Orchard Audio Starkrimson Mono Premium Amplifier Measurements

Link: reviewed by Matt Bonnaccio on SoundStage! Hi-Fi on August 1, 2024

General information

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

The Starkrimson was conditioned for one 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 Starkrimson is a one-channel amplifier with one balanced (XLR) input and one set of speaker level outputs. An input of 740mVrms was required to achieve the reference 10W into 8 ohms.

Our typical input bandwidth filter setting of 10Hz to 22.4kHz was used for all measurements except the FFTs, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a 10Hz to 1MHz input bandwidth. Because the Starkrimson is a pulse-density modulated (PDM) amplifier that exhibits considerable noise above 20kHz (see FFTs below), the 22.4kHz bandwidth setting was maintained for THD versus frequency sweeps as well. For these sweeps, the highest frequency was 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 Orchard Audio for the Starkrimson compared directly against our own. The published specifications are sourced from Orchard Audio’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 740mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Starkrimson was able to sustain about 342W into 4 ohms (~1.5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (34.2W) 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 top of the Starkrimson was barely warm to the touch.

Frequency response (8-ohm loading)

In our frequency-response (relative to 1kHz) plot above, measured across the speaker outputs at 10W into 8 ohms, the Starkrimson exhibits a near-flat frequency response across the audioband (0/+0.1dB at 20Hz/20kHz). The dip at low frequencies is due to the 10Hz low-pass filter we used inside the AP analyzer (DC coupling the Starkrimson caused issues). There is a rise at high frequencies, peaking at +1dB at about 80kHz (into 4 ohms, there is a dip instead of a rise—see RMS level versus load plots below) . The -3dB point is beyond 150kHz.

Phase response (8-ohm loading)

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The Starkrimson does not invert polarity and exhibits, at worst, about -10 degrees of phase shift at 20kHz. The phase shift at lower frequencies is likely due to the 10Hz low-pass filter used inside the AP analyzer.

RMS level vs. frequency vs. load impedance (1W)

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 4-ohm load and no-load to be around 0.04dB. Beyond 2kHz, the deviations increase significantly. This is an indication of a very high damping factor or low output impedance below 2kHz. With a real speaker, the deviations from 20Hz to 2kHz were roughly the same (0.04dB), and about 0.25dB between 4-5kHz and 20kHz.

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 plot is at 1W output into 8 ohms, purple at 10W, and pink is just past the rated power (164W with a 3Vrms input). The 1W and 10W data are close, ranging from 0.0002% to 0.0003% from 20Hz to 1kHz. Beyond 1kHz, the 1W data remained around 0.0002%, while the 10W data rose to 0.0007% at 6kHz. The 164W remained impressively low, at 0.003% from 20Hz to 1kHz, then up to 0.02% 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 Starkrimson as a function of output power for the analog line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.001% at 50mW down to 0.0002% from 1-5W, then up to 0.003% at the “knee,” at roughly 150W. The 4-ohm THD data were 5-10dB higher, with a “knee” at roughly 250W. The 1% THD thresholds were reached at 203W and 352W 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 Starkrimson as a function of output power for the analog line-level-input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.01% down to 0.0005% at 30W. The 4-ohm data ranged from about 0.015% down to 0.001% at 10-20W.

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

The chart above shows THD ratios measured at the output of the Starkrimson as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields roughly 50W at the output into 8 ohms (blue), 100W into 4 ohms (purple), and 200W into 2 ohms (pink). The 8-ohm data ranged from 0.0008% at 20-200Hz, down to 0.0005% at 1-2kHz, then up to 0.0015% at 6kHz. The 4-ohm THD data ranged from 0.002% at 20Hz up to 0.03% at 6kHz. The 4-ohm data results were somewhat inconsistent compared with the measured THD versus output power plots above, which indicate a potential issue with the measurement (we could not repeat the measurement due to a failure of the amplifier during a subsequent test). The 2-ohm data ranged from 0.005% from 20Hz to 1kHz, then a steady rise to 0.01% at 6kHz. This shows that the Starkrimson is perfectly stable into 2-ohms, with very low THD ratios even at 200W.

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

The chart above shows THD ratios measured at the output of the Starkrimson 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 (0.025%). Generally, from 40Hz to 6kHz, THD ratios into the real speakers were higher than the resistive load, varying widely from 5 to 30dB, but remaining, in absolute terms, low and below the 0.01% level.

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

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Starkrimson 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 ranges from 0.0003% at lower frequencies, up to 0.002% at 20kHz. The results were much higher into real speakers, ranging from 0.002% to 0.02%.

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

The chart above shows IMD ratios measured at the output of the Starkrimson 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). The IMD results into the resistive load remained constant between 0.001% and 0.002%. Into real speakers the IMD results were higher, from 0.002% up to nearly 0.01% for the three-way speaker.

FFT spectrum – 1kHz

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), third (3kHz), and sixth (6kHz) harmonics dominate at -115dBrA to -120dBrA, or 0.0002% to 0.0001%. Other signal harmonics can be seen below the -130dBrA level, or 0.00003%. There are essentially no noise related harmonics above the very low -150dBrA noise floor. This is a very clean FFT result.

FFT spectrum – 50Hz

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) peaks are again the signal’s second (100Hz), third (150Hz), and sixth (300Hz) harmonics at -115dBrA to -120dBrA, or 0.0002% to 0.0001%. There are no noise-related harmonics above the very low -160dBrA noise floor.

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 the very low -120dBrA, or 0.0001%, level, while the third-order modulation products, at 17kHz and 20kHz, are higher at -100dBrA, or 0.001%.

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

Shown above is the FFT of the speaker-level output of the Starkrimson 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 very low -130dBrA, or 0.00003%, level.

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 Starkrimson’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Starkrimson’s relatively 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 the 750kHz switching frequency from the pulse-density modulation riding on top of the 10kHz squarewave.

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

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms, this time with a 250kHz input bandwidth on the analyzer to filter out the 750kHz switching frequency, as well as other high-frequency artifacts. Now we see a relatively clean squarewave reproduction, with only some mild over/undershoot in the corners.

FFT spectrum of 500kHz switching frequency relative to a 1kHz tone

The Starkrimson’s amplifier relies on a switching oscillator to convert the input signal to a pulse-density modulated (PDM) signal before sending the signal through a low-pass filter to generate an output signal. The oscillator switches at a rate of about 750kHz, 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 750kHz peak is quite evident, and at -40dBrA. In addition, there is a rise in the noise floor from 25kHz to 200kHz, peaking at -120dBrA. The peak at 750kHz is a direct result of the switching oscillators in the Starkrimson GaN amp module. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and 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. We find very high damping factor values, nearing 600 from 20Hz to 2kHz. Above 2kHz, there is a dip in damping factor, reaching 40 at 20kHz. This is a strong damping factor.

Diego Estan
Electronics Measurement Specialist