The β22 Stereo Amplifier

Specifications and benchmarks

All measurements were done with a β22 amplifier with the following configuration (except where noted):

Test conditions
Amplifier configuration 3-channel active ground
Output stage quiescent current set to 160mA on both channels
Output dummy load 33Ω 5W resistor between each stereo channel to output ground
Volume control50KΩ Alps RK40 set to maximum
Parts selection As specified in the parts list

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. Protek 6510 100MHz analog dual trace oscilloscope
  4. Fluke 95 50MHz digital dual trace ScopeMeter
  5. Fluke 187 digital multimeter
  6. Toshiba Satellite A65 laptop computer with Celeron 2.8GHz CPU and 768MB RAM
  7. M-Audio Firewire Audiophile mobile interface
  8. RightMark Audio Analyzer 5.5 software
  9. dummy loads 330Ω/33Ω/8Ω/none
  10. other custom test jigs, cables and software

Here are the measured results:

Measured results (σ22 PSU ±30V)
Input impedance with 50KΩ input volume potentiometer set to maximum
47.6KΩ
Output impedance less than 0.01Ω, 20Hz - 20KHz
Maximum output voltage
at 1KHz, prior to onset of clipping		 43Vp-p (15.2Vrms) unloaded
38Vp-p (13.4Vrms) into 32Ω
Power output 0.75Wrms into 300Ω
5.6Wrms into 32Ω
18Wrms into 8Ω
Frequency response at 0dBV output
0Hz - 2.5MHz, +0dB -3dB
Rise time 100KHz square wave, at 43Vp-p output, 10% to 90%
175nS
Slew rate 100KHz square wave, at 43Vp-p output
198V/µS
Noise level
Dynamic range
Total harmonic distortion
Intermodulation distortion
Stereo crosstalk		 see RMAA test results below

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 anothers with the β22 amplifier added to the chain with 330&Omega, 33Ω or 8Ω dummy loads connected to the β22's output.

The β22's volume control knob is set to maximum for these tests. This is worthy of mention because many others test their amplifiers with the volume set lower, which would give artificially low noise and distortion results.

Click on the following links to see the results.

Comments on the RMAA test results

Even though the β22's measured results are excellent, it can be seen from the loopback graphs that the β22 added almost negligible distortion to the M-Audio Firewire Audiophile's baseline. Hence, the distortion measured is predominently the sound interface's performance, not that of the β22 amplifier.

The slight rolloff at the low end in the frequency response graph is due to the M-Audio's coupling capacitor. The β22's actual frequency response is flat down to 0Hz. Likewise, the high-end rolloff in the graph is due to the M-Audio's anti-aliasing filters. β22's actual response extends to 2.5MHz (-3dB) in the tested configuration.

The β22's RMAA results remain essentially unchanged whether its outputs are loaded with 330Ω, 33Ω or 8Ω. Many amplifiers' distortion performance deteriorate significantly with lower impedance loads, and the stereo crosstalk behavior degrades. There is no such problem here.

These results amply illustrate the benefits of β22's high-current class A MOSFET output stage, three-channel active ground topology and the use of dynamic cascoding in every stage. See the Technical highlights section for details.

Oscilloscope waveforms

The oscillogram waveforms of the β22 amplifier are shown below. All input waveforms are produced by a Wavetek 188 4MHz sweep function generator.

The following is the 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 and fall times as well as the peak-to-peak output amplitude is shown on the right hand side of the display.



Below are additional waveforms as measured with a Protek 6510 100MHz analog oscilloscope. In all graphs except the Lissajous waveform, the top trace is the input and the bottom is the output.

The square wave graphs show that there is minimum slewing and ringing at the leading and falling edges. The 100KHz sine, triangle, and Lissajous graphs also show very small amount of phase shift between the input and output. Within the 20Hz to 20KHz audio band, there is no measurable phase shift.



Main: β22 Main | Prev: Other options | Next: β22 gallery