Denon AVR-4800 THX Ultra digital surround receiver Measurements
Measuring the Denon AVR-4800 took rather longer than I had anticipated, due to the sheer number of options it offers its owner. I ended up looking at the receiver's amplifier performance using either its CD inputs or its External discrete-surround inputs. But first, using a two-channel TosLink datastream, I examined the receiver internal D/A conversion. The power amplifiers clipped with the volume control set to "–07" with a full-scale digital signal, by the way.
The receiver locked on to an incoming datastream with a 96kHz sample rate. Fig.1 shows the spectrum of the receiver's analog output while it decoded 44.1kHz data representing a dithered 1kHz tone at –90dBFS. The upper trace was taken with 16-bit data, the lower trace with 24-bit data. In both cases, the peak at 1kHz lies around 2.6dB below the –90dB line, implying a smidgen of negative linearity error. Some power-supply components can be seen at 60Hz, 120Hz, 180Hz, and 300Hz, but these are all below –110dB. Nevertheless, increasing the word length drops the noise floor by only 5dB or so, and then only in the higher frequencies. The plot of linearity error vs amplitude (fig.2) confirms the negative linearity error at low levels.
Fig.1 Denon AVR-4800, digital input, 1/3-octave spectrum of dithered 1kHz tone at –90dBFS, with noise and spuriae, 16-bit data (top) and 24-bit data (bottom).
Fig.2 Denon AVR-4800, digital input, departure from linearity, 16-bit data (2dB/vertical div.).
Turning to the analog domain, the AVR-4800 behaved quite differently depending on whether it was assessed in two-channel mode via the regular inputs or via the discrete surround inputs. The top trace in fig.3 shows the small-signal frequency response measured through the latter: the response is a negligible 0.4dB down at 20kHz, sensibly reaching –3dB at 60kHz. Playing back SACDs or DVD-As with 96kHz or 192kHz sample rates through the Denon's External inputs will give its owner the full recorded bandwidth. More important, however, is the broad, 0.3dB-high shelf visible in the bass; this will be audible.
Fig.3 Denon AVR-4800, frequency response at 1W into 8 ohms, CD input (bottom) and External input (top). (0.5dB/vertical div.)
The lower trace in fig.3 shows the response through the regular inputs. The low frequencies are rolled-off, due to the receiver's default setting of Small Speakers when it is first turned on. The output is 3dB down at 80Hz, optimal for use with a separate powered subwoofer, but this high-pass function can be turned off via one of the AVR-4800's many onscreen menus. But at the higher frequencies, some ripples can be seen in the top audio octave, followed by a very steep low-pass rolloff. Both the ripples and rolloff are due to the fact that unless the Direct option is selected, the AVR-4800 digitizes its regular analog inputs, so that it can perform all the necessary signal processing in the digital domain.
As a result of the digitization, the receiver's reproduction of a 1kHz squarewave (fig.4) features the usual symmetrical ripples on the tops and bottoms due to the digital low-pass filtering. (The sloped nature of the tops and bottoms in this graph is again due to the speaker outputs being set to Small, and should be ignored.) As the digitization cuts off all the harmonics of a 10kHz squarewave other than the fundamental, this signal is reproduced as a sinewave (not shown).
Fig.4 Denon AVR-4800, CD input, small-signal 1kHz squarewave into 8 ohms.
Although the Direct option bypasses the A/D conversion for the regular inputs, I was concerned that the discrete surround inputs, which would be used with a DVD-Audio player, might also be digitized, which would negate the advantages of the higher sample rate and longer word length of the DVD-Audio medium. However, Denon has sensibly arranged that these inputs remain in the analog domain. Fig.5, for example, shows the shape of a 1kHz squarewave when fed through these inputs. It has an excellent square shape, with just a very slight slope on the tops and bottoms, this associated with the bass shelf seen in fig.3. The 10kHz squarewave response through these inputs is similarly excellent (fig.6).
Fig.5 Denon AVR-4800, External input, small-signal 1kHz squarewave into 8 ohms.
Fig.6 Denon AVR-4800, External input, small-signal 10kHz squarewave into 8 ohms.
The AVR-4800's input impedance was a moderately high 44.5k ohms, while its output impedance was 0.11 ohm at low and middle frequencies, rising to a still low 0.16 ohm at 20kHz. There will be little interaction with the loudspeaker's impedance curve. The receiver didn't invert signal polarity. The gain with the volume control set to "00" was 29.8dB, a 1kHz input of 100mV raising 3.1V into 8 ohms (1.3W). With the volume control set to its maximum, the output was 15V into 8 ohms with the same 100mV input, equivalent to a gain of 43.5dB. The volume control operated in steps close to 1dB.
Distortion levels were very low through the External inputs, but rose to around 0.15% through the regular inputs. This appeared to be due not to distortion as such, but to the presence of some low-level ultrasonic ringing. Unfortunately, while I was testing this aspect of the Denon's performance I inadvertently drove the receiver with some high-level, high-frequency tones with the test load set to 2 ohms. The receiver's cooling fan immediately switched on, but both left and right amplifier outputs shut down before I could hit the "kill" command on the test computer's keyboard. All the internal fuses seemed intact, but from that point on I could test only through the center and surround channels.
Driven from the External input, the center channel's small-signal THD+noise was very low at around 0.008%, and was predominantly third-harmonic (fig.7, where the level was 60W into 4 ohms to raise the distortion products out of the noise floor). This can also be seen in fig.8, which shows a spectral analysis of the receiver's output while it drove a 50Hz tone at 140W into 4 ohms. The third harmonic lies at around –83dB (0.007%), the second at –94dB (0.002%). Although a 120Hz AC-supply harmonic can be seen at this high level, this is still 90dB down. Intermodulation distortion was also low in level (fig.9, taken just below visible clipping with this demanding signal).
Fig.7 Denon AVR-4800, External input, 1kHz waveform at 60W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Denon AVR-4800, spectrum of 50Hz sinewave, DC–1kHz, at 140W into 4 ohms (linear frequency scale).
Fig.9 Denon AVR-4800, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 162W into 4 ohms (linear frequency scale).
Because of the apparent damage to the left- and right-channel amplifiers, I did not measure the AVR-4800's output power using continuous tones. However, on low-duty-cycle toneburst tests with just one channel driven (fig.10), the receiver proved to be a powerhouse. No less than 254W were available into 8 ohms (24dBW, black trace) at the usual clipping point of 1% THD (magenta line). This increased to 462W into 4 ohms (23.6dBW, red trace). However, into 2 ohms (blue) and 1 ohm (green), the receiver's protection circuit shut it down before the 1% THD point was reached. Even so, 392W (19.9dBW) were available into 2 ohms, equivalent to 14A! Simultaneously driving the three remaining channels with this toneburst dropped the 8 ohm clipping power to 240W (23.8dBW), which is still well above the AVR-4800's specified power.—John Atkinson
Fig.10 Denon AVR-4800, one channel driven, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).