Kinki Studio Vision THR-1 Headphone Amplifier

Link: reviewed by Mark Phillips on SoundStage! Solo on April 1, 2021

General information

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

The Kinki Studio Vision THR-1 was conditioned for 1 hour with 2Vrms at the output (into 300 ohms) before any measurements were taken.

The THR-1 offers one set each of balanced (XLR, Ch2) and unbalanced (RCA, Ch1) inputs, one set of variable line-level balanced (XLR) outputs, one 4-pin XLR headphone output (Phone1), and two ¼″ TRS headphone outputs (Phone2 and Phone3). There is a button on the front panel to select between Ch1 (RCA) and Ch2 (XLR) inputs, as well as a switch to select between the headphone outputs (Phone, all three always active when selected), or the preamp outputs (Output).

There were a couple peculiarities with the THR-1 that merit pointing out. The balanced inputs and outputs are not actually balanced; they are single-ended connections with balanced XLR connectors. For the XLR input, pin 1 is connected to ground, pin 2 is connected to the input, and pin 3 is connected to a 50k ohm resistor then ground. For the XLR line-level output, pin 1 is connected to ground, pin 2 to the output, and pin 3 to ground through a 100-ohm resistor. For the XLR headphone output, pins 1 and 3 are connected to the output, while pins 2 and 4 are grounded. There are no differences between Phone1 (XLR) and Phone2 (¼″ TRS, also labelled High) outputs other than the physical connectors. Pins 1/3 (L/R) from Phone1 are electrically connected to the tip/ring (L/R) of Phone2. The only difference between Phone3 (also labeled Low) and outputs Phone1 and Phone2 is that 1 and 2 have a 50-ohm output impedance, while Phone3 has a 100-ohm output impedance.

The next peculiarity is that the THR-1 inputs and line level outputs are not buffered and are directly connected to the 10k ohms potentiometer used as the volume control. This means that the input and output impedances of the THR-1 are variable. When used as a preamplifier, the variability of the input impedance will depend on the input impedance of the amplifier following it; the higher the amplifier input impedance, the closer the input impedance of the THR-1 will appear as a constant 10k ohms. The line-level output impedance will vary depending on the volume position. When set near minimum, it’s 110 ohms, at 2 o’clock 2.6k ohms, and at maximum, 564 ohms. When used as a headphone amplifier, the input impedance varies between 10k ohms (volume minimum) to 775 ohms (volume maximum). The effect this will have on overall system performance will depend on the source’s (connected to the THR-1) output impedance and ability to drive loads below 1k ohms (if the volume pot is used near its maximum setting). If the source’s output impedance is high (e.g., 1k ohm), raising the volume (i.e., reducing the input impedance) in the THR-1 will have two opposing effects: raising the gain inside the THR-1, as well as lowering the signal output level out of the source component.

Unless otherwise specified, the balanced input connection was used for all measurements. Most measurements were done with the volume set to unity gain (about 2 o’clock), with the exception of the signal-to-noise ratio (SNR), where volume was set to maximum. Outputs 1 and 3 (Phone1 and Phone3) were used as outputs in the tables below, while Output 1 (Phone1) was used for the charts below. When used as a preamplifier (which is actually passive and just a 10k ohm potentiometer), gain (volume maximum) was measured at -0.5dB (-6.5dB for the balanced input) into the Audio Precision’s 200k ohm input load. Decreasing the input impedance of the next device in the audio chain will decrease the gain, which is the nature of a passive preamp.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Kinki Studio for the Vision THR-1 compared directly against our own. The published specifications are sourced from Kinki Studio’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the balanced line-level inputs (unless specified, assume a 1kHz sine wave, 2Vrms output, 300-ohm loading, 10Hz to 90kHz bandwidth):

Frequency response

In our measured frequency response plot above, the THR-1 is essentially flat within the audioband (20Hz to 20kHz). Kinki’s claim of ±dB from 20Hz to 300kHz is not corroborated by our measurement, as the THR-1 is at +1dB at 140kHz. However, this is of zero audible consequence. The THR-1 can be considered a high-bandwidth audio device because of its high-frequency extension past 100kHz. In the chart above and most of the charts 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

Above is the phase response plot for the THR-1 from 20Hz to 20kHz. The THR-1 does not invert polarity, and there is virtually no phase shift throughout the audioband.

Frequency response (600-, 300-, 32-ohm loads)

The chart above shows RMS level (relative to 0dBrA, which here is 2Vrms at the balanced output into 300 ohms) as a function of frequency for the left channel only. The blue plot is into a 600-ohm load, the purple is into a 300-ohm load, and the orange is into a 32-ohm load. Here we find a deviation of about 7.5dB between 600 ohms and 32-ohms, which is a reflection of the relatively high output impedance of 51 ohms.

THD ratio (unweighted) vs. frequency (600-, 300-, 32-ohm loads)

The chart above shows THD ratios at the THR-1’s Phone1 output as a function of frequency (20Hz to 20kHz) for a sine-wave stimulus at the balanced line-level input for a 2Vrms output. The blue and red plots are for left and right channels into 300 ohms, purple/green into a 32-ohm load. THD into a 600-ohm load was also measured but yielded effectively identical results compared to the 300-ohm data, so those results were omitted for simplicity and clarity in the the chart. There is up to a 15dB improvement in THD ratios into 600 and 300 ohms versus 32 ohms. Into 600 and 300 ohms, the THD values are low from 20Hz to 2kHz, between 0.002% and 0.0005%. At 20kHz, THD ratios reach 0.1%. Into a 32-ohm load, THD values start at 0.005% at 20Hz, down to 0.001% at 200Hz, then steadily increase up to just over 0.1% at 20kHz.

THD ratio (unweighted) vs. output power at 1kHz (300- and 600-ohm loads)

The chart above shows THD ratios at the THR-1’s Phone1 output as a function of output power for the balanced line-level input for an 600-ohm load (blue/red for left/right) and a 300-ohm load (purple/green for left/right). At 1mW, THD values are just below 0.001% for both 600- and 300-ohm data. Above 20mW, the 300-ohm THD values are slightly lower than the 600-ohm values by 2-5dB. At the “knees,” the 600-ohm THD value is at around 0.04%, nearing 2W, while the 300-ohm THD value is at around 0.03% at about 3W. The 1% THD value for the 600-ohm data is at 2.2W, while the 300-ohm data is at 3.5W.

THD+N ratio (unweighted) vs. output power at 1kHz (300- and 600-ohm loads)

The chart above shows THD+N ratios at the THR-1’s Phone1 output as a function of output power for the balanced line-level input, for an 600-ohm load (blue/red for left/right) and a 300-ohm load (purple/green for left/right). At 1mW, THD+N values are between 0.01% and 0.02% for both 600- and 300-ohm data, then dipping down to as low as 0.003% at 50mW.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across a 300-ohm load at 2Vrms. We see that the second harmonic, at 2kHz, is just below -100dBrA, or 0.001% (relative to the reference 0dB signal), while the third harmonic, at 3kHz, is at -110dBrA, or 0.0003%. The fourth and fifth harmonics are even lower, and are difficult to distinguish amongst the power-supply-noise harmonic peaks. Below 1kHz, we see noise artifacts, from the 60Hz fundamental at -105dBrA and the second harmonic at 120Hz at -100dBrA (left) and -105dBrA (right), as well as a long series of visible higher harmonics between -105 and -120dBrA.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across a 300-ohm load at 2Vrms. 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 second harmonic of the 50Hz signal (100Hz) is at -125dBrA, or about 0.0006%, while the third harmonic at 150Hz is even lower at -135 dBrA, or 0.00002%, for both channels. Here again, the noise artifacts due to power-supply noise dominate, between -100dBrA and -120dBrA, which are respectively 0.001% and 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

Shown above is the FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone at the balanced input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) into 300 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at about -110dBrA, or 0.0003%, for the right channel; -125dBrA, or 0.00006%, for the left channel; while the third-order modulation products, at 17kHz and 20kHz, are at and just above -80dBrA, or 0.01%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response of the THR-1 into 300 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the THR-1’s slew-rate performance. Rather, it should be seen as a qualitative representation of the THR-1’s extended 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. In the case of the THR-1, the square wave is reproduced cleanly, with mild overshoot and undershoot, which can be seen by the triangular-shaped spikes in the corners of the transitions.

Diego Estan
Electronics Measurement Specialist