SoundStage! Measurements - Song Audio SA-1 Preamplifier (12/2004)

Home Audio
Equipment Measurements

December 2004

Song Audio SA-1 Preamplifier: Measurements

All preamplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test preamplifiers there. All measurement data, including graphical information displayed below, is the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

Additional Data

Measurements were made at 120V AC line voltage, and on the channel with most distortion and most noise, unless otherwise noted.
Gain, volume at maximum:
IHF loading, Lch/Rch = 8.75X, 18.8dB / 8.74X, 18.8dB.
Instrument loading, Lch/Rch = 9.65X, 19.7dB / 9.62X, 19.7dB.
IHF sensitivity, input volts for standard IHF output of 0.5V:
IHF loading, Lch/Rch: 57.1mV/57.2mV.
Output noise versus bandwidth and volume-control position, wideband/A weighted in uV:
    At maximum = 182.9/25.7
    Worst case = 202.2/35.7
    Unity gain = 182.4/28.0
AC line current draw at idle: 0.31A.
Output impedance at 1kHz: 909 ohms.
Input impedance at 1kHz: 50k ohms.
This preamplifier does not invert polarity.

Measurements Summary

General

The Song Audio SA-1 line-level preamp utilizes three 12AX7 tubes in its signal circuitry. The separate power-supply unit uses a 5AR4 tube rectifier along with solid-state regulation for the high-voltage and tube heaters.

Chart 1 shows the frequency response of the SA-1 with the volume control set for unity gain for 0.5V input and IHF loading. The two channels are tracking within 0.3dB at this point on the volume control. Also shown in Chart 1 is the frequency response with an instrument load. As can be seen, there is some definite out-of-band high-frequency peaking as opposed to the IHF load.

In charts 2A-D, the tracking of the volume control is shown as a function of four selected settings: full on, -6dB, 15dB below unity gain, and at -70dB. The preamplifier output is loaded with the IHF load. In Charts 2A-C, the high-frequency roll-off is caused by high-frequency slewing, where the preamp output stage can’t drive the IHF capacitance linearly at the output level asked for. As the level is decreased, the high-frequency response improves so that at typical listening levels, as shown in chart 2C, full output capability up to 200kHz is achieved. To put this in a little perspective, even in chart 1A, performance is good up to about 40kHz. In reality, there is not going to be any full-level music signals up in this frequency range. This data is a indication of the circuit’s objective performance in these regards. In regard to the volume-control tracking, which this data is mainly designed to show, tracking is good down to about the –60dB level (not shown) that covers virtually all cases of listening levels. Only at the -70dB point does the tracking of the channels really start to diverge.

Chart 3 shows how total harmonic distortion varies with input level and frequency for both IHF and instrument loading. For the more typical instrument loading, distortion is noise dominated up to about 1V output at 20kHz, which is quite good. With the IHF loading, 20kHz distortion starts to show above about 0.5V. Still, at a level that would be driving most power amps to 50W or more (assuming a typical power amplifier gain of 26dB), at 1V output, the distortion is a very low 0.03% at 20kHz and much lower at lower frequencies.

A spectrum of the distortion and noise residual of a 1kHz test tone at 0.5V output with IHF loading is plotted in Chart 5. Aside from a low amount of AC-line harmonics, the signal distortion consists of a barely visible 0.0004% of second harmonic. This is a very good-measuring tube circuit.