The Mini³ portable stereo headphone amplifier

Specifications and benchmarks

Measurements were done on Mini³ high performance edition and Mini³ extended runtime edition amplifiers with a default gain of 5. A fully-charged Maha 9.6V 8-cell NiMH battery was used for all tests that ran on battery power.

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/none
  10. other custom test jigs, cables and software
Here are the measured results:

Measured Results Mini³ high performance edition Mini³ extended runtime edition
Opamps L&R: AD8397ARDZ
L&R: LMH6643MA
G: LMH6642MA
Input impedance 10KΩ nominal
Output impedance less than 0.5Ω
Maximum output voltage
per channel, 1KHz, prior to onset of clipping
9Vp-p (3.2Vrms) with battery power
11Vp-p (3.9Vrms) with external power
Power output 200mWrms into 33Ω
30mWrms into 330Ω
120mWrms into 33Ω
30mWrms into 330Ω
Frequency response
at 0dBV output
0Hz - >4MHz; +0, -3dB
(limited by function generator)
Rise time
100KHz square wave, at maximum output, 10% to 90%
166nS 54nS
Slew rate
100KHz square wave, at maximum output
52V/µS 162V/µS
Quiescent current
(battery current draw while power on)
25mA 10mA
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 Mini³ amplifier added to the chain with 330&Omega or 33Ω dummy loads connected to the Mini³'s output.

The Mini³'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

The measured performance with 330Ω load is excellent for both Mini³ editions, exhibiting extremely low THD and IMD distortion, low noise floor and low stereo crosstalk through the entire audio frequency range. There is virtually no degradation of performance on the high performance edition when the load is decreased to 33Ω The extended runtime edition shows increased THD and IMD with 33Ω load (due to its lesser output current capability), but these results are still very good compared to many other amplifiers, particularly those of similar size to the Mini³.

The stereo crosstalk in the high frequency range for both editions was not impacted as the load is decreased from 330Ω to 33Ω illustrating the benefit of the 3-channel active ground topology. See the Tech highlights section for more details.

The slight rolloff at the low end in the frequency response graph is due to the M-Audio's coupling capacitor. The Mini³'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. Both Mini³ editions have high frequency response that extends to 10MHz and beyond.

These tests were run with the amplifier operating on battery power. The same results were obtained when the amplifier ran on external power via an unregulated AC adapter. In particular, no change was observed in the noise floor, showing that the Mini³'s onboard voltage regulation is very effective.

Oscilloscope waveforms

The oscillogram waveforms of the Mini³ amplifier are shown below. All input waveforms are produced by a Wavetek 188 4MHz sweep function generator.

The following are the 100KHz square wave rising edge response of the Mini³ high performance and extended runtime editions, as tested with a Tektronix TDS2014B 100MHz digital storage oscilloscope. The rise time as well as the peak-to-peak output amplitude is shown on the right hand side of the display. As could be seen, the extended runtime edition's rise time (and thus slew rate) is actually faster than the high performance edition.

Mini³ high performance edition Mini³ extended runtime edition

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 100KHz square wave graphs show no overshoot or ringing. The lissajous (X-Y) graph shows no discernable phase shift.

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