This section contains some ancillary measurements in tabular form. Measurements here include preamplifier gain and sensitivity, output noise, AC-line power draw at idle, input/output polarity, and input and output impedance at 1kHz.

The main purpose of this section is to give pertinent details that correspond directly with the charts and help readers interpret the visual data. Salient points about the chart results and additional measurements are included, along with other pertinent comments on the preamplifier’s behavior.

• Purpose: Gives an indication of how flat and uniform the frequency response of the preamplifier is and how this response varies with output loading. This test is done by setting the volume control for unity gain with IHF loading. A standard IHF input signal level of 0.5V is applied.

What it tells you:  The comparative response with IHF loading (10k ohms in parallel with 1000pF) and with the instrument loading (100k ohm in parallel with 300pF) gives an idea what to expect with different power-amplifier input impedances and interconnect capacitance. Some preamplifiers have relatively small film output coupling capacitors and if connected to a power amplifier with low input impedance, some loss of low-frequency response will occur. If a long run of high-capacitance interconnect from preamplifier output to power amplifier input is used, the high-frequency response with the IHF  load will be relevant.

• Purpose: Shows how the frequency response and channel balance vary with volume-control setting. Volume control is set to maximum and a reference level is established as 0dB.  Output loading is IHF. Measuring the left channel, the volume is reduced to -6dB, to 15dB below unity gain, and then to the -70dB level.

What it tells you: With most designs, the high-frequency response will change with volume setting because the source resistance that the power amplifier sees is a function of the volume-control setting. The highest resistance occurs at -6dB of attenuation, and, in most cases, this will cause the high-frequency response of the preamplifier to be less than with the volume at maximum or down at lower working attenuations of perhaps 15dB below unity gain, where most use occurs. In many instances, the preamplifier can’t sustain linear output at high frequencies at the elevated output level with the volume control near full up with the standard input level of 0.5V. In such cases, the high-frequency response will appear to roll off much sooner than at lower levels due to the phenomena of slewing, where the output waveform looks more like a triangle shape than the proper shape of a sinewave. For the reference input level of 0.5V, there may be output clipping or high-frequency slewing at the maximum clockwise level in some cases if the unit under test can't put out 5V with the usual preamp line level gain of 20dB (10x).

• Purpose: Illustrates how preamplifier distortion varies with output voltage, frequency, and output loading. Measurements are made at frequencies of 20Hz, 1kHz and 20kHz as a function of preamplifier output voltage at unity gain and with IHF and instrument loading.

What it tells you:  With most solid-state designs, there is not much difference with IHF or instrument loading. With most tube designs, there may be considerable difference in distortion versus output loading.

• Purpose: Shows a plot of the distortion and noise of a 1kHz test signal at an output voltage of 0.5V for 0.5V input and IHF loading.

What it tells you: At standard output of 0.5V and IHF loading, this plot shows how much hum and noise are present along with the distortion spectra of the 1kHz test signal.

• Purpose: To plot the tone-control characteristics.

What it tells you: The shape and range of boost and cut of the tone controls.

• Purpose: To plot the accuracy of the phono stage's’s RIAA equalization.

What it tells you:  How well the circuit design properly implements the RIAA equalization function.

• Purpose: Illustrates how the phono-circuit distortion varies with output voltage, frequency, and output loading.  Measurements are made on a pre-equalized basis at frequencies of 20Hz, 1kHz and 20kHz as a function of preamplifier output voltage and with IHF and instrument loading at the tape output.

What it tells you: With most solid-state designs, there is not much difference with IHF or instrument loading. With most tube designs, there may be considerable difference in distortion and low-frequency response with output loading. In the case of these measurements at tape output, the loading effect would be relevant to the input characteristics of whatever recording device or other device connected to the tape output connectors.