Audioclinic (Q and A) (Jan. 1970)

Two Uses for an Oscilloscope

Q. I would like to connect an oscilloscope to the tuner section of my stereo receiver to determine signal strength and multipath conditions.

I would also like to use it for observing channel separation of the section.

What would be the best point of connection?

-Sol Kohen, Philadelphia, Pa.

A. Multipath indication can be derived by connecting the horizontal plates of an oscilloscope to the "live zero" detector output. The vertical plates are connected to the input of the first limiter. The two signals will deflect the beam both vertically and horizontally. If the signal is a straight line, there is no multipath distortion.

If there is a dip, or droop, in the line at .any point, you do have some multipath distortion. The greater the dip, the more multipath is indicated.

The scheme just described is a method used by H. H. Scott for this purpose.

If you do not have enough signal from the detector and limiter to cause sufficient beam deflection (and this is likely) when connecting the signal to the plates, feed the signals to the d.c. inputs of your oscilloscope amplifier. This amplifier must have good high frequency response, to at least 100 kHz.

Stereo separation can be shown on a 'scope by connecting the vertical plates to one channel and the horizontal plates to the other. You might need to use the a.c. input of your 'scope amplifier to increase sensitivity.

You must set the 'scope to have the same gain for each channel.

If the trace on the 'scope is a diagonal line, the signal is mono. This brings us to something quite interesting. Once you know the direction of the diagonal line with a correctly phased signal, you can then detect out-of-phase mono simply by noting that the direction of the diagonal has shifted 90 deg.

The more the signal becomes stereo, the more the trace departs from a true diagonal. There will be a number of traces which will proliferate on the "scope face, determined by the difference in phase of the two channels at any instant and at various frequencies of which the program material might be composed.

Speaker Phasing

Q. There was an article in April 1969 issue of AUDIO on "Don't Let Speaker Phasing Faze You." In it the author mentions that speaker phasing meters are available at some audio dealers. I have not been able to locate a dealer carrying these meters. Can you send me an address of a dealer or dealers who might have these units?

-Don Davis, Waverly, N.Y.

A. A couple of years ago RCA sold a phasing system to radio and TV dealers. However, I never saw it and I cannot vouch for its effectiveness. I wonder if perhaps some of you reading this might have some information about these devices which they would be willing to pass along to all of us.

In the absence of such a meter, this brief discussion of speaker phasing may be helpful. Phasing is not all that difficult to determine. First of all, there are some listening rooms which have such severe problems with standing waves that correct phasing does not bring about much improvement in sound reproduction over that which is heard when phase is reversed. Also, there are listeners who cannot hear the difference between proper and improper phasing. If no difference is noticeable to the listener, there's probably no reason to be concerned about the problem. Of course, if this listener invites some other audio buffs to hear his system, it would be well to have the speakers phased. Sometimes this inability to determine the difference in sound between what is correctly phased and what is not is actually a matter of understanding what to listen for.

To phase two speakers in a stereo system all that is necessary is to play a mono source, or at least have your amplifier set to a position whereby stereo is converted to mono, with both speakers operating. Balance the sound between the speakers. If the phasing is correct, the sound will appear to be coming from a virtual source between the two speakers. However, if the phasing is incorrect, the sound will appear to be coming from both speakers, as it indeed is. This sound will be accompanied by a peculiar "pulling" of the ears. The effect is similar to that of reduced pressure on the eardrum. It is a distinctly unpleasant sensation.

Minimum Speaker Driving Power

Q. In the April 1969 AUDIO there is a long list of speakers. One item covered in this list is "minimum amplifier power required in watts." Just what does this mean?

-H. J. Klumb, Rochester, N.Y.

A. When a loudspeaker manufacturer refers to the "Minimum Amplifier Power in Watts," he believes that this is the minimum power at which the speaker will give satisfactory volume as heard by the listener. Thus, your amplifier must be able to supply at least this minimum power to provide full, room-filling volume.

You may need to walk around the room a bit to find the best listening position from which to observe the correctness of speaker phasing.

To correct an out-of-phase condition, reverse the leads connected to one of the speakers.

IC's Installed in Tonearms

Q. If one were to take advantage of the fact that an integrated circuit can be placed in the tonearm, would it be better construction to feed it into the phono input or into an auxiliary input?

- Dan Derbick, New York, N.Y.

A. Unless there is a good reason to add an integrated circuit to your tone arm you should not do so. If the leads between the phonograph and preamplifier are short and if the cartridge you are using has sufficient output to provide good signal-to-noise ratio, you won't require the use of the IC. Of course, under this latter condition, the IC can boost the voltage sufficiently to supply the necessary output to drive your preamplifier properly from the auxiliary input.

Because the object of such circuits is to provide a small amount of gain rather than equalization, you still will want to put the output of the IC into the phonograph input in order to obtain proper RIAA compensation.

Another purpose for employing an IC in a tonearm is to enable the listener to place his turntable at some distance from the preamplifier. The output impedance of most of these devices is fairly low, and hence, it is possible to use many feet of shielded cable without hum pickup or the loss of high frequencies which would result when this greater length of cable is added directly between the cartridge and the preamplifier. In such circumstance, it is not necessary for the circuit to provide gain, but rather, an impedance stepdown. (I guess I really ought to say that the stage must only supply current gain, not voltage gain.) A single transistor, connected as an emitter follower, should serve just as well as IC in this application.

(Audio magazine, Jan. 1970)

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