Balanced Audio Technology REX 300 Stereo Amplifier Measurements

Link: reviewed by Jason Thorpe on SoundStage! Ultra on March 1, 2025

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Balanced Audio Technology (BAT) REX 300 was conditioned for 1 hour at 1/8th full rated power (~25W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The REX 300 is a two-channel amplifier with two balanced (XLR) inputs and two sets of speaker level outputs. An input of 500mVrms was required to achieve the reference 10W into 8 ohms.

Our typical input bandwidth filter setting of 10Hz-22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a DC to 1MHz input bandwidth.

Of note is that BAT claims no global negative feedback for the REX 300. Our measurements corroborate this claim, as two clear consequences of this design can be seen: a low damping factor (high output impedance) and high THD.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by BAT for the REX 300 compared directly against our own. The published specifications are sourced from BAT’s website, either directly or from the manual available for download, or a combination thereof. Assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz:

Our primary measurements revealed the following (unless specified, assume a 1kHz sinewave at 500mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the REX 300 was able to sustain about 250W into 4 ohms (~1.5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (25W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the sides and top of the REX 300 were hot to the touch. Of note, just above the 250W mark, the REX 300’s protection circuit was engaging almost immediately.

Frequency response (8-ohm loading)

In our frequency response (relative to 1kHz) plots above, measured across the speaker outputs at 10W into 8 ohms, the REX 300 exhibits a near-flat frequency response across the audioband (0/-0.1dB at 20Hz/20kHz). The REX 300 appears to be DC-coupled, as it is perfectly flat down to 5Hz. The -3dB point is at 150kHz.  In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so perfectly overlap, indicating that the two channels are ideally matched.

Phase response (8-ohm loading)

Above are the phase response plots from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The REX 300 does not invert polarity and exhibits, at worst, only -10 degrees of phase shift at 20kHz, due to its extended bandwidth.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms, or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 10Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find the maximum deviation between an 8-ohm load and no-load to be nearly 1dB. This is an indication of a very low damping factor, or high output impedance. With a real speaker, the maximum deviation from 20Hz to 20kHz was roughly 0.6dB, which may be audible.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue and red plots are at 1W output into 8 ohms, purple and green at 10W, and pink and orange at 150W. At 1W and 10W, the left channel outperformed the right by as much as 10dB from 300Hz to 20kHz. The 1W left channel data ranged from 0.05% at 20Hz, down to 0.02% from 100Hz to 20kHz. The 10W left channel data ranged from 0.06% from 20Hz to 4kHz, then up to 0.15% at 20kHz. The 150W THD data are higher, ranging from 0.2% from 20Hz to 100Hz, then up to a very high 7% at 20kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms 

The chart above shows THD ratios measured at the output of the REX 300 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green). The left channel outperformed the right up to the “knee” for both loads by as much as 10dB. The 8-ohm data for the left channel ranged from 0.003% at 50mW, with a steady climb to 0.2% at the “knee,” at roughly 100W. The 1% THD mark was hit at 165W, and at the rated output of 200W, THD ratios measured around 3%. The 4-ohm data for the left channel ranged from 0.005% at 50mW, with a steady climb to 0.4% at the “knee,” at roughly 200W. The 1% THD mark was hit at 284W, and at the rated output of 400W, THD ratios would have (the plot stops just shy of 400W) measured around 4%.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the REX 300 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green). The left channel outperformed the right up to the “knee” for both loads by as much as 5dB. The 8-ohm data for the left channel ranged from 0.05% from 50mW to 2W, then a steady climb to 0.2% at the “knee,” at roughly 100W. The 4-ohm data for the left channel ranged from 0.1% from 50mW to 5W, then a steady climb to 0.4% at the “knee,” at roughly 200W.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the REX 300 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded roughly 50W at the output into 8 ohms (blue), 100W into 4 ohms (purple), and 200W into 2 ohms (pink). The 2-ohm data is not present because the protection circuit was initiated almost immediately after the start of the sweep. The 8-ohm data ranged from 0.1% at 20-2kHz, then up to 0.4% at 20kHz. The 4-ohm THD data ranged from 0.2% at 20-2kHz, then up to 2% at 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the REX 300 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At very low frequencies, the two-way speaker yielded the highest THD ratios (0.6%). Between 40Hz and 10kHz, the THD ratios into all three loads were within roughly 5dB of one another, ranging from roughly 0.01% to 0.1%. At the highest frequencies, the three-way speaker yielded the highest THD ratios (0.08% at 20kHz), nearly 15dB higher than the two-way speaker and the resistive load.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the REX 300 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The IMD results into the resistive load range from 0.03% at low frequencies, down to 0.002% at high frequencies. The two-way speaker IMD results were flatter, ranging from 0.01% to 0.006%, while the three-way speaker ranged from 0.02% to 0.005%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the REX 300 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three plots are essentially identical and constant right around 0.1%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at a very high -60dBrA and -80dBrA, or 0.1% and 0.01%. Power-supply noise-related harmonics, and what are likely IMD products between those noise peaks and the signal and its harmonics, are significant and can be seen throughout the FFT at levels of -70dBrA, or 0.03%, and below. This is an extremely poor FFT result. It should be noted, however, that the noise floor and noise peaks are far more significant with the REX 300 when a signal is present. This can be seen in our main measurement table where signal-to-noise ratios are respectable because the noise is measured separately from the signal. It can also be seen by comparing the noise levels in the table with and without a signal. Noise levels with the signal present are very high for a modern solid-state amplifier using a line-level input, and even higher that an average noise level from a solid-state amplifier’s phono input. Noise levels with a signal are 20 to 30 times higher than the noise measured without a signal present.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most dominant (non-signal) peak is the signal’s second (100Hz) harmonic at -70dBrA, or 0.03%. Again, power-supply noise-related harmonics and IMD peaks can be seen throughout the FFT at 10Hz intervals at -80dBrA, or 0.01%, and below.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -70dBrA, or 0.03%, level, while the third-order modulation products, at 17kHz and 20kHz, cannot be distinguished from the densely packed noise peaks rising up to -70dBrA, or 0.03%. This is an extremely poor IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the speaker-level output of the REX 300 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are around the -80dBrA, or 0.01%, level and below.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the balanced analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the REX 300’s slew-rate performance. Rather, it should be seen as a qualitative representation of the REX 300’s wide bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. In this case, we see a very clean result, with no ringing in the corners and only very mild softening.

Damping factor vs. frequency (20Hz to 20kHz, two-channel mode)

The final graph above is the damping factor as a function of frequency. We find very low damping factor values, hovering around 18 to 19 from 20Hz to 15kHz, and 17 at 20kHz. This is a poor damping factor result for a solid-state amplifier.

Diego Estan
Electronics Measurement Specialist