The Cool Sound of Tubes
Distortion under test
Since much of the rationale for the continued use of tubes in audio equipment is based on distortion and noise, we decided to compare how several representative tubes and transistors performed in this regard. Using basic circuit designs in which these devices are typically used, we tested two tubes against four types of transistor--respectively, a medium-mu triode and a pentode against a low-voltage bipolar transistor, a low-voltage junction FET (JFET), a high-voltage bipolar transistor, and a high-voltage MOSFET. The figures show the distortion and noise spectra of each device.
All the measurements were made with an Audio Precision System 2 with its analog oscillator set to 1 kHz. The oscillator output level was adjusted to give an rms voltage of 2.00 V at the output of the test device. The output impedance of the oscillator was either 20 (omega) or 600 (omega), depending on the device under test.
The test device was monitored by the System 2's fast Fourier transform (FFT) function using its 20-bit analog-to-digital converter running at 48 kilosamples per second. The FFT was synchronous (no windowing), and was averaged over 16 samples. The residual harmonics were at least 120 dB down. The input impedance of the System 2 was set to 100 k(omega). Both input and output were floating, with the test fixture separately grounded to the System 2.
In the vacuum tube test fixture, the filament voltage was dc, regulated to 6.3 V. The output of the tube was monitored through a 10:1 compensated divider so as to reduce ac and dc loading.
The summary of second-harmonic distortion levels follows (click on any of these images to enlarge):
6AU6A pentode: Noise floor is about 120 dB below the fundamental; second harmonic is 48 dB down [red curve, above]. Operating conditions were taken from the resistance-coupled amplifier tables in RCA tube manual RC-21, 1961, p. 438.
Click on image for larger view.
2N5457 low-voltage junction FET: Unlike the high-voltage MOSFET, the JFET has excellent noise performance (-140 dB) but poor distortion (second harmonic is only 30 dB down). Because of the lower I dss of available junction FETs, the drain resistor was raised to 10 k(omega); the bias was adjusted to give about 1/2 V DD at the output.
Click image to enlarge.
IRF822 high-voltage enhancement MOSFET: When substituted for the 6SN7GTB, with bias adjusted to give the same operating point as the tube, the MOSFET exhibited excellent distortion characteristics, which were compromised by its noise floor of -100 dB--about 30 dB above the tube's. Second-harmonic distortion is 41 dB down, which is only 59 dB above the noise.
Click on image to enlarge.
Click on image to enlarge.
Although this is not intended to be an exhaustive examination of all available semiconductors or tubes, the resulting frequency spectra lead us to some conclusions that experienced audio designers have often remarked upon in the past.
- Transistors operating on low-voltage supplies tend to have higher spectral distortion components than tubes.
- If we go to high-voltage transistors, operating on supplies comparable to those of the tubes, the distortion products are less objectionable. Unfortunately, the noise floor of such devices is much higher. The IRF822 was very triode-like in distortion yet suffered from a noise floor some 30 dB higher than that of the triode.
- No other active device possesses both the low distortion products and the low noise floor of the medium-mu triode--albeit at the expense of voltage gain.
- The distortion products of transformers are much lower than those of active devices, yet quite different in character. Note that the odd-order harmonic products tend to be higher in level than the even-order products--exactly the reverse of the tubes and transistors.
It should be obvious that these simple circuit designs can be improved upon, by using differential topologies with constant-current loads and negative loop feedback.
It should also be obvious that the same techniques can be applied to transistors or to tubes; and if this were done, the triode would continue to enjoy some advantages over the semiconductors--and the pentode, for that matter.
--E.B. with John Atwood
John Atwood is a consultant on tubed audio design and owns One Electron Co., Santa Clara, Calif.