Audioclinic (June 1978)

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Graphic & Parametric Equalizers

Q. Describe and define the difference between graphic and parametric equalizers. What is the advantage of each of these devices?

-Martin Smith, Grants Pass, Oreg.

A. A graphic equalizer divides the audio spectrum into many separate slices, and because of the arrangement of the controls, you can see at a glance what the frequency response is. The controls are essentially a graphic representation of the frequency response produced by the equalization you have selected.

With a parametric equalizer the spectrum is also divided into sections, and there are usually not as many controls for accomplishing this as are found on a graphic equalizer. Where a good, one-octave graphic equalizer will have 10 sets of controls, a parametric equalizer might only have four sets.

With the graphic equalizer, the controls boost or cut at the frequencies shown as well as some distance above and below these designated frequencies. The controls on parametric equalizers have three different capabilities.

First, associated with each frequency control is another control which can shift the center frequency over some given range, perhaps one octave in either direction; this means that we can either use the controls as marked or shift their operating frequency range. This means that a few boost and cut controls can do the work of many.

The philosophy behind this is that, in compensating for room acoustics, defects in recording quality, etc., it is probably necessary to only touch up about three or four points in the audio spectrum.

Another control associated with each boost and cut control is a Q control. What this does is to adjust the bandwidth covered by the selected boost and cut. The conventional graphic equalizer is arranged so that all boost and cuts cover a specific bandwidth. However, the parametric equalizer is arranged so that the bandwidth can be made either very sharp or very wide, depending upon whether the sound corrections require a narrow notch of correction or a broad, smooth slope.

The one-octave graphic equalizer can be operated with excellent results with only a little practice by most users. The parametric equalizer, on the other hand, will take some getting used to, but once mastered, provides a degree of control not possible with a graphic equalizer.

Matching Output Tubes

Q. I have a vacuum-tube amplifier.

The instruction manual specifies that the output tubes should be replaced with matched pairs only. Can you please tell me how I can match tubes myself?

-Name withheld.

A. It is best to use a mutual conductance tube checker to match vacuum tubes. The tube checkers usually found in stores only check cathode emission, and this is only one parameter of tube operation. Mutual conductance is also important in achieving a proper match.

Some tubes are sold in what are, at least, said to be matched pairs. These are more expensive than buying two loose single tubes with the hope that they will properly match up. I have seen cases, however, where the match of a pair was not too well made.

The need for matched tube pairs is less in an amplifier which has a means for adjusting both bias and a.c. balance. The bias adjustment should be so arranged that it can be set up for each tube individually, rather than for the pair of tubes only.

Drum Miking

Q. I play drums with a few groups and I tape record all our sessions.

Could you tell me what type of mikes are best for recording the drums?

-Steve Johnson, Paulsboro, N.J.

A. You need mikes with a good low frequency response. Probably, they should be omnidirectional to pick up the reverberant as well as the direct sound. You probably will want the microphone to be sturdy, which seems to call for a dynamic.

If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, sell-addressed envelope.

(Source: Audio magazine, June 1978; Joseph Giovanelli)

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