AB-class amplifier schematic analysis & optimization - Page 1
Forum section: electronics / beginners

author: ratatax
author_rank: Regular Contributor
timestamp: May 18, 2019, 11:15:44 am
content: HelloFor my sound generator project I want to add headphones amplifier to it.I planned initially to use a digital amplifier IC but since we want the ability to drive high impedance headphones (600 ohms) with studio-quality someone told me it would be better to use a classic approach with a ~1W class-AB amplifier, powered with +15V/-15V rails.EDIT : See more info about my needs & project on my reply post :https://www.eevblog.com/forum/beginners/ab-class-amplifier-schematic-analysis-optimization/msg2425455/#msg2425455We found this schematic, fromhttp://sound.whsites.net/project113.htm:The original design is very old-school, using through-hole components and big 100uF caps.We'd like to modernize it to integrate into the product we're making. No problems, resistors diodes and small caps are easy, we found SMD transistors equivalents to BD193/BD140, but big caps are expensive and take a lot of board space.I understand the role of some of them in the circuit but I have no idea what C2L, C3L & C4L do.Also I feel this design is old and may use some overly-big capacitors to compensate for a slow linear power supply, but now it's full SMPS, much faster.C2L seems in reverse polarity to me. I'd like to avoid any cap bigger than 10uF to stay with small ceramic ones, and maybe modernize other things if you find flaws... Can you help me ?Edit: after looking at different class-AB amplifiers on google, most of them don't have the caps equivalent to C2L, C3L & C4L. Maybe they are meant to optimize something ?

author: Audioguru
author_rank: Super Contributor
timestamp: May 18, 2019, 03:37:09 pm
content: C2 has a very small input offset voltage across it. It is used so that the amplifier does not amplify this DC offset voltage which would cause some DC voltage at the amplifier output. Headphones move the cone to one side if there is DC on them.The capacitors parallel with the diodes reduce distortion a little.

author: dietert1
author_rank: Super Contributor
timestamp: May 18, 2019, 05:21:54 pm
content: The circuit you show is something like a school example. For a real product there need to be:a) Bandwidth limitingIf there is a SMPS nearby, for example 1 nF caps to Gnd on both inputs of the OpAmp. Then a 39pF parallel to R4 to get a compensated divider. Pulse response needs to be checked for stability.Also there should be a 20 or 30 Hz high pass somewhere. Modern media include subsonic audio down to 5 or 10 Hz and sound quality will suffer if you pass that to the headphones. C1 could be 470 nF instead and C2 may be reduced to 10 uF as well to get something like a second order filter. That's a matter of taste.b) Short circuit protectionFor example sensing the voltage on the two 10 Ohm output resistors R7 and R8.c) Turn-on and turn-off transient suppression (load/listener protection)This can be done with a pair of low Rdson mosfets per channel and a supply voltage supervisor including a small turn on delay.Two years ago i tested 150 uF multilayer smd caps. You can use them for C3 to C4, which only see small voltages.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2019, 12:19:46 am
content: C2 lowers the gain of the operational amplifier at low frequencies to 1 so it does not amplify its own offset. Because of AC coupling from C1, it never sees significant negative voltages.C3 and C4 provide a low AC impedance path between the operational amplifier output and the bases of the transistors so it can better control them.

author: ratatax
author_rank: Regular Contributor
timestamp: May 20, 2019, 01:08:46 pm
content: Ooh I see ! The polarity of C2 confused me since it will see positive and negative voltages, but it's not the first time I see electrolytics used like that, it doesn't seems to matter when the current/voltage is limited enough with a resistor. I was planning to replace them all with 22uF ceramic caps.@dietert1 how do you calculate the value of 39pF for the capacitor to be used with R4 ?a) Bandwidth limitingthe DAC output already has a lowpass output filter and the datasheet mentions a DC blocking cap isn't needed, so I guess there is already a high-pass integrated but I'd need to check its cutoff frequency.b) Short circuit protection => you're right. Since the circuit is ready to drive already high-impedance headphones (600+ ohms), isn't the simplest solution simply to replace the 10 ohm resistors by a higher value ?c) Turn-on and turn-off transient suppression (load/listener protection) => we need to think about that. I think the easiest solution would be to have mosfets controlled by a pin of our microcontroller, so there isn't any need for a hardware-based delay

author: Yansi
author_rank: Super Contributor
timestamp: May 20, 2019, 01:15:10 pm
content: This is a B class amplifier to begin with.Decoupling capacitor C1 placed wrong. Should be split against GROUND. This configuration of decoupling will not help much.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 20, 2019, 01:35:53 pm
content: Quote from: ratatax on May 18, 2019, 11:15:44 amIf the NE5532 is used, as suggested, the polarity of C2 is wrong, it will be biased by the opamp input current (about -0.3uA for NE5532) x 22K feedback resistor or about -6.6mV on pins 2(6). It would be better to reverse the C2 polarity with NE5532.Quote from: Yansi on May 20, 2019, 01:15:10 pmThe C1 capacitor is essential for the NE5532 or other opamps with a similar topology (NE5534, NJM2114) and should be placed as close to the supply pins 4 and 8 as possible. Decoupling to ground caps are C4 and C5.The amp is technically Class AB, as there would be a small idle current present through the output transistors.CheersAlex

author: drussell
author_rank: Super Contributor
timestamp: May 20, 2019, 02:08:20 pm
content: Quote from: ratatax on May 18, 2019, 11:15:44 amThat is sound reasoning and a good idea. I love my old Sennheiser HD400s (the original 1970/80s ones, with the yellow foamies) but many modern devices' headphone amplifiers simply won't drive them to any reasonable level with their 600 ohm impedance without an additional external amplifier.QuoteThere is nothing even remotely "studio quality" about that amplifier schematic.I would start with something better rather than trying to modify that design. Almost anything would better than an op-amp driving a couple transistors.That kind of simple design isgreatfor learning about the basics of amplifiers and makes it very easy to experiment with things like amplifiers where you take the feedback from the current through the voice coil to smooth out the impedance curve of the driver, etc. but is really only appropriate for experimentation or general purpose rather than a high-sound-quality headphone amplifier.If you're looking for something simple and inexpensive, I would try starting from something more like this:https://www.audioxpress.com/article/Low-Cost-Headphone-Amp...or start working backwards from one of thereallygood quality circuits, explore real class A amplifiers to remove crossover distortion (we're only talking about a headphone amp here, so unless you're working battery-powered-only, the bit of power consumed in true class A stages including the output stage really shouldn't be too much of a concern...) etc.If you REALLY want to make one from ICs and don't care as much about the budget, there is also always this approach:https://www.audioxpress.com/assets/upload/files/HighQualityPreampAXJun2013.pdf

author: Yansi
author_rank: Super Contributor
timestamp: May 20, 2019, 02:31:02 pm
content: You say there is nothing studio quality about the simple amp, then you slap here a discrete one that will be likely even worse due to having rather lower open loop gain. And then I see C4:"C4 has been added to provide a greater internal gain."... making the transfer function of the voltage amplifier stage inherently highly nonlinear. No real measurements of that amplifier provided.I will not comment on the other one with a dozen of paralleled buffers, that is just one typical audiophoolright there. I have seen this done even with a dozen of paralleled current output audio DACs.Also, no real measurements of that amplifier provided.If you want fancy pancy, look for a diamond buffer, these work decent. Not all of them, but here you get an idea:

author: drussell
author_rank: Super Contributor
timestamp: May 20, 2019, 02:56:21 pm
content: Quote from: Yansi on May 20, 2019, 02:31:02 pmBeing a beginners' thread, my point was to direct the OP to a type of design that allows more in-depth tweaking and experimentation with the various aspects of the building blocks of the discrete circuit than the one originally provided, rather than provide a "build it this way and be done" circuit.If the goal is not to learn how to design, modify and tweak a circuit for an individual application, then the OP would probably be best to go with a module or essentially a decent power-op-amp.Without more information on precisely what they're trying to accomplish, what their target specifications are and what measurement equipment the OP might have available to them, I can't provide much more specific information or pointers.QuoteMost audiophoolery is based on at least a grain of truth.Paralleling the outputs like that is an effective way to create a medium-power op-amp from individual building blocks while retaining the characteristics of the individual blocks.Is it messy? Sure! Does it work and perform well? Yes.Would I buildmyheadphone amplifier using exactly either of those circuits or the OP's? Heck no.QuoteIndeed, however, I have a feeling that the OP is looking for something reasonably simple and straightforward while providing reasonable fidelity. The OP will need to chime in with more information to be able make a rational decision...As for performance data, even small changes in component types can obviously make large differences in the circuit performance and sound quality. The OP will need to be able to make some measurements on the actual unit to be able to quantify at least some of these aspects, regardless of the circuit topology chosen if they expect to be able to tweak for the desired performance level.

author: drussell
author_rank: Super Contributor
timestamp: May 20, 2019, 03:07:49 pm
content: Quote from: ratatax on May 20, 2019, 01:08:46 pmYou should try to avoid using ceramic capacitors in the audio signal path. Their capacitance tends to be highly non-linear with changes in the applied voltage. Best to use film when possible if permitted by space / cost constraints.For decoupling and stabilization type uses they're great.QuoteIf you go too high, you then remove the ability to drivelowimpedance, low efficiency headphones to high level, although with your original circuit that can theoretically supply multiple watts, that may not be an issue.On some older typical hi-fi stereo amplifiers / integrated receivers, the headphone jack was just run right off the main speaker power amplifier output with, say, 390-560resistors.

author: dietert1
author_rank: Super Contributor
timestamp: May 20, 2019, 03:22:54 pm
content: If you use 1 nF parallel to R3 then for a compensated divider C = 1 nF * 1 KOhm / 22 KOhm = 45 pF should be parallel to R4. By looking at the transient response you can determine whether a 47 pF or a 39 pF works better. The 1 nF close to the OpAmp inputs can make a big difference on sound quality in a RF poisoned environment.Concerning the 20 or 30 Hz high pass, this is an problem i have seen in many audio solutions. Engineers think the lower Hz they handle the better, but this may just not be true, when the speaker/headphone system starts producing lots of intermodulation. I did not propose to use ceramic capacitors in the audio path, but only for C3 and C4, that are uncritical in this respect (effects eliminated by negative feedback). The 10 uF i proposed for C2 can be film, maybe 2x 4u7 50V.Concerning short circuit safety: It's fairly easy to test how long your final design survives a short cicuit at reasonable audio levels and how it dies. Certainly you don't want a fire.Regards, Dieter

author: drussell
author_rank: Super Contributor
timestamp: May 20, 2019, 04:31:30 pm
content: Just a few quick additional notes...Regarding the Elektor magazine example I posted,Quote from: Yansi on May 20, 2019, 02:31:02 pmIndeed, there are no published curves or in depth measurements provided for the unit as-built, only the specifications table according to the original article:https://archive.org/stream/ElektorMagazine/Elektornonlinear.ir2011-01#page/n47/mode/2upQuoteAnd, then to the OP, regarding this advice from:Quote from: dietert1 on May 20, 2019, 03:22:54 pmThese are all very good points. I agree wholeheartedly!

author: ratatax
author_rank: Regular Contributor
timestamp: May 20, 2019, 11:47:01 pm
content: Thanks for all those informations !Now I have more for you.Yeah I'm looking for something 'quite' simple, it's for my product which is a synthesizer I plan to build for my upcoming business, so I can't go crazy on the costs. I'm only in the design phase currently, but the budget for heaphones amplifier is only a small part of the whole product so I think I need to keep it under $5 (this is not including the +15V/-15V power supply and connectors. Price is for buying components in qty ~100 on LCSC).That's why I'm looking either for an amplifier IC or a quite simple but good enough solution with opamps (TL072 series) and transistors. Amplifier IC are almost always specified for 8ohms loads in their datasheet, I lack the knowledge here to understand if they would work well enough for higher impedance loads. I thought it was only a power thing, but I heared frequency response may also be affected if the amplifier isn't designed for high impedances ? (even if it is powerful enough)I'm not searching for the Audiophile thing, if it has noaudible/obvious distorsion or noise it's fineI routed the schematic to see how it looks on the board :yup it's a TL074 with two unused modules in this implementation... !22uF ceramic caps here but I may have to switch back to aluminium caps @drussellcurrent transistors are MMBT100 / MMBT200I have seen opamps with decoupling caps between V+/V-, some others between V+/GND and V-/GND. Best practice would be the latter ?

author: Audioguru
author_rank: Super Contributor
timestamp: May 21, 2019, 12:34:10 am
content: Will the tiny little surface mounted output transistors survive the heating in this amplifier?

author: ratatax
author_rank: Regular Contributor
timestamp: May 21, 2019, 12:54:11 am
content: I have no idea, those puppies are rated for 300mA :https://datasheet.lcsc.com/szlcsc/1902210930_ON-Semicon-ON-MMBT100_C274690.pdfRMS power will be way below 1W (maybe peaks could reach that).I can go for the BCP56/BCP53 which comes in SOT-223 package with a small heatsink, but those are rated for 80V 1A, it seems crazy overkill :https://datasheet.lcsc.com/szlcsc/Nexperia-BCP56-115_C22239.pdf

author: D Straney
author_rank: Frequent Contributor
timestamp: May 21, 2019, 05:51:09 am
content: Be careful about reading too much into the "300 mA" or "1A", as the current spec on transistors can be very misleading: I've seen people assume that because a bipolar (or MOSFET, or diode, or LED...) says "5A" in big letters on the datasheet, that they can put 5A through it under any condition - smoke usually ensues!Think of the current rating more as a way to compare transistors, not as an absolute metric. Having blown up plenty of power devices myself, and hopefully learned something from it, the things that actually matter are:1. How easily the transistor can get rid of the heat: this is more about the package than anything else. I don't know how much power you're planning on allowing your output transistors to dissipate, but I'm going to make up some numbers here, just for an example. Let's say the amplifier is putting out 1W at 50% efficiency: this means it's dissipating 1W too, mostly in the output transistors, 0.5W for each one. Now looking at the datasheet for the SOT-23 parts, it shows 357 C/W junction-to-ambient thermal resistance - that means that the temperature of the transistor's die is going to be 0.5W*357 C/W=179 C above ambient temperature...if the ambient air temperature around the transistor is maybe 60 C inside a box on a hot summer day, that bit of silicon (rated only to 150 C, by the way) is going to be at 239 C! Instant explosion.On the other hand, if you use the other transistor, because the package has a big metal tab its junction-to-ambient thermal resistance is about half of the other one (192 C/W), which means that the temperature rise above ambient will be only half as big. That's only some of the benefit: the metal tab also provides a good place to conduct even more heat away either into an external heatsink, or to a copper area on the PCB as a cheap heatsink. Check out the thermal conductivity list on page 6, and notice that the thermal resistance (junction to ambient) drops from 192->125->93 C/W as you go from no extra copper -> 1 cm^2 extra copper -> 6 cm^2 extra copper.Again, most of this is applicable to pretty much any transistor in a SOT-23 package vs. a SOT-223 package, not just these two particular part numbers. There's some variation between manufacturers, etc. but one package type is usually pretty consistent. Be careful about footnotes and test conditions: the BCP56 datasheet has very good info about thermal resistance, while some "too good to be true" thermal resistance numbers I've seen on other datasheets for some tiny SOT-323 package turn out to be only when using a special ceramic (high-thermal-conductivity) PCB with 4 cm^2 pads on every pin, once you dig into the footnotes.2. Other effects like "secondary breakdown" (essentially non-uniform conductivity causing hot spots which cause thermal runaway) on bipolar transistors: check the device's "safe operating area" (SOA) diagram for this limit.Apologies if you know all this already: just looked like a common misunderstanding was about to happen, and hoped to maybe save some unnecessary smoke

author: ratatax
author_rank: Regular Contributor
timestamp: May 21, 2019, 01:04:17 pm
content: You're right pointing that, I was forgetting that those voltages and current handling are absolute max in ideal cooling conditions (my developer background isn't helping here!). Since the sot-23 package probably can't dissipate even 0.1W without getting hot, we'll go for the SOT-223 with some pcb aera forming a heatsink.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2019, 01:31:17 pm
content: Quote from: ratatax on May 21, 2019, 01:04:17 pmIn the ear phone range there are quite some amplifier chips available. Depending on the power needed (too much is not good for the ears anyway), one could even consider 2 channels of the NE5532 in parallel. No real need to go for a discrete design at this power level. AFAIK it is not that uncommon to have some series resistor (100 Ohms range) at the output, so that a 600 Ohms and 32 Ohms speak could produce a similar sound level.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2019, 02:23:12 am
content: Quote from: Yansi on May 20, 2019, 01:15:10 pmThe diodes between the bases forward bias the base-emitter junctions and the emitter resistors limit the current. Essentially it is a simplified diamond buffer with diodes replacing two of the transistors. It is not as adjustable as replacing the two diodes with a Vbe multiplier but it still works out to class-AB.If I was designing a simple headphone amplifier, I might do something like that shown below but modified for single supply operation. One advantage of this configuration is a rail-to-rail output range.

author: Yansi
author_rank: Super Contributor
timestamp: May 22, 2019, 11:48:33 am
content: I am not sure if two diodes are enough for any bias to flow through the output stage, are they? Such current will be mostly ill-defined.

author: David Hess
author_rank: Super Contributor
timestamp: May 23, 2019, 03:30:59 am
content: Quote from: Yansi on May 22, 2019, 11:48:33 amThey are, especially if the right diodes are selected, but you are right, the current will be ill-defined. A 60 millivolt difference yields a 10 fold change in current. An alternative for better matching is to use an extra set of output transistors as the diodes and if you do that, you might as well configure the output circuit as the full diamond buffer.Diode connected transistors:https://www.edn.com/design/analog/4363991/High-speed-buffer-comprises-discrete-transistorsDiamond buffer:https://www.tubecad.com/2012/09/blog0244.htm

author: dietert1
author_rank: Super Contributor
timestamp: May 23, 2019, 07:35:01 am
content: The tubecad log is interesting but who wants to make a completely discrete circuit nowadays? I think the proposal to bypass the two 10 Ohm output resistors with a capacitor is valid, but it's application isn't really practical with SMD only.I think the Rod Elliott circuit shown in the beginning is a little more rational. Depending on the OpAmp it will certainly outperform all those diamond circuits.I would use a modern audio OpAmp like OPA2134 instead of a TL082. By the way, the OPA2134 is specified for a 600 Ohm load (+/- 35 mA output current). So for headphones you may not even need an extra output stage.Regards, Dieter

author: ratatax
author_rank: Regular Contributor
timestamp: May 23, 2019, 08:44:49 am
content: I agree I'm don't like very much to design a discrete circuit, as it's a bit like reinventing the wheel and taking so much extra risk in design flaws, but I'm stuck with all the confusion in the amplifier world... So much nonsense informations everywhere, I can't believe finding a modern headphones amplifier schematic is almost impossible, all searches always brings very old designs without any explanations about performances except "it sounds good"I've looked at the OPA2134 and the fact the datasheet mentions 600ohm load and +/-18V VCC is very interesting.However it mentions the voltage output to be max +/-2.5V at 600 ohm, which is I=U/R = 4mA. 4mA @ 2.5V is about 10mW, which seems low for those purposes. In real world I don't know, a typical headphones with high impedance would be about 90dB/mW so it seems OK, but without much margin

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 23, 2019, 09:29:58 am
content: The best IC for a headphone amp I've tried is the LM7171 , however it is not an easy opamp to deal with, requires a good layout and P/S decoupling, plus some careful filtering on the input and the output.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 10:09:20 am
content: Quote from: Alex Nikitin on May 23, 2019, 09:29:58 amThat's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble.Speed is good, but having a lot of it just makes life harder, with no benefit.It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.

author: magic
author_rank: Super Contributor
timestamp: May 23, 2019, 10:25:41 am
content: Quote from: Alex Nikitin on May 20, 2019, 01:35:53 pmNot a big deal given that elcos can safely take up to 500mV of reverse voltage (IIRC) but I am actually autistic enough that I would do the sameQuote from: Kleinstein on May 21, 2019, 01:31:17 pmCertainly not common on modern low voltage portable devices driving modern low impedance headphones as it would unacceptably limit power output. And people say that headphones these days are designed to be driven from zero impedance, so there is that.And it is not without side effects, because dynamic headphones and speakers are quite reactive due to diaphragm inertia and suspension compliance. Think of it as a DC motor: voltage controls speed (and air pressure), current is drawn in proportion to force required to move the diaphragm. At resonant frequency, force/current is close to nothing and impedance shoots to the moon.Therefore voltage division between the driver and the series resistor varies with frequency and boom, unequal frequency response.And yes, you are totally right that it is a problem with certain headphone amplifiers.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 10:36:50 am
content: Quote from: magic on May 23, 2019, 10:25:41 amThe problem is increased distortion, even if the capacitor isn't harmed, although it's probably negligible in this case.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 23, 2019, 10:46:03 am
content: Quote from: Zero999 on May 23, 2019, 10:09:20 amThe LM7171 is the best integrated solution I've heard. On the noise side - it is not a problem in a line level amp, it is not a phono stage! The current noise is about 7pA/√Hz at 30Hz, not 60, and 2pA/√Hz at 1kHz, so a 10K resistor on the input will only increase the noise by about 3dB on top of the voltage noise, which is fine. I've designed a respected headphone amp using this chip about 20 years ago, nobody ever complained about the noise level.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 11:31:59 am
content: Quote from: Alex Nikitin on May 23, 2019, 10:46:03 amYes, I got the decimal point wrong when looking at the current noise graph. I must've been looking at the scale for the voltage noise!I agree, if it's not providing much voltage gain, then the noise is negligible. Still I think using a video amplifier IC for a headphone amplifier is asking for problems with oscillation. No doubt it can be done, but why bother? There are far more suitable solutions which are far more stable and are better suited to the application.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 23, 2019, 12:29:14 pm
content: Quote from: Zero999 on May 23, 2019, 11:31:59 amThe reason is very simple - the sound quality when used with various real world headphones.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 12:33:33 pm
content: Quote from: Alex Nikitin on May 23, 2019, 12:29:14 pmToo bad if there's a risk of RFI, especially with long cables.There are far more suitable solutions, offering the same/better sound quality and a much lower risk of turning into an unintentinal radio transmitter.

author: dietert1
author_rank: Super Contributor
timestamp: May 23, 2019, 01:38:08 pm
content: When i read claims that an amplifier sounds different, my first thought is: Maybe it got a problem. Any amplifier with low output impedance, a reasonably flat frequence plot and less than 0.1 % THD up to 10 KHz at somewhat elevated output levels sound the same even for a trained listener. That amplifier is easy to build using modern parts.Most tube amplifiers sound "interesting" since they don't fulfil these criteria (higher output impedance and THD more like 1 or 2 %). Higher THD simulates higher sound level..Regards, Dieter

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2019, 02:08:30 pm
content: Quote from: dietert1 on May 23, 2019, 01:38:08 pmI mostly agree but beware of THD figures which don't always tell the full story. Some types of distortion are noticeable at low levels of THD, such as crossover distortion, whilst others such as slew rate limit distortion aren't noticeable until much higher THD numbers. In this case, my suspicion is the LM7171 could be ringing at certain frequencies which could be seen as sounding brighter, boosting treble response.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 23, 2019, 03:43:04 pm
content: Quote from: dietert1 on May 23, 2019, 01:38:08 pmLet's say my experience is different from yours.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 02:54:24 am
content: Quote from: dietert1 on May 23, 2019, 07:35:01 amIn this case, the diamond circuits are meant to be used within the feedback loop of an operational amplifier and act as a current amplifier.My preference would be to use something based on the earlier examples I gave because full power bandwidth is increased, gain-bandwidth product is increased, and the output operates closer to the supply rails for increased available output power. And this can be done with no significant increase in complexity compared to the class-ab output stage.The disadvantage is that circuit operation is more complex and less applicable to learning how common audio power amplifiers work.Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection. Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.Another solution which avoids using discrete parts is use several audio operational amplifiers in parallel to get the needed power dissipation. If rail-to-rail output parts are used, then increased output power for a given supply voltage will be available also. There are several ways to configure operational amplifiers in parallel.

author: Audioguru
author_rank: Super Contributor
timestamp: May 24, 2019, 03:49:22 am
content: Quote from: ratatax on May 23, 2019, 08:44:49 amNo. the datasheet says the voltage loss when driving a 600 ohm load is the negative supply plus 2.2V to the positive supply minus 2.5V. So if the supply is +/-15V the output swing into 600 ohms is 25.3V p-p which is 9V RMS which is 135mW which would be very loud. Without the positive swing clipping a little more than the negative swing then the output power into 600 ohms with extremely low distortion is 130mW.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 07:51:09 am
content: Quote from: David Hess on May 24, 2019, 02:54:24 amThe LM7171 in a DIP8 package can output about 0.7W RMS into a 100 Ohm load continuously with a +/-15V supply, do you even need that much for a headphone amplifier? On the other hand, driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life (or put it differently, why measurements don't tell a complete story) and why so many different amplifier designs exist.CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 08:45:32 am
content: Quote from: Alex Nikitin on May 24, 2019, 07:51:09 amAny measurements should be performed with the speaker or headphones connected to the amplifier, via a suitable length of cable. I'd say this is even more important when you're using an IC out of specification, as would be the case for the LM7171 driving headphones, rather than the purely resistive load it was designed for.Yes, different amplifier designs exist for different purposes, which is why using a video amplifier for audio is daft, even if some people like the colourful ringing and distortion it might add to the sound.Quote from: David Hess on May 24, 2019, 02:54:24 amYes, the LM7171 isn't suitable for use as a headphone amplifier. There are much better alternatives. Just because it can be done, it doesn't mean it's the best way.QuoteYes, it also results in lower noise and distortion.

author: magic
author_rank: Super Contributor
timestamp: May 24, 2019, 08:53:03 am
content: Quote from: Alex Nikitin on May 24, 2019, 07:51:09 amTrue that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.I'm not aware of any instances of such demonstration actually being done. I suppose it would be all over the Internet if it happened, given the amount of controversy.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 09:07:58 am
content: Quote from: Zero999 on May 24, 2019, 08:45:32 amI personally find this amusing. I have shared some information which I consider valuable, as it is the result of many hours of work at the time I did that design. I am not trying to sell anything. I have a practical experience using that particular chip for that particular application in a production headphone amplifier which was produced for many years and in many thousands of units. And I get theoretical arguments from people who most likely never tried to work with that particular IC and not even read the data sheet carefully (as it clearly states the LM7171 can be used for driving ADSL lines and transformers for instance). ADSL drivers in general are not a bad choice for headphone amplification (low distortion, high output current and ability to work on difficult loads) and there are designs which use them*.CheersAlexP.S. Worth noting that TI actually sells the THS6012 Dual Differential Line Driver as a "High Fidelity Headphone Amplifier" chip under a different name (TPA6120A2).

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 09:12:23 am
content: Quote from: magic on May 24, 2019, 08:53:03 amIt is getting completely off-topic, however I will only ask a simple question: can you provide a link to an example of a statistically valid controlled DB listening test with apositiveresult?CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 10:15:05 am
content: Quote from: Alex Nikitin on May 24, 2019, 09:07:58 amI don't doubt that it's possible to build a headphone amplifier with the LM7171, just that it'll make life harder, with no benefits, over ICs specifically designed for it.What "valuable information" have you posted? You've not posted any details of your design: schematic or layout. All you've done is said "I've used the LM7171, an IC which is not designed to be used as a headphone amp, as a headphone amp" and are wondering why people are questioning it. You haven't provided any solid reasons to use it, over the alternatives, in the face of valid arguments not to use it, as a headphone amp. This is not helpful, especially in the beginners section as someone with far less experience might try the LM7171 and wonder why it doesn't work!Quote from: Alex Nikitin on May 24, 2019, 09:12:23 amI'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 10:23:12 am
content: Quote from: Zero999 on May 24, 2019, 10:15:05 amSo, you can not provide a link to a statistically valid DBT with apositiveresult (meaning that there is a difference in the sound confirmed in the test) ? So, you have a "gold standard" with negative results only, don't you think it is funny?CheersAlex

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 11:01:42 am
content: Quote from: Alex Nikitin on May 24, 2019, 10:23:12 amI still don't know what you're going on about. Please provide a more detailed explanation of what you're asking.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 12:09:33 pm
content: Quote from: Zero999 on May 24, 2019, 11:01:42 amOK, let's go back to your earlier post:Quote from: Zero999 on May 24, 2019, 10:15:05 amThe DBT is a tool. To use any tool you should understand its capabilities and limitations, otherwise you can not trust the results. My question is very simple - could you provide an example of a statistically valid DBT with a positive result, i.e. where the difference between the sound from two pieces of equipment was registered. Clearly, if you have 50% distortion on one of the sources it should give a good positive result, but no one will bother with this kind of test. However unless we have examples of positive results it is not clear where the test sensitivity border line is. Saying that it is "industry standard", "gold standard" and such doesn't prove anything, it is only a convenient smoke screen. DBTs in audio are notoriously insensitive, that is why their use is essentially a biggest can of a snake oil in this industry, IMHO. Well, again, could you provide a link toan example of a statistically valid audio DBT with a positive result?CheersAlex

author: ratatax
author_rank: Regular Contributor
timestamp: May 24, 2019, 12:54:03 pm
content: I know your suggestions here may be using higher quality parts, but what about this one :http://www.ti.com/lit/ds/symlink/rc4580.pdfIt's cheap, has some oomph to be used as a headphones amplifier, has low enough THD/noise... and the implementation seems dead easy.I'm a musician but I like when choices are made with measurable, scientific proofs, since audio as it is perceived by our ears is way too much subjective. Some opamps with thousand of V/uS slew rate and hundred of Mhz bandwidth may sound very good but I feel it's just unneeded complications (unneeded costs too), like you can buy a race car to drive at 30mph in a city...but WHY ?

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 12:57:17 pm
content: Quote from: Alex Nikitin on May 24, 2019, 12:09:33 pmYes, a DBT is a tool. If the results are that no one can differentiate between two amplifiers, any better than chance, then the two amplifiers both sound the same. This is a scientific fact. If no one can tell the difference between the two amplifiers, then saying the they are sonically different is lying. The same is true of a quack claiming their new spiritual healing treatment cures cancer: without a DBT it's total lies.No doubt two amplifiers can sound different and will give a positive result when subject to a DBT. This is likely to be the case if the amplifiers are over-driven or genuinely do produce distortion in normal operation. Some people might like certain types of distortion, so keeping THD figures down isn't always the goal.The idea people can hear things which can't be measured is snake oil. It's true the human ear is more sensitive to some types of distortion than others, so THD figures alone aren't always that useful, but everything which is audible is measurable and is the whole point in having expensive test equipment.No doubt tests have been done to determine what level of distortion is audible, tolerable, and immediately noticeable, in order to develop efficient lossy audio compression algorithms and determine the bare minimum standard for an audible transparent audio amplifier.To answer the question: no, I've not seen an example statistically valid DBT with a positive result, comparing two amplifiers, but that doesn't mean to say that DBTs aren't valid. It most likely means that all decent amplifiers quality sound the same, as long as they're operated within their specifications.I have watched Youtube videos demonstrating what levels of THD, of different types of distortion, people can hear and it would be interesting to see some more scientific DBTs.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 01:09:42 pm
content: Quote from: Zero999 on May 24, 2019, 12:57:17 pm1) Please note that you "have no doubts" but you couldn't provide even a single link, only generic blah-blah. You should try to find an example I asked for, you might be in for a surprise.2) Nowhere I said that "people can hear things which can't be measured" , that is you putting words in my mouth, and I disagree with this idea. I like measuring things, moreover, my background in electronics is in the test and measurement.CheersAlexP.S.Quote from: Zero999 on May 24, 2019, 12:57:17 pmAnd that is not science, but a pure act of faith.

author: dietert1
author_rank: Super Contributor
timestamp: May 24, 2019, 01:16:45 pm
content: Can't you just continue that discussion in private communication? I am afraid you are trolling this thread with your audiophoolisms.Regards, Dieter

author: ratatax
author_rank: Regular Contributor
timestamp: May 24, 2019, 01:27:07 pm
content: Any audio-related discussion inevitably ends like that

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2019, 01:52:26 pm
content: Quote from: ratatax on May 24, 2019, 01:27:07 pmYes it does get on my nerves how audio threads invariably turn to audiophoolery and trolling. I apologise for my part in it and am no longer responding to it.Quote from: ratatax on May 24, 2019, 12:54:03 pmThat's a good idea. I'll add that to my list of decent op-amps.How about the MC33078 or use two channels of the MC33079 in parallel?The NJM2060 also looks good.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 02:12:41 pm
content: Same, I am sorry for this off-topic, stops here.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 07:45:10 pm
content: Quote from: Alex Nikitin on May 24, 2019, 07:51:09 amThe reactive part of the load changes the phase relationship between the current and voltage which can be seen in a class-b design where the crossover distortion will move away from zero volts. But in a well designed amplifier, it makes no practical difference assuming that the reactive load characteristics were designed for.A more important effect is that the reactive load shifts the current to voltage across the output devices which can result in violation of their breakdown rating. The protection circuits and output device derating need to take this into account.Quote from: magic on May 24, 2019, 08:53:03 amI have seen differences in amplifiers with otherwise similar characteristics in two areas:1. Harmonic and intermodulation distortion tests will not reveal low full power bandwidth which will create additional distortion in real content. Production designs should never have this problem.2. Overload recovery after clipping can be significantly different. This is especially a problem with naive designs which use frequency compensation to control noise and bandwidth and I suspect this is where negative feedback in audio amplifiers gets a bad image.As far as the like of rigorous scientific tests, maybe they are allergic?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 24, 2019, 10:04:00 pm
content: Quote from: David Hess on May 24, 2019, 07:45:10 pmThe main problem is not that loudspeaker is a reactive load (at least that is a reasonably measurable and easy to deal with effect) , but that it is a very non-linear load, and not only load but a source as it acts as a microphone picking up vibrations and sounds. All this rubbish gets back to the amplifier and enters the feedback loop, as well as other drivers in a multi-way speaker.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 11:55:01 pm
content: Quote from: Alex Nikitin on May 24, 2019, 10:04:00 pmI have difficulty imaging that being a problem with the low dampening factor at the feedback point of a solid state amplifier combined with the cable and speaker coil resistance.Speakers are specifically designed to be linear because otherwise excessive intermodulation distortion is produced by the speaker itself. This takes the form of using a narrow voice coil within a large field of magnetic flux or a wide voice coil with a narrow field of magnetic flux. Woofers present the most difficulty because they must support a large throw to produce good loudness at low frequencies and base-reflex enclosures help considerably in this respect by lowering the impedance seen by the driver.Interaction between drivers should be prevented with active or passive crossovers.But I would agree that a lot of speakers intended for the consumer market are trash. But hey, the customer knows what sound they want.

author: dietert1
author_rank: Super Contributor
timestamp: May 25, 2019, 07:14:06 am
content: Yes, i agree there is a huge difference between somebody ignorant using some sound system and a technical person using a sound system designed or combined by himself. Sound causes feelings! I know lots of well educated technical people who go crazy about horn speakers and the selection of the right foil capacitor.But human hearing is extremely flexible.Think about the differential analog transmission still used in professional audio. The input stage can be made such as to yield -6 dB when one of the two contacts in the connection fails. Or it can be made to yield -60 dB in that case. Now you may wonder how many of those connections in the real world are jumping between 0 dB and -6 dB without anybody taking notice.Regards, Dieter

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2019, 08:37:37 am
content: Quote from: David Hess on May 24, 2019, 11:55:01 pmYes, speakers and room acoustics are the weakest links in the chain as far as distortion is concerned and dominate over even mediocre amplifiers. The audio amplifier is a problem which has been solved a long time ago. The only improvements made recently involve reductions in sized and increased power efficiency.Headphones are better than speakers distortion-wise, because they're so small and the path between the ear drum and transducer is short (especially for in ear buds), transmission line effects are far less apparent, so it can be treated as a diaphragm changing the pressure levels inside the ear canal, especially at lower frequencies.Try this. Wearing a decent pair of headphones, put some music on with a decent bassline and set the volume to a safe, comfortable listening level. Notice the deep thumping bass. Now take the headphones off, hold them around 30cm away from your ears and crank up the volume until you can probably hear the music. Notice how tinny they sound: the bass has magically gone! To get bass, there needs to be a decent seal between the headphones and the ear drums, otherwise the changes in pressure which produce the bass escape and are lost.Quote from: dietert1 on May 25, 2019, 07:14:06 amSound causes feelings, so many of the perceived differences in amplifiers are often down to the placebo effect, rather than anything real. There is nothing wrong with this, until people start to make extraordinary claims, which cannot be proved.

author: magic
author_rank: Super Contributor
timestamp: May 25, 2019, 08:55:29 am
content: Quote from: Alex Nikitin on May 24, 2019, 09:12:23 amI can't because I don't care, but of course such tests must have been done if there are people talking about "scientifically established" thresholds of audibility of various defects.But that's not the point, I don't really insist that they were valid or anything. I only remarked about the suspicious tendency of audiophiles to show up everywhere and say a lot about what they hear while making zero effort to convince that what they hear is determined by the actual sound rather than by their beliefs about what produced the sound.Say that I swapped a few paralleled NE5532 into one of those LM7171 amplifiers of yours, would you tell it apart?After all those years, somebody could perhaps show something like that. Maybe a vendor looking for publicity, maybe hobbyists looking to stick the finger to nwavguy and his religion, anyoneAnd the problem of people hearing their beliefs rather than sound is obviously real. Gold plated USB plugs, compact disk demagnetizers, NE5532 relabeled as LM4562, all that junk sells and fetches positive feedback from satisfied customers.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2019, 07:49:14 pm
content: Let's not continue the off-topic stuff.I'm surprise no one has done a simulation.The quiescent current was around 25mA, which seems a little high, although may not reflect reality.Overcurrent protection could easily be added by connecting some back-to-back diodes between the output (the emitters of Q1 and Q2) and the op-amp's output. This will limit the voltage drop across R7 and D8 to the diode's foraged voltage. Ordinary diodes will clamp at 0.6V, giving a current limit of 60mA, although there may be some distortion before then. The output current will be a little higher, than that because the op-amp's output current will be added to the output, which will make around 100mA, for the NE5532. A couple of red LEDs connected back-to-back or use a bipolar LED, will limit the current to a couple of hundred mA.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2019, 10:52:36 pm
content: Quote from: magic on May 25, 2019, 08:55:29 amI could; the LM7171 will produce noticeably more hiss because of its greater input noise. This might not be noticed in a noisy environment however.

author: magic
author_rank: Super Contributor
timestamp: May 25, 2019, 11:34:43 pm
content: I will add an LM7171 amplifying its own noise and mix it with the input signal, just for you

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2019, 12:17:59 am
content: Quote from: magic on May 25, 2019, 11:34:43 pmThe noise (hiss) only bothers me (becomes noticeable) with no input signal.Another configuration is to use the low noise audio operational amplifier, NE5532 or whatever, to drive the LM7171 which acts as the high current output stage. Now the LM7171 can operate at lower gain and its noise is suppressed by the excess gain of the NE5532's feedback loop.The noise of an LM317 regulator can be suppressed using the same type of circuit.

author: Yansi
author_rank: Super Contributor
timestamp: May 26, 2019, 08:23:13 am
content: 317 is quite low noise by itself. Doubt it will have any major contribution to the amp output noise.

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2019, 12:54:08 pm
content: Quote from: David Hess on May 25, 2019, 10:52:36 pmWith a gain of 23? Possibly if you have some very sensitive earbuds.In all fairness the proponent for the LM7171, as a headphone amplifier, didn't give any details regarding their design i.e. the gain.The LME49720 seems like a good candidate for a headphone amplifier, but has a relatively high input current noise, so will only be any good, if the source impedance is low. It also has a large enough bandwidth to cause problems, if insufficient care paid to the layout.http://www.ti.com/lit/ds/symlink/lme49720.pdf

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2019, 06:03:01 pm
content: I added the current limiting diodes and reran the simulation. The actual limit is a bit higher than I predicted at 130mA. Note that there's a point when the limiting kicks in where the op-amp is contributing a significant amount to the output current: see the Ix(U1:OUT) trace. It's a good idea to keep the current below this level, even if it isn't clipping, as there will be some distortion.

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2019, 09:52:49 pm
content: Quote from: Yansi on May 26, 2019, 08:23:13 amIt would not matter for audio applications unless the amplifier had an unusually low power supply rejection. I just mentioned it as an example of the same idea and some applications do require lower noise than an LM317 can provide alone.

author: ratatax
author_rank: Regular Contributor
timestamp: May 27, 2019, 08:55:48 am
content: Thanks a lot for the simulated schematic, i'll try LTSpice to see what I can get from it