Is an op amp an op amp an op amp?

[Hartley118]

Originally Posted by Radio Wrangler

I wondered which sort of golden-eared people Martin meant.

He could have used the words in a derogatory sense meaning those who are only lightly burdened by reality.

But he could equally well have meant it appreciatively, meaning people experienced in listening critically. Orchestral conductors, record producers, mix-down operators... People who could say "The cymbal sounded better recorded with the mike plugged into channel X than it did into channel Y" without trying to tell you they could hear which direction the capacitors were wound in.

When the BBC did listening tests they used a mixture of general public and professional listeners in the form of musicians as well as folk from their own studios. Cult figures from the hifi press didn't seem to be invited.

AFAIR the 'golden ears' were just studio staff whose livelihood depended on producing high quality audio results. They were accustomed to careful listening and, unlike some consumer pundits, knew what the live music source sounded like.

The test was blind because we soon forgot which of the test channels contained the 741s!

Martin

[Radio Wrangler]

Yes, the second sort are more likely to be found in a recording studio.

The former sort are strongly phobic about any form of blind listening, because if they don't know what they're listening to/through then they won't know what they're supposed to say and might say something heretical in error.

We've been living with a deafening background of dodgy opinions shouted loud (like children who fear that if they don't express things loudly they might not be believed) for almost four decades now and it's easy to forget that there were and still are people skilled at listening in an unbiased way and who know what the original sounded like.

We need to value these people and defend them from ravaging hordes of audiophiles.

From the language point of view, what makes an amplifier an operational amplifier was originally a matter of what you do with it, which means sticking it in an analogue computer. Since then, amplifiers designed for those purposes have spread into many areas. You could also argue that a preamp (or power amp) is an analogue computer.

David

[Craig Sawyers]

Originally Posted by Radio Wrangler

The former sort are strongly phobic about any form of blind listening

When I worked at Wharfedale, we had a blind listening room. Basically an acoustically transparent black curtain with speakers behind it. There was an amp (nothing particularly special or high end), and a switch that operated relays you could operate to select what unseen speaker to listen to. This was an ABC test and not ABX, but valid nontheless. You wrote down your observations on the sound of each speaker. Kind of like wine tasting.

I recall it was sometimes difficult to hear significant differences, but often the differences were subtle - such as changing the value of a crossover component. But you knew nothing of the purpose of the test up front - just listen and write what you hear for each speaker you select.

But the ear is wonderful at adapting in the same way the colour vision does. Edwin Land (founder of Polaroid) did a lot of work on colour vision, which he called the Retinex theory in 1971 (a portmanteau word combining Retina and (visual) cortex). Basically the eye normalises a scene to a perceived white. Even when there is an significant overall colour cast applied to the scene (like a sodium street lamp), all the colours remain identifiable. Looked at this very seriously when developing a medical endoscope using two-colour frame sequential colour back in the 80's because full colour CCD's were not available at that time.

Anyhow, the purpose of this ramble is that the ear is a plastic organ, and I'm convinced that in a careful blind listening the difference in sound quality from a 741, an LM4562 and an emitter follower are probably very difficult to discern.

[Craig Sawyers]

Just came across the compendium of opamp applications from Tektronix in 1965

http://w140.com/tek_opamps_and_applications.pdf

[Al (astral highway)]

Originally Posted by Craig Sawyers

When I worked at Wharfedale, we had a blind listening room. ...You wrote down your observations on the sound of each speaker. Kind of like wine tasting.

Ahaha! I love it! What a peach aspect of your job! I feel a mixture of joy for you and slight jealousy!!

[Hartley118]

Yes Al, this sort of stuff can be fascinating, but also nailbiting if your commercial reputation depends on the users' perceptions of the audio quality of your products.

Martin

[Synchrodyne]

Given that an operational amplifier was originally designed by its intended function rather than its form, I guess that the form was not bounded, and that any suitable topology that did the job was eligible for the moniker.

In general use though the impression I have – as a layperson looking in from outside the tent, as it were – is that the term operational amplifier usually means either an IC that is described as being an opamp, or a discrete circuit whose form essentially mimics what is inside one of those ICs.

I am not sure that many, seeing for example Ambler’s two-transistor circuit “unlabelled” as it were, would immediately think “operational amplifier”. But seeing it for the first time labelled as an opamp might cause hesitation, but perhaps not much in the way of sustained objection.

The Cambridge P40 circuit was much-lauded at the time for its pickup input overload capability, a parameter coming under the spotlight in the later 1960s. I think though that the P40’s performance in this regard might have resulted in the hi-fi reviewing fraternity’s developing unreasonable expectations. I don’t recall that the higher noise drawback of the Cambridge circuit was much mentioned at the time. That didn’t come up until the early 1970s Linsley Hood-Walker debates over the series or shunt feedback issue in the pages of Wireless World. Cambridge meanwhile had changed to a different input configuration for its P50 – a Darlington emitter follower, I think, feeding a low impedance virtual earth circuit.


Cheers,

[Radio Wrangler]

The first opamps were valved and used for analogue computational tasks like motion control systems in machinery..... servo systems positioning guns based on RADAR returns, steering missiles and such niceties. They were replaced by discrete transistor designs as soon as they could be made because of the improvements in power, size and robustness. Rob Pease wrote about his time with Philbrick. Then these circuits were turned into monolithic form as the next stage of shrinkage. The early monolithics mimicked the topologies of the discrete amplifiers, only later did the designers learn the tricks possible because of the opportunities created by matched and thermally tracking active devices offsetting the problem of getting accurate absolute values of resistance. There was an order of causality and progressive development. Pease bracketed the valve-discrete-monolithic and subsequent improved monolithic eras, and he wrote about it in an entertaining fashion. What he got wrong, sadly, was the VW beetle.

************************************************** *****************

A good friend left HP to go work for a Glaswegian hifi firm, where for a couple of decades he ran the product development side of things. He's a thoroughly good engineer and deeply interested in all things engineering from the fundamental maths to the final appearance. How he handled the amount of hype ladled onto their products by the pundits (and a somewhat larger-than-life CEO) I do not know. I'd have opened my mouth and said something which would have rocked the boat. I think he chuckled internally and just got on with ensuring that the engineering involved was absolutely solid.

The hifi market is too fickle for me to want to rely on it for something I want to be as dependable as I prefer my income to be. It only takes one pundit to throw his rattle out of his pram and something perfectly good becomes as saleable as influenza. The marketing people want special circuit features they can wrap prose around. There is pressure to design things by fashion. Douglas Adams' theories about shoe shops come to mind.

How do you design things where (some) details of the design have to be trumpeted to the public gaze and you know they'll then get re-interpreted through pseudo science? ('Science' as carried out by pseuds?)

I've read about the Cargo Cults and how "Kerosene lamp bilong Jesus Christ him bug***-up finish complete" is one variant of pidgin english for a total eclipse of the sun but those are innocent attempts to interpret a different world to what the people involved understood at the time.

If I pick up any hifi magazine since the end of the 1970s, I don't know whether to laugh or cry. They are either funnier than Monty Python, or just very, very sad. I don't know what to feel, so I carefully pick up detective novels at airports to see me through long waits.

David

[MrBungle]

This thread gets more interesting by the minute. It's nice to know there are sane, rational people out there

My first HiFi was some car door speakers, a terribly matched Realistic amplifier that had some serious problems with a 4 ohm load and an FM tuner from a jumble sale that had valves in it and smelled of concentrated death wish. Wiring was door bell wire. Sounded pretty good, especially when I put the speakers in a cardboard box. I'd love to have put that in front of the golden eared lot in a blind test.

[Radio Wrangler]

I suppose one defining aspect of opamp techniques is the use of large amounts of forwards gain controlled by feedback.

It is usual to refer to the difference between the input signal and the attenuated feedback signal as the 'error'. The 'error' is amplified strongly and becomes the output.

People who didn't entirely understand what was going on seized on the word 'error' and freaked out. 'Error' sounds wrong, very very bad. And all the output is amplified error! Aaaargh! They really, really didn't want anything using feedback!

Meanwhile in the rational world, if our forwards amplifier happens to be linear, then the difference between the input signal and the fed-back output is linearly related to the input and linearly related to the output and the output is linearly related to the input.... and there's not a lot wrong with that! if our forwards amplifier is a bit non-linear, then the ideal state of affairs is lost, but the action of the feedback partially cancels the non-linearity of the forwards amplifier. Partially, only partially. But the more feedback the better until you hit other problems like stability.

Mr Leak worked hard to design his amplifiers to make the raw amplifier as linear as he could and achieved about 1% but he then employed as much feedback as he could and that reduced the overall non-linearity below his target of 0.1% To be able to use so much feedback, he had to pay a lot of attention to phase shifts in his output transformer, and in the valve stages. Good science. Good engineering. Good result. and good marketing too.

But Mr leak was making products to sell to much more rational people than today's equivalent high-end manufacturers must satisfy.

The Leak Point One is a feedback amplifier. The LM741 only becomes one when fitted into a feedback circuit, but it is pretty much useless otherwise. The word 'operational' is of no help. It's a hanger-on meaning the original opamps were employed with feedback arrangements which instantiated chosen mathematical operations.

So 'operational' in this context doesn't mean 'it is working!' but rather that its purpose is to simulate mathematical operations in a system used to solve equations by analogue simulation.

David

[Craig Sawyers]

Originally Posted by Radio Wrangler

People who didn't entirely understand what was going on seized on the word 'error' and freaked out.

It is not limited to this topic. There is a next-generation fusion tokamak being built in Caderache in France called ITER. It builds on the knowledge from JET in Oxfordshire.

ITER stood for International Thermonuclear Experimental Reactor. Three of those terms gave serious negative connotation. Thermonuclear - EEK - like a hydrogen bomb? Experimental - double EEK - an experimental hydrogen bomb? And Reactor just made the whole thing even worse.

So in an attempt to damage limit, they eventually found that the Latin word iter meant "The Way"! So they kept the word, but morphed its meaning.

But Mr leak was making products to sell to much more rational people than today's equivalent high-end manufacturers must satisfy.

Everyone back then did that. Peter Walker at Quad was in the same era, Graham Banks when he set up KEF (and moved them from crop sprayers to speakers) and of course Gilbert Briggs at Wharfedale. A bit of a golden age in the sense that manufacturers and reviewers rarely listened to products, and simply used measurements.

So 'operational' in this context doesn't mean 'it is working!' but rather that its purpose is to simulate mathematical operations in a system used to solve equations by analogue simulation.

Indeed. Back in '62 Tektronix had the "Type O" oscilloscope plug-in, which was two valved operational amplifiers. Each had a selector switch so you could choose resistors and capacitors to connect to each amp. Using those and schematics in the manual you to do funky things like plotting hysteresis loops, measure capacitors, do gated integrations and so forth.

Price in 1963 was $525, which was a sizable chunk of an annual salary, or 12% of an average house.

But the same is true of the classic era amps and speakers. If you index link Quad and Leak gear from the 50's/60's to today's prices you end up well north of £1k per item. So audio was always a well-heeled hobby.

[Craig Sawyers]

Type O opamp schematic.

[Al (astral highway)]

Originally Posted by Radio Wrangler

As the comments about 741s prove, you can go about this business in many different ways and still achieve a successful result if you are careful.

we're in it for fun, so pick something which will be indistinguishable from the best, but which is also fun.

Wonderful advice, David ,and what a thread! I'm enjoying the rich blend of technical history and evolution as well as the collected memories of forum members who have a direct relationship with this history from inside the industry. Wonderful stuff!

[Craig Sawyers]

I'm just looking to build a unit to measure transistor (bipolar and FET) noise with very high sensitivity. To look at how others have done this, I was looking through the manual for the HP 4470B Transistor Noise Analyzer.

It's operation depends on opamps. But this was 1970 - which means that all the opamps are discrete. There is the odd ua709C in there where speed or voltage rails were not an issue, but basically the whole instrument is chock full of discrete circuitry. And relays for signal switching. Buckets of them.

[Radio Wrangler]

Relays with gold contacts are quite low noise. There's probably not a great deal you can do better than the 4470B if you're interested in low frequency noise unless you want to measure your devices in-application. If you're interested in above 10MHz, then there is the HP 8970 followed by the Agilent N897x family and its accompanying N4000 family of noise sources. I'm fairly familiar with these.

David

[Craig Sawyers]

Relays with gold contacts are quite low noise. There's probably not a great deal you can do better than the 4470B if you're interested in low frequency noise

Agreed. If I could find one to buy that was not "ask for a quote", or "what is your target price". Or cost far more than I'm prepared to pay.

[MrBungle]

Originally Posted by MrBungle

I just dropped him an email and have asked if he's still got the mixer and can I borrow it. I'm going to see if it's got 741's in it. I bet it has. It was from Tandy. I'm sure that the reproduction off a digital PCM synthesizer, guitar, drum machine, routed through a Realistic mixer and into a cheap ADC, then burned to CD really has so much more detail, depth than it had when they were recording it

Replying to myself here, just got an email from my brother confirming that the mixer he had went to the tip about 15 years ago. Boo

[Radio Wrangler]

Dunno. MC1458/1558 were a whisker cheaper than 741s and we are talking Tandy, here.

David