contains some ancillary measurements in narrative form. Some of the measurements include
amplifier gain, signal-to-noise ratio, AC line power draw at idle, input impedance at
1kHz, and output impedance at 50Hz. We measure output impedance by injecting a constant 1
amp of current with frequency into the output of the amplifier under test and measuring
the resulting voltage developed accross the amplifier output. Since the current is 1 amp,
the measured voltage is equal to the magnitude of the output impedance in ohms. This
value is checked against the output impedance calculated from the voltage drops measured
in Chart 1 at the 1-watt output level and is generally in good agreement for the vast
majority of tested amplifiers.
output with 1kHz test signal
This section indicates the power output of the
amplifier at 1% and 10% distortion levels and 8-, 4-, and 16-ohm impedances when the
amplifier is driven by a 1kHz test signal. In the case of tube amplifiers with
stated 8-ohm output terminals, the data was taken for that output. Otherwise, the data is
taken on the single pair of output terminals provided. For solid-state amplifiers, no data
is provided for output at 16 ohms; all well-designed solid-state amps should have no
output-related issues at such a high impedance.
In cases where the 1% and 10% powers are close together, this generally is indicative
of a design with quite a bit of negative feedback, typical of solid-state amplifiers.
Where the 1% and 10% powers are quite separated, this is typical of tube or solid-state
amps with little or no negative feedback. Generally speaking with the tube amplifiers, the
best load match for lowest distortion is the impedance that gives the highest power at 1%
The main purpose of this section is to give
pertinent details that correspond directly with the charts, to help readers interpret the
visual data. Salient points about the chart results and additional data measurements are
included, along with other pertinent comments on the amplifiers behavior.
- Frequency Response as a
Function of Output Loading
- Purpose: Gives an indication of
how flat and uniform the frequency response of the amplifier is and how this response
varies with output loading. The response with the dummy speaker load suggests how much
variation in frequency may occur when the amplifier is actually driving a loudspeaker.
For the sake of completeness, here is a plot of the dummy load we use. Impedance magnitude
is represented in red; phase is represented in magenta. The vertical scale is in ohms
(disregard each "m").
What it tells you:
Four measurements can be seen on this chart: frequency responses with open circuit, 8-ohm
load, 4-ohm load, and dummy speaker loading at the amplifiers output. For tube
amplifiers with multiple impedance output connections, the 8-ohm output connector is used.
The lower the output impedance, the less the output will change with loading and therefore
the flatter the response delivered to a speaker load. On the chart, the lower the output
impedance, the closer the three resistive loaded curves are to each other on the chart.
|Chart 2 - Distortion as a Function of Power Output and Output Loading
- Purpose: Shows how the amplifiers
distortion (signal components in the output not present in the input) varies with amount
of output power and output loading.
What it tells you: Three or four measurements are displayed: 1kHz total harmonic
distortion plus noise vs power output for 16-, 8-, and 4-ohm resistive loading on
the 8-ohm tap for tube power amplifiers, plus SMPTE IM distortion with 8-ohm loading on
the 8-ohm tap. For solid-state amplifiers, the 16-ohm loading is omitted. Typically,
solid-state amplifiers will have low distortion up to the start of clipping where the
amount of distortion will abruptly rise. Tube amplifiers generally have higher amounts of
distortion and merge into clipping more smoothly.
|Chart 3 - Distortion as a Function of Power Output and Frequency
- Purpose: Illustrates how amplifier
distortion varies with frequency.
What it tells you: Four measurements are displayed here: Total harmonic distortion vs
frequency at four power levels ranging from 1W to a value at or near the rated power of
the amplifier. Output loading is shown for either 4- or 8-ohms, as indicated.
|Chart 4 - Damping Factor
as a Function of Frequency
- Purpose: Shows how the
amplifier's damping factor varies with frequency.
What it tells you: Damping factor is the value of the output impedance at
a particular frequency divided into 8. With tube amplifiers this measurement is made on
the 8-ohm output tap if available. In a similar manner to Chart 1, this parameter measures
the amplifiers ability to deliver a flat frequency to the load: The higher the
damping factor -- and the lower the impedence, th etwo being inverses of each other -- the
flatter the response is into a speaker load. The value of output impedance at 50Hz is
given in the Additional Data section.
|Chart 5 - Distortion and
- Purpose: Plot of the harmonic
distortion spectrum of a 1kHz test signal at an output power of 10W into an 8-ohm load.
What it tells you: Since the frequency axis is logarithmic, it also allows a
measure of the leakage of power supply line harmonic frequencies into the amplifier
output. Generally, it is preferable if the harmonics decrease rapidly after the second