Amplitude Response Audio Amplifier
The report shows the measuring
technique of the amplitude response for an amplifier. A software supported
measurement in connection with precision circuit analyzers and a sine
level generator is presented.
means amplitude response?
amplifier has the task the voltage applied at the input in such a way to
amplify that the voltage at the output has the accurate same process.
The user has in the hand whether the voltage applied at the input is to
appear multiplied by a factor at the output. This adjustable factor is
the volume control. It is of elementary importance that this multiplying
factor remains always with all input voltages, frequencies and loads the
amplitude response shows, the multiplying factor is how constant over
the frequency? There are many representation possibilities, the fantasy
is set no borders. A function representation is usual in the XY
coordinate system, whose axles are however often also linear shown
logarithmically to the basis 10. The professional among the readers,
asks now immediately for the phase response. Correctly, both are two
inseparable brothers and sisters. It is forgiven me that the phase
response is measured with another measuring system and treated therein
what is that?
in principle a simple affair, which is mathematically fast avowed. It is
with an amplifier a useful characteristic, but something misleading, in
particular for a layman.
range is completely primitively said a guideline for the speed of an
amplifier,: the more highly the range the faster. The range is
recognizable from an amplitude response. It is a size, which is often
wrongly regarded in purchase to an amplifier. The range shows the point
in the amplitude response, at which the output voltage dropped to
approximately 70.7% (1/square root 2) to target the output voltage. Now
the large matter of price: would you like to use an amplifier with a
frequency, with which the amplifier causes already 100%-70.7% = 29.3%
errors? Assumed the range amounts to only 20 kHz, such an amplifier is
- Bad - you can save your time to read a test report. To
power on mesurement devices for something like that - no - only you want
to see the proof, it's always the same lyre. Therefore all measuring
instruments to start is pure waste of time, it is one wants exactly
negatives to confirm, what one can already foresee. - Yes, - the
defender of its noble pieces comes now with this popular argument: the
human hearing can hear only about 16 kHz -20 kHz. There say I
nevertheless easily against it 29.3% error with 30 kHz. With 20 kHz
there is then already too much (is too idle me to calculate because of
critics for 20 kHz and a low-pass of certain order). A lameness in the
amplifier draws a tremendous chain of other negative effects behind
itself ago. Which correctly decent begins with approximately 100 kHz
range and at the same time high open loop under full load conditions.
Not only fast by high range, but also precisely by the high open loop
pictures show the results of the system from two HP3457A (here 6.5
places) and a HP3336B frequency Sythesizer. The measurement happened in
a friendly calibration laboratory, which put the devices to me at the
disposal. In addition I wrote an automatic program sequence under
utilization of the GPIB interfaces.
shows the ganging of both meters connected to the same signal source.
the very constant signal source amplitude response measured with the input
Graph shows the very constant signal source amplitude response measured
with the output voltmeter
of the signal source is fine, but not essential as long as the meters
doing their job satisfactory. The 3336 together with the option 05
(precision amplitude attenuator) has an amazing constant amplitude. The
decreased amplitude at higher frequencies has his origin in the
amplitude response of the meter itself. And of course may be also from
the generator (I don't think so much) or the cables or a marginal wrong
termination. It's not cleary why, but for audio applications
The picture left shows the measurement consists of two HP3457A
and a generator HP3336B. The cables are connected like a mirror
and terminated with 75 ohm. For best results the resistor should
be on the end of the cable, it's difficult here to do it without
an special soldered shared construction, so I missed to do it,
sorry. Note the frequency is adjusted to 1 MHz.
to get a higher accuracy?
to settle the claim of "Ultra" there is some additional work
- a fresh calibrated instruments (constant 0.003 dB
could be reduced)
- same temperature during calibration and measurement, as possible
measure direct after calibratiion
- use for both meter same cable types and length and manufactorer
- as possible ideal refexion free termination of the generator
- calibrate over many points as possible, put the values in a
computer, do a fit function and use this function to correct future measured values
- set instrument resolution to 7 1/2 digits (leds to a long
- do many measurements at same frequency and calculate mean
More handling ways for a better accuracy:
- if you have the change use a HP3458A meter (up to 8 1/2 digits).
The 3457A is already one of the best meter HP builts in the past,
but the 3458A that's really taking "the biscuit of
accuracy". Both meters having a high accuracy.
- Ask the calibration laboratory particularly conscientiously to
work. The laboratory is to measure as much as possible points, in
order to produce a family of characteristics, which can be
out-corrected computationally. Thus the accuracy is close at the
Does it make sense to do this stress of an
improved accuracy because of audio measurements?
- the measurement time should be not too long, the device under test
(audio amplifier) will drift with his amplitude response vs. time
and room temperature certainly more than the accuracy of the
- for audio amplifier (also the best ones) is this system already
very good with enough room for accuracy.
showing details of some measurement ranges. Most meters having their
highest precision under a maximum utilisation of their range. But these
instruments still have a good accuracy even under a low range
Nice to see
the meter works fine in any range.
These are the
voltages applied to the input meter
applied on the output meter
of the 3 volts range. Good result under any applied voltage
applied to the input meter
applied to the output meter
results of the 30mV measurement range
applied to the input meter
applied to the output meter
will happens using the 30V range together with the 300mV range?
be foreseen - very similar results. So far however yet tested, does not
need a frequency-compensated precision voltage divider plus GPIB capable
generator for higher constant tensions plus much time. Since I can
predict the behavior for such a combination of measured the so far, I
would like to do without such a measurement, even if such a combination
for audio is meaningful applications. The accuracy reserves are still
very well enough for Hifi applications.
what I'am using this system?
course to measure the amplitude response of hifi amplifier.