Theta Digital Citadel monoblock amplifier Measurements
Following its 60-minute preconditioning period at one-third full power into 8 ohms, the big Theta Citadel's aluminum casework was warm, though the inset black grilles on the top panel were too hot to keep my hand on. Although the specified voltage gain is the usual 6dB higher from the balanced input, due to the doubled voltage swing, the measured gain at 1kHz was actually the same from both balanced and unbalanced input terminals—you need to flip a switch to select the correct jack—at a fairly low 25.35dB into 8 ohms. The typical 2V maximum output from a CD player will therefore not be sufficient to drive the Citadel to clipping. The amplifier didn't invert signal polarity from either input and its input impedance was a high 129k ohms balanced, 64.3k ohms unbalanced.
At 0.36 ohm across the audioband, the Citadel's output impedance was higher than we usually see from a solid-state design; this is presumably due to its balanced-bridged topology, with which the speaker is driven in push-pull between two amplifier outputs. As a result, there was a moderate degree of interaction between the amplifier and Stereophile's simulated loudspeaker, resulting in ±0.25dB of response modification (fig.1, top trace). This graph also reveals a response that is flat throughout the audioband and rolls off by 1dB at 180kHz. This was with an 8 ohm load; the small-signal bandwidth decreases a bit with decreasing load impedance. Fig.1 was taken using the balanced input jack; the behavior via the unbalanced jack was identical.
Fig.1 Theta Citadel, balanced input, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).
Fig.2 shows the Citadel's reproduction of a low-level 10kHz squarewave; the risetime is suitably short, and there are no signs of ultrasonic instability. The 1kHz squarewave (not shown) was essentially perfect. The Citadel was pretty quiet, with an unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms) of 85dB, which is equivalent to a very wide dynamic range of 111.5dB with respect to the amplifier's clipping point. A-weighted, the S/N ratio improved by 10dB.
Fig.2 Theta Citadel, small-signal 10kHz squarewave into 8 ohms.
Small-signal THD into 8 ohms was low: 0.001% or below across the audioband (fig.3). But as this graph shows, the THD rises by a factor of three every time the impedance halves, and at 2.83V into 2 ohms, the distortion hovers at around 0.1%. At low power levels, the harmonic content is a mix of low-order products (fig.4), but at high powers appears to be almost pure third-harmonic (fig.5). This behavior was identical from both balanced and unbalanced inputs.
Fig.3 Theta Citadel, THD+N (%) vs frequency at 2.83V into (from bottom to top at 1kHz): simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms.
Fig.4 Theta Citadel, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.5 Theta Citadel, 1kHz waveform at 104W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.6 shows the spectrum of the Theta's output while it drove a low-frequency tone of 50Hz at 100W into 8 ohms. Despite this level being 6dB below the amplifier's specified clipping point, the third harmonic lies at a quite high -56dB (0.15%). The second, fourth, and fifth harmonics all lie at around the -80dB point (0.01%), however, and while some 120Hz power-supply content can be seen, this is not, at -100dB (0.001%), going to be audible. Halving the load impedance doubled the level of the third harmonic to 0.3%; increasing the power to 367W into 8 ohms, while below the specified clipping point of 400W, gave almost 2% of third harmonic.
Fig.6 Theta Citadel, spectrum of 50Hz sinewave, DC-1kHz, at 100W into 8 ohms (linear frequency scale).
There was also a slightly higher level of intermodulation distortion than I was expecting. Fig.7 shows the spectrum of the Citadel's output while it drove an equal mix of 19kHz and 20kHz tones into 4 ohms at a level close to clipping. The 1kHz difference component is acceptably low, at -74dB (0.02%), though the higher-order components at 18kHz and 21kHz rose above the -60dB line (0.1%).
Fig.7 Theta Citadel, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 147W into 4 ohms (linear frequency scale).
Fig.8 plots the percentage of distortion and noise in the amplifier's output against continuous output power. An amplifier's clipping point is usually defined as 1% THD+noise, at which point the waveform, viewed on an oscilloscope, is visibly squared-off. The Citadel failed to meet its specified output power of 400W into 8 ohms at the 1% clip point, raising just 250W (24dBW). It did deliver 400W into this load (26dBW) when the specified clipping point was relaxed to 2% THD+N, as s[ecified by Theta.
Fig.8 Theta Citadel, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.
At this level of distortion, the Theta delivered 610W into 4 ohms (24.8dBW) and 715W into 2 ohms (22.5dBW). In case it might be thought that the Citadel's failure to meet its specified power was due to a low AC voltage from the wall, my test bench has its own 20A circuit, and I do monitor the mains voltage. The supply voltage was a high 125V when the Citadel clipped into 2 ohms.
The Citadel's balanced-bridge design meant that I couldn't test its behavior with low-duty-cycle tonebursts using the Miller Audio Research analyzer. But perhaps most significant regarding the Theta's effect on sound quality is the very gentle way in which it clips, as shown by the traces in fig.8. The low noise floor means that the point at which the measured THD begins to reflect the distortion content is quite low: around the 200mW mark into 8 ohms, and below that into lower impedances. The distortion stays at around the same level until the amplifier is delivering a few tens of watts, at which point it starts to gently climb, without the usual sharp "knee" in the trace. While high-power transients will be accompanied by highish levels of distortion, these will disappear as soon as the transients are over. In this respect, the Theta Citadel behaves more like a traditional tube amplifier.—John Atkinson