OK there's just too many opamps.  Help me choose for a difficult audio job? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: October 02, 2023, 03:04:54 pm
content: (from the "Why doesn't this product already exist?" department)I'm building a box to allow use of oscilloscope probes with my audio test equipment. Because it's a front end for test gear I'd like to be as clean and quiet as practical.My thought is to passively terminate the probe with 1M and use an AC coupled unity gain buffer to send the signal along. But this buffer has gotta be clean and it's gotta be quiet. We're feeding test equipment.I'm hoping to use an opamp for this but I'm numb from the sheer quantity available. I need something that is simultaneously really quiet with a 1 Meg input Z, happy used as a follower, & has solid audio performance. It's the noise shopping that's difficult, the Internet does not readily hand over noise current info.I'm thinking one of the 327 offerings from TI/Burr Brown but damn which one? It's an embarrassment of riches these days.What would you use in this application? And thank you for your input.

author: moffy
author_rank: Super Contributor
timestamp: October 02, 2023, 03:49:50 pm
content: I would go for a JFET input opamp because with the 1M input impedance current noise is more important than voltage noise. I think you already understand that and the OPA827, which I think you meant, looks like a good alternative with 4nV/Hz^0.5 and 2.2fA/Hz^0.5.

author: dietert1
author_rank: Super Contributor
timestamp: October 02, 2023, 03:57:25 pm
content: It's not just the selection of a magic part but having a noise free supply, maybe differential output. I'd guess for experimentation any low noise amplifier will do.You could also start with an OPA140: Low power consumption, pretty low noise, precision (THD=0.00005% buffering 1 KHz 3.5 V RMS), high input impedance. It needs input protection.Regards, Dieter

author: blackdog
author_rank: Frequent Contributor
timestamp: October 02, 2023, 05:14:28 pm
content: Hi cvanc,Both the OPA140 and also the OPA827 are nice opamps.Both have fet inputs and the characteristics are almost the same.If you want a little more bandwidth, choose the OPA827.Do choose your resistors well that determine the gain.You probably need a small capacitor from the output to the inverting input.Let the stage amplify 10x, that compensates for the probe attenuation, you will still keep about 500KHz bandwidth flat with the OPA827.It does depend on how well you perform the feedback resistor compensation.Another tip, include at the output a series resistor of 49.9 Ohms driving the coax to your scope or measuring instrument.You can protect the input with two 1N4148 diodes from the +input to the supply lines.The best is if you connect the diodes directly on the + input and from there with 100 Ohm to your input circuit.The 100 Ohm limits the peak currents through the diodes.Of course decouple the supply lines well, with at least 100uF.That seems a lot , but just think where the energy goes when you overload the input, yes that's right depending on the polarity of the signal the supply voltage is lifted.If i have some more time this evening i want to draw a schematic, so you have something to start with.Kind regardsBram

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2023, 05:18:33 pm
content: I would consider the OPA1641: relatively similar to the OPA140 with AC performance, but with some extra circuitry, so that the input capacitance does not change much with the DC level.Many JFET input amplifier have a much stronger voltage dependence of the input capacitane.The voltage dependence of the input capacitance can be an issue with some high impedance source (e.g. a 1:10 probe).

author: blackdog
author_rank: Frequent Contributor
timestamp: October 02, 2023, 06:08:52 pm
content: Hi Kleinstein, :-)You are right about the non-linearity of Fet Opamp inputs.Only I do not know which of the three opamps mentioned here presents best at high input impedances, here around 500K with say 25pF in parallel.All three opamps I have in house and also an Audio Precision test set, just too little time to make a test circuit, maybe later this week, looks like a fun test. :-)Good of you to bring it to our attention, I hadn't thought about it...Kind regardsBramPSThis triggert my memory, maybe this document helps also with modern FET Opampshttps://www.analog.com/media/en/technical-documentation/application-notes/742022599AN232.pdf

author: cvanc
author_rank: Frequent Contributor
timestamp: October 02, 2023, 06:21:16 pm
content: Apologies for using the number 327, it's meant to imply "too many catalog entries" - not a specific part. I was asking for your blank page opinion.Anyway yeah FET input seems inevitable. However it seems none of the vendor-provided "sort our opamps" pages have a column for sorting noise current (nor noise 1/f corner frequency, do I care about this, does it vary much?).And without that I'm down to essentially guessing which datasheets to individually read. I'm sure I'm missing some better way here...?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2023, 06:27:51 pm
content: Most of the manufacturers have parametric filter an sort options. By now there are not that many high grade manufacturers left: mainly Ti and Analog Devices with Renesance and ST and ONS as 3 smaller ones.

author: macboy
author_rank: Super Contributor
timestamp: October 02, 2023, 06:53:28 pm
content: Are you thinking 10x probes? Forget that; the input impedance will be around 1 MOhm, giving very high noise relative to what you want for audio. At audio frequencies, the benefits of 10 MOhm and ~10 pF input impedance are essentially non-existent, but the drawbacks are highly damaging. Those cheap 1x/10x switable probes might be usable in 1x mode, however. Typical audio line inputs are 10 kOhm to 100 kOhm, but they also depend on the low impedance of the source to result in an overall low impedance for low noise.I'd look at TI devices. They have a few nice opamps acquired from Burr-Brown, and other more modern ones acquired from National Semi. Sadly, some of the best former NS ones (like LME49990MAX) are no longer made. IIRC, something about a specific fab being shut down.https://www.ti.com/amplifier-circuit/op-amps/audio/overview.htmlhttps://www.ti.com/amplifier-circuit/op-amps/audio/products.html

author: tszaboo
author_rank: Super Contributor
timestamp: October 02, 2023, 07:09:12 pm
content: I'm a bit puzzled, why you want a 8 bit ADC to be connected to a 16-24 bit source. Oscilloscope is really not the product for audio noise analysis, only to figure out why something doesn't work. Let's say you use a NE5532. That's 3nV/sqrt(Hz), with 20M bandwidth, you still only have 13uV of noise from the opamp. While the scope has about a mV of noise easily.Or you amplify it, and feed it into the 50 Ohm input, but then you are hardly measuring the DUT, you are measuring the amplifier.(I know the NE5532 will not do 20MHz, just to have some numbers)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2023, 07:23:05 pm
content: There can still be some use for a x10 probe, e.g. probing the putput of a power amplifier or maybe a tube amplifier and thus very high levels. Also not all measurements are about super low noise.

author: jonpaul
author_rank: Super Contributor
timestamp: October 02, 2023, 07:42:45 pm
content: Done audio/pro /studio/cinema since 1970s, NEVER needed any such opamp frt end.TEK 10X probe >><TEK scope.Occasionally needed X1 probe.Have may FET/DIFF actives probes, seldom used or neededj

author: cvanc
author_rank: Frequent Contributor
timestamp: October 02, 2023, 08:08:34 pm
content: Thanks all, more info:I'm building this give my audio analyzer access to the full dynamic range offour thousand Voltscope probes. That's why I'm slow walking here, I can see how this project wants to go wrong.The probes I care about are 100:1, have a series resistor of 99 Megs, and are open to ground at DC. The input of my box will look like an oscilloscope to make the probe happy and the rest of the box basically exists to keep my analyzer alive and measuring well.I'm not loving having to provide power to this, it'll probably end up being two 9V batteries.Thank you all for your comments.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 02, 2023, 09:09:57 pm
content: It usually does exist but its impossble to find now.Once upon a time the bloke in the shop knew, now you have to ask a FWit bot.

author: tszaboo
author_rank: Super Contributor
timestamp: October 02, 2023, 09:34:35 pm
content: Quote from: Kleinstein on October 02, 2023, 07:23:05 pmOh, I think I cannot read properly. So he wants to connect a scope probe to the input of an audio analyzer, and not to connect an oscilloscope to some audio gear.Still the 9-99MOhm, the lossy coax, and the probe compensation is not really optimal. But then, the audio analyzer has probably a 10KOhm input, so that's going to load the probe less than a scope would. OK, now it makes sense.

author: jonpaul
author_rank: Super Contributor
timestamp: October 03, 2023, 05:59:23 am
content: Op amp selection guideshttps://www.analog.com/en/parametricsearch/11070https://www.ti.com/amplifier-circuit/op-amps/products.html100 X probe: Most audio even valves (tubes) has a max plate V <400V so any good scope and 10X easily are within ratings.We have used 100X probes and 1000X like P6015A TEK on analog or digital scopes WITHOUT any added amps.100X >>Scope 1M inputPost process eg ADC is done vua a Y or CH 1 aux output avail on most analog scopes.see classic Tek Oscilloscope circuit concepts books and learn.https://www.w140.com/tekwiki/index.php/Concepts_SeriesProbe circuitshttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfProbe measurementshttps://w140.com/tekwiki/images/1/19/062-1120-00.pdfEnjoy,Jon

author: Smokey
author_rank: Super Contributor
timestamp: October 03, 2023, 07:28:07 am
content: Quote from: blackdog on October 02, 2023, 05:14:28 pmSolid post. thanks!

author: cvanc
author_rank: Frequent Contributor
timestamp: October 03, 2023, 10:27:09 am
content: I appreciate the conversation, all. Thank you.I'm working backwards from a real signal I need to measure. It requires the entire dynamic range of a 4KV probe.This amplifier clips at 2.5KVp-p and the output sits on a high DC bias +1.65KV. It's from an electrostatic speaker. So 100:1 probes are mandatory here.

author: jonpaul
author_rank: Super Contributor
timestamp: October 03, 2023, 11:08:08 am
content: Electrostatic spks have transformer faults and plate shorts.Do not leave connected when not in use.use 100X or 1000x probe>>scopej

author: magic
author_rank: Super Contributor
timestamp: October 03, 2023, 02:20:31 pm
content: Quote from: cvanc on October 02, 2023, 08:08:34 pmBeware that general purpose 100:1 scope probes are not rated for THD.High value resistors are quite notorious for their poor linearity (voltage coefficient of resistance, i.e. resistance charges with applied voltage, i.e. current is not proportional to applied voltage, and so on). They also have not so great thermal coefficient and may heat up and cool down with signal. This means a voltage divider consisting of a dubious 99MΩ resistor and a more ordinary 1MΩ may easily produce distortion, and you won't even know how much of it before you measure at the DC and AC voltage level of interest, which may be a chicken-and-egg problem for you.I have no idea how people measure such amps. I think that maybe a DIY capacitive divider could be more accurate.

author: dietert1
author_rank: Super Contributor
timestamp: October 03, 2023, 02:55:09 pm
content: Which high voltage probe are we talking about, i mean which brand and product?To be a bit more optimistic: For a high voltage probe there should be a medium voltage range where it can be used for measurement. Of course you don't want to use it near its physical limits. Same with capacitors: If i want to use a capacitor for precision purposes it should remain within 10 % of its specified voltage in order to limit dielectric absorption.Regards, Dieter

author: cvanc
author_rank: Frequent Contributor
timestamp: October 03, 2023, 04:08:56 pm
content: Hmmm, I'm liking those OPA1641s...The exact probe doesn't really matter as we want our input to look like a generic oscilloscope. We must provide the usual 1Meg || a few pF.But for clarity mine are GE3425 from Cal Test and they have been inside these exact amps for years, if only to feed oscilloscopes....And I get there are imperfections and nonlinearities that will creep in here. 'The best we can do' is the best we can do. It will be interesting running a loopback test on this pig when I finish putting lipstick on it ":^)

author: magic
author_rank: Super Contributor
timestamp: October 03, 2023, 04:24:42 pm
content: The real test will be whether OPA1641 is any better than TL071 with those probes.BTW, that looks like a good choice of chip. Relatively inexpensive too.I expect such probes will be noisy as hell (128nV/rtHz) up to a few kHz, until their capacitance will filter out the Johnson filth.I suspect all serious measurements of e-stat amps are done with custom probes.

author: jonpaul
author_rank: Super Contributor
timestamp: October 03, 2023, 04:27:02 pm
content: Electrostatic spkr like the Quads need a very low load probe.Use TEK X1000 P6015/A, 1000M not X100 100Mj

author: dietert1
author_rank: Super Contributor
timestamp: October 03, 2023, 06:44:36 pm
content: Quote from: cvanc on October 03, 2023, 04:08:56 pmLooking into their manual, there is a table showing that the GE3425 is a good selection. It is specified with a voltage coefficient of < 0.5 ppm/V. The have other HV probes GE3421 and GE3422 with a < 30 ppm/V spec.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 07:26:46 pm
content: Quote from: magic on October 03, 2023, 04:24:42 pmThe TL07x datasheets usually don't show the input capacitance. Ti as an Appl. note to show the voltage dependent capacitance. Looks like the typcal JFET op-amps shown in the OPA1641 DA could be a TL071.https://www.ti.com/lit/an/slyt595/slyt595.pdf?ts=1696360631980&ref_url=https%253A%252F%252Fwww.google.com%252FIf one is after low distortion also for the higher frequencies, this is likely not good. The cross over from resistive to capacitive in the scope input is usually at some 10 kHz and thus still in the audio range.With a 1:10 or 1:100 proble the amplifier noise is largely set by the 1 M resistor and amplifier noise would be a minor issue. So the not so much liked CMOS version TL071H may be good enough too.

author: magic
author_rank: Super Contributor
timestamp: October 03, 2023, 07:33:35 pm
content: Quote from: dietert1 on October 03, 2023, 06:44:36 pmOr up to 0.1% at 2500Vpp.A good amp could be better than this probe. Although there is the question how its feedback divider is made...

author: cvanc
author_rank: Frequent Contributor
timestamp: October 03, 2023, 08:05:58 pm
content: I have owned four of these probes and two of these amps for half a decade. They know each other VERY well, all too well, and get along nicely. I just wish I didn't have to keep taking everybody out for a walk ":^)I frequently have all 4 stabbed into the various nether regions of these amps, running rolling histograms on a scope. Works fine.Schematic attached, these aren't Quads but Beveridges. Thanks all for playing along, good bunch in here.

author: dietert1
author_rank: Super Contributor
timestamp: October 03, 2023, 10:46:20 pm
content: Amplifier negative feedback divider seems to be R1 7.5 MOhm and R112+R114 5.52 KOhm. About 180x less impedance than the probe system.I would rather trust a good microphone for THD analysis. As far as i remember electrostatic speakers produce significant THD already at moderate sound levels, even with a transformerless amplifier.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2023, 08:37:03 pm
content: The OPA1641 would be one of my top choices for low input current noise no matter what circuit configuration you use, however as mentioned, the Johnson noise from a properly terminated x10 probe will overwhelm the noise of any low input current noise operational amplifier:0.13 * Sqrt(910k) = 124 nV/SqrtHzAnd *that* is with a signal which was attenuated by 10, so multiply that by 10 for input referred noise.Noise from a x1 probe will almost entirely depend on source resistance, but might perhaps be 1 to 4 nV/SqrtHz, so the OPA1651 is still a reasonable choice.Quote from: magic on October 03, 2023, 04:24:42 pmDistortion will be better, but noise will not be any different.Quote from: Kleinstein on October 02, 2023, 05:18:33 pmI did not know that there was any difference. I figured the OPA1641 was an OPA140 which failed bias current and DC precision grading.

author: cvanc
author_rank: Frequent Contributor
timestamp: October 05, 2023, 02:28:40 pm
content: It sounds like agonizing over the exact opamp is a fool's errand, noise-wise. I'm gonna bang this out with OPA2134 as I have them on hand. I'll post back here how it turns out.Noise is one of the chewier things to discuss, I think. Thank you for the conversation.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2023, 02:50:10 pm
content: Quote from: David Hess on October 04, 2023, 08:37:03 pmThe lower grade brother to the OPA140 is the OPA141.AFAIK the OPA1641 was availble a little before the OPA140. In the specs there are also minor differences in the CMRR and open loop gain - so there seems to be a some improvement in the CMRR at the cost of maybe a little less gain (the min values are still the same). The OPA16xx are marketed as audio amplifiers and the OPA140 is a more general use precision part.The constant input capacitance is due to the SOI type construction and this should also apply to the OPA140 and likely also OPA827.

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2023, 05:02:24 pm
content: Quote from: Kleinstein on October 05, 2023, 02:50:10 pmThe OPA141 has different 1 kHz noise and bandwidth specifications. None of the three match exactly, and if anything the OPA1641 is more similar to the OPA141 because of input capacitance. The only really different one is the OPA145 which operates at a significantly different power level.

author: magic
author_rank: Super Contributor
timestamp: October 05, 2023, 07:29:41 pm
content: Quote from: cvanc on October 05, 2023, 02:28:40 pmNoise is dominated by probe resistance, unless you roll your own low noise probes. Even TL071 would be good enough.Distortion is less clear and TL071 may or may not be good enough due to variable input capacitance. OPA2134 is disturbingly similar to it, per Samuel Groner's measurements.Here's more information on this. In particular, some TI parts were worse when made before circa year 2010. I suppose this is why Groner also got bad results with OPA827, which should be better today.https://www.diyaudio.com/community/threads/distortion-and-source-impedance-in-jfet-input-op-amps.264797/page-3TL;DR: you can use anything you want. But you will see distortion and you will have no idea if it's the amp, or the probe, or your input buffer.Good news is that opamps are easily swappable with one another, so no problem starting with whatever you have on hand.

author: cvanc
author_rank: Frequent Contributor
timestamp: November 01, 2023, 12:02:03 pm
content: So I got around to building this thing and it came out well.I used what I had in stock, the buffer ended up being OPA2134. The most important feature is the 3rd prong safety ground. One photo shows it calmly measuring the weird amps I built it for.I appreciate the conversation, noise is a very chewy topic to me. Thanks!

author: cvanc
author_rank: Frequent Contributor
timestamp: November 06, 2023, 10:54:26 am
content: Loopback results...