norman Posted November 20, 2019 Posted November 20, 2019 (edited) I decided to bite the bullet and build, not buy, a transformer-fed 12 volt DC regulated linear power supply with half wave rectification for my phono stage to drop the noise floor as far as I can, and get away from the wall wart that came with it. The design is from the "Short Circuits 3" book. I decided to try a kit for the solid state regulator component, and a nice multi-tapped transformer as the AC source. The trafo provides AC +12 V positive, and the rectifier needs at least 2.5 volts more than this to work. How do we get the extra volts out of a 12 volt transformer? The answer it seems is that the AC voltage from the "12 volt" transformer is of course RMS, not peak voltage, which is actually 1.414 x the RMS voltage, so around 17 volts AC at its peak. I have bought a biggish case to fit it all into - plenty of air space and ventilation, and lots of room to keep the high voltage and I presume noisy transformer away from the regulator board. I have already drilled out and fitted stand-offs, and fitting bolt holes, and supply and output fitting openings. Tonight I will start soldering the board and components in place. The regulator only has a few components including a rectifier, 2 diodes, 2 capacitors, and a resistor. I do have a question - the supplied smoothing capacitor is a 1000 uF 25 volt polarised electrolytic in the design. Would I be better to substitute a bigger capacitance but same 25 volt value smoothing capacitor?- I can get a 2200 uF 25 volt low ESR cap or a 4000 uF ordinary cap that will fit right in? I understand that regulator circuits like low ESR components best, so is the 2200 uF part better? Or should I stick to 1000 uF and get a low ESR cap? The lower the ESR, the lower the ripple and therefore noise. I have seen similar pre-made regulator boards\ with 4000 uF caps, hence my question. Another question - is it OK to twist wires pairs together? For example the wires that will lead from the regulator board to the output connection? Would this have any impact on noise? Thanks in advance. Edited November 20, 2019 by norman
andyr Posted November 20, 2019 Posted November 20, 2019 (edited) Good on you for wanting to 'improve' your phono stage, @norman - however, there are a couple of 'warning signals' in what you've written, AFAIAC. 1. What voltage output are you aiming for (ie. what voltage does your phono stage want)? .... I ask this because 12v (in a phono stage) is low - this doesn't provide much in the way of headroom for the output, to cope with the increased output signal resulting from transients & crescendos. .... If it's 18v - then your 25v caps are fine. .... but if it's higher then I would suggest you need to use higher-voltage electros. 2. To minimise noise leaking into the power supply from the power transformer, I suggest you should do 2 things: .... a. isolate the power transformer with a steel case-divider .... b. put a case-earthed shield over the secondary wires going to the rectifiers. 3. Multiple smoothing caps would be better than just one - as this reduces ESR. If the PCB doesn't allow for this ... yes, a 2200uF low-ESR cap would be better than a 1000uF one. Good luck! And definitely twist pairs of wires together, for minimum noise. BTW, I would bet that using an SLA bank with, say, a 47uF film cap across the output ... would give you lower noise than any mains-powered regulated PS. What you then have to provide is a charger connection on the back of the case ... and a digital voltage display on the front. I use: a single 12v SLA in the AKSA 'Paris' head amp I sell - see here: 2x 12v SLAs, to deliver 24v (for each channel), in my "Muse' phono stage: Andy Edited November 21, 2019 by andyr
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