Classé Sigma 2200i integrated amplifier Measurements

Sidebar 4: Measurements

I measured the Classé Sigma 2200i with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Out of the box, I ran into problems. Despite Classé's excellent manual, I couldn't at first define one of the Sigma 2200i's digital inputs to accept USB data. Eventually, I got the input to behave by defining the input, then turning the amplifier off and back on again. Then, I couldn't get the Sigma 2200i to accept a balanced signal at the input marked "XLR L1/R1." But then I remembered that, when I was setting up the USB input with the onscreen menu, I'd had to select a specific input connector for that input, in that case USB. Checking the setup menu for the XLR input, it had been set by a prior user to accept a signal from the unbalanced "Line 2" RCA jacks. Once I'd set it to accept a signal fed to the balanced XLR jacks, all was well—except that the amplifier now went into standby after running for 19 minutes. The manual says that "A power-save feature is enabled that puts the Sigma 2200i into standby after 20 minutes without a signal at its input." In my case, even with a signal present, the amplifier continued to turn itself off every 19 minutes.

I measured the Sigma 2200i's performance via its digital inputs using either S/PDIF signals fed from the SYS2722, or WAV and AIFF test-tone files played with Pure Music 3.0 by my MacBook Pro running on battery power and fed via USB. All measurements were taken at the speaker terminals. For the measurements taken to characterize the Classé's performance as a conventional amplifier, I fed a single-ended analog signal to the Line 2 inputs or a balanced signal to the XLR inputs.

Looking first at the Classé's performance in the digital domain: Apple's USB Prober utility identified the amplifier as the "Sigma 2200i 22A5A7" from "B&W Group Ltd," and confirmed that its serial number was "2270631." The USB input operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI app indicated that the Classé operated at all PCM sample rates up to 192kHz via USB, with integer bit depths of up to 24. With the volume control set to "0.0," the amplifier's output level into 8 ohms with a 1kHz sinewave at –20dBFS was 9.415V, meaning that it would clip with full-scale digital data. I therefore set the volume control at "–13.5" for most of the digital input tests, increasing it to "0.0" for only the low-level tests.

The Classé's impulse response with data sampled at 44.1kHz (fig.1) indicates that its reconstruction filter is a conventional linear-phase type and that the amplifier as a whole inverts signal polarity. Tested with 44.1kHz-sampled white noise at –4dBFS (footnote 1), this filter can be seen to rapidly roll off the output above the Nyquist frequency (half the sample rate, shown by the vertical green line in fig.2), but there is an increase in ultrasonic noise above that frequency, this presumably due to the Sigma 2200i's PWM output stage. The harmonics of a full-scale 19.1kHz tone (fig.2, cyan and blue traces) are a little higher than I was expecting, with the third harmonic lying at –54dB (0.2%).


Fig.1 Classé Sigma 2200i, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).


Fig.2 Classé Sigma 2200i, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

Fig.3 shows spectral analyses of the Classé's output while it decoded dithered 16- and 24-bit data representing a 1kHz tone at –90dBFS. With 16-bit data (cyan and magenta traces), the spectrum is dominated by the dither noise. However, with 24-bit data, while the noise floor has dropped by 24dB, suggesting resolution of at least 20 bits, harmonics of the fundamental tone are visible. This graph was taken with TosLink data; USB data gave an identical result. But with its low noise floor in the audioband, the Sigma's reproduction of an undithered 16-bit signal at exactly –90.31dBFS was superb (fig.4), with the three DC voltage levels described by the data and the ringing of the reconstruction filter clearly visible. Note that even though the traces in fig.4 were taken at the speaker terminals, there is zero DC offset visible. This is rare with amplifiers having a conventional output stage.


Fig.3 Classé Sigma 2200i, 44.1kHz S/PDIF data, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).


Fig.4 Classé Sigma 2200i, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

Tested for its digital inputs' rejection of word-clock jitter with 16-bit, 44.1kHz data, the Sigma 2200i didn't accentuate or attenuate the odd-order harmonics of the J-Test's low-frequency, LSB-level squarewave (fig.5, green line). However, the spectral peak that represents the high-level tone at one-quarter the sample rate has widened at its base, which suggests the presence of random low-frequency jitter. Also visible is a patch of spurious low-level tones between 9 and 10kHz that were present with 24-bit J-Test data (fig.6). A wider spectral analysis reveals these more clearly (fig.7), as well as an idle tone of unknown origin at 15kHz.


Fig.5 Classé Sigma 2200i, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.


Fig.6 Classé Sigma 2200i, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.


Fig.7 Classé Sigma 2200i, spectrum of analog output signal, 1Hz–24kHz, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data (left channel blue, right red).

Also in fig.7 can be seen the start of the PWM output stage's ultrasonic noise floor. Because of the presence of this noise, when I measured the amplifier's performance using analog input signals, I used Audio Precision's high-order AES17 high-pass filter when measuring distortion, as otherwise the reading would be obscured by the noise.

The Sigma 2200i's overall gain via its analog inputs with the volume control set to "0.0" was 27.4dB balanced and 33.4dB unbalanced, the 6dB difference being in the opposite direction to what I usually find. All the analog inputs inverted absolute polarity, and the unbalanced input impedance was 46k ohms at 20Hz and 1kHz, 35k ohms at 20kHz. The balanced input impedance was a high 90k ohms at 20Hz and 1kHz, 84k ohms at 20kHz.

The analog inputs are converted to 96kHz-sampled digital, so the Sigma 2200i's frequency response measured via these inputs rolls off sharply above 40kHz (fig.8). The output impedance was 0.1 ohm at low and middle frequencies, but rose to 1.5 ohms at the top of the audioband. The response therefore peaks a little between 10 and 40kHz into higher impedances, which in turn results in overshoot with a 1kHz squarewave (fig.9). But with 2 ohms, the high output impedance at high frequencies results in an output that is down by 4dB at 20kHz (fig.8, green trace). Although there is very little change in the amplifier's frequency response below 1kHz with our standard simulated loudspeaker (fig.8, gray trace), the top-octave sound of the Sigma 2200i will vary more than usual, depending on the loudspeakers used. With a 10kHz squarewave, the 96kHz sample rate gives a band-limited response (fig.10) that also reveals the presence of the ultrasonic noise.


Fig.8 Classé Sigma 2200i, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (0.5dB/vertical div.).


Fig.9 Classé Sigma 2200i, small-signal, 1kHz squarewave into 8 ohms.


Fig.10 Classé Sigma 2200i, small-signal, 10kHz squarewave into 8 ohms.

The effects of the Sigma 2200i's treble and bass tone controls, set to their maximum and minimum settings, is shown in fig.11. The boost and cut are sensibly chosen. I didn't test the effect of the Tilt control, as I couldn't manage to set this correctly using the onscreen menu. Channel separation was good, at >80dB L–R and >90dB R–L below 3kHz. Though the output was commendably free from power-supply–related spuriae (fig.12), the wideband signal/noise ratio, ref. 1W into 8 ohms and taken with the analog input shorted to ground and the volume control set to "0.0," was affected by the output stage's ultrasonic noise and was just 29.7dB (average of both channels). Restricting the measurement bandwidth to the audioband increased the S/N ratio to 58.1dB; A-weighting further increased it, to 75.75dB.


Fig.11 Classé Sigma 2200i, frequency response at 2.83V into 8 ohms with bass and treble controls set to: "0" (left channel blue, right red), "+6" (left cyan, right magenta), "–6" (left gray, right green) (2dB/vertical div.).


Fig.12 Classé Sigma 2200i, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right channel red, linear frequency scale).

I then plotted how the percentage of THD+noise varied with output power, using the AES17 low-pass filter set to 20kHz. The Classé offered very low distortion and high power below clipping into 8 ohms (fig.13) and 4 ohms (fig.14). These graphs reveal that the amplifier clipped at 215Wpc into 8 ohms (23.3dBW) and 410Wpc into 4 ohms (23.1dBW), both powers slightly higher than the specified 23dBW.


Fig.13 Classé Sigma 2200i, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.14 Classé Sigma 2200i, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.15 shows how the THD+N percentage varied with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The AES17 low-pass filter was again set to 20kHz, which is why the graph ends at 5kHz rather than the usual 20kHz, in order not to eliminate the higher harmonics. The distortion is very low into 8 ohms (blue and red traces), with a slight rise in the treble. The THD+N percentage rises more at high frequencies into lower impedances, with the 2 ohm measurement higher than 0.1% above 1kHz (trace incomplete because the Classé went into standby at that point). The distortion at this power level seems primarily to be the third harmonic (fig.16), accompanied by some lower-level, higher odd-order harmonics (fig.17). Assessed with my usual mix of equal-level 19 and 20kHz tones, the intermodulation distortion was very low (fig.18). However, you can see in this graph the start of the rise in the ultrasonic noise floor, examined in more detail in fig.19.


Fig.15 Classé Sigma 2200i, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right channel red), 4 ohms (left cyan, right magenta), 2 ohms (gray).


Fig.16 Classé Sigma 2200i, 1kHz waveform at 100W into 4 ohms, 0.017% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.17 Classé Sigma 2200i, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (linear frequency scale).


Fig.18 Classé Sigma 2200i, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).


Fig.19 Classé Sigma 2200i, HF intermodulation spectrum, DC–50kHz, 19+20kHz at 50W peak into 8 ohms (linear frequency scale).

Its use of a PWM output stage doesn't seem to have compromised the Classé Sigma 2200i's measured performance, and the amplifier offers high power in an attractively lightweight package. But I don't think it would be a good match for loudspeakers that would present it with impedances as low as 2 ohms.—John Atkinson

Footnote 1: My thanks to Jürgen Reis, of MBL, for suggesting this test to me.
5070 Franáois Cusson
Lachine, Quebec H8T 1B3
(514) 636-6384

georgehifi's picture

"I used Audio Precision's high-order AES17 high-pass filter when measuring distortion, as otherwise the reading would be obscured by the noise."

Hi JA, was this filter also used when measuring/presenting the (fig 9) 1kHz and (fig 10) 10kHz square wave screen shots?

Cheers George

John Atkinson's picture
georgehifi wrote:
was this filter also used when measuring/presenting the (fig 9) 1kHz and (fig 10) 10kHz square wave screen shots?

No, the squarewave images were taken with just the external AP0025 filter as the AES17 brickwall filter would have affected the amplifier's reproduction of squarewaves. The AP0025's rolloff is sufficiently high in frequency not to have an effect other than eliminating RF noise.

John Atkinson
Editor, Stereophile

georgehifi's picture

Ok it's a different AP in line filter, this 20 Hz to 20 kHz passband filter you used
Has a -50db rolloff rate, I take it doesn't handle much power hence the "small-signal" square wave testing you can only do with it in line.

Myself, I would still prefer to see in tests with and without this filter in line, to see what's being sent to the speakers.

Cheers George

caphill's picture

Hello Thomas,
Thanks for the review of the Classe Sigma 2200i integrated. It was interesting to read your comments about the Classe Sigma 2200i being warm sounding tonally. I personally haven't heard the Sigma 2200i but have demoed the Sigma SSP + Sigma Amp5 combo paired with B&W speakers and Wilson Sabrina multiple times and they were very neutral tonally and very transparent imo. The Sigma combo were spectacular sonically. I demoed in both stereo for music in analog / digital bypass mode as well as for home theater (surrounds).

I would assume that the Sigma 2200i would perform very similar.
Did you get to listen to the Sigma separates by any chance?

Richard D. George's picture

I own the Sigma SSP and Sigma Amp 5. They sound spectacular for both home theater and two-channel audio. I have a Bluesound Vault2 connected to the Sigma SSP and high-resolution two-channel audio files sound terrific.

Richard D. George's picture

I also heard the Sigma 2200i demonstrated with Bowers & Wilkins 804 d3 speakers at a 2016 Music Matters event at my local dealer. Sounded fantastic. Heard four or five cuts (all Redbook format, interestingly, no high res) over the course of 40 minutes.

I don't hear the "not neutral" part. Must be my faulty ears.

caphill's picture

Hi Richard,
How did the Sigma 2200i sound compared to your Sigma SSP + Sigma Amp5 separates? What speakers do you have at home?

Richard D. George's picture

Too many variables to directly compare. I thought both sounded good, with no obvious flaws.

In that particular system my speakers are Sonus Faber Venere 3.0 and Venere Center with two REL S/5 subs connected speaker level with Longbow. I may eventually upgrade to Sonus Faber Olympica for L/R/C speakers, or perhaps Bowers & Wilkins 804 d3. Surround speakers are high-end Bowers & Wilkins two-way in-ceiling speakers (don't recall the model number)

caphill's picture

I think both the B&W 804 D3 and the SONUS Faber Olympica are great speakers imo but I think the B&W 804 D3 will have better synergy with your Classe Sigma Amp5.

I myself am using the Classe SSP 800 pre pro with the Classe CA-M 300 monoblock amps (3×) for L/R/C speakers. Using the Classe CA-2300 stereo amp for my surround speakers. My front speakers are the B&W 802 D3 with the matching HTML1 center speaker. My surround speakers are the 804 D3.