Replacing the 4.7uF C now with a 22uF one lowers the frequency and amplitude of the noise peak, though it still is there (it is even there with the 220uF cap, if you look closely). This much lower value of capacitance reacts with the 317's rising output impedance and creates a pretty sharp noise peak at about 8kHz. Above plot was made with a 4.7uF Rubycon ZA cap (an ultra-low impedance component similar to Sanyo's OSCON, 540mOhms ESR) substituted for the 220uF one. See, the behaviour of an LM317 depends a lot on the capacitance at its output. All in all quite appaling, this.īut things get even more interesting. Coupling the sense amplifier to my Cyrus One integrated and listening in with AKG K-400 headphones revealed a steady loud roar, quite different in timbre between 317 and 337. What can we say but that output noise is fairly high, about 250 uV RMS over the 20kHz bandwidth,and perhaps more importantly, noise spectra differ markedly between both polarities. Both 317 and 337 were used without adjustment bypass cap. On both rails a 220uF Rubycon ZL capacitor was used, a medium-quality elcap with an ESR of about 75 milli-Ohms. In the above plot the pink trace is the output noise spectrum of our 12V LM317 power supply, and the blue trace is that of the LM337 negative supply. Let's start with a look at the LM317/337 as employed without adjustment capacitor, in which case it comes pretty close to those other 3-terminal stallwarts, the 78-series and 79-series of fixed voltage regulators. The input filter is for ripple suppression only, so we'll ignore it for the time being. The basic circuit dispenses with the R19/C10 input filter, and the C9 adjustment pin bypass capacitor. The above schematic comprises almost anything you can put around the ubiquitous LM317 positive 3-pin regulator (LM337 for negative voltages, 117 and 137 military versions). Simple Voltage Regulators Part 1: Noise - Simple Voltage Regulators Part 1.2: Tests & Plots LM317/LM337 Series Regulator
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