single supply to split rail loses all balance under load - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: March 20, 2021, 11:28:35 pm
content: I'm attempting to put together a split rail supply to power a TDA 7264 gain clone amp that I built a while ago. The 7264 works great when powered by my bench supply so I know that works okay. I built this single-to-split rail circuit that I found on youTube. The designer used this circuit with a wall wart to power a gainclone amp as well. And mine works perfectly as long as I don't attach it to the 7294. I should add that I used 2000 uF on each leg and instead of a single, 1 W resistor I used 4 1/4 watt 5k resistors in parallel.When I attached the power out rails to the 7264 circuit about 95% (18.1 VDC) of the supply voltage to the splitter circuit (19.3 VDC) shows up on the positive rail. That's without any input to the 7264. I also tried it with an 8 ohm resistive load on each side of the split supply and the voltage was evenly balanced. When I power the 7264 directly with my triple output bench supply (it's an older analog linear beast) the voltage on the rails stays evenly balanced as well. I also tried driving the splitter with my bench supply using the single supply out mode and the splitter did the same thing when hooked up the the 7264. It's only when I use this splitter circuit, powered by a switching laptop supply, that the rails lose balance.At first I suspected the issue had something to do with the switch-mode supply not playing nice with the split-rail circuit. I have another 24VDC out switch-mode supply that shuts itself down when hooked up to the splitter circuit. But then I remembered that the designer of this splitter circuit used a wall wart successfully to drive a gainclone amp. So not sure what might be the issue and I've tried everything I can think of to isolate the problem. It seems to only happen with laptop power supply, the one I was hoping to use because it has the highest output voltage of my stash of wall warts.

author: ajb
author_rank: Super Contributor
timestamp: March 21, 2021, 08:00:52 pm
content: Does the laptop adapter you're using have a grounded mains plug that is tied to the negative side of the dc output? That would explain the behavior if your amplifier has another connection to ground via an audio input or even a scope probe.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 21, 2021, 08:25:36 pm
content: Quote from: ajb on March 21, 2021, 08:00:52 pmI did a continuity test and there does not appear to be any connection between either the positive or negative output of the power supply and the mains ground on the input side.There is a smaller wire on the output that I assume is a signal wire for feedback from the laptop to the charger to limit current when the battery is nearing full charge, etc.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 08:59:27 pm
content: Such "splitter" works only for low power circuits. Even 1W audio amp output will have low frequency distortions because splitter figuratively saying - can't hold middle point stlil. Find two powerful enough 12V supplies and drop splitter nonsense.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 21, 2021, 09:07:55 pm
content: Quote from: ogden on March 21, 2021, 08:59:27 pmGood to know. Thanks for the suggestion!

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 09:24:43 pm
content: Other option would be to build dual-output supply from dual-output transformer and two bridge rectifiers. Just random search hit, but relevant:https://www.tnt-audio.com/clinica/ssps3_e.html

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 21, 2021, 09:41:09 pm
content: That circuit is not a rail splitter, it’s a voltage divider. It is not a method that can be used to create independent positive and negative power supplies from a single supply.Any difference in the current drawn by the positive and negative rails will result in a voltage shift at the “virtual ground” point. The magnitude of that voltage shift is proportional to the magnitude of the difference and the resistance of the two resistors. The lower the resistor values, the lower the voltage shift for a given difference in current but the more power that is wasted.As already mentioned, build a proper positive and negative supply. Two wall warts can do the job or build your own dual output linear supply which is fairly easy to do with fixed 78xx and 79xx voltage regulators - so long as your current requirements are less than a amp.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 21, 2021, 09:47:27 pm
content: Quote from: ogden on March 21, 2021, 09:24:43 pmWas thinking about this as well. I have surplus bridge rectifiers and conditioning caps (4 x 8200 uF) from another project. The 7264 is really a very clean amp capable of 25 w per channel if supplied right. My calculation is that I would need in the neighborhood of 120VA to drive it that hard. Those are around US$50 new but looks like I might be able to find it on ebay for around $30. Right now it's working okay with a pair of 12V wall warts but as you can imagine the power out is pretty low into my 8 ohm bookshelf speakers.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 10:02:31 pm
content: Quote from: WattsThat on March 21, 2021, 09:41:09 pmDon't. That would be another wrong way of building power supply for 2*25W audio amplifier.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 21, 2021, 10:38:55 pm
content: Quote from: ogden on March 21, 2021, 10:02:31 pmIs that because of the efficiency issue with the linear voltage regulators? Just curious...I wasn't thinking I would do it that way just because I have most of the parts I need to build this: [ Specified attachment is not available ]I built one of these for an LM3886 power amp and it works really well.

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 10:48:35 pm
content: Quote from: hummusdude on March 21, 2021, 10:38:55 pmWhen power supply has mains transformer, it is enough to have bridge rectifier with big enough filtering capacitors. Linear regulator in such case is superflous because amplifier itself will easily reject mains low frequency ripple, essentially doing job of linear regulator itself. When you have switching supply, you already have regulation. And lastly - never forget about power dissipation when designing/suggesting linear regulators. You did exactly that.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 24, 2021, 02:24:33 am
content: I'm using 2 12 V wall warts (one of them only puts out 11.5 V) and both are rated at 1.5 amps. The low end is really pathetic. I built the 7264 circuit with 1000 uF on each power rail at the input as per the designer. When he tested it he used his bench supply and had a large capacitor bank on the power input. But he didn't say how much capacitance. I added 3000 uF and the bass seemed to get a little better using the wall warts...still not as deep as just powering the amp with my linear bench supply. I think I mentioned it's kind of burly...GW GPC-3020.I was reticent to add any more capacitance to the input because of in-rush current. If the power supplies are regulated does that mean they likely also have protection against excessive current? I have some 8200 uF caps that I could add but I'm not sure how safe that would be. There's not that much current either way but I wouldn't want to blow the wall warts if i can avoid it.

author: fordem
author_rank: Regular Contributor
timestamp: March 24, 2021, 12:57:57 pm
content: Don't complicate things more than you need to - a 120V to 24V transformer with a center tapped secondary (12-0-12), a full wave bridge rectifier, and two filter/smoothing capacitors is all you really need.

author: hummusdude
author_rank: Regular Contributor
timestamp: March 24, 2021, 02:57:38 pm
content: Quote from: fordem on March 24, 2021, 12:57:57 pmI know it's a bit unwieldy the way I'm going about this. That's partly because I don't have the right transformer and partly because I enjoy exploring the arcane details of how and why things work the way they do.I ended up hooking up the extra 3000 uF capacitor bank to the power supply and it didn't seem to help but I didn't notice any issues with the wall warts so I guess in-rush current isn't an issue at that amount of capacitance. On the DMM I see about 0.1 VDC drop when I push the volume near to clipping and honestly I expected poorer regulation from the admittedly under-sized switching supplies.I'm about done tinkering on this one for now so I may try the thrift stores to see if I can find a transformer to repurpose.