Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on April 15, 2021

General information

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

The LINEb was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The LINEb offers two sets of line-level unbalanced (RCA) inputs, four sets of line-level balanced (XLR) inputs, and two sets of balanced (XLR) outputs. The volume control is implemented using relays and a discrete high-precision resistor ladder. The RCA inputs yield 6dB more gain than the XLR inputs, with a range from –51.4dB (volume position 1 on the display) to +11.8dB (volume position 64). The XLR inputs range from -57.4dB to +5.8dB. The volume control offers 1dB steps from 1 to 56, 0.5dB from 56 to 57, 1dB from 58 to 61, 1.5dB from 61 to 63, and 2dB from 63 to 64. Unity gain (+0.1dB) is achieved at position 60 for the XLR inputs, 54 (-0.1dB) for the RCA inputs. Channel volume tracking is superb (see table below).

There is an Audio Gnd switch on the LINEb back panel. Presumably, this switch disconnects audio ground from chassis/earth ground. I found no differences in noise performance with the switch in the off or on position. It was left on for the measurements.

I found effectively no difference in THD+N values between the RCA and XLR inputs for the same output voltage. I attempted to optimize the volume position to achieve the best signal-to-noise (SNR) and THD+N measurements; however, I found only small differences with the volume at various positions (for the same output voltage). Most measurements were made with the volume set to unity gain (60) using the XLR inputs.

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

Volume position Channel deviation
1 0.009dB
5 0.009dB
10 0.008dB
20 0.009dB
40 0.023dB
50 0.026dB
60 0.025dB
64 0.024dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Karan Acoustics for the LINEb compared directly against our own. The published specifications are sourced from Karan Acoustic’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.

Parameter Manufacturer SoundStage! Lab
Input impedance 30k ohms 57k ohms*
Output impedance 90 ohms 180 ohms*
Maximum output level (1% THD+N, 600 ohms) 18Vrms 15.5Vrms
Maximum output level (1% THD+N, 200k ohms) 18Vrms 20.6Vrms
Gain 6dB 5.8dB
Frequency response (20Hz-20kHz) ± 0dB ± 0dB
Frequency response (1.5Hz-3MHz) -3dB -0.2dB at 200kHz
THD (20Hz-20kHz, 2Vrms, 200k ohms) <0.003% <0.0002%
IMD ratio (18kHz and 19kHz stimulus tones, 2Vrms, 200k ohms) <0.003% <0.00023%
SNR (2Vrms output, unweighted, 200k ohms) >120dB 109dB
SNR (18Vrms output, unweighted, 200k ohms) >120dB 128dB

* The discrepancy in balanced input/output impedances may be due to Karan specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs/outputs on the balanced connector together. Treated separately, our measurement would be halved, or, respectively, 28.5k ohms and 90 ohms for the input and output impedances.

Our primary measurements revealed the following using the balanced line-level inputs (unless specified, assume a 1kHz sine wave, 2Vrms output into 200k ohms load, 10Hz to 90kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -108dB -109dB
DC offset <-0.7mV <-0.4mV
Gain (default) 5.84dB 5.82dB
IMD ratio (18kHz and 19kHz stimulus tones) <-113dB <-115dB
Input impedance 57.6k ohms 57.3k ohms
Maximum output voltage (at clipping 1% THD+N) 20.6Vrms 20.6Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 15.5Vrms 15.5Vrms
Noise level (A-weighted) <5.8uVrms <5.4uVrms
Noise level (unweighted) <19uVrms <15uVrms
Output impedance 179.7 ohms 179.9 ohms
Signal-to-noise ratio (A-weighted) 111.1dB 111.6dB
Signal-to-noise ratio (unweighted, 20Hz to 20kHz) 109.1dB 109.8dB
THD (unweighted) <0.000064% <0.000060%
THD+N (A-weighted) <0.00028% <0.00027%
THD+N (unweighted) <0.00093% <0.00072%

Frequency response

frequency response

In our measured frequency-response chart above, the LINEb is perfectly flat within the audioband (20Hz to 20kHz) and beyond. These data partially corroborate Karan Acoustics’ claim of 20Hz to 20kHz +/-0dB, 1.5Hz to 3MHz (-3dB). However, since the Audio Precision can only sweep to just past 200kHz, we cannot verify the -3dB at 3MHz claim portion. The LINEb is at 0dB at 5Hz, and at about -0.2dB at 200kHz. To state that the LINEb is a high-bandwidth audio device would be an understatement.

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 overlap perfectly, indicating that the two channels are ideally matched.

Phase response

phase response

Above is the phase-response chart from 20Hz to 20kHz. The LINEb does not invert polarity, and the plot shows essentially no phase shift within the audioband.

THD ratio (unweighted) vs. frequency

thd ratio unweighted vs frequency

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus. The blue and red plots are for left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values are very low, about 0.00004% into 200k ohms from 20Hz to 1kHz, and, most impressively, even lower at about 0.00003% into a 600-ohm load. There is a rise in THD values above 1kHz, where at 20kHz, the 600-ohm data are about 0.0003%, and the 200k-ohm data are lower at about 0.0002%, which are still extremely low figures.

THD ratio (unweighted) vs. output voltage at 1kHz

thd ratio unweighted vs output voltage 1khz

The chart above shows THD ratios measured at the output of the LINEb as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.007%, dipping down to around 0.00003% at 3Vrms. The “knee” occurs at around 18Vrms, hitting the 1% THD just past 20Vrms. It’s important to note here that the LINEb’s extraordinarily low THD values are approaching the limits of the Audio Precision analyzer, which, when measured in loopback mode (generator feeding analyzer) measures about 50% lower than the LINEb (at 3Vrms), at about 0.000015%. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps into full power.

THD+N ratio (unweighted) vs. output voltage at 1kHz

thd n ratio unweighted vs output voltage 1khz

The chart above shows THD+N ratios measured at the output the LINEb as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD+N values measured around 0.1-0.2%, dipping down to around 0.0002% at 10Vrms.

FFT spectrum – 1kHz

fft spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load. The red is the right channel, the blue the left. We see that the signal’s second harmonic, at 2kHz, is at a vanishingly low -140dBrA, or 0.00001%, while the third harmonic, at 3kHz, is just slightly above -140dBrA. Below 1kHz, we see some noise artifacts, with the 60Hz peak due to power supply-noise barely perceptible on the left channel below -140dBrA, and the 180Hz (third harmonic) peak just above -140dBrA. The right channel does not appear to show any noise peaks above the very low -150dBrA noise floor.

FFT spectrum – 50Hz

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output into a 200k-ohm load. 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. Here again, there are barely any noticeable peaks. We find the third harmonic of the signal (150Hz) just peaking above the -150dBrA noise floor, or 0.000003%, and the left channel showing the third-harmonic noise peak (180Hz) just above -140dBrA, or 0.00001%.

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

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are at -130dBrA and -125dBrA, or 0.00003% and 0.00006%, respectively. These extraordinarily low harmonic peaks are reflected in the IMD values in our primary table of -113/-115dB, which represent the sum of the second- and third-order intermodulation product peaks.

Square-wave response (10kHz)

square wave response 10kHz

Shown above is the 10kHz square-wave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this chart should not be used to infer or extrapolate the LINEb’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high 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 square-wave representation. As mentioned above, the LINEb is a very high-bandwidth component. Correspondingly, the LINEb’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot or overshoot.

Diego Estan
Electronics Measurement Specialist