Link: reviewed by Dennis Burger on *SoundStage! Access* on November 1, 2023

**General Information**

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

The Ampster BT II was conditioned for one hour at 1/8th full rated power (~3W 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 Ampster BT II offers one analog input (RCA), one digital optical (S/PDIF), and one Bluetooth input, plus a subwoofer output (RCA) and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: optical (S/PDIF) and the analog line-level unbalanced (RCA) input.

Based on the different results at various volume levels of the left/right channel matching (see table below), the Ampster BT II volume control is likely operating in the analog domain but is digitally controlled. The volume control offers a total range from -62dB to +28dB with step sizes ranging from 1 to 4dB.

Most measurements were made with a 1.1Vrms line-level analog input, or a 0dBFS digital input. We avoided our typical standard 2Vrms analog input level because this caused severe distortion at the input. We found that a 1.5Vrms analog input yielded 1% THD at the output of the Ampster BT II, while maintaining a modest 1W into 8 ohms. This could be considered a significant design flaw, as most modern DAC outputs are between 2 and 2.2Vrms, with some even exceeding 4Vrms for a 0dBFS digital input. This means that a typical DAC connected to the Ampster BT II’s analog input, while decoding a digital track that is recorded with little digital headroom (peaks at or approaching 0dBFS, which are common for modern music), would cause the Ampster BT II to clip regardless of volume position and load.

The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 25W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 25W output.

Because the Ampster BT II uses digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz–90kHz was necessarily changed to 10Hz–22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted-bandwidth setting.

**Volume-control accuracy (measured at speaker outputs): left-right channel tracking**

Volume position | Channel deviation |

min | 0.06dB |

20% | 0.057dB |

40% | 0.073dB |

60% | 0.025dB |

80% | 0.021dB |

100% | 0.007dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Tangent for the Ampster BT II compared directly against our own. The published specifications are sourced from Tangent’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter | Manufacturer | SoundStage! Lab |

Rated output power into 8 ohms | 25W | 28W |

Rated output power into 4 ohms | 50W | 49W |

Our primary measurements revealed the following using the analog/optical input (unless specified, assume a 1kHz sinewave at 1.1Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Maximum output power into 8 ohms (1% THD+N, unweighted) | 28W | 28W |

Maximum output power into 4 ohms (1% THD+N, unweighted) | 49W | 50W |

Maximum burst output power (IHF, 8 ohms) | 28W | 28W |

Maximum burst output power (IHF, 4 ohms) | 49W | 50W |

Continuous dynamic power test (5 minutes, both channels driven) | passed | passed |

Crosstalk, one channel driven (10kHz) | -67.6dB | -60.6dB |

Damping factor | 45 | 44 |

Clipping no-load output voltage | 16Vrms | 16Vrms |

DC offset | <-31mV | <26mV |

Gain (sub-out, 80Hz) | 5.07dB | |

Gain (maximum volume) | 28.4dB | 28.4dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-47dB | <-48dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-50dB | <-57dB |

Input impedance (line input, RCA) | 8.9k ohms | 9.9k ohms |

Input sensitivity (for rated power, maximum volume) | 540mVrms | 540mVrms |

Noise level (with signal, A-weighted) | <360uVrms | <410uVrms |

Noise level (with signal, 20Hz to 20kHz) | <540uVrms | <560uVrms |

Noise level (no signal, volume min, A-weighted) | <240uVrms | <240uVrms |

Noise level (no signal, volume min, 20Hz to 20kHz) | <340uVrms | <350uVrms |

Output impedance (sub-out, 80Hz) | 3.67k ohms | |

Signal-to-noise ratio (25W, A-weighted, 1.1Vrms in) | 95.8dB | 95.6dB |

Signal-to-noise ratio (25W, 20Hz to 20kHz, 1.1Vrms in) | 92.3dB | 92.3dB |

Signal-to-noise ratio (25W, A-weighted, max volume) | 95.8dB | 95.9dB |

Dynamic range (30W, A-weighted, digital 24/96) | 93.0dB | 91.9dB |

Dynamic range (30W A-weighted, digital 16/44.1) | 89.2dB | 88.5dB |

THD ratio (unweighted) | <0.063% | <0.055% |

THD ratio (unweighted, digital 24/96) | <0.060% | <0.065% |

THD ratio (unweighted, digital 16/44.1) | <0.061% | <0.065% |

THD+N ratio (A-weighted) | <0.072% | <0.061% |

THD+N ratio (A-weighted, digital 24/96) | <0.066% | <0.071% |

THD+N ratio (A-weighted, digital 16/44.1) | <0.067% | <0.071% |

THD+N ratio (unweighted) | <0.064% | <0.056% |

Minimum observed line AC voltage | 123VAC | 123VAC |

For the continuous dynamic power test, the Ampster BT II was able to sustain about 52W (5% THD) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (5.2W) for five seconds, for five 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 of the BT II were only slightly warm to the touch.

**Frequency response (8-ohm loading, line-level input)**

In our measured frequency-response (relative to 1kHz) chart above, the Ampster BT II is nearly flat at the low end of the audioband (-0.1dB at 20Hz), but deviates from flat at high frequencies (+2.5dB at 20kHz). The -3dB point is just past 60kHz. 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 they perfectly overlap, indicating that the two channels are ideally matched.

**Phase response (8-ohm loading, line-level input)**

Above is the phase response plot from 20Hz to 20kHz for the analog input. The Ampster BT II does not invert polarity and exhibits a about +10 degrees of phase shift at 20Hz, and less than -5 degrees at 20kHz.

**Frequency response vs. input type (8-ohm loading, left channel only)**

The chart above shows the Ampster BT II’s frequency response (left channel only) as a function of input type. The green trace is the same analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same across digital input types (and the analog input)— -1.5dB at 5Hz. The behavior nearing 20kHz for the 16/44.1 digital input is a brick-wall-type attenuation, with a -3dB point at 21kHz, but +0.45dB at 17-18kHz. The -3dB point for the 24/96 data is at 47kHz, and 56kHz for the 24/192 data.

**Frequency response (bass and treble controls, line-level input)**

Above are two frequency-response plots (relative to 1kHz) for the analog input, measured at 10W (8-ohm loading) at the speaker outputs, with the treble and bass controls set at both minimum and maximum. They show that the Ampster BT II will provide a maximum gain/cut of approximately 5dB centered around 150Hz and 9-10kHz. Due to the Ampster BT II’s inherent rise in frequency response at high frequencies, with the treble set to maximum, we measured +8dB at 20kHz.

**Frequency response (subwoofer output)**

Above is the frequency response (relative to 20Hz) plot for the analog input, measured at the line-level subwoofer output. The -3dB point is near 600Hz.

**Digital linearity (16/44.1 and 24/96 data)**

The chart above shows the results of a linearity test for the optical digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the speaker outputs of the Ampster BT II. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. The 16/44.1 data overshot the ideal output signal amplitude by about 10dB at -120dBFS, but yielded perfectly flat results from -90dBFS to 0dBFS. The 24/96 data undershot by 10dB at -110dBFS (left) and overshot by 10dB at -120dBFS, but yielded perfectly flat results from -80dBFS to 0dBFS. Interestingly, the 16/44.1 data outperformed the 24/96 data, because that usually does not happen.

**Impulse response (24/44.1 data)**

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the speaker outputs (10W 8-ohm). We can see that the Ampster BT II utilizes a reconstruction filter with minimal pre-ringing and significant post-ringing.

**J-Test (optical)**

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8-ohms) of the Ampster BT II. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (*i.e.*, 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave. In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (*e.g.*, 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBFS and below. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

We see several and clear peaks in the audioband at -105 to 130dBFS. This is an average-to-mediocre J-Test result, and an indication that the Ampster BT II DAC may have poor jitter immunity.

**J-Test with 10ns of injected jitter (optical)**

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8 ohms) of the Ampster BT II, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/- 2kHz jitter signal) manifest at -70dBrA. This indicates that the DAC in the Ampster BT II has poor jitter immunity.

**J-Test with 100ns of injected jitter (optical)**

The chart above shows the results of the J-Test test for the optical digital input measured at the speaker outputs (10W into 8-ohms) of the Ampster BT II, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are very clear again, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/- 2kHz jitter signal) manifest at -50dBrA. This is yet another indication that the DAC in the Ampster BT II has poor jitter immunity.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (optical input)**

The chart above shows a fast Fourier transform (FFT) of the Ampster BT II’s speaker outputs (10W into 8-ohms) with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at -1dBFS, both fed to the optical digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are several low-level aliased image peaks in the audioband at -90dBrA and below. The main 25kHz alias peak is at -45dBrA.

**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 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. Zoomed in, we can see a maximum deviation within the audioband of nearly 0.4dB from 4 ohms to no load, which is an indication of an average damping factor, or average output impedance. The maximum variation in RMS level when a real speaker was used is less at about 0.2dB within the flat portion of the response.

**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 (20Hz to 6kHz) for a sinewave stimulus at the analog input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 25W (rated power). The power was varied using the volume control. Between 20Hz and 200Hz, all three THD plots are relatively flat and similar, hovering around 0.04% to 0.1%. From 200Hz to 6kHz, the 1W and 10W data ranged from 0.04% to 0.2% (left, 10W at 6kHz), while the 25W THD data yielded higher results, from 0.1% to 0.5% at 6kHz.

**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 Ampster BT II as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Although there are many fluctuations below the “knees” (roughly 25W into 8 ohms and about 45W into 4 ohms), both the 4-ohm and 8-ohm data are relatively close, from 0.2% down to 0.02%. It’s the right channel that generally outperformed the left channel, by as much as 10dB. The exception is the right channel into 4 ohms from 4W to 20W, where THD ratios reached 0.3%, 10-15dB higher than the other data at these power levels. The 1% THD levels were reached at 28W and 49-50W into 8 and 4 ohms.

**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 Ampster BT II as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). The plots are virtually identical to the THD vs. output power plot above, which means, even at low power levels, it’s the THD ratios that dominate, and the Ampster BT II is a relatively quiet amplifier.

**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 Ampster BT II as a function of load (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find identical THD ratios between the 8- and 4-ohm loads, hovering around 0.05%, and up to 0.15% at 6kHz. Since the Ampster BT II is not designed to drive 2-ohm loads, predictably, THD ratios were higher into 2 ohms, at 0.1% to 0.2%. Nevertheless, the Ampster BT II was stable into 2 ohms, and did not shut down due to a protection circuit.

**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 Ampster BT II 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). All three results are similar enough to be judged identical, yielding THD ratios around 0.05% from 20Hz to 6kHz. The exception is the two-way speaker at 20kHz, which typically yields higher THD results in most amps, here at 0.25%. While a strong result in this test is one where the real speaker THD ratios are very close to the THD ratios into a dummy resistive load, as seen here, in this case, since THD results are already high into a dummy load, this result should be taken with a grain of salt.

**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 Ampster BT II 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). All three results are similar enough to be judged identical, yielding relatively high IMD ratios from around 0.05% up to 0.1%.

**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 Ampster BT II 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 results are similar enough to be judged identical, yielding relatively high IMD ratios around 0.2% across the frequency sweep.

**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 analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at -70dBrA, or 0.03%; the remaining signal harmonics are at or below -80dBrA, or 0.01%. Below 1kHz, we see power-supply noise-related peaks at the fundamental (60Hz), and second (120Hz), third (180Hz) and fourth (240Hz) harmonics, at a low -105 dBrA, or 0.0006%, and below. Other lower-level noise-related peaks can also be seen. There is a significant rise in the noise floor just above 20kHz, typical for many class-D amps. It’s clear from the FFT above that THD related peaks dominate with the Ampster BT II, while noise levels are relatively low.

**FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)**

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 optical digital input, sampled at 16/44.1. The main differences compared to the analog input FFT above are the fourth (4kHz), fifth (5kHz), and sixth (6kHz) signal harmonic peaks that are higher here, at or near -70dBrA, or 0.03%.

**FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)**

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 optical digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with the 16/44.1 FFT above, but with a slightly lower noise floor due to increased bit-depth.

**FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)**

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the optical digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at just slightly below the correct amplitude, and no visible signal harmonic peaks seen above the -145dBrA noise floor. The second (120Hz) and fourth (240Hz) power-supply-related harmonic peaks are slightly below -130dBrA, or 0.00003%.

**FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)**

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the optical digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at just below the correct amplitude, and the signal’s third (3kHz, left channel) harmonic can be seen at a very low -140dBrA, or 0.00001%. Power-supply-related harmonic peaks are similar to what is seen in the 16/44.1 FFT above.

**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 analog 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 predominant (non-signal) peak is the signal second (100Hz) and third (150Hz) harmonics at -70dBrA, or 0.03%, with other signal harmonics can be seen below -80dBrA, or 0.01%. Very small power-supply-related peaks can be seen, for example, at 60Hz at -105dBrA, or 0.0005%, and 120Hz at -110dBrA, or 0.0003%.

**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 analog 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 -80/-90dBrA (left/right), or 0.01/0.003%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -60dBrA, or 0.1%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)**

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 digital optical input at 16/44.1. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at 70dBrA, or about 0.03%, while the third-order modulation products, at 17kHz and 20kHz, are at -55dBrA, or 0.2%.

**Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)**

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 digital optical input at 24/96. We find that the second-order modulation product (*i.e.*, the difference signal of 1kHz) is at 70dBrA, or about 0.03%, while the third-order modulation products, at 17kHz and 20kHz, are at -55dBrA, or 0.2%.

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

Shown above is the FFT of the speaker-level output of the Ampster BT II 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 below the -100dBrA, or 0.001%, level, below 6kHz.

**Square-wave response (10kHz)**

Above is the 10kHz squarewave response using the analog 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 Ampster BT II’s slew-rate performance. Rather, it should be seen as a qualitative representation of its average 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. Here we find considerable overshoot in the corner, which may be due to the Ampster BT II’s non-linear frequency response above 10kHz. In addition, we can see the 400kHz switching oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

**Square-wave response (1kHz–250kHz bandwidth)**

Above is the 1kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 400kHz oscillator. Here we seen a relatively clean squarewave response, with the exception of the overshoot in the corner, which again may be due to the Ampster BT II’s non-linear frequency response above 10kHz.

**FFT spectrum of 400kHz switching frequency relative to a 1kHz tone**

The Ampster BT II’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Ampster BT II oscillator switches at a rate of about 400kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 400kHz peak is quite evident, and at -35dBrA. There is also a peak at 800kHz (the second harmonic of the 400kHz peak), at -60dBrA, and at 1.2MHz (the third harmonic) at -75dBrA. Those peaks are direct results of the switching oscillators in the Ampster BT II amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and is therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

**Damping factor vs. frequency (20Hz to 20kHz)**

The final plot above is the damping factor as a function of frequency. Both channels track very closely, and range from about 44 from 20Hz to 2kHz, then a dip to 6.5 at 20kHz. This dip in damping factor at high frequencies is typical of inexpensive class-D amp modules.

*Diego Estan*

Electronics Measurement Specialist