Small and quite +/-8v power supply/regulator - Page 1
Forum section: electronics / projects

author: CuriousAbe
author_rank: Newbie
timestamp: July 24, 2019, 09:37:05 pm
content: So I've come pretty far in my summer project to clone a certain old analogue drum machine and now it's time for the power supply.The original unit is powered by a good ol transformer, 2x 2200uf caps, 2x 10uf caps, a full bridge rectifier and a 7808 and 7908 voltage regulators, needless to say it's pretty noisy.For various reasons I have decided not to supply the kit it with a linear psu and a transformer involving mains wiring.So I'm looking for a reasonably small and quite +/- 8v supply/regulator design, requirements:*Must fit on a board that is max 145x45mm (50mm height)*+/- 8v output*Around 200-300ma output*No mains wiring, preferably input should be with a ac/dc power adapter (ac/ac works too)*Switching design is fine by me as long as it's reasonably quite*Preferably all through hole components

author: ogden
author_rank: Super Contributor
timestamp: July 24, 2019, 10:02:10 pm
content: Why don't you like original power solution of linear supplies? Don't spoil your *analog* build with switching supplyYou can't avoid mains because wall plugs or brick supplies, usually are single output. You could use power module, but it requires AC wiring & AC fuse holder: AC->DC two output +/-12VDC *module* like Raccom RAC06-12DC. Obviously with two linear regulators behind it. Don't use prehistoric 7808 and 7908. Use LT3045 and LT3094 instead. Pricey but low noise as you wished, will not regret that choice.

author: iMo
author_rank: Super Contributor
timestamp: July 24, 2019, 10:25:55 pm
content: LM317/337 are cheap and quiet too (with a capacitor in the divider).

author: CuriousAbe
author_rank: Newbie
timestamp: July 24, 2019, 10:28:41 pm
content: Quote from: ogden on July 24, 2019, 10:02:10 pmWell I will be selling the clone as a kit (pcb's+case) and I don't want people who are not comfortable handling mains wiring doing so, and I know people who are not comfortable with mains will be buying this kit.Yea I was thinking on the small module route, however there seems to be a gazillion different small ac/dc modules to chose fromWill keep the LT3045 and LT3094 in mind!Another idea is to make something like this, maybe I could redesign it to use the LT3045 and LT3094 regulators instead.http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.phpInput is a 12VAC power adapter (problem here is that these are not that usual to come by for cheap in europe).

author: ogden
author_rank: Super Contributor
timestamp: July 24, 2019, 10:42:03 pm
content: Quote from: CuriousAbe on July 24, 2019, 10:28:41 pmDefinitely overkill for DIY kits. Better use mentioned LM317/337.QuoteWhy your specified current is so high, 200mA? Anyways for 12VDC power adapter (wall-wart) there's solution for negative rail: isolated DC-DC 12V->12V converter like Murata NXE2S1212MC-R13Through hole: SPU01M-12 from MeanWell, but it's 80mA only, very noisy on both sides, input & output. Make sure to use LDO's with good PSRR.

author: langwadt
author_rank: Super Contributor
timestamp: July 24, 2019, 11:01:51 pm
content: Quote from: imo on July 24, 2019, 10:25:55 pmafaict from a quick look at datasheets LM317 is much higher noise than LM7808

author: mariush
author_rank: Super Contributor
timestamp: July 24, 2019, 11:26:44 pm
content: You can get PCB mounted transformers, here's an example:https://www.tme.eu/en/details/bvei4811118/pcb-transformers/hahn/bv-ei-481-1118/5$ for two 9v secondaries, in 68.2x42.3x34.6mm package. Leaves you plenty of room for a SIP style bridge rectifier (GBU/GBJ etc) , two regulators and a couple capacitors. You could place rectifier and the two regulators on same heatsink even.Alternatively, why not just add a switching regulator in inverting mode .. for example aLM2574should do -8v at 0.2..0.3A with something like 18..24v input (like a laptop adapter). You can add a pi filter on output to get the output smoother. Easy to use DIP package, low 52kHz frequency so normal through hole inductors and electrolytics, good documentation in datasheet.Or LM2596 if you want 150kHz and smaller/cheaper inductors and diodes :https://www.onsemi.com/pub/Collateral/LM2596-D.PDF- page 17 of datasheet even shows example circuit for -12v @ 0.7A with 12v+ input voltage.They're not expensive, 2$ each in 1pcs, 1.6$ if you get 100...https://www.digikey.com/product-detail/en/on-semiconductor/LM2596DSADJG/LM2596DSADJGOS-ND/2003290Inductor, diodes etc would probably cost 2$ more.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 24, 2019, 11:48:31 pm
content: Quote from: langwadt on July 24, 2019, 11:01:51 pmThe 7808 may have a bit better output noise figure, but the LM317 has better ripple rejection (with the appropriate capacitor), and with a typical noisy input power such as here coming from rectified AC, this is usually what matters most, especially for audio applications.Another argument is that the 7808 is flagged obsolete (and often unavailable) from the usual distributors...

author: ogden
author_rank: Super Contributor
timestamp: July 25, 2019, 12:05:13 am
content: Quote from: langwadt on July 24, 2019, 11:01:51 pmYes you are right. @imo:Well.. LM7808 is obsolete. Modern version is UA78M08, 40uV RMS noise. LM317 have whooping 240uV RMS. MC7808 (from ON semi) have 10uV RMS.Always doublecheck info from public forum where anyone can convincingly say anything.

author: mikerj
author_rank: Super Contributor
timestamp: July 26, 2019, 11:43:42 am
content: Quote from: CuriousAbe on July 24, 2019, 09:37:05 pmIn what way noisy? A linear supply should be far quieter than a switching one, do you have a problem with dead capacitors causing high ripple or oscillation?

author: CJay
author_rank: Super Contributor
timestamp: July 26, 2019, 11:49:11 am
content: I did a bit of reading about regulators when building a power meter with an AD8307, what I found was quite surprising to me, the 78xx and 79xx devices stack up pretty well against 'low noise' regulators, are you absolutely certain it's the regulators causing the problem and if you are, are you absolutely certain it's not your implementation?

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2019, 03:54:24 pm
content: The LM7808 and LM7908 should be quiet enough. If it's noisy, then it's because it's a bad design or there's a faulty component.Replacing the LM7808 and LM7909 power supply with something else will make no difference.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 26, 2019, 04:45:16 pm
content: Quote from: ogden on July 25, 2019, 12:05:13 amWell, imo's comment was quick and inaccurate, but as I said above, there's still a reason why the LM317/337 was a favorite in many audio applications compared to the 78xx line, so maybe that's what he was remembering.Yes the typical RMS output noise of the LM317 is higher (although I think they don't state a max value in many datasheets, whereas the max noise of the original 78xx, 200µV, is close to the typical of the 317...), so imo's short statement was wrong. But again ripple rejection was typically 8dB better in the LM317. Ripple noise (often typically harmonics of mains frequency) would be much more annoying in many audio applications compared to a linear reg's own noise, which is more wideband. So the choice would depend on the specific application and of course on the PSRR of the powered circuitry.I think the LM317 load regulation was also slightly better. So all this was only to be considered maybe 2 decades ago or more.Either way, for this level of noise to matter, the analog circuit would have to have high gain, and poor PSRR. Otherwise it would not be noticeable.But yes there are now dozens of alternative regulators for this. The UA78M08 has better specs than the original LM7808, and similar ripple rejection than the LM317, and it's cheap, so it's better overall. It can put out 500mA only, but that would be a good fit in the OP's case.To get back to what the OP said, they would have to explain what "noisy" means. If it's a matter of ripple noise, then yes ripple rejection would matter, but if it's so noisy as to be noticeable, a few dB better ripple rejection would probably not make a lot of difference. Maybe they could just post the original schematic. I suspect the input (rectified) voltage was very poorly filtered, or something like that. Or maybe the layout is a disaster. Or probably both.

author: iMo
author_rank: Super Contributor
timestamp: July 26, 2019, 05:13:48 pm
content: What is the rms noise of the LM317/337 with the capacitor (say 100u) in the divider (as in my post)??

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 27, 2019, 04:16:30 pm
content: Quote from: imo on July 26, 2019, 05:13:48 pmFrom memory, and then taking a look at the datasheet again, it was mainly to improve ripple rejection (what I'm talking about since first post), whereas ogden was talking about the RMS output noise voltage figure, which I take it as the regulator's own noise, which is a separate source of noise from ripple. That's how I interpret those figures anyway. I haven't seen anything regarding the RMS output noise figure with this additional capacitor in the DS I have. Do not hesitate to show the info if you have it.The optional cap between the ADJ pin and GND is indeed to improve ripple rejection as I stated. I'm not sure how it affects the regulator's own noise. I admit this figure is not necessarily trivial to understand, and I'd be interested in seeing what kind of setup is used to characterize it independently from noise coming from the input voltage (haven't seen the test setup in the DS I looked at.)As I already said, ripple noise in a typical PS stage coming directly from rectified AC (so often horrendous ripple even with big caps) would often matter more than the typical linear reg's own noise, especially for audio stuff, because its own noise is more wideband and thus more spread out spectrally.Now there is probably some dependance between the two sources of noise, so again that would be nice to see how exactly they are characterized (if anyone has the info).And of course if I have missed anything, do not hesitate to point that out.

author: iMo
author_rank: Super Contributor
timestamp: July 27, 2019, 06:04:29 pm
content: There are hundreds of posts on LM317/LM337 noise at, for example, the diyaudio forum.The 240uVrms somebody here posted comes from the DS (ie. TI), where it states the rms value (10Hz to 10kHz) is 0.003% of the output voltage.With a blocking capacitor at the divider the noise is much lower, for detailed measurement results please read the above forum, for example.

author: Gyro
author_rank: Super Contributor
timestamp: July 27, 2019, 06:23:54 pm
content: @CuriousAbe:You could usefully add a 10nF ceramic cap across each of your rectifier 1N4004s (CR1 and CR2) to snub any recovery induced switching noise. In sensitive audio circuits, this can manifest as a twice mains frequency low level buzzing.

author: mariush
author_rank: Super Contributor
timestamp: July 27, 2019, 09:08:18 pm
content: Quote from: imo on July 27, 2019, 06:04:29 pmLM1084 advertises the same 0.003% of Vout RMS output noise (page 6):http://www.ti.com/lit/ds/symlink/lm1084.pdfIf you don't mind less than 200mA of current, an option may be ADP7118 with 11uV rms :https://www.digikey.com/product-detail/en/analog-devices-inc/ADP7118ACPZN-R7/ADP7118ACPZN-R7CT-ND/6568642

author: CuriousAbe
author_rank: Newbie
timestamp: August 11, 2019, 01:30:04 pm
content: Thanks for all the help and useful suggestions!I will probably go with some variation of below since it seems pretty much perfect for what I'm after:https://www.diyaudio.com/forums/power-supplies/340731-lm317-lm337-power-supply-pcb.html