Krell KSA-100S power amplifier Measurements

Sidebar 2: Measurements

I measured the Krell KSA-100S in its balanced mode, repeating selected measurements in the unbalanced configuration.

Following the 1/3-power, one-hour preconditioning test, the KSA-100S's heatsinks were only moderately warm.

The KSA-100S's input impedance measured 47.3k ohms unbalanced, and just under 95k ohms balanced; its output impedance varied between 0.095 and 0.11 ohms, depending on the frequency and load (with the higher values at 20kHz). Voltage gain into 8 ohms measured 26dB, balanced or unbalanced. DC offset, though fluctuating slowly with time, hit maximums of about 6.9mV (L) and 6.7mV (R). Signal/noise (unweighted at 1W into 8 ohms) measured 85dB in the balanced mode, 83dB in the unbalanced configuration. The KSA-100S is noninverting in the unbalanced mode; in the balanced, pin 2 is the positive leg, pin 3 the negative.

Fig.1 shows the frequency response of the KSA-100S driven from its balanced inputs at 2W into 4 ohms. The response into 8 ohms was virtually the same. Note a slight added HF rolloff in the unbalanced mode; its audible significance should be nil. The response to a 10kHz squarewave in the balanced mode is shown in fig.2: it has a short risetime, no overshoot, and only the slightest rounding on the leading edge. The 1kHz squarewave response (not shown) was perfectly square.

Fig.1 Krell KSA-100S, frequency response at 2W into 4 ohms, in balanced mode (top) and unbalanced mode (bottom) (right channel dashed, 0.5dB/vertical div.).

Fig.2 Krell KSA-100S, 10kHz squarewave at 1W into 8 ohms.

The crosstalk shown in fig.3 indicates nearly identical performance in balanced or unbalanced configuration, and requires no comment.

Fig.3 Krell KSA-100S, crosstalk (from top to bottom at 2kHz): unbalanced L–R & R–L; balanced L–R & R–L (10dB/vertical div.).

The THD+noise vs frequency for the balanced mode is plotted in fig.4. While the results aren't the lowest we've ever measured, they're nonetheless unlikely to affect the amp's audible performance. The THD+N for the unbalanced configuration was marginally higher, but not by more than 0.03% (not shown). The 1kHz distortion waveform (fig.5) is dominated by low harmonics. The result for a 2 ohm load, which is shown, wasn't visibly different into 4 and 8 ohms.

Fig.4 Krell KSA-100S, THD+noise vs frequency at (from top to bottom): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms in balanced mode (right channel dashed).

Fig.5 Krell KSA-100S, 1kHz waveform at 4W into 2 ohms (top); distortion and noise waveform with fundamental notched out (bottom).

The spectrum of the KSA-100S's output, reproducing 50Hz at 133W into 4 ohms (2/3-rated power at that load) is shown in fig.6. The distortion components here are all at or below 0.1% (–60dB), with the third-harmonic being the highest in level. Fig.7 shows the spectrum of the amplifier's output reproducing a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—at 108W into 4 ohms. The largest artifacts here are at 18kHz and 21kHz (–55dB and –54dB, respectively— .2% or less). The component at 1kHz is –68dB, or about 0.04%. The 19+20kHz artifacts at 55W into 8 ohms (not shown) were similar, though slightly lower—particularly at 1kHz (0.008%). These power levels—108W into 4 ohms and 55W into 8 ohms—were the highest attainable with this input signal prior to visible signs of clipping.

Fig.6 Krell KSA-100S, spectrum of 50Hz sinewave, DC–1kHz, at 133W into 4 ohms (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level at –63dB (0.07%).

Fig.7 Krell KSA-100S, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 108W into 4 ohms (linear frequency scale).

The 1kHz, THD+N vs level curves are shown in fig.8. The general shapes of the curves here are very similar to those of the KSA-300S (measured in my review in Vol.17 No.1, p.92), rising to a virtual plateau somewhere between 1W and 10W, then remaining at or near that value until reaching the knee of the curve—the point just prior to clipping. The latter, of course, occurs at a much higher level in the larger KSA-300S. The KSA-100S's discrete-clipping levels (at 1% THD+N) are shown in Table 1. The front-panel bias LEDs illuminated at 700mW (first), 15W (second), 49W (third), and 93W (fourth), both channels driven.

Fig.8 Krell KSA-100S, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

Table 1: Krell KSA-100S Clipping Power (1% THD+N at 1kHz)

Both Channels DrivenOne Channel Driven
LoadW (dBW)W (dBW)
8103 (20)103 (20)118 (20.7)
4201 (20)201 (20)227 (20.6)
2391 (19.9)

This is a fine set of test results. Measurements aren't everything, of course, but there's certainly nothing here that should keep the KSA-100S from being, within its power rating, an excellent-sounding amplifier.—Thomas J. Norton

Krell Industries
45 Connair Drive
Orange, CT 06477-0533
(203) 799-9954