Linear audio amplifier IC suggestion (12V supply, few W into 4ohm load) - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: January 08, 2024, 10:43:18 pm
content: Hello,just got to realize, that there is not many if any at all linear audio amplifiers being made nowadays. Trying to build some rather simple measurement gizmo, where a small output power is required, say a watt or two into a 4ohm or even slightly lower load impedance.First wanted to search for a modern SMT packaged device (rest of the device will be SMT in a low profile enclosure), but found none suitable. If anything, it has only a headphone power, I need watts, not milliwatts.Then I was like oh well, lets just stick there a good ole TDA20xx amplifier chip, it could be bent and soldered horizontally too. But sh!t was I wrong! None of the well known TDA ICs are being made anymore! TDA2030, 2005, 2003, nothing. Obsolete, no replacement, apart of some questionable garbage with pricing that they can shove them down their ass!https://www.tme.com/us/en-us/details/nte1380/rtv-audio-integrated-circuits/nte-electronics/So what I am looking for: 12V supply,low distortion, must do couple W into 4ohm load. By low distortion i mean to not suggest stuff like LM386. Too weak, and THD level garbage.I was not expecting I would need to design a discrete component amplifier for this simple task...Thank you for any suggestions.Y.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 08, 2024, 11:00:46 pm
content: Is externally buffering an opamp out of the question? That's usually my goto technique to boost power from an underpowered, but otherwise ideal amp.

author: mariush
author_rank: Super Contributor
timestamp: January 08, 2024, 11:04:27 pm
content: TDA2050 is still made by asian companies :https://www.lcsc.com/search?q=tda2050example datasheet :https://datasheet.lcsc.com/lcsc/1809201023_UTC-Unisonic-Tech-TDA2050L-TB5-T_C84890.pdfST datasheet with example circuit board layouts for both single power supply and split power supply :https://www.st.com/resource/en/datasheet/cd00000131.pdfIt needs at least 9v ( or +/- 4.5v) to work, up to 50v / +- 25v , so it would work with 12v.... and would probably do 3-4 watts with 4 ohm speaker.tda7266 is in stock at digikey :https://www.digikey.com/en/products/detail/stmicroelectronics/TDA7266/677596TDA7266M (single channel version of the above) is still stocked at digikey, last time buy though :https://www.digikey.com/en/products/detail/stmicroelectronics/TDA7266M/1665067other stuff... maybe tpa1517 but not so great graphs, thd just a bit below 1% on higher frequencies :https://www.digikey.com/en/products/detail/texas-instruments/TPA1517DWPR/374558

author: moffy
author_rank: Super Contributor
timestamp: January 08, 2024, 11:12:50 pm
content: The LM4950 doesn't look terrible, 0.03%THD@1kHz@3W into 8R:https://www.ti.com/lit/ds/symlink/lm4950.pdf?ts=1704755347041&ref_url=https%253A%252F%252Fwww.ti.com%252Faudio-ic%252Famplifiers%252Fspeaker-amplifiers%252Fproducts.html

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2024, 11:14:16 pm
content: Go to ti.comThey have plenty, both from the National portfolio as well as their own.

author: Yansi
author_rank: Super Contributor
timestamp: January 08, 2024, 11:39:58 pm
content: Hey guys (and girls), please read carefully. 4ohm load, 12V, couple W, low distortion.Quote from: Andy Chee on January 08, 2024, 11:00:46 pmWas thinking exactly the same. But once you add all the bells and whistles (bias current, over-current protection, etc..) it gets quite complicated. And due to the opamp not being usually really a RRIO one, you get so much voltage drop together, you can barely get a watt or two to the load at the end. I don't find this really great, although still about usable.mariush: Almost all if not all TDA chips have chinese/taiwanese/various clones. Don't want to rely on those. Plenty available on LCSC.Quote from: moffy on January 08, 2024, 11:12:50 pmGood catch. How could I miss the LM495x series?Benta: Been there, done that, although probably used too tight filtering in the product table, so some did not get enough attention they would deserve. Anyway, still did not find anything amazing yet.So far looks like the LM4950 could be usable, but the THD is not anything to brag about, is about the level of THD i could get from the common TDA chip, which I was willing to try.Wondering if I could still beat it with a simple discrete built amplifier

author: wasedadoc
author_rank: Super Contributor
timestamp: January 08, 2024, 11:41:29 pm
content: Quote from: Andy Chee on January 08, 2024, 11:00:46 pmYou are joking, are you not?

author: Andy Chee
author_rank: Super Contributor
timestamp: January 08, 2024, 11:52:20 pm
content: Quote from: wasedadoc on January 08, 2024, 11:41:29 pmWell if the OP is desperate enough to resort to discrete component circuit design for their solution, then why not?Of course the devil is always in the detail that is missing from the OP. "Low distortion" isn't exactly the most precise language for an engineering specification.

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2024, 11:59:26 pm
content: Try looking at the LM4752. Seems to fit the bill.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 09, 2024, 12:10:12 am
content: Quote from: Yansi on January 08, 2024, 11:39:58 pmYou may be able to help the THD along ?The data suggests lower gains have better THD, and of course it varies with load resistance, so an 'overpowered' choice might be good enough ?

author: langwadt
author_rank: Super Contributor
timestamp: January 09, 2024, 12:19:14 am
content: power opamp, something like an OPA549/OPA547 ?

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 12:27:39 am
content: Quote from: Benta on January 08, 2024, 11:59:26 pmPerformance wise seems similar to the LM4950, maybe slightly worse even. Seems 4950 could have less distortion. I compared 12V 4ohm SE operation. (Need load against ground). Also like the external gain setting possibility and low/unity gain stability it offers. 4752 has way too high gain to my liking.Quote from: Andy Chee on January 08, 2024, 11:52:20 pmDon't see much point, as obviously, there's not much to choose from to begin with. Higher performance would require discrete solution and possibly wiser supply voltage choice. Now just trying to balance cost, complexity and performance.Quote from: langwadt on January 09, 2024, 12:19:14 amOPA549 possibly would work at 12V into 4ohm with less THD, but is HELLAEXPENSIVE. Way too much.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 09, 2024, 12:35:51 am
content: Quote from: Yansi on January 09, 2024, 12:27:39 amWell you could at least give us a bigger hint as to what your measurement gizmo application is! It would at least give us an indication on distortion and other performance limits. For all we know, you're being way too picky!

author: moffy
author_rank: Super Contributor
timestamp: January 09, 2024, 01:01:05 am
content: If you want to improve the LM4950 THD you could try Andy Chee's idea of a composite amp but use the LM4950 in low gain as the output buffer, but it depends upon the phase margin and how you configure/compensate the first amp. The detail would be important.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 07:23:18 am
content: The composite amp idea is actually a very good idea. I like it, it could also solve problem of the LM4950 being inverting amplifier.Total overall gain needed is probably 4x (12dB) or slightly less. The DAC is providing me 2Vpp, and for target 2W into 4ohm I would need 8Vpp exactly (4Vp, 2,83Vrms).Thank you for the composite amp idea with the 4950 or similar!

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 06:36:41 pm
content: I guess, by a composite amp something like this was thought of in this case (imagine U2 is the LM4950):Unfortunately, no SPICE model is available for the LM4950. Just tried to verify the topology, using just two TL07x opamps, which i will likely end up using in the design. Hopefully, it will not be too difficult to get it stable, once I will have LM4950 available to play with.The only question is whether it makes sense to keep the 4950 at the minimum gain of -1, or for example to split the gain equally (total gain needed is 4x, so each amp would provide gain of 2x) to extend the bandwidth of the amplifier maximally.Y.

author: Benta
author_rank: Super Contributor
timestamp: January 09, 2024, 07:28:30 pm
content: I totally fail to see what this would achieve, except increased parts count.

author: magic
author_rank: Super Contributor
timestamp: January 09, 2024, 07:38:27 pm
content: Quote from: Yansi on January 09, 2024, 07:23:18 amThe topology you posted next still seems overall-inverting, so what did you mean by "solve"?BTW, can't the DAC input be inverted or the load connected backwards?Quote from: Benta on January 09, 2024, 07:28:30 pmTHD can be reduced.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 08:09:07 pm
content: Yes, THD reduction is the goal. LM4950 is used just as a relatively cheap protected power stage.Inverting or not, you can feed it from the other input of the U1 to get non-inverting action. However, I do not like it, as I am not sure in this case the input impedance would behave purely resistive. (Feeding signal into R3 should provide a well behaved resistive load to the source DAC).Software is unfortunately already written (not by me and not by anyone willing to modify it for me - it is an old commercial measurement suite), so it may not be as simple to invert the signal. However, adding an extra opamp inverter still should be of a far less issue, than a shitty amplifier at the end.Note: As noted inpost #15,U2 in the schematic is supposed to be the LM4950. There is no SPICE model available for LM4950, hence for a simple test replaced by another TL074.

author: Fryguy
author_rank: Regular Contributor
timestamp: January 09, 2024, 08:42:58 pm
content: Just out of curiosity - how many units are you planing to make ? If i needed half a dozen nice TDA20xx chips to make a single unit for myself or two , i'd go to the scrapyard and pick up a couple of old crappy car radios ! Millions of good working TDAs are waiting there . . .

author: langwadt
author_rank: Super Contributor
timestamp: January 09, 2024, 08:56:01 pm
content: Quote from: Yansi on January 09, 2024, 12:27:39 amyeh it is getting old, how about OPA564 ?

author: Benta
author_rank: Super Contributor
timestamp: January 09, 2024, 09:11:49 pm
content: Quote from: magic on January 09, 2024, 07:38:27 pmI fail to see how adding amplifier stages reduces distortion. But perhaps there's a Floobyduxt factor involved.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 09:21:06 pm
content: What is different in using a discrete power (current) amplification stage enclosed in a global feedback, from a solution where the powerstage is more integrated and enclosed within a global feedback?

author: Benta
author_rank: Super Contributor
timestamp: January 09, 2024, 09:37:14 pm
content: Define "more integrated".IC power amps have exactly the same architecture as discrete (or semi-discrete) amps:input stage with voltage amplification, driver stage with voltage/current amplification and level shifting, and current amplifying output stage.You can add (unnecessary) amplfication stages in between, but why?

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 10:56:02 pm
content: Because today I try to be patient, my repeated hint is here: The power amplifier stage is within a global feedback loop.Please have a look for example here:https://www.analog.com/media/en/analog-dialogue/volume-53/number-3/composite-amplifiers--high-output-drive-capability-with-precision.pdfLinearity is dictated by AMP1 (or U1 in my case respectively). AMP2 (U2, the LM4950 in my case) serves just for power amplification.Does not matter if U2 will be discrete complementary pair of power transistors, or another power opamp. In this design, I chose LM4950, because of its simplicity, protected output and integrated biasing... or prove us wrong otherwise please.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 09, 2024, 11:06:55 pm
content: Quote from: Yansi on January 09, 2024, 10:56:02 pmI disagree. For example, the configuration proposed earlier using complementary emitter followers without bias will not have good linearity.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2024, 11:13:41 pm
content: In AMP1 is ideal amplifier, then yes, it is true.For example: Using just the unbiased complementary pair will have awful linearity (lots of crossover distortion). When put into a global feedback with an opamp, it starts to behave much better, doesn't it? If bias is added and the complementary pair will operate in class AB, would't that improve some more?Now tell my why compositing the LM4950, with a more precise amplifier, will not improve upon its distortion. I still think it will improve. However, without a SPICE model for the 4950, hard to make estimations. Once I get samples, I will try to measure that improvement.

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2024, 11:17:18 pm
content: Quote from: Andy Chee on January 08, 2024, 11:00:46 pmIt will work, but downside is high crossover distortion.Pass transistors are generally better because the op-amp's internal transistors work at low currents and the external ones only turn on, when necessary. R1 and R2 must be low enough to ensure the output transistors don't turn on, due to the op-amp's quiescent current. R3 and R4 aren't really needed, but improve stability and will offer some level of over-current protection, if the output is shorted.

author: magic
author_rank: Super Contributor
timestamp: January 09, 2024, 11:18:36 pm
content: To be exact, U1 has its own tight feedback through the C1/R7 divider, so it's the closed loop gain of this loop which appears in the forward path of the amplifier, not full open loop gain of U1.The argument is simple: any distorted differential voltage appearing at U2 input is divided by closed loop gain of the U1 block before appearing at the differential input of U1, where it is amplified by noise gain of the global feedback network as usual. Without U1, U2 error voltage itself would appear at the output amplified the same.In this configuration U1 produces substantially the same output swing as U2 and it could produce similar distortion on its own, but not if:a) U1 is more linear than U2b) U1 is identical to U2 but less loaded (frankly, a special case of a))As far as I understand, further improvement is possible by reducing the output swing of U1, i.e. increasing local closed loop gain around U2.You don't exactly get "the unloaded linearity of U1 combined with the load driving ability of U2", but it can be better than either of them working alone.

author: langwadt
author_rank: Super Contributor
timestamp: January 09, 2024, 11:36:22 pm
content: Quote from: Zero999 on January 09, 2024, 11:17:18 pmwith R1/R2 you can set a quiescent current in the output transistors

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 09, 2024, 11:41:28 pm
content: Quote from: langwadt on January 09, 2024, 11:36:22 pmTrue, but with terrible temperature stability.There is merit in the broad idea, it just needs better thermal tracking.One plus of this design, is it has very good rail-rail swing, whilst emitter follower designs lose voltage at both rails.I've been pondering what a 'few watts' discrete solution might look like, in 2024.This circuit from another thread, takes steps to fix the temperature issues, of theOpAmp power pinstopology. (of course, it needs part scaling for +/- 2A drive)In 2024, parts like PHPT610035 & DXTC3C100PD give nice SMD packages with about the right thermal power, and they have two die in one package,for very good thermal tracking.The current mirror needs to be adjusted to not be 1:1.In the SCH above they use diode drive limiters with RE's to set a max current.

author: langwadt
author_rank: Super Contributor
timestamp: January 09, 2024, 11:49:46 pm
content: Quote from: PCB.Wiz on January 09, 2024, 11:41:28 pmor maybe the question is why would you want to, when you get an integrated solution then also includes protection ..

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 12:00:24 am
content: Quote from: langwadt on January 09, 2024, 11:49:46 pmYes, it's mostly an academic exercise, for those cases where the generic parts distortion is not good enough.

author: Yansi
author_rank: Super Contributor
timestamp: January 10, 2024, 12:17:39 am
content: PCB.Wiz: This actually looks interesting to me. I have known the "power supply pin solution", however never seen this variant used.If I understand this right, the power stage bias current is a mirrored and multiplied copy of the bias current of the opamp itself. (leading to about 10mA bias for Q1/Q2 output pair).If I think about the diodes D1 D2, are they part of a current limiter? Clamping the voltage at the emitters to about 0,6V, meaning output has a limit of +-60mA or so.But what I can not fully understand is the voltage divider against the output of the opamp.Could you please share the thread where this was discussed, or the source of the schematic? This is making more sense to me to do, than to sodomize LM4950 with external opamp, expecting improvement.//EDIT: Just found this doc, with explanation of the circuit:https://www.eevblog.com/forum/beginners/low-impedance-amplifier-with-broad-frequency-range-suggestions/?action=dlattach;attach=617647

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 12:33:44 am
content: Quote from: Yansi on January 09, 2024, 06:36:41 pmYou may need some gain just above unity, in the output stage, as the preamp side is not designed as rail-rail.There are also parts like TDA2030 / TDA2050 that could be worth trying ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 12:58:24 am
content: Quote from: Yansi on January 10, 2024, 12:17:39 amYes, it took a while to find one that did more correct thermal tracking.Quote from: Yansi on January 10, 2024, 12:17:39 amBroadly, yes. You scale things for higher currents.Quote from: Yansi on January 10, 2024, 12:17:39 amR2 is needed to give variable current on the supply pins. ie the OPAMP needs a lowish load, in order to get enough base drive current flowing.R1 & C1 help stability. If they are removed entirely, light loads can oscillate.Sim suggests > ~300pF is where C1 settles things down.Quote from: Yansi on January 10, 2024, 12:17:39 amIt was more mentioned in passing, than discussed in depth.https://www.eevblog.com/forum/chat/favorite-op-amp/msg2349186/#msg2349186

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 01:41:39 am
content: Quote from: Yansi on January 08, 2024, 10:43:18 pmWhat distortion do you target, at what frequency ?When you look at the old power-amps vs new opamps, the power-amps have a significant slope on THD at the low frequency end, I guess that's 'thermal ripple' on the die itself.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 10, 2024, 05:32:42 am
content: Quote from: Zero999 on January 09, 2024, 11:17:18 pmCrossover distortion is minimised due to the buffer transistors being included within the feedback loop. Of course, the degree to which it is minimised is entirely dependent upon the transistor and opamp's characteristics.

author: Yansi
author_rank: Super Contributor
timestamp: January 10, 2024, 03:09:58 pm
content: PCB.Wiz: Unfortunately, seems all TDAs dead, no longer made. And I do not want to play with the Chinese clones. And I did not hoard a good stash of those. Could probably find a one or two used TDA2030 pulled from some old junk, but that's it. Wouldn't use for a device I plan making more than one piece.In any way, thank you very much for providing some detailed insight into the operation of the boosted opamp circuit.Could you please elaborate more on the reason of the voltage divider against the opamp output? It seems this circuit exist in say, two or three variants:1) Output of opamp directly connected to output. What may be the drawback here?2) Output of opamp terminated to GND with low resistance.3) Output of opamp terminated to the voltage divider from output.I understand the C2 (as referenced in your schematic, the cap between an output of the opamp and the load output) is part of frequency componesation. Would be probably worth playing with it in the simulator, to see exactly what it does.In case the voltage divider (option 3) is used, how is the suitable ratio determined? Does it decrease the gain of the transistor buffering stage? It seems, the voltage divider is working against the change of supply current. If opamp wants to go more positive, voltage is increased at the node, increasing the supply current. But as the output also increases in voltage, part of the current to the node is supplied by the boost stage.By the way, where doesthis document containing the circuitcomes from? Looks like to be a part of some book.

author: langwadt
author_rank: Super Contributor
timestamp: January 10, 2024, 05:56:00 pm
content: Quote from: Yansi on January 10, 2024, 03:09:58 pmdigigkey et.al. has plenty of +4W audio amps and +1A opamps, what's wrong with those ?

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2024, 06:41:15 pm
content: Quote from: Yansi on January 09, 2024, 07:23:18 amHow about the TDA7269A?It appears to be still manufactured and is available in RS Components and Digikeyhttps://www.digikey.co.uk/en/products/detail/stmicroelectronics/TDA7269A/715880https://uk.rs-online.com/web/p/audio-amplifier-ics/7959233?gb=sOh maybe not. It's not specified for gains under 25dBYou could try setting a much higher the noise gain, perhaps at an ultrasonic frequency.Refer to the following thread.https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/msg2401983/

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 06:51:29 pm
content: Quote from: Yansi on January 10, 2024, 03:09:58 pmI've only seen that in symbolic circuits, simplified ones, not real working amplifiers.It makes the gain more dependent on load impedance, and it also compromises the full swing voltage.Quote from: Yansi on January 10, 2024, 03:09:58 pmI would do a design as shown, sim shows the C is certainly helping light load stability.The 499 R is more optional, but it does provide a defined lightest load.Quote from: Yansi on January 10, 2024, 03:09:58 pmIt defines the highest voltage gain, and provides some defined load for the amplifier.Sim gives a guide, but exact values would be determined on the test bench with a good scope.

author: Yansi
author_rank: Super Contributor
timestamp: January 10, 2024, 09:23:27 pm
content: Zero999: Thank you for a suggestion, however I'd like not to use a THT part of such size. I would like to fit to a flat small enclosure, where I have about 13mm clearance over the PCB top side. Hence why I was liking the flat SMT packaged LM4950.I was familiar with the concept of the resistor increasing the gain "arbitrarily", but did not know it was called a "noise gain", although it makes sense, as the noise gain of the amplifier really is bigger, due to the higher gain it operates internally.Keeping the high gain at ultrasonic frequency only was however a trick I did not know about. I came across this trick when bridging audio power amplifiers, where its original gain must be kept for stability, but you need gain of just -1 to make the inverted channel, without additional inverting amplifier in front. (You just slapped a single resistor from output of one to the inverting node of the second).PCB.Wiz: Thank you for explanations. I have already played with it for a bit in the simulation, however found a few snags, that I am not sure how to solve fully. See my current schematic below:First snag: The resistor loading the opamp output to ground goes to ground. Oh wait, I have just a single polarity 12V supply. I can only see this solved by tying it against a capacitor, kept at 1/2 supply by a voltage divider. It won't work for DC, sure, but I do not need DC. Just a matter of large enough cap I guess.Snag no. 2: Not enough current drive for power transistors. The opamp can deliver say up to 15mA, meaning that will not be enough to drive a power transistor, whose gain may fall well below a 100. I need at least 1.5A of output current. That way I need more than 15 mA of drive current for sure.A logical solution would be to use a sziklai pair, to keep somewhat low voltage drops (or say lower than with standard darlington pair). However, with the sziklai, I can not probably keep the bias stable. Seems not even able to bias it properly at all. Probably not the greatest topology for when high output power is needed, eh... Any ideas please?Please find also the sim file attached with all the models. BD139/140 used only as a temporary placeholder, need to select something in SMT package. Was thinking about KTC2020/KTA1040 (nice and fast) or plain old boring MJD44H11/45H11. My home stash can offer only MJD31/32, which does not have enough current capability.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 11:38:52 pm
content: Quote from: Yansi on January 10, 2024, 09:23:27 pmYou can trial a second half of an opamp as the mid-bias point, and when you have things stable, see if you can lower R4 and maybe even remove R3, and still get good THD.Quote from: Yansi on January 10, 2024, 09:23:27 pmYes, it's marginal with low drive opamps.I was focused on higher drive opamps, with lower distortions, ( eg OPA1655 ) but you do pay for the performance.Quote from: Yansi on January 10, 2024, 09:23:27 pmI'm not following ?Do you mean temperature stability of quiescent current ?That should be ok, as your Q3/Q5 are the tracking parts.You do need care to ensure IQ is only via Q5, and not Q2, (to keep VBE correct) so R10 may need to be lowered.If you need a higher IQ for lowest distortion, that may need a larger package for Q3/Q5. Maybe the PHPT610035 * ?Addit: The PHPT610035 is in a nice package, but I see Nexperia did have better parts like PBSS4041Sx, Dual in SO8 with HFE IC = 2 A > 250 ~450, but NRND, and they seem to have no replacements ?There are onsemi parts NSS40301MDR2G & NSS40300MDR2G & NSV40301MDR2G & NSV40300MDR2G = Dual, Matched in SO8, still showing as active. Those will do > 2A.

author: Yansi
author_rank: Super Contributor
timestamp: January 11, 2024, 12:14:32 am
content: Quote from: PCB.Wiz on January 10, 2024, 11:38:52 pmI was having trouble forcing it into any bias current at all. Maybe a mistake somewhere in the sim? Will have a look again.You are correct, that if bias will flow only through the driver transistor (Q2 Q4 not opening at all), it should be stable. I was thinking that even the power transistors need a bias for it to mitigate the crossover distortion.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2024, 06:40:37 pm
content: Quote from: Yansi on January 10, 2024, 09:23:27 pmYou won't get compact with a class AB amplifier, because the heat has to go somewhere.The LM4950 looks very good. Note that the THD is better at low gains and it's loaded to well below the point of clipping.As as mentioned earlier in the thread, compounding can work. The only disadvantage is it complicates frequency compensation which increases the risk of instability.You should be able to get less distortion by separating the power stage from the input, because there's less thermal feedback to the input stage.

author: Yansi
author_rank: Super Contributor
timestamp: January 11, 2024, 11:51:03 pm
content: Quote from: Zero999 on January 11, 2024, 06:40:37 pmSure, but we are talking 2W maximum required power output, which is easily heatsinked into the PCB. I don't certainly need a 30mm high THT part sticking out of the PCB for that.I still would like to try both the LM4950 composite amp and the other more discrete topology. Am only afraid that bias stability will not be trivial to solve with the discrete one.Am also working my way into obtaining an injection transformer and Analog Discovery, so I should be able to likely verify the stability of a real sample. And a fully equipped BODE 100 is available for me at work. However I still need to work on a solution to measure the THD (need getting at least a higher quality PC soundcard).Will be ordering parts likely on Sunday, so I could start cobbling together stuff shortly.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2024, 09:32:39 am
content: Quote from: Yansi on January 11, 2024, 11:51:03 pmHow many are you units making? If it's a small number, then the leads could be reformed, so it lays flat on the PCB.

author: moffy
author_rank: Super Contributor
timestamp: January 12, 2024, 10:39:43 am
content: Yansi, just a note about a serious drawback to the Boosted_Amp configuration, you cannot decouple the opamp because that will short its effective output current, and medium to high BW opamps tend to oscillate when not properly decoupled.There was a previous discussion where I suggested something similar:https://www.eevblog.com/forum/projects/dso-awg-based-curve-tracer/msg4446133/#msg4446133Not the same as it's transimpedance, HV and precision, also you wouldn't need most of the darlingtons but drawing its output from the opamp power pins, the second diagram shows a way to decouple the opamp.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 12, 2024, 08:47:16 pm
content: Quote from: moffy on January 12, 2024, 10:39:43 amYou can add a cap across the opamp power pins, as those signals nominally track.I've seen values like 47nF or 220nF used in circuits, decided experimentally. Some circuits show no cap.Other things can mitigate this effect too* Keep the power pin load impedance low (higher voltage designs use emitter loads as level shifters )* Modern opamps, with much higher PSRR figures, will be more tolerant of small supply pin signals, allowing a smaller cap.

author: Yansi
author_rank: Super Contributor
timestamp: January 13, 2024, 01:17:57 pm
content: Zero999: Not really many of those, maybe 5 or 10 at best. I will try the LM4950 composite amp first, as it seems the most straightforward to do.moffy: Lets see once I build a prototype. Planning to put some stuff on PCB this weekend and order some components for it.And regarding the measurements, I have a QA403 analyzer on my way. Hated doing measurements with PC soundcards ever since, so finally decided to invest in a little improvement and a proper measuring device.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 13, 2024, 11:47:44 pm
content: The circuit in #30 included current limiting clamping, but that does compromise choices on the current mirror, so it may be better to manage current limit separately.A saturating current mirror seems to work well, in allowing a defined and stable IQ, but reducing the current losses at high drive .The matched power devices like OnSemi NSS40301MDR2G & NSS40300MDR2G & NSV40301MDR2G & NSV40300MDR2G should be good enough, (with a 'good' opamp) but the choices are thinning out on tracking pairs.An alternative is to use MOSFETS, drive is simpler, but they have caveats tooThe reflected impedance on the opamp pins is higherAuto-adjust of IQ is less practical, so some trim or DAC setting would be needed.The tempco of MOSFETS varies - ROHM have quite good data, and they have values from -3mV/℃ & 2.9mV/℃ for higher current parts to -4.7mV/℃ & 3.7mV/℃ for 4A parts.With a VGS(th) in the 1.75V region, a part like SD103ASDM has 4 x 300mA STKY diodes, that drop ~300mV at 10mA, so a choice of up to 4 diodes in series can do a rough tracking match.That leaves a final setting of IQ (done using R3 in the example below)I also see that a TO252-4 package is popular at lcsc, giving P+N MOSFETS with a shared DRAIN pin, some quite cheap ~10c+/150, that will be very good thermally.https://datasheet.lcsc.com/lcsc/2212160808_HXY-MOSFET-HXY3012GD_C3033441.pdf

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2024, 02:17:06 pm
content: Oh, that is some very clever stuff. Need to dig a bit into it. I like the biasing idea for the mosfet output variant. That dual mosfet with common drain looks awesome too. I have never came across such a device.Just drawing the PCBs for first prototypes, so hope I will finish them today.

author: Yansi
author_rank: Super Contributor
timestamp: January 16, 2024, 12:45:58 am
content: So first prototype PCB with the LM4950 composite amp finished and ordered. Lets see where it will bring us. QA403 also on the way.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 16, 2024, 03:16:13 am
content: looks nice.I played about with MOSFET temperature correction.Simple cascaded multiple schottky can work, but this interacts with the threshold and load resistance, and has finite steps.More promising, is a slightly more complex single schottky and sense OpAmpThis has gain to generate any mV/K and then an injection R means the sense opamp does not need to drive close to the rails, and the Dopamp load resistor is set by Vgs/I(Dopamp)In a practical system, there will be delays and offsets in the temperature of MOSFET die and schottky diode, so adjustable gain has appeal.

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2024, 09:01:38 am
content: Quote from: PCB.Wiz on January 13, 2024, 11:47:44 pmDon't forget that the threshold volage adds to the saturation voltage of the output stage, because the MOSFET only turns on, when the voltage across the resistor in series with the power supply to the op-amp exceeds the threshold voltage.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 16, 2024, 10:29:34 am
content: Quote from: Zero999 on January 16, 2024, 09:01:38 amThat’s true of source follower design, however this design has the drains joined.

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2024, 11:41:03 am
content: Quote from: PCB.Wiz on January 16, 2024, 10:29:34 amIt's still the case for the common source topology, with the drains connected, because the op-amp's power supply voltage is reduced by VGS.EDIT:I got this wrong. I neglected the effect of R9 and R10, which reduce negative feedback, causing the pass transistors to have more voltage gain. This means the output voltage swing of the op-amp is less than the sources.https://www.eevblog.com/forum/index.php?action=dlattach;topic=409632.0;attach=1980652;image

author: Yansi
author_rank: Super Contributor
timestamp: January 16, 2024, 02:12:18 pm
content: With low supply voltage (as with my case, just single 12V), it really could get us in trouble. Hard to guess what kind of Vgs voltages will be required to achieve biasing. But expecting Vgs-th could be as high as 4V, you would have just 4V (12 - 4 - 4) left to supply to opamp at idle, which may be too little already. I think that is a good thing to keep in mind.Another potential I can think of may be the gate drive current is very assymetrical. One way the opamp can pull the gate, the other way only the G-S resistor discharges the gate. It could potentially get even into a serious cross-conduction at higher frequencies, depending on the G-S resistors used and Cgs capacitances.

author: Yansi
author_rank: Super Contributor
timestamp: January 20, 2024, 09:47:20 am
content: Isn't it actually the other way round? R9 R10 makes the transistor gain more defined? (less gain, than without those resistors and less variable upon the load connected? I.e. R9 R10 adding local negative feedback)And the supply voltage of the opamp still gets lower by the Vgs(th) from each side.//Note: PCB samples from JLC already on the way here.EDIT: This a reaction to Zero999's updated post.

author: Yansi
author_rank: Super Contributor
timestamp: January 22, 2024, 07:17:18 pm
content: Hello,so after not even a full week (just 6 days!) i've received the assembled PCBs via DHL. I have assembled all the missing components (THT and some SMT) and tried the first power-up. The USB part with the audio codec from TI works fine, no surprises there. However the LM4950 with the "THD improver" was very stubborn and did not want to turn on. I was almost suspecting a fake IC got assembled.Please find below the schematic of the relevant amplifier part. Please ignore IC3A IC3B, these are lowpass filters for the DAC. IC2A/IC2B is the "THD improver".I have had huge trouble making the LM4950 go from power down up to a running state. Seems there is some half-documented smartness, that should prevent the turn-on "pop" when the amp is used in stereo single-ended mode (charging of output caps). This smartness seems to hate absolutely my external attachment, that the IC design did not account for.I admit, that stealing the 1/2Vcc bias from the BYPASS pin of te LM4950 was a stupid half-thought idea. So, I fixed it by connecting all those opamps preceding the LM4950 to a separate 1/2Vcc bias generator. There is already one in the design. So just cut a trace and a piece of wire and voila, there I fixed it. LM4950 now turns on.The likely problem is, that if the input voltage of the LM4950 is kept too low after /SHTDN signal is released, it will never turn on. So when the opamps are biased externally, this does not happen. However, in shutdown state, IC2 opamp now starts charging the output capacitors through the feedback network of LM4950. Not nice.Originally I was thinking of really using the /SHDN pin to switch the amp off when the USB will be in suspend mode, however, it will not spare that much power. Rest of the stuff draws more, than the LM4950 bias itself. So we could get rid of the shutdown function alltogether. Need to think about it some.Values of components on the PCB are as drawn in the schematic. Seems the IC2 configuration is stable like this, however no further checks were done. Only ave noticed that when I disconnect the 12V power, it will make some slight oscillation buzz, something not happy there.Todo:1) borrow Audio Precision from my colleague, cause my QA403 will probably take a longer while to arrive2) verify stability of the composite amplifier3) measure THD4) adjust schematic and PCB accordingly.(After that, we could try the discrete solution and compare it with this one.)For schematic click here: