+15V/-15V power supply design question - Page 1
Forum section: electronics / projects

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 03:02:11 pm
content: Am building a ps for an opamp based preamp that uses +15V/-15V. Everything is good except the LM7815 regulates at +14.99V and the LM7915 regulates at -15.31V. Both have 18V going into the regulator so input voltage is good. Tested each one with direct power supply voltage and upping voltage made no change. So, put an adjusting circuit in and bump up the + side to +15.31V, no problem. It now draws an extra 15mA but that shouldn't be an issue.Question is, is it common for these regulators to be off like this? I know it is within specs but everything else I recall building was pretty spot on. Second, I assume op amps would function the best with equal +/- supply rails? Maybe I have my undies in a twist over nothing...I learned a lot building this, particularly the capacitance multiplier addition which reduced the 120Hz hum to the inaudible range. If anyone has any comments or suggestions on my design, please feel free to opine, looking to learn.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 09, 2022, 03:04:17 pm
content: They're not off, they're spot on for their specs. Why do you want such accurate supplies?

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 03:10:11 pm
content: Like I said, maybe I am just being too picky.Was under the assumption with opamps circuits, you want the supplies to match. This is the preamp schematic, tiz for a microphone. Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2022, 03:24:36 pm
content: You need to read the data sheets.With respect to tolerance on the +/- 15 V supplies for op amps, the relevant spec is the DC PSRR and CMRR, which allow you to calculate the shift in DC input offset voltage for a change in the DC voltages.PSRR is relevant to the magnitude of the total voltage between the supplies, and CMRR is relevant to the mismatch in absolute values.High precision in DC values is only relevant to normal op amp applications if a precise DC output is required when the DC signal input is derived directly from the supplies.More important for normal applications is the hum, noise, and ripple (AC components) of the supply voltages, since PSRR and CMRR fall with increasing frequency.Still, this is a quantitative question.

author: MikeK
author_rank: Super Contributor
timestamp: July 09, 2022, 03:43:23 pm
content: What's the purpose of the pass transistors in front of the regulators?

author: themadhippy
author_rank: Super Contributor
timestamp: July 09, 2022, 03:51:18 pm
content: change out the fixed regulators and use lm317 and 337's,supposedly there quieter for audio work,and you can tweak them to match precisely,or leave alone as its good enuff for rock n roll

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 03:57:24 pm
content: Quote from: TimFox on July 09, 2022, 03:24:36 pmHere is a screen shot of the data sheets. Not exactly sure how to interpret it?

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 03:59:04 pm
content: Quote from: MikeK on July 09, 2022, 03:43:23 pmCapacitance multiplier, not in the correct spot?

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 04:05:31 pm
content: Quote from: themadhippy on July 09, 2022, 03:51:18 pmWas wondering about those. I have some 317's but no 337's, will have to add them to the next order. It's pretty quite now but can always improve, thanks!

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2022, 05:22:31 pm
content: Quote from: Tim88 on July 09, 2022, 03:10:11 pmYou don't need the supplies to match. It won't make any difference. The circuit attached to the original post can be trimmed down considerably and will still work perfectly.

author: iMo
author_rank: Super Contributor
timestamp: July 09, 2022, 05:50:36 pm
content: With opamps you do not need exact matching as it was said above.Anyhow, your above schematics is not good with the 7815 level adjusting - it should be wired as below..PS: the 7815 output voltage will be always higher (with trimmer resistance set >0 ohm) than without the trimmer, 15V plus 0 .. 1.7V with your values.You want to add up only 0.32V, therefore I would use 3k3 instead of your 1k and you'll get those 0.32V in aprox. the middle of the 100ohm trimmer setting..

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 06:46:41 pm
content: Rule #1 for beginners:Do not try trimming 78xx/79xx regulators like that. The ground pin current varies wildly with load current and input voltage.The LM317/337 are the right parts for doing that.Live with the 78xx/79xx voltages as they are. They are 1970s devices.

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2022, 07:45:03 pm
content: For some reason people think exact +/-15V is required and trimming to exact values = better fidelity.You don't have enough input voltage 18VDC will not work (especially with the c multiplier) if there is a mains sag and give a blast of hum. I use a 28VCT transformer, 25VCT was not enough for some venues. Others recommend more.Elliott Sound Products - Power Supply for Preamplifiersis the bare minimum andPower Supply for Preamplifiers (Revision D)uses LM317/LM337 w/full protection diodes.Note your circuit can blow the SSM2019 if phantom power is on and any shorts (or plugging in wrong mic) at the XLR will dump C4, C5 into the IC's input and it's dead. It's common to have clamp diodes there.

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 07:54:51 pm
content: Quiescent Current(ground current) changes very little for 78xx regs78xx regs have no trouble with dropout voltage with 18V supplies, unlike 317 reg types.

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2022, 08:21:49 pm
content: Quote from: SuzyC on July 09, 2022, 07:54:51 pmNo, they both use Darlington pass transistors giving dropout voltage of 1.5-2.3V depending on load, and OP has extra cap multiplier losses.Having only 1V of headroom means a mains sag of a percent to two gives you a very loud blast of hum out of the mic preamp.

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 08:42:44 pm
content: Quote from: SuzyC on July 09, 2022, 07:54:51 pmI'd say that 0.5 mA change at 3 mA ground current is significant. Works out at ~17%.

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 08:42:59 pm
content: Quiescent Current specsheet(above) show geriatric 78xx quies cur to be 8ma, not 3ma.-------------------------Nope, specs for Dropout V are better than that.2 per-centsag of 18V =.36V 18-.36V= 17.64VDropout V spec=1.1V or 16.9V Vin to Reg which will dropout at high current output beginning at 16.1VIt is uncommon for a preamp to draw so much current like 1A or so..it is not a P.A. system.

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 09:04:21 pm
content: Some great posts, much appreciated, will address them when I get done cooking supper. One thing of note, the preamp draws less than 80 mA so I knew I had a lot of headroom on the PS. The link to the project is here if anyone is interested:https://www.instructables.com/Build-the-Four-Channel-SSM2019-Phantom-Powered-Mic/Thanks again!From the website Q &A:"DJJulesAnswer 1 year agoAbsolutely. The SSM2019's draw at the most 10ma each and the OPA2134's about the same so all in 80ma all in so you are easily covering that with 850ma"

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 09:05:39 pm
content: You're showing a LM7905 which is not the subject here.The LM7815 has atypicaldrop-out voltage of 2 V. Unfortunately no max. is listed in the data sheet, but thinking 2.5...3 V is not unrealistic.

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 09:16:58 pm
content: Nope, I first posted above both 78xx and 79xx specs(Sorry, they do not show dropout V, so I post the date 2000 Nat Semi Spec Sheet page showing dropout here)From looking at the dropout graph at 80mA, the Nat Semi 78xx spec sheet clearly shows less than 1.5V.. but not 2.5..3VNotice how the dropout voltage decreases with temperature, but o.k. about 1.5V with 80mA OP's current draw. The hotter the device, the better the dropout voltage spec.------------Assume that the OP's idea is to correct the .23V difference in output voltage between the positive and negative regulated output voltage:I will assume that the quiescent cur for the 78xx series is typically 6mA(I measure this often).Assume .5ma difference in Quies Cur will result in about an output voltage change of 15V +/- (8% of .23V) due to correction circuit change).do the math..not so much is it?

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 09:19:11 pm
content: Quote from: Zero999 on July 09, 2022, 05:22:31 pmThis is the first pre-amp I built. I used a PS design like the one you posted (just the + rail part at 12V) and it produced horrible line hum. I added the capacitance multiplier and it cleared most of it up. The mic sounded very high treble and just plain bad. Wasn't sure what was going on with that amp design so went to this other one. Admittedly, I did not test it minus the cap multiplier, maybe it will sound fine, will definitly test it.

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 09:24:08 pm
content: Quote from: imo on July 09, 2022, 05:50:36 pmAppreciate the advice and will change that resistor to a 3.3k. I just used a 1k per the spec sheet recommendation. Am gonna test the setup with the voltage inputs unbalanced and see if it sounds any different. Probably doesn't as most have said but it's a learning deal for me. Thanks!

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 09:27:24 pm
content: Quote from: Benta on July 09, 2022, 06:46:41 pmGood info on the 317/337's, much appreciated. This pre amp draws very little current, <80mA, so I figgered I was safe on trimming the 7815. Thanks!

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 09:29:49 pm
content: You can't trim the 7815 lower..only higher in output voltage.

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 09:33:04 pm
content: Quote from: floobydust on July 09, 2022, 07:45:03 pmSome great info there, thank you. I have been to the sound-au.com site but somehow missed that good article. Will add the clamp diodes. Had seen them in some othere schematics and wasn't quite sure their purpose.

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 09:42:47 pm
content: Quote from: SuzyC on July 09, 2022, 09:16:58 pmOdd. You seem to have different data sheets to the ones on the TI or ON web sites.Note the last specification: "input voltage required..."

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 10:05:46 pm
content: But your Snosbitol vender for regs, their specsheet for this part?? Maybe they ain't so good at making these regs.Name sounds like a stuffy nose fix. I always find these parts from Nat Semi, Fairchild and trust their datasheet.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2022, 10:18:56 pm
content: Quote from: Tim88 on July 09, 2022, 09:19:11 pmThat circuit is designed to work off a 9V battery, which is a very clean power supply, not the mains.The 2.2µF capacitor on the input is either the wrong way round, or the 10k biasing resistor should go to 0V.To run off the mains, the potential divider should be bypassed with a capacitor and another resistor added to bias the op-amp's input at half the supply voltage.

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 10:28:11 pm
content: Quote from: SuzyC on July 09, 2022, 10:05:46 pmErm... seems like you're a decade behind the curve. Nat Semi was bought by TI (Texas Instruments) in 2011. Fairchild was bought by ON (today onsemi) in 2016.The web sites are:www.ti.comandwww.onsemi.comThat's where you'll find the official documentation.So who's got the "Snosbitol" data sheets now? Not me.

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2022, 10:37:51 pm
content: Quote from: Tim88 on July 09, 2022, 03:02:11 pmThe regulators are well within their specifications.When it is important for the positive and negative supplies to match, then a dual tracking regulator is used instead. This makes any variation and noise on one supply match the other which aids common mode rejection.

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 10:40:10 pm
content: Quote from: Zero999 on July 09, 2022, 10:18:56 pmYa, was wondering about that 2.2 cap polarity. Mic sounded good from a hum perspective with the 9V battery but still sounded like crap, no bass. I tried a lines ps just to learn about making one for it, obviously much more to learn.This new amp design even with my crappy sounding voice actually makes me presentable to the interweb. Gonna try a few iterations of ps design this evening and post the results. My guess is you all are right and a simple design will be just fine, we shall see. Thanks!

author: SuzyC
author_rank: Frequent Contributor
timestamp: July 09, 2022, 10:49:44 pm
content: Benta: Latest 2016 78xx T.I. datasheet attached..looks something like the same to me as I've already posted.

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 10:59:41 pm
content: Quote from: SuzyC on July 09, 2022, 10:49:44 pmYes. Exactly this one:https://www.ti.com/lit/ds/symlink/lm7800.pdfWhich shows nothing about the 1.1 V dropout voltages that your previous posts did. But shows the 17.7 V minimum for regulation.But obviously Snosbitol.Over and out.

author: MikeK
author_rank: Super Contributor
timestamp: July 09, 2022, 11:11:10 pm
content: Quote from: SuzyC on July 09, 2022, 09:29:49 pmWhy can't you do this, but with 7815/7915 and the resistors of appropriate values?

author: julian1
author_rank: Frequent Contributor
timestamp: July 09, 2022, 11:21:03 pm
content: QuoteLooks wrong to me. Try another part eg. mc7915, check for oscillation.The old-school regulators, have high dropout (npn darlington/ ) versus pnp types. But the tradeoff is often better ripple-rejection/ CMRR - from mains EMI that comes across the mains transformer. from memory mc78xx/mc79xx some 70-80dB out to 10kHz. That stands unless one spends money on exotic LDOs.The fusing between the secondary and the rectifier might cause trouble, as the caps charge at turn on. Even with slow blow. Also both rails should be fused.

author: Tim88
author_rank: Regular Contributor
timestamp: July 09, 2022, 11:48:01 pm
content: Quote from: julian1 on July 09, 2022, 11:21:03 pmGot two from Digikey, both regulate at 15.3V even powering them with my HP power supply from 17.7V up to over 20V...

author: Benta
author_rank: Super Contributor
timestamp: July 09, 2022, 11:58:51 pm
content: Quote from: julian1 on July 09, 2022, 11:21:03 pmWhy? 7815/7915 regulators are +/-5%, in other words: 14.25...15.75 V. Do you expect 0.01% precision here? Price point is wrong.

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2022, 12:12:54 am
content: Quote from: SuzyC on July 09, 2022, 08:42:59 pmAnd where did this "1.1V" dropout number come from? It's competition and datasheet copypasta... going back 44 years wow."The LM320 is the only negative fixed voltage regulator available today." released in 1973. History readEDN July 1973amidst Fairchild competition, Voltage Regulator Handbook 1978 it's 1.1V typ. max 2.3V for the uA7915 with Vout -5%. But other -ve regulators like 79M15, 79L15 have higher dropout voltage so it's likely specmanship. For some reason the LM340/LM320 lost out to the 78xx/79xx.In reality, as well as low line you have to look at the +ve regulator's 2V dropout and something called ripple voltage... as well as the voltages and currents are not pure DC or sinusoidal. So math doesn't work well and you compensate by having extra input voltage at the expense of heat.OP's circuit would work due to the fact that small power transformers have poor regulation. At light loads they can easily be +20% high almost 30VAC. Even a multimeter reading is misleading because the true RMS values are not the peak values the rectifier, for what you will see at the filter caps.Hammondtransformers seem to be designed for the good old days of 115VAC- when I measure 122VAC at home.

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 01:30:10 am
content: Drum roll... Tested 3 power supply configurations:First on the pic is the original PS from the guy that created the preamp, no appreciable noise.Second from a suggestion here, no appreciable noise.Third is the one I came up with.Those are screen captures of Audacity spectrum analysis of quite spots on test recordings.This is what I was dealing with in the first preamp:Not sure what kind of craziness was going on there but it was quite the cluster.Balancing the input voltages didn't seem to matter though I still may put the adjustment circuit in, I got an issue with symmetry...Comments?

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2022, 01:40:04 am
content: Important is your circuit layout - need single-point (star) grounding for the power transformer center tap, filter caps, Vreg's and output COM take off especially. There are very high ripple charging currents so any resistance (i.e. daisy chain there can add hum to the output unexpectedly.The fuse comes first, not after C1 and is best on the primary. I think the cap multiplier provides no value as the IC's already have a lot of ripple rejection, or just upsize the filter caps to 2,200-3,300uF. Avoid the cheapola single gauge (thin) tab 78/79xx, and some manufacturers 3-terminal vregs are quite noisy. I would be looking at that on the scope.

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2022, 01:36:13 pm
content: Quote from: Tim88 on July 09, 2022, 10:40:10 pmThe volume control should be on the input, rather than the output.Be careful with just relying on how it sounds to you, because it's subjective. It will tell you if something is really wrong, but not for subtle changes in noise an distortion, which can only be identified by measurements.

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 04:08:08 pm
content: Many, many thanks to all that contributed to my PS project. This is what I have pending any screw ups...

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2022, 04:17:34 pm
content: Quote from: Tim88 on July 10, 2022, 04:08:08 pmThat's fine except I don't see why you need the adjustment, which only works upwards.EDIT: Why not make R5 axial then you don't need the wire jumper and the trace can be routed underneath it.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: July 10, 2022, 04:20:34 pm
content: It will tell you if something is really wronghello,Could you say something about the idea to remove the 27K in parralel with the switch ?

author: iMo
author_rank: Super Contributor
timestamp: July 10, 2022, 04:37:05 pm
content: As it was said above - the fuse F2 should be placed at the primary of the transformer, not secondary (primary side is the left hand side of the transformer symbol on your schematics).Also your pcb layout - it was recommended to you to make a "star configuration" of the ground wires - it means the ground connections from all particular parts on your pcb should have their own tracks, which should connect together in a single point, if possible..

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2022, 04:40:36 pm
content: Quote from: imo on July 10, 2022, 04:37:05 pmIt depends on the transformer. If it has built-in thermal protection, then a primary fuse is unnecessary.I'd make the traces thicker to save etchant and lower the impedance.Quote from: Coordonnée_chromatique on July 10, 2022, 04:20:34 pmIt alters the gain. Look up non-inverting amplifier.https://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifier

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 04:56:49 pm
content: Quote from: Zero999 on July 10, 2022, 04:17:34 pmYa, the adjustment probably is not necessary but can't hurt balancing the inputs. Your idea on the resistor is brilliant and will be modded in the layout!

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 05:01:39 pm
content: Quote from: Coordonnée_chromatique on July 10, 2022, 04:20:34 pmI can't remember what that was there for, I recall it was a tone resistor or something. I didn't want it in my breadboard circuit so I just marked an X on it to remind me.

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 05:15:42 pm
content: Quote from: imo on July 10, 2022, 04:37:05 pmMy bad on the fuse, forgot to move it, will mod it accordingly. From what I understand, the star ground is the most desirable but would one not accomplish the same concept by routing all the ground tracks to a higher gauge wire (say a 1.5" length of 16 gauge wire) of relatively short length? Routing everything to one point would require soldering in extension wires to reach that point and I figgered that would negate any benefit? Using the PS layout with my cheap Chinese breadboards didn't produce any noise so thought we would be good. Am learning so dress me down if my assumptions are incorrect.

author: mariush
author_rank: Super Contributor
timestamp: July 10, 2022, 05:15:51 pm
content: Unless 1n4002 is special in some way, it's probably fine to just get 1n4007 or S1B-S1M (ex:https://www.digikey.com/en/products/detail/onsemi/S2M/1049068) or S2B-S2M (ex:https://www.digikey.com/en/products/detail/onsemi/S1G/965464)Most likely overkill and not needed, but you could add some snubber capacitors / resistor+capacitor across the rectifier diodes ... ex seehttp://www.hagtech.com/pdf/snubber.pdfandhttps://www.diyaudio.com/community/threads/bypass-capacitors-on-diodes.133916/Don't really like that led with a 2.7k resistor .. you're dropping 30v+ on that resistor and around 0.3w on that resistor, pointlessly heating up the board in that area and the led could be very bright at 10mADidn't see anyone mentioning minimum loads... some datasheets say specifications of 7815 and 7915 only valid from 5mA and higher. Not sure about the internal resistors used to set the output voltage, but maybe it would be a good idea to have a led on each output as a sort of minimum load instead of that single led before the input of the regulators.Yeah, star ground or as close to star ground as possible could help.Maybe think if it's worth arranging the two linear regulators in such a way as to be on same heatsink, so that they'll heat up evenly ? Probably won't matter. Also, one of them would have to be insulated, because I guess one regulator's tab would be ground and the other could be -V

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2022, 05:18:19 pm
content: Quote from: Tim88 on July 10, 2022, 04:56:49 pmThe adjustment might not even work, because it's one way i.e. it can push the voltage up, but not down.Suppose the negative voltage regulator outputs -14.8V and the positive 15.2V? The adjustment will be useless, since it can only increase the voltage and you'd need to reduce it.

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2022, 05:25:10 pm
content: Quote from: Zero999 on July 10, 2022, 04:40:36 pmHaving one secondary fuse does not cover two rails - if it blows you still get 1/2 wave output.

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 05:38:33 pm
content: Quote from: mariush on July 10, 2022, 05:15:51 pmThe diodes were just random rectifier diodes I had in my stash, I agree.You do bring up a good point on the led. I haven't experimented with that design yet, just borrowed that layout from another interweb design. Am open to any ideas on how to add the led. I had one project where adding a led added noise to the circuit so unsure on best design option. It is just a luxery that can be removed if necessary.Didn't plan on using heat sinks on the regulators. Only about 80mA current and during testing they did not get even slightly warm.Great input, thanks!

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 05:41:07 pm
content: Ok, latest mods posted, off to do some demolition on a house I am tearing down. You guys have been great, enjoy your Sunday!

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: July 10, 2022, 06:48:23 pm
content: Quote from: Zero999 on July 10, 2022, 04:40:36 pmAnd the 2.2µF in DC on the input ?

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2022, 07:08:27 pm
content: Quote from: Tim88 on July 10, 2022, 05:41:07 pmYou've completely missed my point about the adjustment? What happens if the negative rail is 14.5V? How do your adjust the positive rail down, or the negative rail up? You need to either have adjustment on both regulators, or neither.Quote from: Coordonnée_chromatique on July 10, 2022, 06:48:23 pmIt's an AC coupling capacitor. It allows the op-amp to be biased at half the power supply at DC and the AC signal to be injected at the input, so it swings around half the supply voltage.

author: Tim88
author_rank: Regular Contributor
timestamp: July 10, 2022, 09:43:54 pm
content: Quote from: Zero999 on July 10, 2022, 07:08:27 pmI do see your point and it makes perfect sense if I were gonna make a bunch of these. This, for now anyways is just specific to this one build. However,your point should be noted for anyone else that is going to use this PS design. Sorry for the confusion...

author: srb1954
author_rank: Super Contributor
timestamp: July 11, 2022, 07:16:11 am
content: Quote from: mariush on July 10, 2022, 05:15:51 pmIt isn't necessary to have a minimum load on 7815/7915 for them to remain in regulation. They don't have the same circuit configuration as the LM317/LM337 which pass a lot of their quiescent current through to the output pin and therefore require a minimum load to remain in regulation.The 5mA minimum for the 7815/7915 is probably specified by the manufacturers for ease of testing and to give a slightly better load regulation figure.

author: srb1954
author_rank: Super Contributor
timestamp: July 11, 2022, 07:29:52 am
content: Quote from: julian1 on July 09, 2022, 11:21:03 pmThe 70-80dB ripple rejection of the 78xx/79xx is usually specified with a minimum 3V input-output differential across the regulator. The ripple rejection drops off dramatically if the input-output differential drops much below 3V.Also the higher voltage versions of the 78xx/79xx series tend to have poorer ripple rejection figures than their lower voltage brethren.

author: magic
author_rank: Super Contributor
timestamp: July 11, 2022, 07:53:46 am
content: Quote from: Tim88 on July 09, 2022, 03:02:11 pmIt isn't common and to hell with everyone who says the spec is ±5%Your schematic shows only 100nF on the output of the negative reg. This is not within spec, the datasheet calls for minimum 1µF tantalum or 25µF aluminium electrolytic. The IC may be oscillating and this could cause the output to be off. Increase output capacitance and check the output with a scope if you can.Any problem with reducing the supplies to ±12V to eliminate concerns about dropout?

author: Tim88
author_rank: Regular Contributor
timestamp: July 11, 2022, 05:38:46 pm
content: I changed the dropping resistor for the LED to 4.7k, that brings the amp draw down to 8mA and is plenty bright. I also posted the transformer I used, $20 at digikey. I wonder if the going across the +/- rails for the led and not + to gr keeps any noise from it down?

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2022, 08:00:58 pm
content: You might be able to adjust both channels with a single potentiometer. The total voltage will be a little over 15V, but it should be possible to balance them.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2022, 10:07:34 pm
content: That is similar to one configuration of dual tracking regulator where the trimmer is replaced by an operational amplifier which balances the output by driving the common pins:https://www.eetimes.com/use-a-tracking-power-supply-to-improve-signal-chain-performance/