March 2006

Audio Research Reference 3 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.

• Measurements were made at 120V AC line voltage. Measurements made on the left channel and with balanced input and output connections unless otherwise noted.
• Unity gain for instrument loading and balanced I/O is "80" and for unbalanced I/O is "93" on front-panel display.
• This preamplifier does not invert polarity.
• AC line current draw
  • Standby: 0.03A
  • Operate: 1.32A
• Input impedance at 1kHz:
  • Unbalanced input = 52k ohms
  • Balanced input = 108k ohms
• Output impedance at 1kHz:
  • Unbalanced output = 320 ohms
  • Balanced output = 660 ohms
• Gain, balanced I/O, volume at maximum:
  • Instrument loading, Lch/Rch = 3.52X, 10.9dB / 3.30X, 10.4dB
  • IHF loading, Lch/Rch = 3.50X, 10.9dB / 3.27X, 10.3dB
• Gain, unbalanced I/O, volume at maximum:
  • Instrument loading, Lch/Rch = 1.77X, 5.0dB / 1.74X, 4.8dB
  • IHF loading, Lch/Rch = 1.71X, 4.7 dB / 1.68X, 4.5 dB
• IHF Sensitivity, input volts for standard IHF output of 0.5V, IHF loading:
  • Balanced I/O, Lch/Rch = 143mV / 153mV
  • Unbalanced I/O, Lch/Rch = 292mV / 298mV
• Output noise versus bandwidth and volume-control position:
  
  
  • Balanced output, at maximum (103), Lch | Rch, wideband/A weighted =
    56.7uV / 9.5uV | 88.8uV / 10.3uV
  • Balanced output, at unity gain (80), Lch | Rch, wideband/A weighted =
    114.2uV / 11.9uV | 164.3uV / 12.7uV
  • Balanced output, at typical listening level (20dB below unity gain (34)), Lch | Rch, wideband/A weighted =
    48.2uV / 8.7uV | 68.7uV / 9.7uV
    
    
  • Unbalanced output, at maximum (103), Lch | Rch, wideband/A weighted =
    320uV / 36.0uV | 224uV / 35.7uV
  • Unbalanced output, at unity gain (93), Lch | Rch, wideband/A weighted =
    338uV / 37.4uV | 255uV / 36.2uV
  • Unbalanced output, at typical listening level (20dB below unity gain (53)), Lch | Rch, wideband/A weighted =
    341uV / 36.2uV | 243uV / 34.7uV

General

The Audio Research Reference 3 line-level preamplifier is the newest version of the Reference series of tube preamplifiers. A physically large but not particularly heavy unit, it features balanced and unbalanced connections for all inputs and outputs. The front-panel display is large and easy to read from across the room -- a very nice feature. The setting of volume to known reference numbers via the front-panel display is now a reality --- much nicer than the earlier reference units, where volume setting was by guess and by God.

Chart 1 shows the frequency response with the volume control set for unity gain for 0.5V input with instrument and IHF loading and balanced I/O. The two channels are tracking within 0.12dB at this point on the volume control. The bandwidth at this volume setting (as defined by the -3dB points) is less than 10Hz to about 80kHz. Note: with the IHF loading, there is some low-frequency attenuation starting to show due to the size of the output coupling capacitors and their low-frequency cutoff frequency against the 10k of the IHF load.

Now, as with most preamplifiers, especially tube designs, the high-frequency response is usually some function of the volume-control setting. In Charts 2A and B the frequency response of the preamp is shown for both channels with the volume at maximum and at a typical listening position, set at 20dB below unity gain. For these volume settings, the high-frequency bandwidth is about 200kHz. In Chart 2C, for volume set for 40dB below unity gain, the HF bandwidth is even wider yet. And at the last volume position, 1, before mute, the high-frequency response actually starts to boost being up some 5-6dB at 200kHz. Volume-control tracking is very good over the working range, generally being better than 0.2dB. The frequency-response behavior with unbalanced I/O (not shown) is essentially the same.

Chart 3A shows how total harmonic distortion varies with input level and frequency for both IHF and instrument loading for balanced I/O and unity gain. The IHF loading has a bit more distortion, but, like the instrument loading, the distortion is essentially independent of frequency. Charts 3B and 3C illustrate how distortion varies with level and frequency for unbalanced I/O connections. In these charts, 20Hz and 1kHz are essentially the same and overlaid, but the 20kHz distortion is a bit more.

A spectrum of the distortion and noise residual of a 1kHz test tone at 0.5V output at unity gain with instrument loading is plotted in Chart 4A and 4B for unbalanced and balanced I/O, respectively. The unbalanced I/O condition seems to have a low-frequency rise compared to the balanced I/O case. Furthermore, the overall noise level is lower in the balanced I/O mode. Hum harmonics are typical in level for other preamps measured. The dominant signal harmonic is the second for both I/O conditions.

IHF loading
Blue line = left channel
Cyan line = right channel

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 2A - gain at maximum

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 2B - gain at -20dB below unity gain

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 2C - gain at -40dB below unity gain

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 3A - Balanced I/O

Red line = Instrument loading (20Hz, 1kHz, and 20kHz)
Magenta = IHF loading (20Hz, 1kHz, and 20kHz)

Chart 3B - Unbalanced I/O

Instrument loading
Magenta line = 20Hz and 1kHz
Red line = 20kHz

Chart 3C - Unbalanced I/O

IHF loading
Magenta line = 20Hz and and 1kHz
Red line = 20kHz

Chart 4A - Unbalanced I/O

Instrument loading
Red line = spectrum of 1kHz test signal distortion and AC-line harmonics at 0.5V input and output at unity gain.

Chart 4B - Balanced I/O

Instrument loading
Red line = spectrum of 1kHz test-signal distortion and AC-line harmonics at 0.5V input and output

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