The α24 fully-differential line amplifier

Specifications and benchmarks

All measurements were done with two α24 boards, set up for stereo operation, with the following configuration (except where noted):

Test conditions
α24 configuration Line stage, no low-pass filter
Supply voltage ±10V DC
Voltage gain 3 (9.54dB)
Load impedance 100KΩ, 600Ω, 300Ω (test-specific)

The tests were conducted using the following equipment:
  1. Audio Precision System Two SYS2322 Dual-Domain audio analyzer
  2. Wavetek 188 4MHz sweep function generator
  3. Tektronix TDS2014B 100MHz quad trace digital storage oscilloscope
  4. Fluke 45 digital multimeter
  5. Fluke 187 digital multimeter
  6. HP 6236B triple output tracking power supply
Measured results
Input impedance
(nominal)
1MΩ (balanced)
500KΩ (unbalanced)
Output impedance 66Ω (balanced)
33Ω (unbalanced)
Clipping threshold
(1% THD, 1KHz, 100KΩ load)
±10V supply rails:
37.48Vp-p (13.25Vrms) (balanced)
18.81Vp-p (6.65Vrms) (unbalanced)
±12V supply rails:
45.54Vp-p (16.10Vrms) (balanced)
22.80Vp-p (8.06Vrms) (unbalanced)
Frequency response
(at 0dBV output)
0Hz - 2.5MHz, +0dB -3dB
S/N ratio
(20Hz-20KHz, +10dBV, A-weighted)
128dB
Total harmonic distortion + noise
(0dBV tone input, 20Hz-20KHz, A-weighted)
0.00025%
Intermodulation distortion + noise
(SMPTE method 60Hz & 7KHz 4:1, A-weighted)
0.0004%
Common mode rejection ratio (CMRR)
(20-20KHz)
>75dB

Performance graphs

The graphs below are as measured on the Audio Precision audio analyzer. For a "loop-back" baseline of the analyzer's measurement limits, click here.

Click on a graph to see it in full size.

Balanced in - Balanced out Unbalanced in - Unbalanced out
 
Frequency response
0dBV tone input, 10Hz-200KHz sweep
Comment: The input voltage is 1Vrms = 0dBV, with gain of 3, the output voltage is 3Vrms = +9.54dBV, and that's exactly where the flat-as-a-ruler frequency response trace is.
 
Noise floor
1Hz-20KHz linear
Comment: The noise floor is higher than the SYS2322 loopback baseline, due to the input noise being amplified by the α24's gain.
 
FFT
0dBV 1KHz fundamental tone input, +9.54dBV output, 1Hz-24KHz linear spectrum
Comment: Notice that the α24 did not add any significant harmonic distortion. The 2nd and 3rd harmonic spikes are there in the SYS2322 loopback test result.
 
Total harmonic distortion & noise vs. frequency
0dBV tone input, 20Hz-20KHz sweep, A-weighted
Comment: This shows that α24 produced negligible distortion compared to the SYS2322 loopback baseline.
 
Total harmonic distortion & noise vs. amplitude
1KHz fundamental tone input, A-weighted
Note: the horizontal scale in dBV refers to the input amplitude.
Comment: The upward tilt toward the left of the graph shows that as the signal level is decreased, noise becomes dominant over distortion. The fast uptilt toward the right of the graph is where clipping takes place.
 
Intermodulation distortion & noise vs. amplitude
SMPTE method 60Hz & 7KHz (4:1), A-weighted
Note: the horizontal scale in dBV refers to the input amplitude.
Comment: The upward tilt toward the left of the graph shows that as the signal level is decreased, noise becomes dominant over distortion. The fast uptilt toward the right of the graph is where clipping takes place.
 
THD+N vs. frequency: 100KΩ, 600Ω, 300Ω loads
0dBV swept tone, 20Hz-20KHz, A-weighted
Yellow: 100KΩ, Blue: 600Ω, Red: 300Ω
Comment: There is no significant increase in distortion even with a load impedance as low as 300Ω, indicating robust output drive capability from both the balanced and unbalanced outputs.
 
THD+N vs. amplitude: 100KΩ, 600Ω, 300Ω loads
1KHz fundamental tone, A-weighted
Yellow: 100KΩ, Blue: 600Ω, Red: 300Ω
Comment: There is no significant increase in distortion even with a load impedance as low as 300Ω, indicating robust output drive capability from both the balanced and unbalanced outputs.

Oscilloscope waveform

The oscillogram waveforms of the α24 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 scope shows that the α24's impulse response is fast and controlled, with no sign of overshoot or ringing.

Balanced output Unbalanced output



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