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Old 27th Jul 2020, 9:20 pm   #81
bikerhifinut
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Default Re: Very low noise op amp recommendations?

just when you thought it was safe to go back in the water...………..

I was looking at the, admittedly flawed, 2 stage circuit and it occurred to me why not split the EQ between the 2 stages, surely where its applied has no effect on the result at the output?
My thoughts were as there seems to be a possible issue, perhaps to less than optimal design admittedly, with the LF boost in the high gain 2nd stage, why not put that bit of EQ around the 20db first stage feedback loop or even have it as passive although I have my own irrational bias against that. My theory, possibly extremely flawed, is that any large LF bursts say from a scratch or other disturbance will not be amplified to an extent that will cause signal blocking, clipping or other overload issues? This does of course mean there is still the issue of said transient impulse being amplified in stage 2 which would have the HF EQ wrapped around it.
So is there any advantage in this?
and as a supplementary to the query, if the LF EQ can cause such an issue then is it a feasible thing to have it as a passive filter, accepting the gain loss issues, at the output as I imagine it will work by attenuating the mid and high frequencies above its frequency turnover point?
Told you I was simple minded...………
I can only think there is a glaringly obvious reason why not otherwise the commercial designs would do it.
A.

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Old 28th Jul 2020, 12:13 am   #82
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Default Re: Very low noise op amp recommendations?

I think that the closing paragraph in Craig’s post #33, repeated below, indicates why doing the RIAA equalization around the input stage, as compared with doing it later, is beneficial from a noise viewpoint. And lowest reasonably achievable noise does seem to have been an objective here.

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Originally Posted by Craig Sawyers View Post
I implemented as a spreadsheet a table which appears in the National Audio/Radio handbook, Appendix 6, page 15. That divides 25Hz to 20kHz into 10 bands. (This classic tome has been reprinted; I bought mine recently from Amazon)

This calculates SNR, flat and with RIAA for cartridge L and R, capacitive loading and load resistance. With a noiseless amp.

For 50k/sub pF load capacitance, L=600mH and R=1 ohms and 2mV (Walker's parameters) it gives:

Flat SNR 59.5dB, and with RIAA 72.0dB.

Which is exactly the same as Walker's manual calculation for RIAA SNR of five decades ago.

What is rather interesting is that if the noise is dominated by a high gain flat buffer, the effective SNR is degraded as compared with wrapping the RIAA around the input stage. It also says that having a high gain input stage, passive RIAA and then a second flat gain stage is also going to have a degraded SNR.

A pertinent question concerns placement of the 75 µs part of the equalization curve, assuming that the 3180 and 318 µs parts are done in the feedback loop around the input stage. Assuming a perfect or near-perfect amplifier, is there a material difference in noise performance between doing the 75 µs part within the feedback loop (with a passive supplementary pole to maintain the 6dB/8ve downslope) and doing it passively, after the amplifier, as Baxandall did?

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Old 28th Jul 2020, 12:47 am   #83
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Default Re: Very low noise op amp recommendations?

A light has just started to dimly glow...……..Whereas my thinking, in the last post was a bit muddled and was me mulling over the benefits or not of splitting the LF and HF components of the EQ between the 2 stages, The bit that got me there was the argument on noise of feedback vs passive EQ between 2stages.I think this explains why I think my valve phono stage is noisy as its a completely passive EQ between the stages with the extra 75us correction on the output. I previously had an EAR834p phono stage which used feedback EQ and it was to my best memory audibly quieter than the current one.
Ok that was valves and we are talking op amps here but the principle must be the same.
Therefore, it seems to me that in this instance a single stage with the EQ around its feedback loop must be the quietest way and should I need a wee bit more gain, then a subsequent op amp flat amplifying stage will not add any appreciable noise degradation because we're not working with a very low input voltage so the op amps inherent low noise values will keep the S/N ratio pretty much intact?
And any possible overload issues from LF transients can be dealt with by dint of a low value coupling capacitor into the second stage to form a high pass filter that will roll off any excessive non music LF components?
Am I also correct in thinking that the 75us filter can use the "stopper" resistor on the output as part of it?
A.
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Old 28th Jul 2020, 1:47 am   #84
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Default Re: Very low noise op amp recommendations?

Following on from my post #33, above, something that had occurred to me is that some equipment designers may have been distracted by the progressive lowering of opamp noise floors, seeing this as a pathway to ever lower noise floors in RIAA stages, whilst forgetting that with MM cartridges as the input source, the ultimate noise floor was determined by the nature and magnitude of the MM source impedance coupled with its required loading.

Whilst H.P. Walker’s 1972 work focussed on the MM/RIAA case, it actually had a broader mission, encapsulated in the opening statement: “Minimizing transistor noise-figure does not necessarily lead to an optimum low-noise design. In this article the effect of circuit configuration is included with a discussion of optimization with a complex source impedance, and a design procedure is given.” One could substitute “opamp” for “transistor” to bring that statement up-to-date.

Out of curiosity, I looked up one of the “high-performance” MM/RIAA circuits, namely that described in Electronics Today International, Australian edition, 1981 September and October. It was a two-stage circuit, with a flat input stage and equalized second stage, both using NE5534AN opamps. The circuit may be found on page 27 of the September edition, with the specifications on page 25. With respect to the noise parameters, the total equivalent input noise was stated to be 122 nV, A-weighted, and 216 nV flat (presumably meaning unweighted), in both cases with the input shorted. Signal-to-noise ratios were given for 1, 5 and 10 mV input levels, which as expected jumped in 10 dB steps. For 1 mV the numbers were 73 dB flat (unweighted) and 78 dB A-weighted. Source conditions were not stated; perhaps it was intended to be inferred that they were the same as for the equivalent input noise numbers, namely input shorted. But we can be fairly certain that those SNRs were not measured with an MM cartridge or simulated MM cartridge source impedance as the input. Both HPW’s and Craig’s calculations indicate that for a 1 mV input, 66 dB (72 dB for 2 mV less 6 dB for the halving of the input level) is the theoretical best with a noiseless amplifier.

One could ask was the designer unaware of the effect that an MM cartridge has on noise performance, or was/is there some convention in the way that MM/RIAA stage noise numbers were/are presented, or was it a case of choosing the best-looking numbers?

To be fair, focussing on device rather than overall circuit noise characteristics was not something that arrived with low-noise opamps. In the discrete era, some maker used PNP transistors in the first stage of two-or-three-device MM/RIAA circuits, claiming that such were lower noise than similar NPN devices. HPW covered that in his 1972 article. Apparently the PNP noise advantage was there, but realizable only with lowish source impedances, and apparently not in an MM/RIAA circuit. Notably in his 1971 mixer design, HPW used PNP inputs for some of the microphone preamplifiers, but not for his MM/RIAA module. Going further back to the valve era, when MM cartridge outputs were somewhat higher than was normal by the later 1960s, the pentode (often an EF86) with shunt feedback was frequently used for the MM/RIAA stage. Here the limiting factor for noise was the input arm resistor, typically 68k in those days, and not the valve itself. (Some makers actually stated their noise specifications that way.) But there were one of two cases where makers used a cascode double triode with shunt feedback, claiming qualitatively at least lower noise because a cascode is per se quieter than a pentode, even though the shunt feedback circuit cancelled that advantage. In some of the PNP and cascode cases we can give the makers a partial pass, as the same preamplifiers were also used for microphone inputs, where their noise advantages would be realized. But the distinction was usually not made in respect of the “lower noise” claims. With valves, I think that the “standard” route to lower noise than the shunt feedback MM/RIAA circuit was a double triode with series feedback (including the equalization) and DC heating, at least on the first triode.

Circling back to the original question, “Very low noise op amp recommendations?”, if this is restated as “Very low noise opamp-based MM/RIAA stage recommendations”, then all of the arrows appear to be pointing to the NE5534 single-stage circuit.


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Old 28th Jul 2020, 7:21 am   #85
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Default Re: Very low noise op amp recommendations?

Not that many people can do noise calculations for themselves. Making it more difficult is the fact that noise can be modelled/measured/specified in several ways which all come to pretty similar end results, but it's easy to get confused and inadvertently wind up trying to mix and match things across the different approaches. This doesn't work and leads to errors - without those errors being readily apparent unless you already knew what the result should be!

As a consequence, the hifi industry did what it tends to do and fashion crept in. The straight-forwards approach of an affordable opamp like NE5534 with a simple network around it looks too easy and not at all esoteric enough. They needed more impressive looking circuitry which would allow purchasers to feel more exclusive, accountants to see more profit and marketing departments to have product differentiations to point at.

All win-win for everyone, unless you do the real noise analysis, or do some honest measurements with real cartridge impedances on the input. Then you will see what a hard act the NE5534 as an RIAA shaped first stage is to follow.

Flat pre-amplification ahead of an RIAA shaped stage and flat pre-amplification ahead of a passive RIAA shaped filter both are compromised in noise behaviour and are compromised in terms of clipping levels. But, these things are not obvious until you do the analysis. What is obvious is that they look more elaborate on circuit diagrams and it's easy to fall into thinking that passive equalisation must be better than active equalisation - simply because it doesn't have any of those accursedly imperfect active devices in it. Well you need the accursed active devices anyway to produce the required gain so you get their imperfections anyway. Wrapping the equalisation function around them means that they don't have to produce that full gain across the entire audio spectrum and the active devices get an easier life.

Passive equalisation sounds sexy though, and has helped to sell a lot of designs. Human memory of sound levels is notoriously poor and trying to remember the sound of noise levels is particularly so.... also remember that reviewers, even those whose external auditory meati are particularly auric, are also listening via a volume control which they choose to set however they will. Any chance of hearing that this RIAA stage is a couple of dB noisier than that one, went out the window long ago.

You could notice it with careful A/B listening with each carefully set up to produce the same signal level from the same record and cartridge, and convince yourself it was real once you look at the statistics of a group of properly blind tests. But we all know exactly how averse modern reviewers are to such antics. So fashion triumphs over maths, science and engineering, and hardly anyone knows.

It is a delightful illustration that you can't judge performance from component count. This also works in the opposite direction, there are many circuits around for other functions where component count shaving has done to such an extent that the remaining components become critical on some parameters and only selected ones can make the circuit work.

David
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Old 28th Jul 2020, 8:15 am   #86
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Default Re: Very low noise op amp recommendations?

Although this is all about moving coil designs, the architecture is interesting. First, with MC having a flat gain stage first gives the best noise performance because the cartridge is resistive with a few ohms resistance. (Self agrees with this approach).

But what is interesting is the combination of discrete input amp followed by op-amp RIAA stage(s). He tends to split the RIAA over at least two stages.

Finally, he shows a transistor noise measurement schematic and the gain equation to use, but with an error. He calculates the gain as being dependent on R12 only, and misses the point that pot R21 is in parallel via C21. So the noise gain depends on the setting of R21 (collector voltage).

http://www.synaesthesia.ca/LNschematics.html

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Old 28th Jul 2020, 8:41 am   #87
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Default Re: Very low noise op amp recommendations?

Quote:
Originally Posted by Synchrodyne View Post

Out of curiosity, I looked up one of the “high-performance” MM/RIAA circuits, namely that described in Electronics Today International, Australian edition, 1981 September and October.

With respect to the noise parameters, the total equivalent input noise was stated to be 122 nV, A-weighted, and 216 nV flat (presumably meaning unweighted), in both cases with the input shorted. Signal-to-noise ratios were given for 1, 5 and 10 mV input levels, which as expected jumped in 10 dB steps. For 1 mV the numbers were 73 dB flat (unweighted) and 78 dB A-weighted.
That is rather a large difference between flat and A-weighted of 5dB, perhaps because they used a 1k resistor as the input. I suspect that is the case, because an equivalent input noise of 216nV is -133dBu, and that is quieter than a noiseless RIAA amp which is -130dBu with a realistic cartridge input of 610 ohms and 470mH ref 5mV at 1kHz.

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Old 28th Jul 2020, 9:09 am   #88
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Default Re: Very low noise op amp recommendations?

Noise is spectrally-distributed power. Any expression of its level whether as power, voltage or current is meaningless without a statement of the bandwidth involved.

Otherwise, by playing about with bandwidths, you can have any numbers you want.

The native SI units would be Watts/Hertz if it was to be expressed as a density at a spot frequency.

The Maxwell-Boltzmann constant 1.38 E-23 Joules/Kelvin
becomes 1.38 E-23 Watts per Hertz per Kelvin noise power density from an ideal resistor into a matched load.

If the load isn't matched you don't get all that power density.
Some resistors are close to ideal, other types can be 20dB or worse more than the ideal case due to fluctuations in any residual DC flowing in them.... and it doesn't take much DC.

A-weighted measurements at least constrain the bandwidth and start to make sense. The A-weighted curve is an answer to the old question "What noise annoys an oyster?" by modelling how much people with normal hearing object to different frequencies of unwanted sound by representing the change of level versus frequency for equal annoyance. There are flaws in this, especially as the presence of multiple frequencies at once has interaction effects, so it's not sure if single tone tests represent the objectionability of noise. Tests with shaped noise asking which sub band is most noticeable might lead to a more appropriate shape. So there are several competing noise weighting curves.

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Old 28th Jul 2020, 10:38 am   #89
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Default Re: Very low noise op amp recommendations?

Although a lot of the learned discussion is going above my level of understanding and requires several reads of a sentence I'm getting a lot of insight here.
I am not sure where I fit in with the what you can hear and what annoys from Davids last thread but I do know that at least the last hearing test I had for work, I seem to have an unnatural sensitivity to high frequencies, for instance at one point a few years back I was getting very concerned that I was suffering from a very severe degradation of my low level tinnitus. It turned out that I was getting my ears itched by the HF noise from the SMPS powering the computer and some of my other kit. This included at one point a particularly noisy light bulb!
So maybe I am a bit odd in that respect and that's why I find hissy (to my ears) disc stages to be mildly irritating? It's one for the psycho accousticians I think.

Craigs bit there about why a flat gain stage in front of the RIAA works for MC but not MM does fit in nicely with the discussion about how the 2 different technologies produce their inherent noise. And I wonder if that's why at least one highly regarded commercial product uses a completely separate circuit for MC and MM rather than simply tweaking the gain up on a single circuit.
I've generally used transformer step ups in the days when I had and could afford MC...……… I may get lucky on the lottery and be able to indulge in the luxury, who knows? I've got a fairly ancient Pickering MC step up, given by a friend, that is a common base amplifier, battery powered using a pair of power switching transistors because they are lower noise due to their base spreading properties or something like that which I do not understand. what I will say is I found it rather noisy and my modern dedicated MC preamp knocked it for six. And it wasn't a very expensive bit of kit either.

A.
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Old 28th Jul 2020, 10:44 am   #90
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Default Re: Very low noise op amp recommendations?

OK back to practical circuits, I can pick up a pair of these Hitachi chips for a reasonable price from the usual places and hope they are genuine...……..
Supposed to be a dedicated RIAA chip needing a fair bit of external bits round it.
The circuits from the same R A Penfold book but looking at it I think he's just cribbed it from the data sheets but it does look to me as if it won't need an awful lot of refining to make it useful.

I'm tempted.

A.
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Old 28th Jul 2020, 10:53 am   #91
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Default Re: Very low noise op amp recommendations?

I certainly wouldn't go for a relatively obscure chip and design unless there are clear and unambiguous benefits, and even then I'd think twice.
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Old 28th Jul 2020, 10:55 am   #92
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Default Re: Very low noise op amp recommendations?

Those HA12017 can be found from the usual place and I can get 10 for a tenner from one place...……… whats the feeling they may be counterfeit? or hopefully leftover stock?
I don't mind taking a gamble at that price, if anyone else fancies a couple to play with then I'd get the bulk orders as the price seems to plummet in quantity.
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Old 28th Jul 2020, 11:06 am   #93
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I certainly wouldn't go for a relatively obscure chip and design unless there are clear and unambiguous benefits, and even then I'd think twice.
It's not an obscure chip Paul. I wouldn't call Hitachi an obscure producer of audio chips and this was a mainstream device in the 80s. I am betting its found in a lot of Hitachi and other Japanese gear at the quality end of the market.
the design doesn't look obscure it looks fairly standard really, all you are getting is the bits on a single chip as far as I can see, and as it was specifically designed for LP stages I figured what's not to like?

My main concern here if I decided to build it is whether I'd be getting the real thing from the auction site sources.
A.
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Old 28th Jul 2020, 11:11 am   #94
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Default Re: Very low noise op amp recommendations?

I doubt if it's being faked, given the combination of relative obscurity and low selling price. You've probably just found some old stock.

No reason why you shouldn't build it as an experiment of course.
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Old 28th Jul 2020, 11:16 am   #95
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Default Re: Very low noise op amp recommendations?

That's the whole point Paul.
Sorry if I came over a bit *rsey I didn't mean to.
In the meantime I'm going to scrape the parts out of the toybox for that single chip project you linked me to, probably with a couple of tweaks for stability etc.
I have a use for a simple but effective stage for a cheap and cheerful job for my great nephew.

A.
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Old 28th Jul 2020, 11:20 am   #96
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Default Re: Very low noise op amp recommendations?

The fun comes in the future if you ever need to find a replacement.

I much prefer to stick wherever possible to mainstream devices and those with multiple manufacturers. I know exactly which muggins gets called in to wade in and sort things out when some source of supply runs out.

I try to design with currently manufactured parts and leave period parts for servicing period equipment. It seems fairer on the people trying to keep period equipment going.

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Old 28th Jul 2020, 11:39 am   #97
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Default Re: Very low noise op amp recommendations?

The HA12017 was quite heavily lauded in one of Ambit's famous catalogues of the late '70s- whilst I enjoyed reading some of William Poel's insightful and shibboleth-slaying blurb, there's always the chance that they just wanted to sell the things.... It does have unusually high supply voltage capability, no doubt fitting into the then must-allow-vast-pre-clipping-capability tick-box. If it's cheap enough, no reason not to try it and SIL means easy layout!
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Old 28th Jul 2020, 12:14 pm   #98
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Default Re: Very low noise op amp recommendations?

But is it any better than an NE5543?

Assuming your workshop has a reasonable supply of passive components, it would take 30 minutes tops to knock up something like the Self circuit (don't need the sub-sonic filter for a quick test) on a breadboard or even a bit of Veroboard. Why not do that first, before sourcing obsolete ICs?

That will establish a baseline - a reference point. After than, feel free to experiment with different op-amps and different topologies - you never know, you might find something you prefer, even if it's not as good on paper.

I'm worried you're on the edge of an audiophile rabbit hole here. Regular audiophiles get it bad enough, what with all those magazines, forums and sometimes iffy marketing materials, etc, but it's even worse for the DIY audio crowd. Many of them love finding the most obscure and difficult solutions, as if the pain of the journey makes for a better end result - and in their minds, it absolutely does. Which is fine in itself, but unfortunately it often develops into a faith-based believe system to push - sometimes quite forcibly - onto others. Which is why I no longer frequent the DIY audio forums...

To steal the signature of a well-known poster from the Usenet days, music is art, audio is engineering. Wise words
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Old 28th Jul 2020, 1:58 pm   #99
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Default Re: Very low noise op amp recommendations?

I have nothing to add to what Mark's just said.

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Old 28th Jul 2020, 3:34 pm   #100
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Default Re: Very low noise op amp recommendations?

It's irritating in a way, but the fact is that the 5534A is still the best chip you can get for an MM preamplifier, even if it has been about for forty-odd years, and very close in noise terms to the best attainable with discrete devices, an approach which brings other and greater problems. The single stage EQ approach is the optimum, but to calculate the exact component values is non-trivial exercise, because of the complexity of the relevant equations. Still, Self has done the sums, so that's that problem kicked into touch. He's written a tome just about electronics for vinyl, incidentally...
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