The α10 stereo pre-amplifier

Specifications and benchmarks

All measurements were done on AMB's reference α10 pre-amplifier with the following configuration (except where noted):

Test conditions
Supply voltage ±12V DC
Voltage gain 1 (0dB)
Output stage quiescent current set to 30mA on both channels
δ1 volume attenuator set to maximum (0dB)

The tests were conducted using the following equipment:
  1. Wavetek 188 4MHz sweep function generator
  2. Tektronix TDS2014B 100MHz quad trace digital storage oscilloscope
  3. Fluke 95 50MHz digital dual trace ScopeMeter
  4. Fluke 187 digital multimeter
  5. Toshiba Satellite A65 laptop computer with Celeron 2.8GHz CPU and 768MB RAM
  6. M-Audio Firewire Audiophile mobile interface
  7. RightMark Audio Analyzer 5.5 software
  8. other custom test jigs, cables and software

Here are the measured results:

Measured results
Input impedance 25KΩ nominal (δ1)
Output impedance less than 0.1Ω, 20Hz - 20KHz
Maximum output voltage
per channel, 1KHz, prior to onset of clipping
±12V rails: 18.5Vp-p (6.54Vrms)
±15V rails: 24.4Vp-p (8.63Vrms)
±18V rails: 30.3Vp-p (10.7Vrms)
Frequency response at 0dBV output:
0Hz - >4MHz*, +0dB -3dB (gain = 1)
Rise time 100KHz square wave, at maximum output, 10% to 90%:
Slew rate 100KHz square wave, at maximum output:
Noise level
Dynamic range
Total harmonic distortion
Intermodulation distortion
Stereo crosstalk
see RMAA test results below*
* Limited by test instrument.

RMAA test results

Several tests were performed, one with the M-Audio Firewire Audiophile mobile interface running in loopback mode to establish the baseline (lowest limit of measurement resolution), and another with the α10 amplifier added to the chain. The α10 in this test was configured for a gain of 1.

Click on the following links to see the results.

Comments on the RMAA test results

The slight rolloff at the low end in the frequency response graph is due to the M-Audio's coupling capacitor. The α10's actual frequency is flat to 0Hz. Likewise, the high-end rolloff in the graph is due to the M-Audio's anti-aliasing filters. α10's actual response extends to beyond 6MHz (-3dB) in the tested configuration.

The α10's RMAA results are as good as the M-Audio Firewire Audiophile. In some cases the α10 results are better than the M-Audio baseline, probably because α10's low output impedance could drive the M-Audio's line input more effectively than the sound interface's own line output. The measured noise, distortion, etc., are mostly that of the M-Audio sound interface, which does not have enough measurement resolution to test the α10.

Oscilloscope waveforms

The oscillogram waveforms of the α10 amplifier are shown below. All input waveforms are produced by a Wavetek 188 4MHz sweep function generator.

The following is an expanded 100KHz square wave response as tested with a Tektronix TDS2014B 100MHz digital storage oscilloscope. The top trace is the input and the bottom trace is the output. The rise time as well as the peak-to-peak output amplitude are shown on the right hand side of the display. The scope shows that the α10's impulse response is fast and controlled, with no sign of overshoot or ringing.

Below are additional waveforms as measured with the Tektronix scope. In all graphs except the Lissajous waveform, the top trace is the input and the bottom is the output.

There are no surprises here, the α10 shows excellent waveform response, regardless of frequency or shape. At 100KHz, the triangle wave is straight and sharp, the square wave exhibit only slight rounding, and the lissajous (X-Y) graph shows only a small hint of phase shift. However there is no measurable phase shift within the audio band.

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