Audio circuit, replacing linear supply with SMPS, increasing voltage. - Page 1
Forum section: electronics / beginners

author: Chris935
author_rank: Newbie
timestamp: March 10, 2020, 09:24:24 pm
content: I have an audio project that originally ran from a small linear supply using 7815 regulators. I am not comfortable with re-building the mains supply because of physical constraints regarding the chassis/mounting, and intend to supply power from a wall-wart SMPS. It's a BSS AR416 DI box, intended to take high-impedance un-balanced signals and turn them into low-impedance balanced outputs. It uses a transformer at each output.I understand that the SMPS will introduce some noise not present in the original design, and I intend to leave the filter capacitors in place with a resistor in front of them to produce a significant RC filter, and also leave the linear regulators in place. Frequency response graphs for linear regulators show decreased performance at higher frequencies, but my feeling is that I'll take whatever improvement I can get, unless I'm wasting my time (and the 2V).I am also interested in running this at a somewhat higher voltage than the original circuit, but don't have too much scope to add components with the existing PCB layout. In Doug Self's "Small Signal Audio Design" he mentions a technique of applying a small voltage back to the ground pin of a linear regulator so it's sitting above the actual ground. In his example this is done with resistors, which he mentions has the downside of this voltage changing as the current draw changes. My idea is to use a Zener diode to do this, as it should produce a more consistent voltage drop as current fluctuates. Is this correct? Am I potentially introducing distortion here? I would intend to implement this by snipping off the centre pin on the linear regs, soldering the zeners to the tab, and using them as the ground pins.The reason for the voltage increase is to provide more headroom before distortion, and to bias any distortion that does occur towards the transformers rather than the op amps. My idea is that when it does begin to distort I want it to be transformer distortion with the op amps still having some headroom left, just so it's less objectionable sounding and with a more gradual onset.I have temporarily given the circuit 30V in order to get a 26V pk-pk signal to clean up (at 1KHz). Nothing caught fire or had more than it's rated voltage across it, though this brings up power concerns about the transformer. It's rated for 100mW, and at twice the voltage it'll be getting four times the power, won't it? If the audio signal has a high crest factor then this is fine, headroom for the peaks and safe RMS levels, but the pessimist in me says I should make it safe to drive with square waves. How bad an idea is it to put a thermistor in front of the transformer? It's additional non-linearity by definition, but with a relatively slow time constant, it wouldn't be "riding the waveform". I don't think I would actually go near 30V, pending testing at lower frequencies, but the question still stands. The transformers are one OEP a262a2e and three STC 66122x (probably interchangeable and just what they had in stock).I had the output terminated at 600R as a worst-case load, so it was doing 567mW but passing a clean 1K sine wave, more of an issue at lower frequencies? I notice that if I ran this at the original 15V it would only do 11V pk-pk, which as a sine wave would produce 101mW into 600R... Real world loads will be more like 2K to 10K, but I wanted to see how it would handle seeing an old piece of 600R gear. I'm not sure if the maths applies the same way with it not being a resistive load.If the answer is that I should play about with the higher voltages for learning purposes, but run it close to the original 15V for actual use then I'm happy just to do that.Any comments are very welcome.