+/-5V DC from a 9V DC power addapter - Page 1
Forum section: electronics / beginners

author: jwhitmore
author_rank: Regular Contributor
timestamp: July 29, 2020, 08:23:40 am
content: Hello all,I started messing with audio stuff, guitar effects so nothing too advanced. They generally use a Centre negative 9V DC power adapter which is pretty universal. That's all good but I wanted to use a 'Volume' IC which has an analog supply requirement of +/-5V DC and a Digital supply requirement of 0 and 5V DC.I've botched together Switched mode Buck supplies before to step down from 12V to 5V DC, so 9V to 5V DC is the same, but plus and minus supply rails is new to me. I know it's possible as described in  but I'm wondering is the easier solution not to just split the 9V DC supply with a voltage divider, (so connect the 9V to two 10K resistors in series) and tap the center off as Ground and the two ends as +/-4.5V. If the volume IC could cope with a supply down at +/-4.5V all is good but perhaps I'd have to boost the 9V to 10V to get exactly +/-5V. Even then you'd only be using one Boost IC rather then 2 Bucks. Maybe a resistor divider is a very inefficient method of doing it. And given the simplicity it's probably a trap for new players.The other question is the Analog and Digital supply rails. Both require a +5V rail, but I'm sure there's a good reason why you can't use one +5V rail for both purposes. Again simple but probably a trap for new players.As this is engineering there's probably a multitude of ways of doing this and it's all trade offs between them. I think I should be looking at efficiency over simplicity, but simplicity is so tempting. If anybody has any tips which would steer me away from the traps I'd be grateful.https://www.electronicspecifier.com/products/power/creating-a-negative-output-voltage-using-a-buck-converter

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2020, 09:31:54 am
content: You can split the supply with resistors but the impedance of the ground will be very high which tends to make all hell break loose. You will need to buffer it with an opamp or a rail splitter IC. For examplehttps://www.ti.com/product/TLE2426But this is a big problem because the average wall wart is “nominally 9v” which means it can spike higher or drop out depending on mains voltage. More problems.Then there’s the negative voltage charge pumps. Which can’t source much current.Best option I find here is to use an AC wall wart instead of DC in these circumstances and an arrangement of diodes to get +/- whatever volts you need, smooth it and use a linear regular on each rail. Cheap and easy!I had a schematic for a supply I built a few years ago that derived +/-15v from one transformer tap but I can’t find it now. I used one terminal of the transformer as ground and derived the rails from the other terminal. Fine for low current.

author: ciccio
author_rank: Frequent Contributor
timestamp: July 29, 2020, 10:03:46 am
content: I do not reccomend using a switch mode power supply, including boost and buck converters, for powering audio circuits. Switching noise seems to always find a route to the speaker..Using low voltage op-amps, your 9 v dc supply can be transformed to +4.5 - 4.5 using a virtual ground: There is an inexpensive TI IC that does this simply. here is the datasheet :https://www.ti.com/lit/ds/symlink/tle2425.pdf?ts=1596016946818&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTLE2425%253FkeyMatch%253DVIRTUAL%2BGROUND%2526tisearch%253DSearch-EN-everything%2526usecase%253DGPNBest regards

author: kripton2035
author_rank: Super Contributor
timestamp: July 29, 2020, 10:03:49 am
content: you could use these modules :https://usa.banggood.com/A0505S-1WR2-A0505S-DC-DC-Isolation-Power-Supply-Module-Input-4_5-5_5V-Output-5V-p-1519087.html?rmmds=search&cur_warehouse=CNot this one in 2W :https://www.aliexpress.com/item/32968841370.html

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2020, 03:27:42 pm
content: Quote from: jwhitmore on July 29, 2020, 08:23:40 amA resistor divider can be used with a voltage follower to make a rail splitter providing low output impedance while drawing little current. A switched capacitor charge pump can also operate as a rail splitter.QuoteYou can almost always use the same supply for the analog and digital but there may be issues with noise. If there is, then separate decoupling may be sufficient to handle it.QuoteI would not count on the output from a 9 volt wall adapter to be well regulated or noise free, and from experience, my preference is to assume that any external power should be galvanically isolated in an audio application anyway to prevent ground loops; what happens if someone uses the same supply to power two different modules?This immediately leads to using an isolated switching regulator circuit although a good alternative might be an inverter with primary side linear regulation because it could be inherently low noise. If you do this, then you can use whatever supply voltages for your circuits that are convenient, and have truly separate analog and digital supplies.

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2020, 03:46:37 pm
content: Quote from: ciccio on July 29, 2020, 10:03:46 amIf that were true, then probably 99% of modern audio equipment wouldn’t work right. But every computer, smartphone, tablet, TV, DVD player, bluetooth speaker, compact stereo, etc out there today uses switchers. Will some of those have switching noise? Sure. But most don’t, because itispossible to do it correctly and get no noise.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 29, 2020, 04:03:14 pm
content: You are going to receiveat least a couple dozen different responses.Most, if not all of them will be correct, but the optimal choice depends on what aspects of the circuit are important to you.I am focusing on simplicity, so this is my two yen:Use a 7805 to regulate the +9V TO +5V.Then use a charge pump IC like the 7660, to obtain -5V.And yes, the charge pumps are also a little noisy. Although much less than a SMPS.

author: David Hess
author_rank: Super Contributor
timestamp: July 29, 2020, 04:16:05 pm
content: I should have included that most equipment assumes a negative ground so using a rail splitter would shift the ground point by 4.5 volts leading to all kinds of problems if the same supply was used on two interconnected modules.Schmitt trigger's suggestion is the simplest solution and a good place to start.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 29, 2020, 06:28:43 pm
content: I'll second schmitt trigger's suggestion, but with a variation (= more parts).I would put the charge pump right on the 9V in to create ~-9V (actually a bit less), then clean that up with a 7905 linear reg. That's an extra regulator, but would mean both analogue supplies have the benefit of a linear regulator for lower noise.The 7660 is ancient, some newer ones have switching frequency much higher than audio, but often are 5.5V max. Ultimately you'll have to see if noise is a problem, and if your circuit draws too much current on the negative rail. It is unlikely analogue circuitry will draw enough current to tax the charge pump.I don't know what this "volume" IC is, but I can't see it drawing more than 50mA, but if you have the space, I would opt for the TO-220 regulators, the little TO-92 can handle 100mA but not if the input is (-)9V and the output (-)5V.It is worth noting that many guitar pedals are infact powered by +/-4.5V, from a 9V battery/PSU: They create a virtual ground for opamps with a voltage divider, sometimes using a spare opamp as a voltage follower as a buffer. As the inputs and output are AC coupled it doesn't matter to external equipment. Given that you're using an IC that has both analogue and digital parts, best to keep 0V ground, otherwise the digital supply would be floating.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 29, 2020, 07:42:56 pm
content: Excellent idea, the negative regulator would significantly clean up the noise from the 7660.There are several improved devices over the 7660, which must be at least 40 years old.For the LTC1044A its frequency can be boosted close to 100 Khz and can operate up to 12V.

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2020, 08:27:17 pm
content: AnICL7660 has a maximum output current of 20mA and is optimized for just 2-5mA. It’s eminently possible that, especially after running its output through a regulator, that there simply won’t be enough current to run the IC.Besides, given that it’s a charge pump, and thus already a type of DC-DC converter, why not use something more suitable to the job?For what it’s worth, I can attest that TI’sTPS65133 “±5-V, 250-mA Dual-Output Power Supply”DC-DC converter chip works beautifully for audio op-amp circuits. I just replicated their reference design from the data sheet, closely copying the recommended PCB layout.

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2020, 08:46:42 pm
content: Simplest solution as mentioned earlier. Use an AC wall wart and replace the zener regulators with 7805/7905

author: tooki
author_rank: Super Contributor
timestamp: July 29, 2020, 08:58:55 pm
content: Quote from: bd139 on July 29, 2020, 08:46:42 pmExcept that the OP wants to maintain compatibility with the 9V DC standard used by guitar pedals.

author: bd139
author_rank: Super Contributor
timestamp: July 29, 2020, 09:01:04 pm
content: Sometimes it’s easier to throw a constraint in the fire than live with it. Also a lot of guitar kit, at least here, runs on 9v AC already so there’s hardly an established standard...

author: jwhitmore
author_rank: Regular Contributor
timestamp: July 30, 2020, 12:05:55 pm
content: Thank you all for your comments. I'm going to try the charge pump/7905 linear regulator solution as it seems simple enough, and I've never used that charge pump IC tech so I might learn something.This is the volume IC that I'm looking at , which seems to have a typical current requirement of less then 20mA so this should all work out.Thanks againhttps://www.ti.com/lit/ds/symlink/pga4311.pdf?ts=1595878234967&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FPGA4311

author: Saimoun
author_rank: Frequent Contributor
timestamp: July 31, 2020, 09:55:11 am
content: You got a lot of ideas already - but since you're going with the charge pump solution I definitely recommend the LM27762 IC, very cheap and will remove the noise on both your +5V and your -5V (it has an high-PSSR LDO on both):https://www.ti.com/lit/ds/symlink/lm27762.pdfNote that you will still need to convert your 9V to 5V - but with this IC you can use switch mode instead of a linear regulator to be more energy efficient - if for example you plan on running on a 9V battery at some point like many guitar pedals)

author: Buriedcode
author_rank: Super Contributor
timestamp: July 31, 2020, 05:34:04 pm
content: RE: LM27762... WSON package isn't exactly prototype friendly. I realize its pretty hard to completely avoid SMT these days, but SOIC packages are much easier to work with.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 01, 2020, 11:19:26 pm
content: Quote from: Buriedcode on July 31, 2020, 05:34:04 pmExactly!These types of packages assume that you will require a board even at the fact-finding stage.Additional business for the Chinese board prototyping houses!Or purchase an Eval board from TI. Yours for $49 US.https://www.ti.com/tool/LM27762EVM

author: Saimoun
author_rank: Frequent Contributor
timestamp: August 02, 2020, 08:20:47 am
content: True - annoying they do not make a bigger package version