OK audio gurus: What are the highly regarded FET-input op amps these days? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: December 06, 2018, 05:37:55 pm
content: This question is triggered by a repair I recently did where the originals, while still available, were stupid expensive. It occurred to me that a more modern & higher performing part must exist at a lower price - a win-win scenario if ever there was one.Due to the circuit specifics it needs to be FET input, and also unity gain stable. I have not kept track of this category since the LF411/LF412 were considered the good stuff. Surely there's something better by now?Thanks.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2018, 06:00:58 pm
content: It's not that simple, you have to consider what the op-amp is seeing and doing.Line driver? Tone controls? DAC reconstruction filter?The better FET-input op-amps are expensive because they are laser-trimmed and prices can be crazy.I would recommendOPA2134or OPA134 and if you have money theOPA627or OPA637. But $40-57 eachCareful swapping in a higher bandwidth part, they can oscillate in "old" circuit designs and not all parts are unity-gain stable.

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2018, 06:02:52 pm
content: Quote from: cvanc on December 06, 2018, 05:37:55 pmJust replace the original op-amp with one with a similar specification, if it's not possible to find an exact replacement.Old op-amps such as the LF412 and TL072 are more than good enough for 99% of audio applications.Replacing the part with a more modern device could risk messing it up and ruining the performance so lose, lose. Newer so-called audiophile op-amps have a higher bandwidth, than the traditional ones, so are more sensitive to board layout and can easily oscillate. If a circuit isn't designed for such a high bandwidth op-amp, then blinding replacing the op-amp with a faster one is asking for trouble.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 06, 2018, 06:04:46 pm
content: Some references come to mind:OPA2134OPA2604OPA1652

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 06, 2018, 06:06:42 pm
content: A lot of new parts are only available in SMT, which can limit you unless you use adapter boards. The TI OPA164x series looks interesting.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 06, 2018, 06:22:02 pm
content: 1) you may not like the answer (since you provided open question)2) i'm not audio guru, just a repair stuffs, like you mentioned... but...TL072 (from AQ-2000R 2KW Class-D amp)(picture 1)4558D (from PA37-M350 2.5KW monoblock)(picture 2)4558D (from AQ-854 1.5KW Class-AB amp)(picture 3)another i checked 4558D from AQ-2580R 350 x 4W ab-amp(no picture)its looks like 4558D wins but it aint fet opamp, if you want fet, then we have TL072 there appearing once in my statistics. they are not new chip, but they are far from obsolete, even in hung low grey beard market, and they cost like dirt... ymmv.

author: floobydust
author_rank: Super Contributor
timestamp: December 06, 2018, 06:35:37 pm
content: TL072 was relevant in 1978 lol. From the datasheet:•Avoid input voltage values below 1V to prevent phase reversal where output goes high.•Avoid input values below 4 V to prevent degraded VIO that results in an apparent gain greaterthan 1. This may cause instability in some second-order filter designs.Graph is single supply voltage follower. Great for guitar fuzz boxes.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 06, 2018, 06:54:29 pm
content: not forget to mention, that the opamp is just one part of the noise figure; the circuit surrounding it is as important as the opamp itself, so input impedance, feedback networks aso. The lowest noise opamp serves no purpose, if you have a MOhm feedback network.Personally I still like to work with the TL07x if I need a FET input, or otherwise LM833 and NE5532. They are doing just fine for almost all needs. If need super low noise, I use an OPA2134, but as far as I remember this a BJT input opamp.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 06, 2018, 07:06:59 pm
content: Quote from: floobydust on December 06, 2018, 06:35:37 pmin audio world... you supply the opamp with ±12V and input is in mV range from center so, phase reversal is just as bad as sound clipping which is not uncommon, ymmv.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 09:08:06 am
content: Supposedly upgrading an old op-amp with a newer one, in a piece of equipment is audiophoolery. If it really does sound different, then it's because it's oscillating. If it's not possible to get a direct replacement, then use one with a similar bandwidth and output current capacity.

author: helius
author_rank: Super Contributor
timestamp: December 07, 2018, 09:13:52 am
content: Analog IC technology may not be advancing. Smaller feature processes are developed for highly-integrated digital ICs, but have worse analog performance. It is entirely believable that the greatest op-amps were designed in the 1970s.

author: spec
author_rank: Frequent Contributor
timestamp: December 07, 2018, 10:19:59 am
content: Quote from: Mechatrommer on December 06, 2018, 06:22:02 pmApart from not being MOS input, the 4558 is two 741s, which are not good for audio work... sorry

author: spec
author_rank: Frequent Contributor
timestamp: December 07, 2018, 10:28:11 am
content: Quote from: floobydust on December 06, 2018, 06:00:58 pmSecond the OPA2134 and OPA134. Nicely behaved, nice price, and sound nice too.

author: Marco
author_rank: Super Contributor
timestamp: December 07, 2018, 10:34:07 am
content: Quote from: floobydust on December 06, 2018, 06:00:58 pmOPA637 might make some sense if it just fits your application perfectly and component cost is largely irrelevant. Getting an OPA627 instead of an OPA140 is just throwing money away though. OPA627 is a joke at HiFi nutter's expense.

author: Marco
author_rank: Super Contributor
timestamp: December 07, 2018, 10:38:05 am
content: Quote from: helius on December 07, 2018, 09:13:52 amFETs seem exceedingly dependent on the consistency of the process and nothing that old seems to have a decent 1/f, we might have peaked with BF862 though since the market for good quality high area devices is drying up.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 11:24:16 am
content: Quote from: spec on December 07, 2018, 10:19:59 amThe RC4558 is a bit better than the uA741. It's faster and lower noise.http://www.ti.com/lit/ds/symlink/rc4558.pdfhttps://www.st.com/resource/en/datasheet/cd00001252.pdfAnd why are those op-amps no good for audio?Quote from: Marco on December 07, 2018, 10:34:07 amQuote from: spec on December 07, 2018, 10:28:11 amIf this were a new design then I'd agree use the OPA2134, OPA627 etc. but blindly replacing an older LF412 in an existing piece of equipment with them is a very bad idea and the OPA637 isn't even unity gain stable, just no!If you have the schematic for the existing piece of equipment or it can be easily reverse engineered and the layout appears to be suitable for these faster op-amps, then you stand a good chance of being able to do it, without any ill effects, but additional supply decoupling and compensation capacitors may be necessary.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 07, 2018, 11:49:17 am
content: The OPA627 is mainly good for very low input bias - usually no need for this with audio. Noise and distortion wise OPA1641 or OPA827 perform better.It depends very much on the circuit if a faster OP can be used. If there are more complicated parts (especially active parts) in the FB path, a faster OP can be a problem. However typical preamplifier circuits with just some gain or filtering may very well work with an faste OP too.For a better (lower noise) replacement with not much higher GBW, the OPA145 could be an option - though no THT.The OPA134 is not the highest performance, but still available in DIP and not that much faster.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 07, 2018, 02:42:59 pm
content: Thanks everyone for your input and suggestions, they are always appreciated.I'm going to sidetrack my own thread and ask a question now about the various versions of OPA2134. They look nice to me, but Digi-Key stocks 2 slightly different part numbers.Here's the kicker: As far as I can see even the T.I. datasheet (link below) doesn't explain what the difference is. The 2 part numbers read identically in all specifications. They are OPA2134PA and OPA2134PAG4. The 'G4' is never explained in the datasheet.Any idea what it all means? And thanks again.http://www.ti.com/lit/ds/symlink/opa134.pdf

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2018, 04:13:33 pm
content: Quote from: HB9EVI on December 06, 2018, 06:54:29 pmNo it's actually not. "A true FET input stage is incorporated" as stated in its datasheet.

author: Bassman59
author_rank: Super Contributor
timestamp: December 07, 2018, 04:24:55 pm
content: Quote from: SiliconWizard on December 06, 2018, 06:04:46 pmThumbs up to all three.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 07, 2018, 04:25:58 pm
content: Quote from: cvanc on December 07, 2018, 02:42:59 pmSee here:https://forum.digikey.com/t/texas-instruments-e4-g4-suffix-information/95It's related to the lead finish and apparently parts with and without G4 are now equivalent (G4 used to be the "green" finish - lead free - but they are all "CU NIPDAU" now as you can see in the datasheet.) Due to stock reasons, you will find that there can be significant price difference between the two, whereas they are basically the same AFAIK.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 07, 2018, 04:49:16 pm
content: Quote from: SiliconWizard on December 07, 2018, 04:25:58 pmHey, thank you! Clears things right up.

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 05:01:40 pm
content: Quote from: Kleinstein on December 07, 2018, 11:49:17 amI agree, if it's a basic op-amp circuit, then it should generally be OK, even with a faster op-amp. The main risk is the board layout and decoupling might be insufficient for a fast op-amp, but as long as you don't go crazy, it should be fine.Quote from: cvanc on December 07, 2018, 02:42:59 pmYou've not said what the piece of equipment is or what op-amps were originally used?If it's replacing something like the LF351, then the OPA134 is only a little faster so shouldn't cause any problems, unless there are other gain stages in the feedback loop, but if it's something really slow and low power, such as the TL061, it becomes more of a risk and the power consumption will be a higher too.Will you notice any difference in sound quality? Probably not. The only way to tell honestly is by conducting a blind test, to eliminate any cognitive bias you may have. Fit sockets to the PCB, cover the part numbers on the ICs and randomly swap them around to see if you can tell the difference.

author: floobydust
author_rank: Super Contributor
timestamp: December 07, 2018, 06:55:56 pm
content: OP, with audio op-amps most EE's are datasheet designers- using a few listed numbers to compare parts. In that regard, a 5532 is all you'll ever need as Douglas Self proclaims.Going beyond the datasheets, even as far back as 1987 it was known op-amps have many important traits not characterised on their datasheets. Input, output and transfer non-linearity, common-mode slew rate etc. Example graphs of LF351 and LF356 from Audio IC Op-Amp Applications by Walter Jung. I have more but it's to see THD can be up there, for different reasons.I don't think you can "get into it" here, it quickly degrades to psychoacoustics and audiophoolery compared to science. The diyaudio forum might be better suited for posing the question.You can dig on the Internet and find modern op-amp test results, but it quickly crosses over into sonic character and subjective opinion. They'll say the OPA2134 "sounds dark" for example.There have been audio op-amp distortion measurements and comparisons with double-blind listening tests. Paid access:http://www.aes.org/e-lib/browse.cfm?elib=16029

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2018, 09:42:43 pm
content: The NE5532 is all you'll ever need? I don't know about that. It's not very good in applications which require a high input impedance, where a J-FET input op-amp is pretty much essential, although I suppose one could add a discrete J-FET input stage to the NE5532, for those applications.And what about low voltage applications? Old op-amps such as the NE5532 and LF353 are marginal for even 9V applications, let alone V or 3V, then there's the really crappy LM358 with horrific crossover distortion.Valid comparisons between amplifiers can only be made by a double blind test. Decent quality audio amplifiers all sound the same anyway, as long as they're not driven towards the point of clipping. Over 40 years ago a double blind test was done on three amplifier designs: one thermionic and two solid state, with golden eared audiophiles and they couldn't tell the difference!http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/Valves%20versus%20Transistors%20DCD.pdf

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: December 08, 2018, 01:34:49 am
content: I have been using LM4562 for a while now with no complaints. They are VERY linear, quoting an amazing 0.00003 % typ distortion, 55 MHz GBW, unity gain stable and with Ibias of only 72 nA. They are in standard pinout in soic-8 and stk at DK at USF$2.79 for 1.https://www.digikey.ca/products/en/integrated-circuits-ics/linear-amplifiers-instrumentation-op-amps-buffer-amps/687?FV=1f140000&k=lm4562&pkeyword=lm4562&stock=1&ColumnSort=1000011&cur=USD&lang=enThey are characterised for distortion, CMRR, PSRR, etc up the yin yang.

author: Marco
author_rank: Super Contributor
timestamp: December 08, 2018, 02:05:58 am
content: Quote from: floobydust on December 07, 2018, 06:55:56 pmIncreasing the distortion by80 dBto do a "listening" test ... is that to emulate the audio chains you will find everywhere with 200 opamp circuits?

author: GregDunn
author_rank: Frequent Contributor
timestamp: December 08, 2018, 04:19:07 am
content: I have several audio boxes which use either the TL07x or NE553x op amps; some of them are just low or unity gain amplifiers, others are single or double pole filters. I've driven them with millivolt levels (phono preamp) up to 5-10V levels and they behave just fine. All are suitably low noise for audio and transparent. We actually did a double-blind test against a very high end tube preamp about 30 years ago and couldn't tell them apart. Pretty much any op amp in the same category will be more than good enough if you pay attention to the spec sheet when picking them. I think the NE5532 is unity gain stable whereas the NE5534 requires compensation?

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 08, 2018, 05:54:51 am
content: I guess in audiophewlery, first there's blind test or post. And then there's double blind post, and then there's showing real circuit. I see a lot of work was done on filtering, those tantalum and ceramics caps? and then there's discreete power stage AB topology with careful biasings. And then there's speaker box resonant calculator or whatnot. Is it worth arguing something that contributes less than 1% from the whole system? And then there's tube transistor that sounds white, man.. i cant hear 1MHz anyway..

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2018, 07:03:29 pm
content: Quote from: GregDunn on December 08, 2018, 04:19:07 amThe NE5534 is stable in gains of 5 or less, but is faster than the NE5532.Yes, it's all a load of bollocks, the distortion produced by modern op-amps is negligible, compared to speakers and the room acoustics.Quote from: basinstreetdesign on December 08, 2018, 01:34:49 amThe LM4562 is a completely unsuitable replacement for a J-FET op-amp. It's a BJT input op-amp with a high bias current of 72nA, compared to, 65pA typical, 20nA worst case, for a J-FET op-amp such the old TL072 and a 55MHz of GBWP could be far too fast for a circuit and layout designed for one with a GBWP of 3MHz. The input current noise for the LM4562 is 1.2pA/√Hz which will make it vary noisy when the inputs are connected to a high impedance. The old TL072, with an input current noise of just 0.01pA/√2 will be less noisy in applications when the input is connected to a higher impedance than 15k.Note that it's only specified for that very low level of distortion when in unity gain configuration and with an output voltage of 3VRMS, not that you'll notice the increased distortion at lower and higher output levels. It's just a case of the figures being manipulated to make it look good.

author: CodyDowling
author_rank: Newbie
timestamp: December 09, 2018, 02:11:55 am
content: I just designed in the OPAx140 from Texas Instruments. (I'm using the OPA2140, for dual op-amps, but they also have a single and a quad version, IIRC). The specs that I cared about looked good, but without knowing what you're using for it's a little hard to say if it's a good fit. I was worried about relatively low noise, relatively good accuracy, and ridiculously high input impedance with a reasonably low input capacitance. It fits nicely for that application (monitoring audio signal to feed into a DC servo), but it will depend on your application specifics.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 11, 2018, 01:24:58 am
content: Thanks for all the comments. A couple of you asked what the specific repair was. It was an old piece of stereo equipment, a "Quad 34" preamplifier. I'm guessing here, but it's maybe early 80's vintage?The preamp is op-amp based and is populated with TLE2071 and TLE2072 chips (http://www.ti.com/lit/ds/symlink/tle2071a.pdf). This is a part I had not encountered before, and as I said at the beginning you can still get them but they're kind of expensive. The board has many high impedance nodes and a fairly spread out layout - not something to blindly drop a super fast modern chip into. It would probably oscillate.It wasn't really about improving the performance after repair. It was about finding a replacement part that was more common and hopefully less costly.Again, thanks for the advice and opinions.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: December 11, 2018, 01:40:13 am
content: Just for grins, here is the bible on opamps for low distortion applications. The 4562 *is* very good.http://www.nanovolt.ch/resources/ic_opamps/pdf/opamp_distortion.pdfTherearesome applications where these specs do matter. Samuel Groner, for example, designs oscillators with vanishingly low distortion. That is necessary when using said oscillator to measure other equipment.

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2018, 03:40:56 pm
content: Consider what the LF411 and LF412 are. This is not quite straightforward because the two parts are not directly related. The LF411 is a single version of the LF353 dual and the LF412 is a dual version of the LF351 single. So:LF351(S)/LF412(D) 4MHz 13V/us 25nV/SqrtHz@1kHzLF411(S)/LF353(D) 3MHz 15V/us 18nV/SqrtHz@1kHz 1.8mA/chThe only thing which makes these parts suitable for audio is the low transconductance of their JFET differential inputs which allows higher bandwidth and slew rate while maintaining unity gain stability. So replacements will need JFET inputs with class-AB outputs.Modern jelly bean replacements are the TL051/TL071/TL081 and TL052/TL072/TL082. The TL05x series has better precision. The TL07x series used to be the noise tested versions of the TL08x series but today they are the same.JFET Singles:LF351 SO only from STM, EwwLF356 $0.83 5MHz 12V/usLF411 $1.29 3MHz 13V/usTL051 $0.60 3MHz 20V/us 18nV/SqrtHz@1kHz Improved TL071/TL081 Higher PrecisionTL071 $0.55 3MHz 13V/us 18nV/SqrtHz@1kHz Noise Tested TL081TL081 $0.52 3MHz 13V/us 18nV/SqrtHz@1kHzJFET Duals:LF353 $0.60 3MHz 13V/usLF412 DiscontinuedTL052 $0.70 3MHz 20V/us 18nV/SqrtHz@1kHz Improved TL072/TL082 Higher PrecisionTL072 $0.68 3MHz 13V/us 18nV/SqrtHz@1kHz Noise Tested TL082TL082 $0.75 3MHz 13V/us 18nV/SqrtHz@1kHzJFET Quads:TL054 $2.62 3MHz 20V/us 18nV/SqrtHz@1kHz Improved TL074/TL084 Higher PrecisionTL074 $0.64 3MHz 13V/us 18nV/SqrtHz@1kHz Noise Tested TL084TL084 $0.60 3MHz 13V/us 18nV/SqrtHz@1kHzSomewhat improved replacements for these:AD711/AD712/AD713LT1792/LT1793 Single only but lower noise, lower distortion, and lower input capacitance.In his book, Douglas Self discusses a bunch of more modern JFET parts:OPA134/OPA2134 Not much of an improvement.OPA604/OPA2604 Not much of an improvement.OP627 Great improvement.

author: Marco
author_rank: Super Contributor
timestamp: December 11, 2018, 03:49:57 pm
content: A great improvement towards what exactly? When even the venerable AES needs to do something extremely silly like making the circuit amplify the distortion by 80 dB to do a listening test you can easily see HiFi kinda lost the plot.

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2018, 09:09:59 pm
content: Quote from: Marco on December 11, 2018, 03:49:57 pmStudy the datasheets and find out. The OP627 has lower distortion, lower noise, and higher common mode rejection then the jelly bean parts it can replace.There are some really improved JFET parts for audio applications but I limited myself to suitable replacements for the LF411 and LF412 and parts discussed elsewhere. The LT1113 and LT1169 come to mind.As a practical matter, I would stick with the cheap TL07x/TL08x series unless lower noise or lower distortion in a non-inverting configuration is required and instead use better circuit topologies including unloading the output of the operational amplifier.Linear Technology had a great discussion about non-linear input capacitance in JFET operational amplifiers causing distortion in non-inverting configurations but Analog Devices has apparently deleted it.

author: Audioguru
author_rank: Super Contributor
timestamp: December 11, 2018, 09:57:43 pm
content: Quote from: Mechatrommer on December 08, 2018, 05:54:51 amThe audiophools in the double blind listening tests might be old like me (73) with normal for their age high frequency hearing loss. Then they cannot hear missing high frequencies and cannot hear distortion and hiss anyway.Old geezers without hearing aids cannot hear the distortion and hiss of a lousy old 4558 dual opamp. The datasheet does not say a single word about how much distortion it has and lists "typical" noise but lists no maximum amount of noise like audio opamps do.My hearing aids boost the highs to normal without any distortion or hiss. In a noisy mall, street or restaurant their automatic noise reduction mode works very well.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 11, 2018, 10:06:54 pm
content: i'm not entirely sure what you mean, good or bad. anyway what took you so long? the OP was calling for you (look at the topic!)

author: cvanc
author_rank: Frequent Contributor
timestamp: December 12, 2018, 12:06:59 am
content: Quote from: Mechatrommer on December 11, 2018, 10:06:54 pm

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2018, 12:44:52 am
content: Quote from: David Hess on December 11, 2018, 09:09:59 pmDo you think there is anyone in the world who could even hear the difference compared to an OPA140 in an A/B test for a realistic circuit? (ie. not the silly AES one.) Let alone a quality test.

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2018, 02:20:09 am
content: Quote from: Marco on December 12, 2018, 12:44:52 amIn the right application, sure it will be audible. The OP627 will support a higher closed loop gain at the same distortion level as the TL071/TL081; raise the closed loop gain high enough and there will be an audible difference. In high sensitivity applications, the difference in input noise will be audible. The OP627 also maintains lower distortion with lower impedance loads but this should be solved with better circuit design; do not drive low impedance loads with operational amplifiers when distortion or precision matters.

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2018, 03:18:08 am
content: Compared to an OPA140 ...

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2018, 04:10:39 am
content: The OPA140 series should be lower distortion but it is not available in DIP packages. However I doubt the TI datasheet specifications represent reality; it looks like they used a very low source impedance for the distortion tests which is unrealistic for a JFET part. The included schematic does not show an input cascode so distortion from non-linear input capacitance is likely but they do not discuss this. Maybe their JFET capable complementary bipolar process solves this problem but if so, they are not saying.Build a test circuit and do comparisons yourself.