Sidebar 3: Measurements
I measured the Air Tight ATM-300R with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Before doing any testing, I checked the ATM-300R's front-panel meter to ensure that the bias was correctly set for the two 300B output tubes.
The Air Tight's maximum voltage gain into 8 ohms from its High (8 ohm) output-transformer tap, which was how Art Dudley listened to the amplifier, was a high 29.5dB; from the Low (4 ohm tap) it measured 26.9dB. Both output taps preserved absolute polarity (ie, were non-inverting). The input impedance was a usefully high 78.5k ohms at low and middle frequencies, dropping to a still-high 54k ohms at 20kHz. The output impedance from the 4 ohm tap was low for a single-ended tube design, at 0.84 ohm at 1kHz, rising to 1 ohm at 20Hz and 20kHz. From the 8 ohm tap, the output impedance was 1.2 ohms at 1kHz and 2 ohms at 20Hz and 20kHz, these values still low for this kind of amplifier. As a result, the response with our standard simulated loudspeaker varied by ±0.8dB (fig.1, gray trace).
Fig.1 shows that the ATM-300R's audioband response is flat up to 10kHz, with a small amount of rolloff in the top octave reaching –0.3dB at 20kHz into 8 ohms (blue and red traces), but –1.8dB into 2 ohms (green). The top-octave rolloff was lower from the 4 ohm tap, but now a low-level peak was evident around 150kHz, this associated with a small amount of overshoot or ringing with a 10kHz squarewave from this tap (fig.2). A 1kHz squarewave was reproduced with flat tops and bottoms from both taps (fig.3), suggesting excellent output-transformer design.
Fig.1 Air Tight ATM-300R, 8 ohm tap, 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 (red) (1dB/vertical div.).
Fig.2 Air Tight ATM-300R, 4 ohm tap, small-signal, 10kHz squarewave into 8 ohms.
Fig.3 Air Tight ATM-300R, 4 ohm tap, small-signal, 1kHz squarewave into 8 ohms.
The ATM-300R's channel separation, measured at a level close to clipping into 8 ohms, was 87dB L–R and 77dB R–L at 2kHz, but these decreased at the frequency extremes (fig.4). The reduction at high frequencies is presumably due to capacitive coupling between the channels somewhere in the circuit; at low frequencies, it suggests an increasing power-supply impedance as the frequency drops. The wideband, unweighted signal/noise ratio, ref. 2.83V and measured with the input shorted to ground but the level controls set to their maximum, was a moderate 71dB in the left channel and 67dB in the right. These ratios improved slightly, to 71.3dB, when the measurement bandwidth was restricted to the audioband, and to 87.5dB when A-weighted. Spectral analysis of the Air Tight's noise floor (fig.5) revealed spuriae at 60Hz and its odd- and even-order harmonics. All of these spuriae are sufficiently low in level that they will not be audible as hum; also notable in this graph is the absence of the low-frequency random noise that is often present with tube designs.
Fig.4 Air Tight ATM-300R, channel separation ref. 6V into 8 ohms (L–R blue, R–L red, 20dB/vertical div).
Fig.5 Air Tight ATM-300R, 8 ohm tap, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).
The ATM-300R is specified as being able to deliver 8Wpc (9.03dBW), presumably with the load matched to the nominal output-transformer tap. With "clipping" defined as when the THD+noise reaches 1%, fig.6, taken from the 8 ohm tap with both channels driven, indicates that the Air Tight clipped at 3.65Wpc into 8 ohms (5.6dBW). Relaxing the definition of clipping to 3% THD+N, the Air Tight clipped at 9Wpc (9.54dBW). Into 4 ohms (fig.7), the ATM-300R's 8 ohm tap delivered 4.5Wpc (6.5dBW) at 3% THD+N. From its 4 ohm tap, the ATM-300R clipped (3% THD+N) at 9W into 8 ohms (fig.8) and 4 ohms (not shown). Figs. 5–8 reveal that the distortion rises linearly as the power increases, which is typical for an amplifier with a single-ended output stage; however, the distortion at low powers is higher than that of the single-ended but solid-state First Watt SIT-3 reviewed elsewhere in this issue.
Fig.6 Air Tight ATM-300R, 8 ohm tap, both channels driven, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.7 Air Tight ATM-300R, 8 ohm tap, both channels driven, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.8 Air Tight ATM-300R, 4 ohm tap, both channels driven, distortion (%) vs 1kHz continuous output power into 4 ohms.
Like that First Watt amplifier, the Air Tight ATM-300R offered levels of distortion that remained relatively constant with frequency but that increased into lower impedances (fig.9). The rise in THD+N at low frequencies was small, which again suggests an excellent output transformer with a sufficiently hefty core. This graph was taken from the 8 ohm tap; distortion levels from the 4 ohm tap (fig.10) were just half those from the 8 ohm tap.
Fig.9 Air Tight ATM-300R, 8 ohm tap, THD+N (%) vs frequency at 1V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (left gray).
Fig.10 Air Tight ATM-300R, 8 ohm tap, THD+N (%) vs frequency at 1V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (left gray).
As with the First Watt SIT-3, the Air Tight's distortion was predominantly the sonically benign second harmonic, but this time in phase with the fundamental regarding the positive-going half cycles (fig.11). The third harmonic was around 22dB lower in level than the second harmonic (fig.12), but it both increased in level and got closer to the level of the second harmonic as the output stage was asked to deliver more current (fig.13). Tested with an equal mix of 19 and 20kHz tones at a low power into 8 ohms, the amplifier produced levels of higher-order intermodulation products that were relatively low (fig.14), though the second-order difference product lay close to –50dB (0.3%).
Fig.11 Air Tight ATM-300R, 8 ohm tap, 1kHz waveform at 1Wpc into 8 ohms, 0.44% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.12 Air Tight ATM-300R, 8 ohm tap, spectrum of 50Hz sinewave, DC–1kHz, at 1Wpc into 8 ohms (linear frequency scale).
Fig.13 Air Tight ATM-300R, 8 ohm tap, spectrum of 50Hz sinewave, DC–1kHz, at 2Wpc into 4 ohms (linear frequency scale).
Fig.14 Air Tight ATM-300R, 8 ohm tap, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1Wpc peak into 8 ohms (linear frequency scale).
The relatively high level of second-harmonic distortion comes with the territory in an amplifier having a single-ended output stage. Otherwise, Air Tight's ATM-300R appears to be well engineered. But it does need to be used with a loudspeaker with an impedance that doesn't drop below 8 ohms, like the DeVore Fidelity Orangutan O/93 that AD used for his auditioning.—John Atkinson
