B. If when the input of the stage is shorted, the hum decreases, it means that the source of hum is previous to that stage. If the hum increases, then hum cancelling (of insufficient value) is being employed in two or more stages. If the hum remains the same, the hum source is in the same stage. Hum in the same stage may be caused by insufficient plate supply filtering, insufficient grid filtering if negative bias is used as in 2-A, or improper poling of the speaker circuits. Fig. 1. In A the hum introduced in the grid and plate circuits cancels out when hum voltages are in proper relationship. In B the process of hum amplification by the tube is shown. When amplified the hum is reversed 180° in phase. Part C shows how the amplified grid hum which is 180° out of phase with the plate hum, reduces hum to zero. 26 Radio Today