lincoln@eosp1.UUCP (Dick Lincoln) (06/14/84)
> If you want to see some ugly square waves, hook up a function > generator to your amplifier, put some low-amplitude square waves > through your system, and hook the 'scope up to a microphone placed at > any convenient distance from your speakers. That's right, *your* > speakers; I don't care what kind they are or how much you paid for > them. You will see "square" waves that are considerably more > distorted and ragged on top than any CD player produces. Does this > mean your speakers are no good? Of course not, [because musical > instruments don't produce square waves and square wave response is, > at best, a very rough measure of the frequency response of a system.] Low *frequency* square waves at "significant" power levels make a good indicator of power amplifier bass driving and distortion capability even into resistive loads without mic'ed speakers. Try a 60 or 100 Hz square wave fed as "directly" as possible into even a "good" amplifier ("tape monitor" input with "flat" tone controls for a receiver), and use a dual trace sope to show the generator input contrasted to the power amp terminal output at, say, 10% of rated output (you need hefty power resistor loads). You will probably see significant "droop" (lesser trailing portions) in the top and bottom "flat" portions of the wave, indicating significant phase shift in the amp's frequency response over the "active" band (square wave harmonic amplitude drops like 1/f). The demonstrated low frequency phase lag indicates low frequency band width problems in the *overall* feedback loop gain through both the "forward" gain stages and the feedback circuit (unless you are feeding through pre-amp tone controls which are not set "flat"). In 60 Hz square wave patterns with 100% droop (all too common), the feedback loop gain approaches zero at the square wave fundamental frequency, indicating you have no effective damping (because of *several ohms* effective output impedance) and noise/distortion canceling at that frequency in your amp. When you see a significantly "droppy" 100Hz square wave pattern, you have a "sloppy", boomy" bass driving amplifier. Yet if you measured the simple sine wave amplitude response over this same frequency band into the resistive load, it would probably be as flat as a pancake. Square waves whose fundamental and 9th harmonic are within the bandwidth of interest (I can't appreciate less than 10% differences in a scope waveform) are handy test tools because they show simultaneous amplitude and phase relationships very sensitively in one picture over a fairly broad band (1/f drop-off, remember?). A recorded 1KHz square wave would certainly show the alleged CD 3-4KHz response "bulge" (if it exists) unmistakably. 60Hz-5KHz square waves are useful audio test signals.
lincoln@eosp1.UUCP (Dick Lincoln) (06/14/84)
Oops, in my article I claimed that the amplitude relationship between square wave harmonic Fourier components was 1/f (for frequency); what I meant was 1/n where "n" is the harmonic order. I.e., the "third" harmonic has 1/3 the amplitude of the fundamental, and so on. All harmonic components are exactly "in phase" with one another - they all cross the time axis ascending in amplitude at exatly the same periodic points. Sorry 'bout that.