Why use 15V for symmetrical supply in amplifiers? - Page 1
Forum section: electronics / beginners

author: Pferd
author_rank: Contributor
timestamp: September 23, 2021, 03:31:25 pm
content: Hello community!:)I was wondering, why is it so common to use +/-15V symmetrical supply in operational amplifier circuits? Wouldn´t it just be ok in most cases to use +/-5V? The (in my case audio) signal is in the 500mVpp ranges anyway.In my case it could possibly save me one transformer, which is specifically for the operational amplifiers supply in my design.Am I missing something here? Does it has to do with the potential difference to the power rails? (not using rail to rail op amps)Thanks in advance!

author: TimFox
author_rank: Super Contributor
timestamp: September 23, 2021, 04:39:11 pm
content: Operational amplifiers were, as stated in the term, originally designed for analog computer use ("operational"). The older vacuum-tube amplifiers usually had full-scale outputs of +/- 100 V, but the solid-state amplifiers used +/- 10 V full scale outputs for the calculated voltages. To reliably obtain +/- 10 V output, +/- 15 V power supplies were reasonable. Of course, modern operational amplifiers do not have a "ground" terminal, so you can play all sorts of variations on the splitting (on non-splitting) of the power supplies, so long as you remain within the minimum/maximum range of the total voltage from positive to negative supply terminals, and keep the input terminals within the allowed range of voltages with respect to the two terminals. Some amplifiers suitable for audio are perfectly happy with +/- 5 V: check the data sheet. Split supplies are handy for audio, and can avoid extra coupling capacitors with signal voltages referenced to ground.

author: tooki
author_rank: Super Contributor
timestamp: September 23, 2021, 04:57:09 pm
content: Indeed, many audio op-amps are happy with ±2.5V (5V total, with a virtual ground). TI alone has nearly 1200 op-amps that suffice themselves with 5V total or less. 37 of those are expressly categorized as audio op-amps. For consumer line level audio signals (2Vp-p) that more than suffices.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 24, 2021, 04:21:30 pm
content: Unless you need large output swing lower supplies are fine. Nowadays any opamp that runs on over 5V is commonly called "high voltage". When I was a newbie that meant >44V!

author: armandine2
author_rank: Frequent Contributor
timestamp: September 24, 2021, 04:30:41 pm
content: Quote from: TimFox on September 23, 2021, 04:39:11 pmmight be good to see this reasoning in documents of the time - I had a quick look at my recently acquired copy of Roberge (1975)Operational Amplifiers: Theory and Practicebut nothing there.

author: TimFox
author_rank: Super Contributor
timestamp: September 24, 2021, 05:37:26 pm
content: Looking at "many a quaint and curious volume of forgotten lore":R M Howe "Design Fundamentals of Analog Computer Components" (Van Nostrand, 1961) from the vacuum-tube era shows examples of commercial vacuum-tube operational amplifiers designed for +/- 100 V output range.Chapter 19 of R F Shea "Amplifier Handbook" (McGraw-Hill, 1966) states that "operational amplifiers using transistors are much more difficult than those using electron tubes". The examples of commercial solid-state amplifiers do not state explicitly the voltage output range for discrete-transistor designs shown. The Dymec (division of Hewlett-Packard) amplifier with photochopper in chapter 18 uses +/- 16 V supplies, and the other two examples (one from Fairchild) uses +/- 30 V supplies.J Brophy "Basic Electronics for Scientists" (McGraw-Hill, 1966), Chapter 8, shows a discrete bipolar operational amplifier from Burr-Brown with +/- 15 V supplies as a design example, but does not quote the output capability.My remembrance of +/- 15 V supplies for +/- 10 V output go back to the 1970s potted modules from Analog Devices and Philbrick, where the manufacturers discussed analog computation applications.By the time we get to the integrated uA709/LM709 operational amplifier, the guaranteed output voltage into 2 kload resistance is +/- 10 V for +/- 15 V supplies, but analog computation is no longer the prevalent use.

author: magic
author_rank: Super Contributor
timestamp: September 24, 2021, 05:51:51 pm
content: Quote from: tooki on September 23, 2021, 04:57:09 pmNot really, lots of CD players and soundcards output 2V RMS which is ±2.8V peak.Also, you may find tons of opamps that are happy on ±2.5V but many common classics require at least ±5V.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 24, 2021, 06:03:05 pm
content: I did my first electronics course in 1986. We powered the op-amps typically at +/- 15v and all our textbooks had this as the default supply voltage. I'm interested that this is due to a historical artifact from the (if I follow you correctly) general analogue computer's +/- 10V output range.I'm a little surprised that my teacher at the time was wedded to the tradition.I remember photocopying a previous years student's project on analogue computing (not available to me right now - must look it up sometime), which was "old hat" then.

author: TimFox
author_rank: Super Contributor
timestamp: September 24, 2021, 06:33:27 pm
content: Section 3 of this history lecture from Jung at Analog Devices, introducing the history of solid-state op amps, does contain a specific reference to +/- 15 V supplies for +/- 10 V outputs. See pp H31 ff ofhttps://www.analog.com/media/en/training-seminars/design-handbooks/Op-Amp-Applications/SectionH.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 24, 2021, 07:54:38 pm
content: Kind of two or three things:1. Just straight up dynamic range.If you need to work with 10V-scale signals, you need at least as much supply. It can be single-supply, with or without offset, but where everything is zero-referenced, that can get quite annoying, and the offsets really add up after a while. So a bipolar supply is of great help. Notably, audio is AC coupled, so this isn't strictly necessary, but it can be nice-to-have -- for example removing the turn-on *pop* as those coupling capacitors come up to nominal charge.So, also consider other applications, like control circuits, analog computers, etc. Op-amps were used for a lot more, back in the day!This point might not be so important for audio, mostly being in the couple-volts range -- but the extra range does allow for transients, and not having to worry about clipping as signals are amped, filtered and attenuated through a signal path, like in a mixer.2. Common mode range.Especially for EMI/RFI immunity, you need about as much supply voltage as the noise peak you need to reject. In commercial applications this is around 3V conducted, or 3V/m radiated; both of which can be increased by a modest amount due to resonance and antenna effects. (In the RFI range, it's a lot better to filter it off in the first place, especially against op-amps with bipolar input stages that tend to rectify and detect that noise. RF-insensitive amps are becoming more common these days, especially among precision types, where, to achieve their ~uV spec, even without bipolar transistors, low RF at the input is mandatory!)And if you need more immunity, even with proper filtering/damping to avoid the super high RF and resonance effects -- if you need to meet the 10V levels of most industrial gear, for example, you simply need that voltage range.A couple years ago I did a 4-20mA receiver and mux, for an industrial/automotive environment. Put in a cute little +/-15V converter on board. Just a single boost regulator chip did the job, only needed like 10mA total load, plus a bit of filtering to make sure it's not going to interfere with the op-amps.3. Anything else.Especially early op-amps, the output range loses 1-2V from either rail. "Single supply" types are the same way but to just one side, with the other saturating potentially pretty close to zero (but doing so, may require additional assumptions, like only pull-down resistors and other loads, so that the pulling-down output never has to work very hard, preferably not at all*). So you might go with +/-12V rails for example, but only have +/-10V of usable range, give or take.*Especially for types like LM321/358/324, with a class-B output stage that produces very visible crossover distortion if conduction switches polarity. The pulldown is also very weak, being basically to the rail for some 50-100uA, and about a diode drop or more above there. So a doubly good idea to have enough pulldown to keep it behaved.Same goes for the input common mode range (Vicm), which sometimes also caused the output stage to hard-saturate the opposite direction (phase reversal) when overdriven. Modern amps designs are (almost?) all designed to avoid this, fortunately. RRI (rail-to-rail input) types are also quite common, often with a modest worsening of input offset at the extreme (usually +V side) range of Vicm.Or just plain old tradition / inertia. Who needs a reason to do anything? Not us humans! All the appnotes say +/-15V? That sounds good, let's do that! Do we really need it, is it even healthy to do so**? Who cares, manufacturer said it, it must be true!**NE5534 for instance, as I recall just an amped-up 741. This reduces noise and distortion, albeit at great expense to supply current. Which, at the full rated +/-22V, can dissipate up to 350mW just sitting there, and a heck of a lot more under load -- a PDIP-8 is only good for a watt at room temperature, you don't need much load to exceed it!As for modern applications -- we have a lot of low-noise and high performance amps available to us now. It can be quite reasonable to receive analog signals in the 15V range, simply dividing them down to start, with tightly matched resistor dividers to maintain CMRR (which may be internal to the amp, as in some instrumentation amps -- which are modestly well available as single-chip solutions, no need to design your own).Or if that's still not good enough, there are even some weirdo op-amps that work even with their inputs outside (usually above) the supplies -- these are more comparators than op-amps as I recall, and their specs might not be that impressive (particularly input bias current, as I recall these all work by effectively being powered by the input voltage(s), it's just little enough that you can get away with it sometimes), but it's yet another trick that's available.Most analog front-ends (AFE) are going to be feeding an ADC, then the rest is handled digitally, giving vast improvements in performance (bit depth, sample rate; power consumption too!), processing power, and especially configurability (the whole software mixer/effect chain can be rewritten in real time, if needed!).And yes, digital circuitry isso many orders of magnitudeahead of analog, that it is, in fact, worthwhile even just in terms of power consumption, to bring everything into the digital domain, as early and often as possible. Despite using also exponentially more transistors in the process. It's truly amazing.The downside is, all that software is arguably harder to write (more bug prone), and generally slower to write as well (with the tradeoff that, it can be patched/updated after release, while hardware can't be patchedat all). It also costs essentially nothing to reproduce, besides the cost of the chip it runs on; which is often less than the cost of an equivalent analog solution anyway.Tim

author: armandine2
author_rank: Frequent Contributor
timestamp: September 24, 2021, 08:02:50 pm
content: hopefully the right bit of Section H.31 snipped

author: tooki
author_rank: Super Contributor
timestamp: September 25, 2021, 01:37:04 pm
content: Quote from: magic on September 24, 2021, 05:51:51 pmOops yes, sorry I remembered wrong, but I suspect you’re wrong too.I just double checked, and standard (-10dBV) consumer line level is about 0.9p-p, and the higher level consumer line level (0dBV) is 1Vrms (2.8Vp-p).The latter is probably where I got the 1V (misremembered as Vp, which I then doubled to 2Vp-p, rather than Vrms), and where you got the 2.8V from, but which you misremembered as being Vp and not Vp-p.No line level standard, not even the +4dBu pro line level, comes anywhere close to ±2.8Vp (5.6Vp-p).Quote from: magic on September 24, 2021, 05:51:51 pmDuh. I didn’t say anything about “classics”. I simply said that many op-amps that are happy on ±2.5V, including audio op-amps, exist. (Along with the source of that information.)That models exist that do not fit within the criterion (and more specifically, that people erroneously think that it’s rare) is literally why I posted: the prior comment made it sound as though ±5V was already exotic, when in fact even the much tighter ±2.5V is actually quite common.

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2021, 01:56:52 pm
content: Actually, 2 V rms = 5.6 V pk-pk is a common output level for CD players, but there seems to be no standard. See:https://forum.audiogon.com/discussions/standard-output-voltage-for-rca-line-outs-dac-cd-streamers-etc

author: tooki
author_rank: Super Contributor
timestamp: September 25, 2021, 02:23:05 pm
content: Quote from: TimFox on September 25, 2021, 01:56:52 pmThe higher voltages listed there seem to be referring to balanced connections, which double the voltage once more. Nothing there ever refers to 2Vrms.It baffles me why any audio device manufacturers would produce such high levels, given that standardsdoexist.(With that said, years ago I had to build an RCA patch cable with an attenuation pot in it, because the Sony MiniDisc deck I bought had outputs too “hot” for the line level inputs on my Sony stereo system…)

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2021, 03:44:24 pm
content: The third response in that link addresses unbalanced outputs of CD players. I was surprised by these high levels when I built a preamplifier, and added a switched attenuator to the CD input to deal with the output level of my Rotel unit. It’s predecessor had an output level control, so that wasn’t needed. That was my fault: I designed the preamplifier output for nominal 0 dBu drive to the power amplifier, and naïvely thought that CD players had the same output; the manual on the Rotel RCD-1072 explicitly specified 2 V rms maximum output on the unbalanced RCA connectors.

author: magic
author_rank: Super Contributor
timestamp: September 25, 2021, 06:18:07 pm
content: Many soundcards have 2V RMS too and likely many standalone DACs, particularly the higher end ones. Audio DAC chips are usually specified for that level as well. Start reading specs (from those vendors that bother to publish them) and you will see it everywhere.Intergrated audio tends to be ~4Vpp or whatever you can get on a 5V supply with AC coupling. This needs enough amplitude to drive headphones and louder=better.I don't know where you find your "standards" but this is the way things are.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 26, 2021, 03:05:59 pm
content: I'm not entirely sure, but I think that a higher power supply voltage results in higher linearity and thus less distortion, because the proportional biasing voltages changes are less for the same output.

author: dmills
author_rank: Super Contributor
timestamp: September 26, 2021, 04:26:47 pm
content: Quote from: tooki on September 25, 2021, 01:37:04 pmBe careful here, pro level has a standard reference level of +4bBm (ref 1mW into 600R) (Not dBV), which works out to about 1.228V RMS, this being where the 0 on a VU meter was typically aligned.However VU meters do not respond to short term peaks in a meaningful way, and the assumption in pro audio is typically that the level can swing some 20dB higher then this without clipping (+24 to +26dBu is typical, in broadcast +18dBu is typically full scale per EBU standards), thus about 18V across a differential pair (Reasonably easily hit with a +-15V rail), 36V pp!The CD thing is odd in that the 2V is at full scale digital, where for most other things the reference level is some 20dB (give or take) below whatever full scale means for that medium.This is a 'standards are great, there are so many to choose from thing'., Yes I know dB is a power ratio, but modern audio is almost all voltage transfer not power transfer, so it works with voltages and pretends the load is 600R, they even repurposed the dBu reference to specify this, thereby annoying every RF engineer out there.

author: tooki
author_rank: Super Contributor
timestamp: September 26, 2021, 05:18:27 pm
content: Oops, yes, I wrote dBV instead of dBu, fixed now.

author: rstofer
author_rank: Super Contributor
timestamp: September 26, 2021, 05:25:50 pm
content: Quote from: TimFox on September 24, 2021, 06:33:27 pmInteresting tribute to Bob Widlar of Fairchild and then National Semi fame!

author: magic
author_rank: Super Contributor
timestamp: September 26, 2021, 06:19:23 pm
content: Quote from: Doctorandus_P on September 26, 2021, 03:05:59 pmYou aren't wrong, and some rare opamps are officially characterized for that.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 26, 2021, 07:07:58 pm
content: Thanks for the verification.Looks like about a factor of 4 reduction of THD+N for the 17V power supply and for the same output voltage.Somehow I expect distortion to lower with a higher power supply voltage, but noise to increase, because each electron slipping through has a higher energy.I also find those charts suddenly stopping at 100mV instead of going through to 10mV suspicious. It always makes me wonder that something "ugly" may be happening below those 100mV and they do not want to show it. The "wavy" line of the +/- 2V5 graph is also an indication that this LM4562 has some trouble in maintaining internal biasing points at this low supply voltage.The +/- 2V5 graph also shows a sharp clipping point at 800mV output while the +/- 17V graph shows a much rounder curve which is already starting to get worse at around 7V output. I assume this is caused by current limiting on the output. 7V / 600 Ohm is still only 12mA but apparently enough to show some non-linearity.Edit: Oops, it's only a factor of two as "magic" wrote below, and the rising distortion can indeed have other causes too.

author: magic
author_rank: Super Contributor
timestamp: September 26, 2021, 08:07:34 pm
content: I see 2x difference. For example, 1ppm vs 2ppm at 0.8V.Distortion is almost identical at 2kΩ so it could be input stage distortion rather than current limiting. None of it is seen at low supply because the floor is too high and hard clipping occurs earlier.