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Old 19th May 2017, 12:14 pm   #21
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Default Re: Is an op amp an op amp an op amp?

Have a read at this old thread where we tried a 741 vs OPA134 for real. I'm afraid the files are long gone but its interesting reading:

http://www.vintage-radio.net/forum/s...d.php?t=105706
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Old 19th May 2017, 1:16 pm   #22
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Default Re: Is an op amp an op amp an op amp?

Interesting thread - will read that thoroughly when I get some time.
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Old 19th May 2017, 5:50 pm   #23
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Default Re: Is an op amp an op amp an op amp?

A lot of op-amp tests here:

http://www.nanovolt.ch/resources/ic_...distortion.pdf

Unfortunately he doesn't include the 741. Most of the HiFi guru discretes aren't up to much either!
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Old 19th May 2017, 8:21 pm   #24
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Default Re: Is an op amp an op amp an op amp?

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Originally Posted by Hartley118 View Post

We equipped a channel amplifier with 741s and compared it with the adjacent identical channel amp fitted with its usual NE5534s. We did this at George Martin's AIR studios in London using a close-miked 'hi hat' cymbal as a full bandwidth live sound source with lots of HF content with an aggressive slew rate to fully push the limitations of the 741s.
Hey Martin, I just love this whole story, including your choice of the hi-hat cymbal. I'd love to see that on a spectrum analyser (other than my ears). That's made my evening, nice one!
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Old 19th May 2017, 10:40 pm   #25
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Default Re: Is an op amp an op amp an op amp?

There's a lot of snobbery in op-amp choice for audio, but on the other hand we also have some rather good devices which are quite affordable, so why not have some margin, just as long as you don't go silly?

There was a restaurant outside Edinburgh which was famous for being overpriced. The food was good, but not good in proportion to price. Its unique selling proposition was that it was famously expensive, so if you took someone there you would impress them with how much you were prepared to spend on them. The place seemed never short of trade.

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Old 19th May 2017, 11:18 pm   #26
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Default Re: Is an op amp an op amp an op amp?

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Originally Posted by astral highway View Post
Hey Martin, I just love this whole story, including your choice of the hi-hat cymbal. I'd love to see that on a spectrum analyser (other than my ears). That's made my evening, nice one!
Hi Al, Glad to have given you cheer! I think that a conclusion of our trials is that, whilst it's easy to demonstrate the limited slew rate of a 741 on the scope screen, it's really difficult to find an audio source where it's a limiting factor in its performance. We used a top end direct unlimited uncompressed live source. Any recorded or broadcast source will not stress the slew rate anything like as far as that.

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Old 20th May 2017, 6:04 am   #27
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I see I've opened the Audiophool section of Pandora's box here, I didn't mean to. Elektor published several amp design's that used the discrete op amp approach and I've seen it esposed in several other articles but never with the reason why.

As the end point or termination of any recording chain is a set of human ears, which unlike a SA, scope or other measuring device based on scientific principles is attached to a human being, subjectivity and an inherent degree of monkey madness is bound to creep in.

Interestingly Rupert Neve has designed (probably a long time ago, I'm a bit behind the curve) a box of tricks called The Silk that introduces distortion into the recording chain. Good story Martin, must have been a great job.

Re monkey madness, R N sounds less than objective at the end of this interview - https://www.youtube.com/watch?v=AGt0KXW_T1Y at about 1:10:00. At 1:05:00 ish he says the reason op amps were discounted was because an op amps OP is class AB and introduces X-over distortion, however slight

Lastly it's a tad ironic that the result of all this striving for excellence in the design of low noise op amps, top notch recording equipment, and a distortion free recording is then played on an Iplod or other low-fi device.

Ce la vie, A.
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Old 20th May 2017, 7:41 am   #28
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Rupert Neve was always a quirky mixing desk designer, and insisting on a discrete op amp design is typical of the man. Looking at the 5088 desk he is talking about in the vid, it for some reason runs on 90V rails, which cannot be needed for any realistic overload situation. It also uses transformer galvanic isolation at both the input and output of every single channel strip, which is usually an additional distortion mechanism (not quite true - the Audio Precision test gear uses auxillary windings fed from a floating amplifier with negative output resistance to get transformer distortion down to -140dB).

But his assertion of crossover distortion in chip op amps is not really founded in fact. I've mentioned the LM4562 before. Although this was introduced 20 years ago, the distortion is below the measurement threshold of any equipment. TI had to use a trick , which is in the datasheet, in order to measure the 0.00003% (0.3ppm) distortion. I defy anyone to hear that. And I'd love to know what the distortion of Neve's transformer coupled discrete amp channel strip is. I can find no specification for that (quelle suprise)

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Old 20th May 2017, 8:08 am   #29
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There are so many factors in play.

In the nice, safe, objective world it is easy to design a piece of electronics to just work. It is a more difficult undertaking to design something to keep working under adverse conditions. An example would be a radio receiver. As a kid in Huddersfield, my crystal set gave plenty of volume on signals from the local MW station, Moorside edge just across a valley. On getting interested in amateur radio I discovered the difficulty of receiving weak signals in the 40m band with the megawatts of aSW broadcast band thundering away a few kHz higher. Getting more gain for the weakness of the signal was fairly easy, getting the linearity to defend against the big stuff was difficult, cost money and made my receivers power hungry and hot. But it was amenable to engineering and maths. I learned about linearity in amplifiers, mixers and crystal filters. I learned to plan the levels of signals down a receiver strip. Spreadsheets became a useful tool when they appeared on the scene.

In opamp applications, if you can write a comprehensive spec for what you want it to do, you can choose a circuit topology and choose a part to do the job. Most often, the part needed to do the job is a bit more basic than you'd guess would be needed. There will be a wide range of available parts which will drop in and go. It doesn't matter if some are over-specified up to the point where the price becomes noticeable, but beware of design tradeoffs in the opamps themselves. Super-fast opamps often have high input bias currents which can spoil other circuit spec aspects. Super-fast opamps can be unstable in basic circuit configurations. Ultra-low offset voltage parts can hide chopper designs internally and be very slow indeed. Don't exceed one requirement at the price of failing others.

The foundation of all this objectivity lies in having an objective spec to meet.

And where does that spec come from?

Human aspirations.

"To return still pictures of x by y pixels, of z bits of resolution from Mars" is an easy spec. and sensible engineers are attracted to the task.

"To complement my new curtains" is a borderline impossible spec from which any sane engineer will run like hell.

"To sound good" is the opening of an abyss which has sucked in and destroyed a number of people who could otherwise have become good designers. Mars starts to seem like an attractive holiday destination.

As said earlier, many engineering systems terminate in human senses. Human senses are easily misled. That picture seems to have a different colour cast depending on the colour of the wall behind it. This doesn't just affect those endless evenings spent re-arrangeing the Gaugin, but affects your ability to edit a photo by viewing a monitor. Then there are the "Which line is longer" optical illusion puzzles. Hearing has a full set of illusions, too. To all of these add in the shifting sands of memory. At least these are all real effects insomuch as they seem to work similarly on everyone.

But you can shove all these factors on a bonfire once vanity gets in on the act. Conspicuous consumption becomes a big factor, and the illusion of exclusivity helps it along. Take a mundane item, dress its appearance up a bit so it looks unusual and more than a bit extreme. Spend most of your effort/money on choosing a name for it. Spend the rest on packaging. The mind of your well-heeled purchaser will fill in lots of imaginary attributes for you. The minds of those who only aspire to your product will follow like lemmings. Exclusivity is assured because the masses cannot hear what the purchaser and aspirational purchaser think they can hear. Also the masses do not understand the special jargon these people invent to describe their illusory experiences.

If you can't get real unobtainium to make it out of, you can at least synthesise unaffordium just by writing a really big price tag.

But like any fashion, it is fickle. Your gizmo's success lasts only until something new supplants it. It isn't a case of sanity breaking out and the emperor turning suddenly red and quickly cupping his hands over embarrassing areas, rather it is the emergence of a new emperor whose imaginary robes are even grander, hailed by a competing string of pundits.

If it wasn't for the fear that one of the assessors might be an audiophile there would be a scrum of psychology and anthropology students fighting over subjects for PhD dissertations in this area....

OK, that was fun, but there was enough truth in it to make it funny.

The 741 is a very useful opamp. With care, it can be used in all sorts of applications, but you need to make sure the punters don't suspect that they're in there.

Like a plant throwing off colour and perfume to attract insects, a recording studio must exhibit pictures of sumptuous decor and above all a mixing desk with more controls than the whole of NASA. This carries as much weight as the list of which luminaries have parked their cars in the swimming pool. Just a hint, just one hint.....

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Old 20th May 2017, 9:31 am   #30
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Default Re: Is an op amp an op amp an op amp?

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T
The 741 is a very useful opamp. With care, it can be used in all sorts of applications,
David
Quad, always well thought of (even now that it is in the hands of the Chinese) used the TL071 in the Quad 34 preamp. This still sells for very good money on eBay. With Quad's usual eye for detail, they spotted that the '071 clipped asymmetrically (not something that is in the device spec sheet). So they ran it with rails of +8.6V -9.4V to ensure symmetrical clipping.

They were quirky in their own right though. They absolutely never ever listened to their products - if it measured right, it was right. And their head electronics designer, the late great Mike Albinson, confided with me that he had absolutely no interest in domestic audio - his hobby was restoring old motor bikes.

They had a wonderful "cottage industry" feel. Mike used to tape up the circuit on drafting film at x4. To make the prototype, he would masking tape that to the end of a Dexion rack which faced the downstairs toilet, and wait for a sunny day. In the toilet door was a camera lens. He would tape the resist-coated board to the wall of the toilet facing the lens and take the cover off the lens (the magnification was arranged to be 1/4). After a guesstimated time sitting in there he would develop and etch the board. Every classic Quad product that used a circuit board went through that process during product development.

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Old 20th May 2017, 11:07 am   #31
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Craig, your story of the behind-the-scenes development world of such a revered amplifier brand is both fascinating and surreal! Great accompaniment to my morning coffee and OJ! Thank you !
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Old 20th May 2017, 2:16 pm   #32
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I built the prototype of a Neve microphone amplifier in 1991/2 a discrete op amp thing with a switched mode power supply. I questioned the use of an SMPS because of noise, the reply from RN was "it only adds a dB or so of noise", discuss, as they say.

I got paid (the main thing) and a pair of Quad ESL 57's to boot.
 
Old 20th May 2017, 2:55 pm   #33
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Default Re: Is an op amp an op amp an op amp?

I guess it depended what the 'dB or so' of noise sounded like, but a little worrying nevertheless!

In my (70s, 80s) Neve days, I was prejudiced against SMPS largely on grounds of unreliability after an over-hasty introduction on one product back in 1977. I guess they've taken a while to mature.

On the noise theme, op amp input noise figure is of course another topic which probably attracts more audio attention than it deserves. In reality it's probably much more critical in a consumer hifi phono input than it is in most studio applications.

For example, condenser mikes contain their own amplifiers, which therefore dominate the noise scene. Dynamic and Ribbon mikes of course do rely on the desk mic amp, but only rarely do they receive such a low sound level for the electronic noise to be audible. More often they're being shaken by a kick drum or pushed to their limit by a vocalist.

Specmanship nevertheless demands a respectable input noise figure and I wouldn't recommend a 741 . Not because it's particularly noisy, but only because its optimum source impedance would demand a difficult big high ratio mike transformer.

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Old 21st May 2017, 1:58 am   #34
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My recollection is that the NE5534 was about the first opamp that was viewed as being able to match discrete circuitry when it came to RIAA-equalized input amplifiers for moving magnet cartridges in high-performance equipment. The TL071 family, although good for most domestic audio functions, wasn’t quite there, and for the RIAA application required a separate pre-stage or a discrete input stage that was within its feedback loop (e.g. as in the Quad 44). That said, the LM381 IC of around a decade earlier than the NE5534 was intended to be used for the MM RIAA job, and as I recall Hi Fi News ran a construction article for such a circuit. The LM381 was not an opamp as such, but was configured somewhat like an opamp. I suspect though that it did not compete with discrete circuitry cost-wise.

The earliest reference that I can find for the use of discrete opamp circuitry in audio applications is an article by Daniel Meyer in Wireless World for 1972 July, page 309ff, entitled “Audio Pre-amplifier using Operational Amplifier Techniques”. The opening statement was: “It has been obvious for some time that the level of performance that can be obtained from the common two- or three-transistor pre-amplifier circuit is no longer adequate in systems of the highest quality.” I wonder whether or not there was an element of exaggeration in that. One may ask how much better Meyer’s 7-transistor circuit than say H.P. Walker’s implementation of the Bailey 3-transistor of about a year previous. Meyer also used the same discrete opamp as the basis for a Baxandall tone control circuit, and again H.P Walker’s 3-transistor circuit (which looked very similar in form to the Quad 33 tone control circuit) would have been a reasonable basis for comparison. On the other hand there is I think a certain neatness to the idea of using a common multi-device gain block for each stage, configured for purpose essentially by its NFB network. Naim I think adopted the discrete opamp approach in its control units, also circa 1972 I think.

But how were opamps, IC or discrete, viewed by the anti-NFB brigade? With opamp based circuits, NFB is integral to their function, and not just an assisting device employed to obtain lower distortion. Quelle horreur!

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We equipped a channel amplifier with 741s and compared it with the adjacent identical channel amp fitted with its usual NE5534s. We did this at George Martin's AIR studios in London using a close-miked 'hi hat' cymbal as a full bandwidth live sound source with lots of HF content with an aggressive slew rate to fully push the limitations of the 741s.

The surprising result was that none of the 'golden ears' present in the control room, listening on top quality Tannoy Dual Concentric monitors could hear any difference at all between the NE5534s and the cheapo 741s.
I’d also be surprised if those “golden ears” did not come up with a post facto rationalization. Perhaps something along the lines that ears attuned to discerning minute and subtle differences, in somewhat of an audio parallel to a macro lens, cannot focus upon and disceren the gross differences as surely must obviously exist when comparing such vulgar devices as 741s with “high end” opamps. Those who prefer myths over maths and measurements will simply invent a new myth to explain away the apparent “busting” of one of their current myths. No country for mythbusters, one might say, and the “mythers” might be in the majority. In his excellent book “Uncommon Sense”, author Alan Cromer suggested that logical deductive reasoning was very much a minority sport.


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Old 21st May 2017, 6:03 am   #35
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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.

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Old 21st May 2017, 10:20 am   #36
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The earliest reference that I can find for the use of discrete opamp circuitry in audio applications is an article by Daniel Meyer in Wireless World for 1972 July, page 309ff, entitled “Audio Pre-amplifier using Operational Amplifier Techniques”. The opening statement was: “It has been obvious for some time that the level of performance that can be obtained from the common two- or three-transistor pre-amplifier circuit is no longer adequate in systems of the highest quality.” I wonder whether or not there was an element of exaggeration in that. One may ask how much better Meyer’s 7-transistor circuit than say H.P. Walker’s implementation of the Bailey 3-transistor of about a year previous. Meyer also used the same discrete opamp as the basis for a Baxandall tone control circuit, and again H.P Walker’s 3-transistor circuit (which looked very similar in form to the Quad 33 tone control circuit) would have been a reasonable basis for comparison.
Cheers,
Back in '76 I was working as a student at CERN, and in my spare time made a preamp using CERN's circuit board facility. I used the '72 discrete op amp circuit described above. It still exists, in the loft. It was quite radical for the day, and Meyer was right - at the time he was writing the 741/748 were about all that was commonly available, and a discrete op amp was a sensible choice. In point of fact, in hindsight the design was not so good. Distortion was quite high, for example, and the bias conditions were not optimal. I think I spice modelled it a while back - I'll have a look see.

The history of the transistor in an RIAA front end is interesting. A one transistor stage was published by Tobey and Dinsdale, WW Dec 1961. Dinsdale later modified that into a two transistor version in January 1965. Both these used Germanium transistors (OC44, OC75 etc).

Bailey then added an emitter follower to provide isolation and low impedance drive for the RIAA network in the classic three transistor RIAA stage (WW Dec 66) using silicon BC107's. Walker analysed the noise performance of that configuration in WW May '72.

And of course the classic Quad 33 used a two-transistor RIAA, an emitter follower tape buffer, a bootstrapped single transistor to drive the tone controls, and a two transistor output buffer with tone control feedback. 1967 to 1982 with production of 120,000 units.

Douglas Self then comes on the scene in WW Nov '76. His Advanced Preamplifier uses the three transistor Bailey circuit, but as a flat response front end. Next were two discrete op-amps for RIAA and Tone.

Next in WW Feb '79 came his High Performance Preamplifier. This had a 7-transistor flat front end, a three transistor RIAA EQ and a two transistor tone. All these circuits were novel arrangements and owed nothing to any previous topologies.

Finally in WW Oct '83 in his Precision Amplifier, Self went over to op amps after introduction of the 5532/5534.

Even the legendary discrete preamp man John Curl has eventually gone over to using op amps in the last few years, but in his usual cryptic way reveals absolutely nothing of relevance to precisely how.

So discrete RIAA and other associated control tone control circuitry was pretty much entirely discrete right from 1961 to 1980-ish other than quirky designers like Neve and Curl.

Craig
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Old 22nd May 2017, 1:42 am   #37
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The Revox B251 and B252 of the mid 1980s are discrete opamp realisations.

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.

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Old 22nd May 2017, 5:46 am   #38
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In my previous post I had overlooked the case of the Ambler tone balance control, Wireless World (WW) 1970 March page 194ff. This was based upon an ideal inverting amplifier. Ambler showed a complete control unit, using four such amplifiers preceded by a modified Bailey input stage. These ideal inverting amplifiers were described as opamps, and shown as a simple two-transistor single-ended circuit. I think that links back to the original theme of is an opamp an opamp.

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Ambler used this opamp not only for the tone balance stage, but also for a Baxandall tone control stage and as buffers. So one might say that it was a quite early use of discrete opamp audio circuitry.

Hutchinson, in WW 1970 November, page 538ff, apparently inspired by Ambler’s work, described a quite elaborate tone control system with separate lift and cut controls each for bass and treble. This was based upon an SN72709 IC opamp, employed in the non-inverting mode. So here we have an early application of an IC opamp in audio service.

That something better than the Bailey single-transistor circuit was desirable for a Baxandall tone control was the basis for an improved circuit by Quilter (WW 1971 April page 199ff) in which the collector load of the transistor was bootstrapped, using an emitter follower, to increase its open-loop gain. I suppose that one could say that this circuit was heading somewhat in the direction of having opamp characteristics. But in fact it wasn’t new, having been used in the Quad 33 tone control circuit, where the emitter follower after the gain stage served as the output buffer as well as in its bootstrapping role. As Craig has said, the Quad 33 tone control circuit was driven by an emitter follower with a bootstrapped input. I suspect that this was done to provide a very high input impedance for the volume control to look into, and thus to ensure that the tape input, which went to the volume control via a switchable attenuator, saw an impedance that was not materially affected by the volume control setting. (As an aside, the Quad 33 disc input amplifier also had a bootstrapped input, following the 1965 Dinsdale precedent. In this case, I think that the idea might have been to provide such a high impedance that the load that the cartridge saw was essentially defined by the input resistor (68k) (and any added parallel capacitance). Quad claimed that the input was essentially resistive to with ±5 degrees.) H.P. Walker chose a similar bootstrapped collector load transistor for his stereo mixer, WW 1971 June page 295, and explained its virtues. That resulted in a small debate in the pages of WW, but not of the same scale as the series-or-shunt feedback debate.

Ellis (WW 1973 August page 308) went a step further than Quilter, using a cascode pair in place of the single gain transistor, whilst retaining the bootstrapped collector load. Again I think not an opamp in the conventional sense, but with some of its properties. But bearing in mind what Ambler labelled as an opamp, where is the dividing line?

Linsley Hood detailed a second-generation modular preamplifier in WW 1982 October and November, and 1983 January. This followed a couple of articles on opamps in audio applications, 1981 October mostly on the µA741, and 1982 September mostly on the CA3140 and TL071/2. The modular preamplifier was based almost entirely on TLA072s. This disc input amplifier was a two-stage series-shunt design (evidently JLH was not ready to completely abandon the shunt feedback circuit he had previously championed), using a discrete first stage and a TL072-based 2nd stage. I don’t think that the 1st stage was described as being an opamp, but it looked like one.

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I recall that Hi Fi News ran a construction project for what it called a “logical” control unit not long after the Quad 44 was released. As best I can remember it used CD4066 switches and TL072 opamps, but that needs to be confirmed.


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Old 22nd May 2017, 6:00 am   #39
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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.
Thanks! I had assumed the first kind of "golden ears" without giving it too much thought. But upon reflection, the second kind seems equally or even more likely. In which case it is QED - a "lesser" device used well and within its reasonable working envelope can match a "superior" device in some applications.


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Old 22nd May 2017, 6:34 am   #40
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In my previous post I had overlooked the case of the Ambler tone balance control, Wireless World (WW) 1970 March page 194ff. This was based upon an ideal inverting amplifier.
This is all absolutely fascinating stuff!

The earliest "op-amp" I can recall was designed by my friend and mentor Gordon Edge (alas no longer with us) in the original Cambridge Audio P40. He used it in a flat response input buffer back in the mid '60s. The volume control was the feedback resistor. Gordon actually described it as a virtual earth input stage. It isn't quite that because there is not enough open loop gain to ensure that. The advantage was his design goal that you could not clip the preamp before the power amp clipped. The disadvantage is that to achieve that he used shunt feedback, so the noise was limited by the 47k input load.

This is the complete disc input stage.
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Craig Sawyers is offline  
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