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I have two boards (microphone amplifier and speaker driver) that require +/-15 VDC. The max current required for the boards is 500 mA. Since low-noise dual power supply devices are hard to come by, I'd like to modify my system to accept a single power supply. Ideally, what I'd like to do is design the following chain:

  • Input from the AC mains would be via a high-end SMPS (with switching frequency in the MHz range) with an output of 36 VDC.
  • A virtual ground would be provided using a TLE2426. A current boosting BUF634 feedback loop to get up to 250 mA as described in https://tangentsoft.net/elec/vgrounds.html would be used. Note that this can be stacked with a second BUF634 to get up to 500 mA. This would provide +/-18VDC rails.
  • I would then use LT3090 and LT3045 LDO regulators to further drop the voltages to +/-15VDC rails that feed into the board. The LDO regulators are very high PSRR devices with ultra low noise outputs.

Questions:

  • To minimize cross-talk between the microphone amplifier and speaker drivers, each device should have their own power rail splitter and LDO regulators (this is also necessary because of the max current capabilities of of the regulators and buffers). Can they share a common SMPS, or should each have their own?

  • Should this setup work well for powering devices that work in the frequency range 100 Hz to 100 kHz (this is for auditory research experiments at a university, so we have to deal with frequencies above the audible range of humans as well)?

  • An alternate approach would be to use the TPS7A47 ultra low noise LDO (which can accept up to 36 VDC input). In this case, I could then use the following chain: SMPS -> TPS7A47 -> TLE2426 + BUF634 -> board. However, would this be OK or would there be higher noise in the circuit due to placing the TLE2426 + BUF634 after the LDO?

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  • \$\begingroup\$ This does not make much sense. A pair of BUF634 plus heat sinks will cost more than a small transformer and a +/-15V linear supply. Low noise regulators are not relevant for powering opamps which have huge PSRR anyway... \$\endgroup\$
    – bobflux
    Nov 30, 2017 at 20:27
  • \$\begingroup\$ Do you have one in mind? I looked on Mouser and couldn't find a+/-15V linear AC DC converter for less than $60. Even then, the dual-rail linear power supplies are "open-frame" (meaning I need to cut off the end of an AC cable, wire it into the supply, and build a chassis for it). I'm not willing to take the risk of working with AC current directly. There used to be a good series of linear power supplies that plugged into the wall (e.g., WM113S, Elpacs of old, etc.). However, these all appear to have been discontinued. \$\endgroup\$
    – Brad
    Nov 30, 2017 at 20:56
  • \$\begingroup\$ Is this a certification problem for a commercial product, or a personal project? \$\endgroup\$
    – bobflux
    Nov 30, 2017 at 21:03
  • \$\begingroup\$ No, it's not for a commercial product. It's for some equipment we have in the lab. We build our own circuits for experiments, but have historically purchased high-quality linear regulated power supplies with +/-15 or 12VDC outputs. However, these are becoming increasingly hard to find (even on EBay), so we would like to convert our circuits to single-supply mode (our data-acquisition systems have fully differential ADC and DACs so we're not worried about referencing the signals to earth ground). \$\endgroup\$
    – Brad
    Nov 30, 2017 at 21:19
  • \$\begingroup\$ To clarify, we want to find a good, clean, low-noise power supply for work with our circuits which require +/-12VDC and 500 mA current. We're just not sure what the simplest, most straightforward approach is. We're OK taking the output of some "wall wart" and cleaning it up. We're just not sure the best way to do that. Or, if there's another option we're missing we'd love to hear it! Also, note that all circuits are AC-coupled (DC is filtered out). \$\endgroup\$
    – Brad
    Nov 30, 2017 at 21:21

1 Answer 1

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To clarify, we want to find a good, clean, low-noise power supply for work with our circuits which require +/-12VDC and 500 mA current. We're just not sure what the simplest, most straightforward approach is.

Okay!

The simplest is a linear power supply: IEC mains filter if required, transformer, rectifier bridge, caps, and a regulator. Really oldskool.

For your measurement stuff (and microphone preamp) the best is to have low interwinding capacitance in the transformer which reduces 50Hz leakage current through your grounds. For this the best is split bobbin EI core, which conveniently is also the cheapest... Toroid has less magnetic flux leakage but if you shove the power supply further away on your lab bench from the sensitive bits then flux leakage does not matter.

OK, you say you don't want to work with mains, but it isn't a problem if you do it right, make sure there is no easily exposed copper at high voltage. If you use a PCB mount transformer, just tape a nice thick piece of photocopier transparent on the back of the PCB. I always do this. The point is that component pins do not puncture it because it is thick. So if a finger wanders in the hot zone... it is safe. If you use a chassis mount transformer, use heat shrink. Or put the thing in a box.

"Low noise" power supplies are not necessarily worth the hassle either. For example, if you use a good old LM317 you can expect a few mV ripple on the output, at frequencies where your opamps will have 80dB PSRR or more...

Since you mention TPS7A47: this is a very low noise regulator... but if the load is opamps, ripple rejection matters more than noise. Opamps have high PSRR: 1µV noise or 100µV noise on the supply matters little if the opamp has, say, 60dB PSRR, which is a factor of 1000.

Usually you will have a few critical bits of the circuit which require a very clean supply but draw low current (like the microphone preamp) and other parts of the circuit which will work fine with a less clean supply but drive high current (like the loudspeaker driver).

You could build a very complicated, low noise power supply like you suggest. But it would still have a non-zero output impedance, if you include the wires and everything. High currents drawn from the power amplifier would thus influence its output voltage.

A much simpler option is just stick some RC filters with large capacitors on the supply of the really sensitive circuits. Or a local regulator. If your microphone preamp draws 10mA, just put a 1000µF low-ESR cap and a 100R resistor. You lose one volt... it costs 1€...

Please understand that the whole point of modern fancy regulators like LT3045 is that they are fast. This means they work with low output capacitance, which allows a small footprint, low height solution which you do not care about. This also means they have good HF PSRR which you do care about. However you can achieve the same with a passive filter. These regulators are not magical, just another engineering tradeoff (space, cost, height, etc).

Hm... I kinda went on a tangent here...

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  • \$\begingroup\$ Hmm. This is great information, and it sounds like I may be worrying too much about LDO noise where a single RC filter would suffice. Is it possible to push the cutoff frequency too low? For example, can I use 1000 uF with a 1kOhm resistor for a cutoff frequency of 0.16 Hz? It sounds like you still recommend building a DIY power supply off of the AC mains. I'm not too keen on that but will do a little more research. \$\endgroup\$
    – Brad
    Dec 1, 2017 at 20:10
  • \$\begingroup\$ Well if you use 1kOhm on your 15V supply and the opamp draws 10mA there won't be a lot of volts left... Point is, LM317 and other oldskool regulators have very good low frequency rejection, but crummy HF rejection. So a RC filter is a good complement since it provides the exact opposite. You can also put two LM317 in series (like 18V then 15V) if you're extra paranoid. \$\endgroup\$
    – bobflux
    Dec 1, 2017 at 20:17
  • \$\begingroup\$ The biggest advantage of the linear PSU is absence of HF noise, especially common mode, which is harder to get rid of. I'd just recommend picking a readymade schematic for a LM317 based supply off the net. Also you can heat sink a TO220 regulator, not so much for TPS7A47... \$\endgroup\$
    – bobflux
    Dec 1, 2017 at 20:21
  • \$\begingroup\$ Good point. I wasn't thinking about the voltage drop across the resistor under load (i.e., when drawing current). As for your second point, are you advocating for a DIY supply that connects to AC mains, or just telling me to use a LM317-based circuit to clean up whatever DC power supply I use? \$\endgroup\$
    – Brad
    Dec 1, 2017 at 20:28
  • \$\begingroup\$ You can do both but if you need +/-15V 500mA it's easy enough to make a PCB with a a 20-25VA pcb-mount transformer, diodes, caps and regulator... \$\endgroup\$
    – bobflux
    Dec 1, 2017 at 20:38

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