Concept 16 5 thd test 1978

Page 1

I e AMPLIFIER. rest cumc

This graph is important to
you. It shows valuable
information showing the
general condition and

ultimate capabilities of
your amplifier. ..

HOW TIIE TEST IS DONE:

Your amplifier was tested for total harmonic
distortion at its maximum useable continuous
power output. Stereo amplifiers are tested with
both channels operating simultaneously. If your
amplifier did not reach advertised power, it was
tested at a lower power. This distortion measure-

ment was made with audio frequency tones from'

the low bass to very high treble. The amount of
distortion tells how much your amplifier changes
the music you hear.

WHY TIIIS KIIID
0F TEST IS USED:

Many tests are possible. Total harmonic distor-
tion is the single most revealing test since it covers
all audio frequencies, and is essential to locating
and solving problems in an amplifier.

Originated in 1982
to help you better
understand your amplifier.

IIOW DISTORTION
AFFECTS YOUR MUSIC:

HIGH FREQUENCY DISTORTION causes
harshness of sound and listener fatigue.

MID FREQUENCY DISTORTION confuses
the sound image making it difficult to detect the
timbre of various instruments.

LOW FREQUENCY DISTORTION causes bass
instruments to lack clarity, definition and fullness.

SOME TYPICAL CAUSES
OF HARMOIIC DISTORTION:

HIGH FREQUENCY DISTORTION in a
transistor amplifier is usually caused by the inabil-
ity of certain transistors to properly amplify high
frequencies.

MID FREQUENCY DISTORTION can be
caused by circuits that are improperly designed,
allowing the transistors to operate in a non-linear
current range. Defective circuit components can
also cause non-linear operation.

'LOW FREQUENCY DISTORTION can be
caused by inadequate power supply capacitors. At
low frequencies, these capacitors are asked to
provide current over a relatively long time period.
if the capacitors are of inadequate size they will
run out of charge part way through the cycle. Once
this charge depletion occurs, there is an abrupt
power supply voltage change causing ali fre-
quencies to be distorted.

Page 2

THE TESTING PROCEDURE

1.
2.

Your amplifier's tone controls and filters are
set to flat, the balance is set in the middle and
the volume or gain is set to maximum.

The audio outputs of each channel are con-
nected to the isolated precision load resistors.
4 or 8 ohms is selected to represent the im-
pedance of your speakers.

Power line voltage maintained at 120 volts is
supplied to your amplifier by a 20 ampere
variable autotransformer using a peak-
responding AC voltmeter.

The test signals are connected to the inputs of
your amplifier. Flat or auxiliary inputs are used
to insure accurate harmonic analysis. Magnetic
phono inputs are not used for this particular
test since they incorporate RIAA phono equali-
zation. Phono inputs may possibly be tested
for other performance, such as noise, hum, and
equalization accuracy.

The test signal is set to 1000 Hz, approximately
the center frequency of the audio spectrum.
The level of the input test signal is increased
until visible distortion or clipping of the output
waveform is observed on the oscilloscope. The
test is then run at a reduced output just below
the visible distortion point, or at rated power,
whichever is lower.

Both channels of your amplifier are driven
simultaneously. The amplifier outputs are
sampled, one channel at a time, by the distor-
tion analyzer and oscilloscope. The analyzer,
by means of filters, removes all output of the
original sine wave test frequency. All remaining
signals are measured and displayed as total har<
monic distortion, since they appear in the amp-
lifier output butwere not present in the original
test signal. The test is usually performed at
20 Hz, 1000 Hz and 20,000 Hz. Additional
test frequencies are used when required for the
most accurate and correct graphical presenta-
tion.

The distortion measurements are plotted on a
graph of distortion versus frequency.

OUR TEST
GEAR AND
WHY WE
(HOSE IT