Starke Sound Fiera4 Amplifier Measurements

General Information

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

The Fiera4 was conditioned for 1 hour at 1/8th full rated power (~18W 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 Fiera4 is a four-channel amplifier that can be configured for stereo (two-channel) applications. Outputs 1 and 3 can be configured in “BRIDGE” mode (as opposed to “STD” mode), where two channels are configured in parallel to output the same output voltage, but with more current handling, and thus higher power, in particular into lower impedances.

The Fiera4 offers four unbalanced (RCA) and four balanced (XLR) line-level analog inputs. Unless otherwise stated, the Fiera4 was operated in “BRIDGE” mode, where Output 1 was interpreted as the left channel and Output 3 the right channel, using the balanced inputs. To achieve the standard 10W into 8 ohms, 0.75Vrms was required at the input.

Because the Fiera4 is a 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 speaker output 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.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Starke Sound for the Fiera4 compared directly against our own. The published specifications are sourced from Starke Sound’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 extended from DC to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 0.75Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Fiera4 was able to sustain 299W into 4 ohms (>1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (29.9W) for 5 seconds, for 5 minutes, without inducing any protection or shutdown circuits. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the Fiera4 was only slightly warm to the touch.

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

In our measured frequency-response chart above, the Fiera4 is nearly flat within the audio band (20Hz to 20kHz). At the extremes, the Fiera4 is at -0.05dB at 20Hz and +1dB at 20kHz. Starke Sound claims a frequency response of 10Hz-20kHz (-1.2dB), which, although not stated, is likely into 4 ohms. It should be noted that the rise at high frequencies is typical for this type of digital amplifier technology, which exhibits a low damping factor (high output impedance) at high frequencies (see damping factor vs. frequency chart below). When frequency response is measured into a 4-ohm load (see RMS level vs. frequency vs load impedance chart below), there is a dip instead of a rise in the response at high frequencies. 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 are the left- and right-channnel phase-response plots from 20Hz to 20kHz for the balanced line- level input, measured across the speaker outputs at 10W into 8 ohms. The Fiera4 does not invert polarity and exhibits, at worst, about 50 degrees (at 20Hz) of phase shift within the audio band.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the balanced line-level input swept from 5Hz 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. The most obvious feature in these plots is the significant deviations in response at different loads at high frequencies (above 5kHz or so). This is a characteristic of this type of digital amplifier technology, where there’s a rising output impedance (or low damping factor) at high frequencies. This can have the effect of either brightening or dulling the treble response in a system, depending on the characteristic impedance of the speaker being driven. In the flatter part of the curves (below 5kHz), we can see a maximum deviation within the audio band of about 0.2dB from 4 ohms to no load, which is an indication of a mid-level damping factor. The maximum variation in RMS level when a real speaker was used in the flat portion of the curve is a little less, deviating by about 0.1dB.

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 sine-wave stimulus at the balanced line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 132W. The Fiera4 shows a trend of rising THD as the frequency increases, as well as higher THD ratios at higher output power levels. Between 500Hz and 6kHz, at 1W, THD ratios ranged from 0.003% to 0.02%, while at 10W, then at 132W, THD ratios were about 10dB higher at each increased power increment. From 20Hz to 300Hz, THD ratios at 1W and 10W were similar, hovering between 0.002 and 0.003%. At 132W, from 20Hz to 300Hz, THD ratios ranged from 0.005% to 0.015%, with the left channel outperforming the right channel at low frequencies by about 5dB.

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 Fiera4 as a function of output power for the balanced 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). The 8-ohm THD ratios ranged from about 0.03% at 50mW down to about 0.002% at 100mW, followed by a steady rise to 0.04% at the “knee,” at roughly 130W. The 4-ohm data yielded THD ratios roughly 5dB higher. The “knee” in the 4-ohm data occurs around 230W. The 1% THD values are reached at 153W (8 ohms) and 288W (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 Fiera4 as a function of output power for the balanced 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). Since the Fiera4 produces levels of THD that dominate relative to the noise, the THD+N plots look very similar to the THD versus output power plots above.

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 Fiera4 as a function of frequency into three different loads (8/4/2 ohms), for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the balanced line-level input. We find increasing THD values from 8 to 4 to 2 ohms. Into 8 ohms, THD ratios range from 0.002% at 20Hz up to 0.08% at 6kHz. The 4-ohm THD ratios are 10dB higher through most of the frequency range (200Hz to 6kHz), and the 2-ohm data, about 8dB higher than the 4-ohm data. Even into 2 ohms, the Fiera4 manages THD ratios between 0.006% and 0.5% at 80W.

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 Fiera4 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 balanced 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 high frequencies (5-6kHz), the dummy load and both speakers track closely with THD ratios between 0.01% and 0.03%. Below 1kHz, there are significant diffrences in THD ratios between the dummy load and real speakers, by over 30dB at 20Hz, where the two-way speaker THD ratio was 0.15% compared to the 0.002% measured across the dummy load. Between 50Hz and 1kHz, deviations between the dummy load and speakers were within 10dB or so. From this, we conclude that the Fiera4 seems to be sensitive to load variations in terms of THD.

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 Fiera4 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 balanced 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 plots are similar. The IMD ratios range from 0.006% at 2.5kHz up to 0.15% between 10kHz and 20kHz. The largest devations between the dummy load and speakers came at high freqenices, where the two-way speaker measured almost 10dB lower than the three-way speaker between 10kHz and 20kHz.

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 Fiera4 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 balanced 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 track closely, hovering between 0.01% and 0.02%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the balanced line-level input. The signal harmonics are evident with the third (3kHz) harmonic dominating at -80dBrA, or 0.01%. The second (2kHz) and fifth (5kHz) harmonics are between -95dBrA and -100dBrA, or 0.002% and 0.001%. On the left side of the main signal peak, we find virtually no power-supply noise related harmonics. The right channel shows a -120dBrA, or 0.0001%, peak at 60Hz, and the left channel shows a -125dBrA, or 0.00006% peak at 120Hz. The low noise of the Fiera4 results in good signal-to-noise ratio performance (see primary table above).

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the balanced 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 third signal harmonic (150Hz) dominates at -95dBrA, or 0.002%, with other signal harmonics seen at -100dBrA and below. Once again, only a hint of power-supply noise related harmonics can be seen at 60Hz and 120Hz, just above the -130dBrA, or 0.00003%, noise floor. 

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the balanced 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 -100/105dBrA (left/right), or 0.001/0.0006%. The third-order modulation products, at 17kHz and 20kHz, are much higher, at around -65dBrA, or 0.06%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the balanced 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 Fiera4’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Fiera4’s mid-level bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 can see the 450kHz switching oscillator frequency used in the digital amplifier section visibly modulating the waveform.

Square-wave response (10kHz) with 250kHz bandwidth

Above is the 10kHz square-wave response using the balanced input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 450kHz oscillator. We see more evidence here, in the over and undershoot at the square-wave corners, of the Fiera4’s limited bandwidth.

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

The Fiera4’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Fiera4 oscillator switches at a rate of about 450kHz, 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 sine wave. We can see that the 450kHz peak is quite evident, and at -30dBrA. There is also a peak at 900kHz (the second harmonic of the 450kHz peak), at -65dBrA. Those peaks—the fundamental and its second harmonic—are direct results of the switching oscillators in the Fiera4 amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audio band—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 graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 600Hz, right around 92/101 (left/right). Above 600Hz, the damping factor decreases sharply, hitting a low of about 7 at 20kHz.

Diego Estan
Electronics Measurement Specialist