Measuring amplifier noise with scope? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 30, 2020, 02:14:01 pm
content: I've built a differential preamp for a mic. with XLR output, circuit shown below:The mic. has about 220R impedance, and the op amp is 1/2 of an NE5532. The amp. is supposed to have a low freq. voltage gain of 100 (40 dB) and and an upper 3 dB point of about 15 kHz.I'm trying to measure the self-noise at the output with a DS1054Z and I've done the following:1) I've shorted the inputs with a 220R resistor.2) I've set the 20MHz b/w limit on the channel, and the channel and probe are on x1 setting3) I'm measuring the RMS voltage on the channel4) I've set the timebase to 10 ms so that the measured value include frequency components down to 100 Hz.So the question is: given the above, is the RMS figure that I'm seeing on the scope going to be a valid representation of the amp's noise, or have I made some newbie error in my setup somewhere?

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2020, 02:30:01 pm
content: There's an old trick with non-digitising scopes.Connect the signal to both channels; you will see two "fuzzy" traces. Move the traces so the fuzz partly overlaps,then move one until the fuzz intensity of one trace is contiguous to the fuzz intensity of the other, e.g.Switch the input coupling to GND, and the difference between the two traces is the RMS noise.Source:https://www.edn.com/calculate-and-measure-noise-values/

author: exe
author_rank: Unknown
timestamp: March 30, 2020, 02:40:34 pm
content: I'd say measuring noise is a tricky topic. Anyway, here are a few questions that should help. I'd say, one need a low-noise preamp (LNA) because scopes are noisy and not very sensitive devices. So, in order to do any sensual measurements, be sure the signal is above noise noise floor of the scope. So, questions:1. What's the noise floor of your oscilloscope? Is it measurably lower than noise from the opamp?2. When measuring opamp noise, did you short its input?3. Did you use a grounded shield and shielded connectors?4. Is opamp supplied from a clean supply (e.g., batteries)?

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 30, 2020, 03:18:26 pm
content: Much easier to measure amp noise by soundcard (with specialized program, one ore another).As for me, I use both soundcard and microvoltmeter (with real termo TrueRMS converter).

author: magic
author_rank: Super Contributor
timestamp: March 30, 2020, 07:47:20 pm
content: The thing about soundcard measurements is that it requires knowing or measuring its sensitivity (how many volts for full scale output from the ADC). Once this is known, a soundcard is superior.20MHz bandwidth is way too much, you probably only care about noise in the first 15kHz.The scope may not have exactly 20MHz bandwidth if sampling rate is reduced due to long acquisition time and limited sample memory depth.Scopes are noisier than audio gear, in general.Noise of your circuit can be calculated.Johnson noise of the 4k7 resistors: 8.7nV/rtHz per resistorvoltage noise of the NE5532: 5nV/rtHz totalcurrent noise of the NE5532 times input resistance: 0.7pA/rtHz · 4700Ω = 3.3nV/rtHz per input pinAll sources add as RMS sums so the total input referred noise density is: sqrt(2·8.7² + 5² + 2·3.3²) nV/rtHz = some 10~20nV/rtHzIt will be somewhat more at low frequencies.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2020, 02:45:28 am
content: Quote from: tggzzz on March 30, 2020, 02:30:01 pmAlmost all DSOs fail when making tangential noise measurements at this step:or a DSO that emulates an analog scopeThe ones which could work produce a density modulated display like a sampling oscilloscope. But that is acceptable because there are two alternative ways to make the measurement on a DSO:1. The standard deviation is equal to the RMS noise. Just beware that some DSOs calculate this incorrectly on noise. (1) See Dave's video about RMS measurements on DSOs.2. It may be possible to use the FFT function directly however without a noise marker function, interpreting the results may be difficult.https://www.edn.com/dsos-and-noise/(1) I think the problem here is calculating the standard deviation on the processed and interpolated display record instead of the acquisition or processing record.Quote from: exe on March 30, 2020, 02:40:34 pmThat part can be a real problem even with amplification because oscilloscopes, especially modern ones, tend to have very high input noise simply because of device physics and optimizing for highest unlocked bandwidth. Old oscilloscopes used lower noise JFET input stages which were just fast enough to meet the input bandwidth requirements and this resulted in devices which were close to optimal for noise.The easy place to start is watch to see if the displayed noise level increases when the amplifier is connected. If it does not increase by several times, then the oscilloscope's input noise is too high.Magic did the calculation up to 14 nV/SqrtHz. Continuing, 14 nV/SqrtHz * Sqrt(17 kHz) = 1.82uV RMS * 1.6 Shape Factor = 3uV RMS * gain of 100 = 3mV RMS output noise.The oscilloscope input noise is very unlikely to be better than 14nV/SqrtHz (2) so with an input bandwidth of 20 MHz and shape factor of 1.6, 100uV RMS. So the measurement should be possible even if the oscilloscope input noise is several times higher but double check as I discussed above because I have seen some modern DSOs which were much worse than this.(2) Some of my old 100 MHz analog oscilloscopes are a little bit better than this.Quote from: magic on March 30, 2020, 07:47:20 pmSample rate should have no effect unless processing is done during decimation to improve resolution. The DSO will happily return full input bandwidth, or the 20 MHz bandwidth from the analog input filter in this case, even if it violates Nyquist which means that the noise above Nyquist is aliased into the displayed lower frequencies. The sample rate could be 100 kSamples/second and the input noise would still be calculated using the 20 MHz input bandwidth. RF sampling voltmeters take advantage of this to make measurements into the GHz range with sample rates of 10s of kHz.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 31, 2020, 10:11:33 am
content: Thanks for the replies - some interesting info. for me to take in. I shall return to it when I've thought about it all a bit. However, for the moment:1) With the settings as given in the OP, I measure about 490 uV(rms) noise floor for the scope i.e. with probe attached but leads shorted, and I measure about 3.5 mV(rms) of self-noise for the amp. I guess with those figures, the scope should have no problem measuring the noise of the amp?2) At the moment, the amp is made up on breadboard. I've deliberately made no effort at shortening or twisting leads to minimise noise pickup, and I'm indeed seeing a fair bit of what looks like switching noise. Given David Hess's calculation above, I guess that I'm not going to see a vast improvement if I make the circuit up more carefully, or even made a PCB with nice short traces etc? At most it's only going to be about 0.5 mV(rms), no?3) Can someone point me at a noise analysis for a differential amp? I've read section 8.8 in AoE, pages 520ff, but they don't treat the differential case there and I'm not entirely sure how to modify their approach correctly.

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 31, 2020, 10:25:24 am
content: Quote from: David Hess on March 31, 2020, 02:45:28 amI can't really follow your calculation here - for example, I don't know what you mean by a "shape factor". Also, I have 220R across the inputs, so shouldn't there be a shot noise contribution from that? I don't think Magic included that, AFAICS.Also, I was aiming for an input impedance of 4K7/2 =~ 2K3 - shouldn't that be the appropriate resistance to use when doing the Johnson noise calculations?signedA Slightly Baffled Noise Newbie

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 31, 2020, 10:36:13 am
content: Quote from: tggzzz on March 30, 2020, 02:30:01 pmLooks interesting but I didn't have much joy with this - my DSO has different colours for the different traces, and I found it very difficult to align them in any sensible way - I could see where one lot of fuzz began and the other ended too well - it may work better on a grey-scale analogue scope.

author: tggzzz
author_rank: Super Contributor
timestamp: March 31, 2020, 10:47:44 am
content: Quote from: aneevuser on March 31, 2020, 10:36:13 amThat's because you have a digitising scope!You need more than a simple "grey scale" for the technique to work. FFI see David Hess' response.

author: magic
author_rank: Super Contributor
timestamp: March 31, 2020, 11:02:33 am
content: Quote from: aneevuser on March 31, 2020, 10:25:24 amJohnson noise, yes. Even more precisely, without the mic, the input still has 9.4kΩ differential impedance due to the two grounding resistors. So you should count that in parallel with the 220Ω. But none of that makes much difference, see below.Quote from: aneevuser on March 31, 2020, 10:25:24 amNope, because each of the input series resistors generates its own noise and this noise appears across the resistor and adds to the signal, from the opamp's perspective. This happens independently to each half of the differential signal so noise contributions from the two 4k7 resistors add up.If you really want to push noise further you would need to reduce those resistors. And if that results in unacceptable input impedance, add more opamps and use the "instrumentation amplifier" topology.Or just ask yourself how much noise you needWhat are the mic's specs?editAnd by the way, if your NE5532 look like those on the first pic below, they are not NE5532https://www.eevblog.com/forum/projects/whats-inside-the-cheapest-and-fakest-jellybean-opamps/

author: aneevuser
author_rank: Frequent Contributor
timestamp: March 31, 2020, 12:34:40 pm
content: Quote from: magic on March 31, 2020, 11:02:33 amWhat I'm finding difficult to see here is the appropriate equivalent circuit for the resistor network that allows to calculate the noise correctly. If I had each input connected independently to ground via a distinct R network, I believe that the appropriate noise resistance is simply the Thevenin resistance of that network, no?However, here the inputs are not independent - they're connected via the 220R, and it's not at all clear to me how to analyse this from a theoretical point of view.QuoteBy "instrumentation amplifier" topology, do you mean the paralleled-up identical amp approach?QuoteThe noise I'm getting when recording is to my mind unacceptable so I want less. I think that part of the problem is that the mic. (Behringer XM8500) has a low sensitivity and so I need excessive gain (=> more noise) to get the o/p up to consumer line-in levels.Anyway, as you can see, I know sweet FA about noise issues, so I'm using this as opportunity to learn about it.QuoteThanks, but I got this years ago from RS or somewhere vaguely reputable, so I think it's probably OK. I now avoid ebay for components after I bought some fake varicaps there.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 31, 2020, 01:29:54 pm
content: make your own 10X gain post-amplifier opamp module you'll see noise in DSO 10x bigger. maybe TL071 will do? or there are more plenty higher end low noise opamp out there. if you still cant see anything other than DSO noise floor, you can bet your circuit is as good as calm water.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2020, 02:51:29 pm
content: Quote from: aneevuser on March 31, 2020, 10:25:24 amShape factor is a correction for the finite high frequency roll-off of the amplifier or filter. When the noise is calculated up to the bandwidth converting nV/SqrtHz to nV RMS, some of the noise above the bandwidth leaks through from above the -3 dB cutoff frequency. If the filter was perfect, then the shape factor would simply be 1. But for a single pole rolloff of -6 dB per octave, the correction for shape factor is 1.6. Better filters have shape factors closer to 1.In this case there are two corrections for shape factor; your amplifier has a single pole cutoff of about 17 kHz from 470K in parallel with 20 picofarads, which I assumed the NE5532 is fast enough to support at a gain of 100, and the oscilloscope has a single pole cutoff at about 20 MHz.QuoteWe both kept things simple by only including the highest noise sources which make the most contribution. I didn't bother with flicker noise which increases at low frequencies either because it is usually insignificant over audio bandwidths and unspecified for almost all oscilloscopes.Quote from: aneevuser on March 31, 2020, 10:36:13 amIt works great on an analog oscilloscope; better than 5% accuracy is readily achievable. The only thing better is computing noise on a DSO using standard deviation assuming that it does it correctly but how do you know unless you have some way to test it?

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2020, 03:31:12 pm
content: Quote from: aneevuser on March 31, 2020, 12:34:40 pmHe means use a true differential input circuit which lacks those relatively high impedance resistor networks on the inputs and their contribution to noise; the disadvantage is an increase of input voltage noise by 3 dB but the decrease in current noise and Johnson noise from resistors more than makes up for it. The first example here shows it:https://en.wikipedia.org/wiki/Instrumentation_amplifierNow all of the resistors can be lower in value, consistent with the drive capability of the operational amplifiers, without loading the source so their contributions from Johnson and current noise are minimized. Note that only the differential preamplifier stage contributes significant noise because its gain divides the noise contribution of the following instrumentation amplifier stage.Douglas Self's book "Small Signal Analog Design" shows this circuit configuration but with two transistors replacing the two operational amplifiers which allows tailoring the ratio between input voltage and input current noise by adjusting their operating current.And that answers:Quote from: aneevuser on March 31, 2020, 10:11:33 amEssentially the differential configuration increases input voltage noise by 3dB (1.414) because two separate amplifiers are contributing it. A single ended configuration using a transformer followed by non-inverting amplifier would inherently have a 3dB advantage in noise and used to be common for microphone input amplifiers.

author: magic
author_rank: Super Contributor
timestamp: April 01, 2020, 07:21:05 am
content: Quote from: aneevuser on March 31, 2020, 12:34:40 pmFrankly, I identified the offending resistors heuristically: they are in series with the signal source from the opamp's perspective. The lower resistor has its left end connected to ground through (4700||(4700+220)) and the right end through 470k, so essentially all its noise voltage appears on the right side and is indistinguishable from "cold" signal. The upper resistor has its right end maintained at constant voltage by the opamp, so its noise voltage appears as random variation on the left side, which cause random variations in the resistor's current, indistinguishable from "hot" signal.If you want a single number, off the top of my head I would say you should calculate the equivalent resistance of the whole input network between the input pins of the opamp.Quote from: aneevuser on March 31, 2020, 12:34:40 pmWell, simple question, what happens to the recorded noise when you replace the mic with an quivalent resistor?editAs for the noise of the in-amp topology, there are of course better chips than NE5532 out there. Right off the bat, NE5534 has about 1.5x less noise to perfectly compensate for the noisier topology. NJM2068 is a cheap dual about on par with NE5534, though I'm not sure about its distortion performance. NJM2122 is not much pricier andverylow noise. Or pull out the big guns like LM4562, OPA1612, LT1028. Finally, there are dedicated integrated in-amps like the various INA-whatevers from Burr-Brown/TI and people sometimes build mic preamps with them too.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 09:55:25 am
content: Quote from: David Hess on March 31, 2020, 03:31:12 pmOK, that looks promising - I guess it's based on the idea that to minimise noise figure in a composite system, you want high gain/low noise in the first amp.However, it looks a little too good to be true - in my setup, won't I still need a resistance to ground at the inputs of the instrumentation amp, to satisfy input bias current requirements? If so, doesn't that undermine the whole argument that you've made?

author: magic
author_rank: Super Contributor
timestamp: April 01, 2020, 10:08:51 am
content: The 220Ω resistance of the mic will largely mix and equalize the two bias currents before they enter the grounding resistors. The sum of the two bias currents will flow through the parallel combination of grounding resistors producing relatively high common mode noise due to their high resistance, the difference between them will flow through the mic, resulting in much lower differential mode noise which is the noise that matters.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 10:12:29 am
content: Quote from: magic on April 01, 2020, 07:21:05 amHaving thought about this a bit, each input sees the Thevenin resistance to ground of the network as calculated at the appropriate point, and will thus see potentially different resistances from the POV of noise - but these won't be entirely uncorrelated noise sources, so we can't simply add them in the usual way. Hmm, this is all too damn tricky for a clueless noob such as myself.QuoteWell, that's exactly the setup I've got - a 220R directly across the inputs. Or am I misunderstanding what you're saying here? I don't have the XLR cable connected, but I have done that in the past when I was trying to listen for mains hum rejection - I had the XLR cable with the 220R between hot and cold at the mic end.QuoteYeah, I could look at other opamps, but I'd like to push what I've got as far as possible before I do that. The instrumentation amp idea looks promising.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 10:17:28 am
content: Quote from: magic on April 01, 2020, 10:08:51 amThanks, but I can't follow this at all - can you put up a circuit diagram so I can see what you're describing?

author: magic
author_rank: Super Contributor
timestamp: April 01, 2020, 10:40:56 am
content: Look at the schematic herehttps://www.audiomasterclass.com/newsletter/the-famous-5-preamp-everything-you-need-to-knowEach INA input injects noise current into one of the 2k2 resistors R4/R5. However, at audio frequencies, impedance of the coupling capacitor C1/C2 is very low and impedance of the mic is low too so each noise current splits in half and flows through both 2k2 resistors, producing common mode noise which is ignored by the INA.Quote from: aneevuser on April 01, 2020, 10:12:29 amFine, I thought that "recording" meant "with a mic". If you get noise with a resistor, it has to be coming from the preamp, particularly if we calculated that the preamp makes a lot more more noise than a 220Ω resistor.I agree, swapping opamps at this point is futile. The largest source of noise are resistors.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 12:46:39 pm
content: Quote from: magic on April 01, 2020, 10:40:56 amOK, so if I understand you right here, you're saying that we have a single source of current noise that then appears, due to your splitting argument, as common mode noise to the amp, which has good CMR, so we can neglect it?However - and I don't want to seem too dimwitted here - but... don't we have essentially the same front end resistor setup that I currently have - 2K2 at the inputs, so we have precisely the same *Johnson noise* problem as I do currently? Sticking two resistors at the the front end of the instrumentation amp seems to negate the argument given by David Hess, where he said "use a true differential input circuit which lacks those relatively high impedance resistor networks on the inputs and their contribution to noise".Am I confused?

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 12:54:17 pm
content: Quote from: David Hess on March 31, 2020, 02:51:29 pmOK, that makes sense. BTW, is there some classic "Noise Theory for Electronics" text that I should be aware of? I'm quite like to see some derivations of all this stuff from first principles (I'm Mr Theoretical - making real stuff scares me) but I don't know what the literature is like here.QuoteThe quoted GBW is 10 MHz, so with a gain of 100 the upper 3dB point is at 100 kHz - that should be fine, I think?Anyway this is the most incredibly useful thread - I'm amazed by the technical quality of some of the people on this site. Thanks.

author: magic
author_rank: Super Contributor
timestamp: April 01, 2020, 05:14:16 pm
content: For the record, NE5532 has over 30MHz GBW at frequencies that matter and for NE5534 it's over 60MHz and perhaps up to 80MHz from some manufacturers, IIRC.The trick? Gain falls faster that 6dB/octave starting at a few hundred kHz.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2020, 07:41:47 pm
content: Quote from: aneevuser on April 01, 2020, 09:55:25 amSerious audio equipment uses a pair of transistors for the differential preamplifier instead of two operational amplifiers but the result is the same and using operational amplifiers is easier.QuoteThat is right; a DC connection to common is still required at the input to supply the input bias currents however it is the source impedance which determines input noise.Quote from: aneevuser on April 01, 2020, 12:54:17 pmYes, that will be fine. However one advantage of the three operational amplifier instrumentation amplifier is that the total gain is divided up between stages so frequency response and distortion can be better. Effectively you have 3 * 10 MHz GBW = 30 MHz GBW to work with instead of just 10 MHz GBW.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 08:28:21 pm
content: Quote from: David Hess on April 01, 2020, 07:41:47 pmI'll have to come back to all this at a later time, but from what you've said, I'm guessing that the effective noise resistance is the source resistance || bias current resistance (or something similar - not sure of the details)? So I can in fact make the bias current resistances quite large without degrading noise performance too much?

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2020, 08:39:22 pm
content: Quote from: aneevuser on April 01, 2020, 08:28:21 pmThat is right. And it is the differential source resistance which matters because the instrumentation amplifier rejects common mode noise.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 01, 2020, 08:54:22 pm
content: Quote from: David Hess on April 01, 2020, 08:39:22 pmI'm not sure what you mean by "differential source resistance" here. Does this just mean the source resistance across the diff. inputs i.e. in my case the 220R?Anyway, this looks incredibly sweet - I'll put it together tomorrow and see if I can measure any improvement in the noise. Unfortunately I'll have to use a 411 for the "backend" amp - only 3-4 MHz GBW - but I think that'll be good enough given my bandwidth requirements, and the higher noise shouldn't matter there.Oh and I've just noticed another benefit - controllable gain via a single pot. - I don't think that was possible with my original diff. amp. arrangement.

author: David Hess
author_rank: Super Contributor
timestamp: April 02, 2020, 03:49:23 am
content: Quote from: aneevuser on April 01, 2020, 08:54:22 pmYes.QuoteThere are ways to do it but it adds complexity.Douglas Self wrote a book called "Small Signal Audio Design" which has a chapter on microphone input amplifiers. Analog Devices has some application notes on the subject. And check out Walter Jung's book "Op Amp Applications Handbook" which is freely available online.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 03, 2020, 03:27:28 pm
content: OK, after a torrid 24 hours of nothing at all working properly (breadboard eff ups, probably), I've put together an operational version of the following in-amp-topology preamp:with:R_i = 4K7R_f=560R || 10 pFR_g=100R pot.R=1KI've used hand tested 2% resistors to ensure the best matching with my somewhat limited stock of components, and it's once again a Heath Robinson breadboard affair, with no attempt at keeping leads short. In the first version, I was getting lots of oscillation with a 1 kHz i/p signal (and some bizarre o/p clipping at 1V) so I've added some more decoupling after rebuilding and the problems have now gone away. It's powered off 2 9V batteries so I don't have to worry about power supply noise.With the measurement setup as described in the OP, I measure 2.7 mV(rms) self noise for the amp with a 220R source resistance - a smallish but useful improvement from the original 3.5 mV(rms). However, I also now have a measured CMRR of -105 dB (! is appropriate, I think) and I think that makes a more significant difference to the the audible noise. (The CMRR of the purely differential amp was about -60 dB).With the gain set to 70 and with the mic. given a bit of adhoc shock mounting, the noise is now pretty acceptable in recordings - much less noticeable rumble.So, to summarise: instrumentation amplifier topologies for mic. preamps is now very much the bag I'm into, baby. I'm fairly impressed that a clueless noob such as myself can throw something together with such good CMRR from discrete components.If anyone wants to point out any obvious improvements that can be made, feel free to do so.BTW, I have in the mean time discovered the INA163 which is aimed at these sort of applications - would I be likely to see a noticeable improvement if I were to use one of those instead?

author: magic
author_rank: Super Contributor
timestamp: April 03, 2020, 06:45:57 pm
content: Quote from: aneevuser on April 03, 2020, 03:27:28 pmQuote from: aneevuser on April 03, 2020, 03:27:28 pmThis isn't a valid comparison if you haven't preserved the original 15kHz bandwidth limit.I recommend taking a few seconds of recorded noise and feeding it into some spectrum analysis software for comparison. This will show noise density vs frequency, in some unspecified dB units which stay consistent as long as recording volume and analysis software settings are the same. This is important, don't fiddle with any FFT options between measurements.With a bit of calibration, one can even produce numbers in absolute units of nV/rtHz or dBV/rtHz. For example, an NE5532 with 1Ω/100Ω gain network and the input shorted to ground makes a ballpark accurate 5nV/rtHz input-referred noise reference because Johnson and current noise are made negligible. More accurate calibration is also possible from first principles, but it's a subtle business as shown by example of one NwAvGuy whose noise measurements are complete bollocks if naively interpreted in units of dBV/rtHz.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 03, 2020, 07:30:37 pm
content: Quote from: magic on April 03, 2020, 06:45:57 pmHmm. Are you talking about the high frequency roll-off here? If so, yes, it looks like I've screwed up - doubly in fact. There's a typo in the info above - should be 10 nF not 10 pF, but in fact the 10 nF is wrong as it doesn't give the ~15 kHz 3db point. Damn - I'd better look at this again. Thanks. However, it's wrong in the right direction - this should double the noise bandwidth, and if that's good enough, then it can only get better when I fix it.QuoteSounds like a good idea - does Audacity do this? If it does it more accurately than the scope, that would be ideal.

author: magic
author_rank: Super Contributor
timestamp: April 03, 2020, 07:49:56 pm
content: I think it has that option, check the menus.One thing I forgot: I'm not entirely sure if the whole noise floor won't move up or down if you increase/decrease the length of file. Yes, you read that rightWeird problems exist when measuring noise because spectrum analysis divides the whole spectrum in buckets and reports signal energy per bucket. With concrete tones, each tone falls in one bucket and this bucket reports the whole energy of the tone correctly. With noise, bucket width (measurement resolution) determines how much energy falls in each individual bucket. The results may vary with software settings in bizarre ways.

author: magic
author_rank: Super Contributor
timestamp: April 04, 2020, 07:33:33 am
content: Quote from: aneevuser on April 03, 2020, 03:27:28 pmThese things have inherent noise advantage in that they use a very simple current feedback input stage with only one transistor per cold/hot input. This gives them noise performance on par with a normal opamp made of the same low noise transistors, but it would take two such opamps to build an equivalent circuit. Conceptual schematic of a similar part from AD is attached for reference; I suppose BB uses similar input circuitry.But for reasons not entirely clear to me, their differential to single-ended stage is oftentimes horribly noisy. When you look at the plot of input referred noise vs frequency and gain for INA163, you see that it only reaches its full potential at gains greater than 100x. At 100x or less it would be beaten by quiet opamps like LT1028, AD797, OPA1612, maybe even NJM2122. At gains of 10x and less it's on par with what your current NE5532 circuit should deliver.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 04, 2020, 09:13:13 am
content: Quote from: magic on April 04, 2020, 07:33:33 amThanks for this. I think that this has in fact answered another question that I was going to ask. David Hess mentioned the Self audio book - I have a copy of this in fact, though I havent looked at it much. Self has some preamps frontended with a discrete transistor stage, whose topology confused me - they look like they have an almost-long-tailed-pair, with emitters shorted by a resistor - this is simply a discrete transistor in-amp, isn't it?QuoteThey're using 5K resistors there - that seems large to me - could that be part of the reason?BTW, what determines how small the resistors can be chosen on the o/p stage? The o/p impedance of the front end amps will be negligible, so presumably it's not that?QuoteWell, I'm looking for gains of 70-100 with my current mic. - maybe less if I get a condenser mic - so it should be a good match from what you've said.Anyway, I'm still basking in the glow of -105 dB CMRR from a breadboard lash up - eat ya heart out, IC designers!!!!

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 04, 2020, 09:23:41 am
content: Two other points:1) It's just dawned on me that since the noise model for a resistor is a noisy AC voltage source in series with an ideal resistor, then if you want to analyse the noise behaviour of a circuit properly, you need to do it on the AC equivalent circuit. This is probably blindingly obvious to anyone who isn't a noob.2) I'm now beginning to see why the weird audiophile guys get so weird - it's very easy to get painfully involved with trying to hear small improvements in noise, or whatever, and to keep tweaking things to get the best possible result. Anyway, that's enough for now - my delivery of oxygen free copper wire has just arrived - can't wait to see what difference that makes.

author: magic
author_rank: Super Contributor
timestamp: April 04, 2020, 12:10:23 pm
content: Quote from: aneevuser on April 04, 2020, 09:13:13 amObviously, only if the amplifier has very high open loop gain from each input to the output and feedback is arranged in the same manner as here to set a desired closed loop gain.Such resistor arrangement could also be used in any other kind of amplifier to reduce and linearize the differential gain of an LTP stage.Quote from: aneevuser on April 04, 2020, 09:13:13 amHave a look at the JRC MUSES03. It's a rare opamp with OFC pins and it has JFET inputs so you can parallel pretty much as many of them as you want to average out the noise. Bummer that the starting noise is relatively high.Maybe TI will pick up the fad and offer OPA827 with OFC? So far I have only seen one TI analog manager rating on DIYAudio how unfair it is that "a competitor" sells snake oil chips made with 1980s technology and makes money on that.Those guys have no clue

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 04, 2020, 12:40:13 pm
content: Quote from: magic on April 04, 2020, 12:10:23 pmThank you but sadly, since Mouser seems to be asking for about 60 of the finest English pounds each (inc. VAT - they're generous like that) in quantities < 10, I suspect that I'll stick with my discrete-in-amp-o-matic solution, humble as it may be.

author: exe
author_rank: Unknown
timestamp: April 05, 2020, 08:09:12 am
content: My friends, I see discussion of correcting offset voltage, but it seems I missed the problem statement -- why do you think offset is a problem? I believe many well-designed audio circuits don't need it. If one wants, there are many precision opamps with very low offset voltage.It is also possible to externally correct offset voltage, or even build a something like a chopper amp, or with some servo circuitry. But before going that way I'd say it needs to be proven that this is necessary.Update: wrong thread

author: magic
author_rank: Super Contributor
timestamp: April 05, 2020, 08:20:06 am
content: Quote from: exe on April 05, 2020, 08:09:12 amI think you meant this post for some other thread?Quote from: aneevuser on April 04, 2020, 09:13:13 amYou can calculate how much of a reason it is and how it compares to the second stage noise spec from their datasheet. Hint: their second stage is almost identical to the circuit you started withQuote from: aneevuser on April 04, 2020, 09:13:13 amThere are other concerns like the size of output stage transistors required to drive those resistor networks (remember the amp runs on up to ±18V rails), power consumption, heating of the die.

author: exe
author_rank: Unknown
timestamp: April 05, 2020, 08:08:34 pm
content: Quote from: magic on April 05, 2020, 08:20:06 amYes, thanks for telling me, I was wondering what happened to that message.

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2020, 10:07:02 am
content: Quote from: aneevuser on April 04, 2020, 09:13:13 amIt is the two transistor equivalent to the dual operational amplifier differential preamplifier we discussed earlier.

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 08, 2020, 11:54:15 am
content: Quote from: David Hess on April 06, 2020, 10:07:02 amThis confuses me a lot. Self seems to have quite a few discrete transistor frontends for his amps, often with some differential aspect. However, I can't see how this could possibly beat an integrated implementation - IC designers can match transistor sizes accurately, use laser trimmed resistors, slap in a current source at will, and use whatever other tricks I'm unaware of, to make a well matched differential amp. How can a discrete implementation possibly compare with that?

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 08, 2020, 11:59:07 am
content: Quote from: magic on April 01, 2020, 05:14:16 pmI now have time to come back to this. I'm not seeing the trick - can you elaborate? In fact, I'm not sure that I understand what you're saying - is it that effective GBW is a function of overall amp bandwidth, rather than the constant 10 MHz or whatever quoted in the datasheet?

author: aneevuser
author_rank: Frequent Contributor
timestamp: April 08, 2020, 12:07:36 pm
content: Quote from: magic on April 01, 2020, 07:21:05 amThis looks like a promising replacement for the NE5532 - same pinout, low price - but...this datasheet:https://www.njr.com/semicon/PDF/NJM2122_E.pdfquotes only equivalent input noise voltage - which is very good - but doesn't mention input noise current. I'm suspicious that it's appallingly bad. Anyone know what's going on here - AoE doesn't list it so no joy there.

author: magic
author_rank: Super Contributor
timestamp: April 08, 2020, 04:15:57 pm
content: So you discovered the joy of Japanese datasheets. JRC, NEC or Mitsubishi, all that stuff was marketed for audio products and usually only specifies noise in terms of total noise output from some RIAA preamp or other "typical" audio circuit. Oftentimes A-weighted.That being said, this one has a "noise versus source resistance" plot. Of courseit's the total noise over some unspecified bandwidth (lemme guess, 20-20k) with a few weighting schemes to choose from, but you can see the general pattern: it's about 1.5x worse at 200Ω than at 10Ω. Since Johnson noise of 200Ω is 1.8nV/rtHz, it fully explains the increase and no third noise source is apparent.For the record, the minimum limit of current noise is the shot noise of the input bias current. Good bipolar opamps achieve this limit, at least above some tens of Hz. Shot noise density formula due to one Schottky: √(3.2e-19·IB), so 1pA/rtHz for 3.6µA.I actually bought one of those chips, but haven't played with it yet.Quote from: aneevuser on April 08, 2020, 11:59:07 amOkay, I dug out my old notes. Actually, NE5532 had only some 20~25MHz GBW at audio frequencies. The 30MHz figure was for a fake 5532 from an auction site, I have no idea what's inside of that one but (surprisingly) it has to be some good shitIt means that open loop gain at 10MHz is 1x and at 1MHz is 10x, but at 100kHz it's about 230x, and then at 10kHz it's 2300x again.

author: David Hess
author_rank: Super Contributor
timestamp: April 08, 2020, 06:31:46 pm
content: Quote from: aneevuser on April 08, 2020, 11:54:15 amThe two transistor version is a differential tranconductance amplifier with a constant resistance load to convert the output currents into a differential voltage. The differential voltage gain is the collector resistance divided by the emitter resistance and this configuration is commonly used in true instrumentation amplifiers. Check out figure 6 in theAnalog Devices MAT-04 datasheetwhich could work as a pretty good microphone amplifier.The matching of the transistors is important in DC precision applications but less so in AC applications.The operational amplifier version works considerably differently but produces the same result.