All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Notes: Measurements of the unbalanced and balanced inputs of both channels were made at the 120V AC line voltage, both channels driven. Since the Parasound Halo A 23 is switchable from stereo to mono mode, measurements were made in both modes. (The suffix “A” indicates a stereo-mode measurement. The suffix “B” indicates a mono-mode measurement.) Unless otherwise noted, the data reported below are for the unbalanced inputs and the left channel.

### Power output (stereo mode)

- Power output at 1% THD+N: 131.5W @ 8 ohms, 193.5W @ 4 ohms
- Power output at 10% THD+N: 160.0W @ 8 ohms, 227.5W @ 4 ohms

### Power output (mono mode)

- Power output at 1% THD+N: 384.6W @ 8 ohms
- Power output at 10% THD+N: 449.3W @ 8 ohms

### Additional data

- This amplifier does not invert polarity.
- AC-line current draw at idle: 0.6A, 0.68PF, 47.6W
- Gain: output voltage divided by input voltage, 8-ohm load
- Stereo mode: 27.55X, 28.8dB
- Mono mode: 54.0X, 34.7dB

- Input sensitivity for 1W output into 8 ohms
- Stereo mode: 102.8mV
- Mono mode: 52.4mV

- Output impedance @ 50Hz
- Stereo mode: 0.01 ohm
- Mono mode: 0.29 ohm

- Input impedance @ 1kHz

- Stereo mode, unbalanced inputs: 34k ohms
- Mono mode, unbalanced inputs: 34k ohms
- Mono mode, balanced input: 66.5k ohms

- Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch

- Wideband: 0.220mV/0.197mV, -82.2dBW/-83.1dBW
- A weighted: 0.069mV/0.060mV, -92.3dBW/-93.5dBW

- Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch

- Wideband: 0.272mV/0.221mV, -80.3dBW/-82.1dBW
- A weighted: 0.100mV/0.073mV, -89.0dBW/-91.8dBW

- Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms

- Wideband: 0.362mV, -77.9dBW
- A weighted: 0.130mV, -86.8dBW

- Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms

- Wideband: 0.371mV, -77.6dBW
- A weighted: 0.135mV, -86.4dBW

### Measurements summary

The Parasound Halo A 23 is a stereo power amplifier of moderate power output, and the lowest-powered amp in the Halo line. As the A 23 can be switched from stereo to bridged mode, both modes were measured. In the charts, the suffix “B” indicates measurements taken in bridged mode.

Charts 1A and 1B show the frequency response of the A 23 with varying loads. There is more high-frequency rolloff in bridged mode due to the series-connected nature of this mode. This also causes the high-frequency deviation with load to show up more. Since the A 23’s regulation is so good, the IHF dummy load wouldn’t show any significant variation in the audioband.

Chart 2 illustrates how the A 23’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals for loads of 8 and 4 ohms. Chart 2B is for bridged mode into 8 ohms -- the A 23 is not specified for bridged use into loads of 4 ohms. Interestingly, the curves for 1kHz THD+N and SMPTE IM distortions are almost an overlay.

THD+N as a function of frequency at different power levels is plotted in Charts 3A and 3B. High-frequency rise with frequency is moderate, and distortion is quite low through most of the power range in both stereo and bridged-mono modes.

The Halo A 23’s damping factor vs. frequency, shown in Chart 4A, is of a value and nature typical of many solid-state amplifiers: high up to 1-2kHz, then rolling off with increasing frequency. Somewhat puzzling was the measurement of damping factor in bridged-mono mode (4B). Usually, this is about half the damping factor in the individual channels of a stereo amp in stereo mode, but in the case of the A 23 it was much lower. I checked this with an alternate method, comparing the open-circuit voltage vs. the voltage when loaded with 4 ohms, and got the same result.

A spectrum of the residue of harmonic distortion and noise of a 10W, 1kHz test signal is plotted in Charts 5A and 5B. The AC-line harmonics are complex but relatively low in magnitude. Signal harmonics are low enough in amplitude, and consist mostly of a descending series of odd harmonics. Things are similar in bridged mode (Chart 5B), but with magnitudes about doubled.

### Chart 1 - Frequency response of output voltage as a function of output loading

Chart 1A

Stereo mode

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

Chart 1B

Mono mode

Red line = open circuit

Magenta line = 8-ohm load

Blue line = 4-ohm load

### Chart 2 - Distortion as a function of power output and output loading

Chart 2A

Stereo mode

(Line up at 20W to determine lines)

Top line = 4-ohm THD+N

Second line = 8-ohm THD+N

Third line = 4-ohm SMPTE IM distortion

Bottom line = 8-ohm SMPTE IM distortion

Chart 2B

Mono mode

(Line up at 300W to determine lines)

Top line = 8-ohm SMPTE IM distortion

Second line = 8-ohm THD+N

### Chart 3 - Distortion as a function of power output and frequency

Chart 3A

Stereo mode

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 70W

Cyan line = 100W

Green line = 125W

Chart 3B

Mono mode

(8-ohm loading)

Red line = 1W

Magenta line = 10W

Blue line = 0W

Cyan line = 100W

Green line = 300W

### Chart 4 - Damping factor as a function of frequency

Chart 4A

Stereo mode

Damping factor = output impedance divided into 8

Chart 4B

Mono mode

Damping factor = output impedance divided into 8

### Chart 5 - Distortion and noise spectrum

Chart 5A

Stereo mode

1kHz signal at 10W into an 8-ohm load

Chart 5B

Mono mode

1kHz signal at 10W into an 8-ohm load