Measuring Power Supply Ripple and Hum

Building Valve Amplifiers There are many packages that allow computers to perform the FFT. Beware that when you use these, you rely on the linearity of your sound card. The built-in audio stages of a com- puter are not usually very good, and you need a recording-quality sound card, preferably 24 bit, 192 kHz sampling frequency, or better. Measuring power supply ripple and hum Since almost all valve electronics is mains powered, there is always the possibility of mains hum and its harmonics entering the audio signal. Full-power distortion spectra of power amplifiers often reveal power supply sum and difference frequencies around each distortion harmonic. Thus, a 1 kHz distortion measurement might have a 2 kHz harmonic plus lower-level 1.9 and 2.1 kHz frequencies because 1.9 kHz 5 2 kHz 2 2 3 50 Hz (the factor of 2 occurs because full-wave rectification in the power supply produces ripple at double mains frequency). More commonly, we look for hum in the absence of signal, and we typically begin by measuring across the amplifier’s output terminals with the input either short-circuited or terminated in the expected source resistance. The reason that a short circuit might be used is that we want to discriminate between hum generated within the amplifier and hum picked up by external cabling. A digital oscilloscope is the best instrument for measuring hum, although its sensitivity may need to be increased by preceding it with a high-gain amplifier À an audio test set or AC millivoltmeter having a monitoring output is ideal. The reason that a digital oscilloscope is best is that good audio design/construction should have its hum buried in the noise, making it difficult to measure. However, if we trigger the oscilloscope from “line” and invoke “averaging”, we can average out the random noise to leave the repetitive hum, then use an automated measurement to quantify that hum. Having found hum (usually at a higher level than we would like), we want to reduce it, so we need to determine its cause. If the hum is the same as mains frequency (50 or 60 Hz), it is probably caused electromag- netically, and due to transformer induction or a hum loop. If waving a sheet of steel between a power amplifier’s mains transformer and audio transformer changes the waveform’s shape, then induction is likely and the only real cure is to move the two 470 Measuring power supply ripple and hum transformers further apart. A less likely possibility is poor electrostatic screening at the amplifier’s input, and if waving your hand near the input changes the waveform, improve the screening either by eliminating gaps in the screen or reducing the impedance bonding it to chassis. If the hum is double the mains frequency (100 or 120 Hz), it is almost certainly power supply ripple due to inadequate filtering. Although an oscilloscope and associated probe can measure millivolts of HT ripple simply by engaging AC coupling at its input, we might need to measure microvolts of HT ripple without destroying our audio test set. There are various ways in which the sensitive input of the audio test set may be safely connected to the HT, and they all require the HT to be switched off before connection. See Figure 6.28. 330nF 3.3uF 6.8uF 630V 630V 630V I/P I/P O/P to O/P to O/P to I/P 1M 10k 10k test set test set 6.8uF test set 630V (a) (b) (c) Figure 6.28 Connecting a delicate audio test set to the HT to measure hum. Capacitor coupling: This is a single-ended measurement and prone to inaccuracy due to earth loops on the input leads. The capacitor needs to have a DC rating greater than the HT and sufficient capacitance in conjunction with the tests set’s input resistance to keep f23dB, 20 Hz. Given that the test set’s input resistance might only be 100 kΩ, we probably need a 330 nF 630 V capacitor. Some test sets have an input capacitor to protect them from DC, but the capacitor is likely to be rated at only 100 V. The 1 M resistor ensures that all the input DC appears across the capacitor that is rated for it. Single-pole capacitor coupling plus 1:1 line-level audio transformer: This breaks earth loops, and the transformer’s common-mode rejection ratio (CMRR) assists in a quiet measurement. However, if inadvertently connected the wrong way round, although the HT is blocked from appearing across the primary (vaporising it), the HT could appear across the interwinding insulation, and break that down instead. The transformer 471 Building Valve Amplifiers probably needs to be loaded by 10 kΩ to damp high frequency ringing, so the DC blocking capacitor now needs to be 3.3 μF 630 V. Double-pole capacitor coupling plus 1:1 line-level audio transformer: This avoids the cross-connection hazard but requires the primary to have a centre tap that can be earthed to prevent the HT appearing across the transformer’s interwinding insulation. It also restores the earth loop. The two DC blocking capacitors are in series, so they each need double the previous capacitance À we now require a pair of 6.8 μF 630 V capacitors. These are big capacitors, engendering ingenious engineering justifications for why a particular application did not need such large capacitors and why the smaller (but avail- able) capacitors were adequate. When pressed, the author uses a pair of inconveniently large 6 μF 2 kV polypropylene capacitors, but usually uses a pair of much smaller 10 μF 250 V polyesters. The ripple on .500 V supplies is usually so large that it can be measured by an oscilloscope via a high- voltage probe. As WWII flying ace Douglas Bader quoted, “Rules are made for the guidance of wise men and the obedience of fools,” but because he lost both legs as the result of a flying accident whilst showing off, his opinion carries reduced weight. The real value of a rule is to make you think before you break it. Measuring noise We measure noise at the amplifier’s output in the absence of an applied signal but under all other conditions of use. Thus, we either terminate the input of the amplifier with its expected source resistance or a short circuit, and to avoid gain errors due to non-zero output resistance, we load the amplifier’s output with its expected load resistance. The classi- cal assumptions are that noise is random, has a Gaussian distribution, and is white (constant power with frequency), so it doesn’t matter whether we measure a noise band- width of 20 kHz at 500 kHz or 500 MHz. 1/f noise means that the white assumption may not be exactly true for audio, but it is a good place to start. Remembering that: . The mean is the DC component. Any deviation from the mean is the AC component. 472.

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