LTP version is sometimes called modified Lin . My friend Ijaz lived in his town . I wished I had known . Maybe we could have visited him . So stupid I should be there and he was a few doors away .
Oral History Lin Index RCA Germanium Transistors Audio
The topology discussed by D. Self in his book bears no resemblance to Lin's topology. The latter is a single gain stage design, while Self's is a double gain stage design that is due, in fact, to Thompson.
That is unfortunate, really, as "VAS" does not in any way describe what the stage does.
What the second stage in the Thompson topology analysed by D.self does, in fact, is take a current from the input, transadmittance, stage and deliver a voltage at its output. It is, therefore, a transimpedance stage, or TIS.
This is the nearest you will ever get to a H C Lin circuit . How so ? When Harold Leak was accused of steeling the Tobey and Dinsdale design he is reported to have said it was they who should say sorry to HC Lin . I think there is some truth in that .
The last Leak designs grafted on a LTP . Hence modified Lin . The rest is history and the Sinclair Z30 comes into it somewhere ( and RCA as was Naim ) . The Gogny of 1967 also .
The Lin was simply a common emitter stage driving a unity gain output stage (see figure below); nothing at all to do with the two stage design pioneered by Thompson, which consisted of an LTP transadmittance stage driving a transimpedance stage which in turn drives the unity gain output stage.
Sometimes it is really hard to make simple points of fact on this forum.
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Point taken . I never found a real life amp that had the circuit . I am sure some existed with famous names ? This was one I thought qualified as something like .
Simple 40 Watt Power Amplifier
The Gogny is the oldest design I know of that resembles the generic design . I will look out for this modified Lin statement again . He was involved in early op amp design . It is just possible he is the originator ?
Simple 40 Watt Power Amplifier
The Gogny is the oldest design I know of that resembles the generic design . I will look out for this modified Lin statement again . He was involved in early op amp design . It is just possible he is the originator ?
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Unless the Vas/Tis stage includes the load resistor of the prior stage then it can't be a current input. No active devices are current controlled, they are all voltage controlled devices. Some of them have a low (non-linear) input impedance that confuses some people into thinking they are current controlled.
It's semantics - it's what parts you decide to include in your definition of the 'stage', i.e. where you draw the lines between the stages that determines it's function in this respect. So VAS is perfectly valid, with the right topology and TIS is equally valide with the right topology.
The paper by Bruno was a good read - first time I've seen it. I would be interested in his thoughts about the influence of the external load (non-linear) on the effectiveness of the global feedback - or the potential risk of speaker back emf or rf pick up adding error signals from outside the loop - I read about this from time to time quoted as a reason to be cautious of global feedback around the output stage.
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Funny you say this . I had this discussion recently . I said why do valves avoid some problems when having terrible common mode rejection in typical amplifiers . The answer I was given was the valve doesn't mind . He then went on to say the Russians used valves in aircaraft for this reason . Perhaps .
A friend begged me to use a valve as a VAS . One day . What a frightening thought .
It's still a TIS even if its driven from a resistively loaded LTP. All that happens in this case is that some of the TIS input current is shunted away from its input to the rail.
Life, the Universe & Everything.
The 'correct' name isn't the one with the pedantically correct meaning. It's the term which the intended audience readily connects to the intended object. Its about communication, remember?
The definitive treatment of VAS vs TIS is attached to #968 of this thread.
It is far more USEFUL than the misguided, misinformed and downright fallacious statements made by various pseudo gurus and pedants so far.
My apologies Mr. Pearson. I enjoyed having sight of these old circuits which bring back many happy memories of my misspent youth.
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I had the honour of working with Ted Ashley who designed the last of the great LEAK valve amps and the early transistors. Also dealings with Arthur Bailey of Radford, Peter Walker of QUAD and of course Great Guru Baxandall.
Even when they were no longer involved in day-to-day Amp design, these gentlemen were fully au fait with the latest theories in the electronic comics including JAES & IEEE Trans.
Their views were ALWAYS accurate, refreshing and illuminating compared to the ravings of most modern pseudo gurus. It was very educational to hear from them what THEY thought they did well ... and where they might have done better given the technology of the time.
Also which of the wonderful bits we have available to us today, they would like to have had available in their day.
And even harder to move discussion to something which might actually affect Life, the Universe and Everything.Sometimes it is really hard to make simple points of fact on this forum.
The 'correct' name isn't the one with the pedantically correct meaning. It's the term which the intended audience readily connects to the intended object. Its about communication, remember?
The definitive treatment of VAS vs TIS is attached to #968 of this thread.
It is far more USEFUL than the misguided, misinformed and downright fallacious statements made by various pseudo gurus and pedants so far.
My apologies Mr. Pearson. I enjoyed having sight of these old circuits which bring back many happy memories of my misspent youth.
____________
I had the honour of working with Ted Ashley who designed the last of the great LEAK valve amps and the early transistors. Also dealings with Arthur Bailey of Radford, Peter Walker of QUAD and of course Great Guru Baxandall.
Even when they were no longer involved in day-to-day Amp design, these gentlemen were fully au fait with the latest theories in the electronic comics including JAES & IEEE Trans.
Their views were ALWAYS accurate, refreshing and illuminating compared to the ravings of most modern pseudo gurus. It was very educational to hear from them what THEY thought they did well ... and where they might have done better given the technology of the time.
Also which of the wonderful bits we have available to us today, they would like to have had available in their day.
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Even if you take away the resistor and feed the LTP collector directly and only to the base of a VAS transistor the current flow into the base develops a voltage across the internal resistance of the transistor. It is still voltage controlled. The internal resistance is not very linear and this would degrade the performance of the amplifier.
The physics of a bipolar transistor are such that the current flow from collector to emitter is a function of the control voltage - not the input current. The current is a consequence of finite input impedance. It is the distribution of charge in the base region that controls the device. Because bipolar devices have such low input impedance it is important to consider the current that must be supplied and this leads to the viewpoint of current control.
If you always provide enough (even unlimited) current drive to a bipolar device so that it`s never starved then only the base-emitter voltage is important for control.
When you look at the physics, all active devices are voltage to current converters. Only the introduction of load impedances to make a circuit allows us to create a TIS.
The 1970 Sinclair . Perhaps the missing link ? In the old money at 89/6 d ( £ [ 89.5 x 5] /100 ) . A TDA 2050 kit costs about the same today ( $7) , perhaps $70 to 100 now money . Said to have inspired both Naim and Arcam . People fitted 3055's as the outputs were fragile . Some modern outputs might get them to work well . 3055 was a lot slower than the Sinclair devices I beleive ? Naim used BDY56 . I think some will say the Brits liked this design the most . The RCA which it is said to be based on looks different . I would say the book has it's origins here with Sinclair ? One could be forgiven for saying we have been talking about it for 43 years ? I do seem to remember the RCA of circa 1968 was connected with Mr Lin ?
The Naim is not the same by a mile . It is more like the Sinclair than the RCA some say it copied . Rumour has it as a design by Alan Mornington West and might have run along side the Quad 405 ? Rejected and became a Naim .
The Gogny was for driving a ribbon drive unit I think ? 50 watts into 0.8R if memory is correct ? The fuses serve as emitter resistors in the text ( again if memory is correct ) ?
http://www.listenersguide.org.uk/pdf/sinclair-stereo-60-with-z30-power-amplifier.pdf
http://techpreservation.dyndns.org/beitman/abpr/HCLin_Quasi-Compl-Amp_Elec-1956-9134.pdf
Audio Engineer's Reference Book - Google Books
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A point in case is the (35 pages!) personal letter of Peter Baxandall to Doug Self which is reproduced in Linear Audio's "Baxandall and Self on Audio Power". It is very rare to see such well-argued, clear and well-explained technical narrative.
jan
Ross Walker who I am sure was not as able as his father said to me " There are only three ways to hook up a transistor and one of those is wrong " . His point being it is difficult to claim originality or defend a patent . I had phoned him out of the blue and asked him about how he would view me possibly infringing one of his fathers patents ? Rather than getting angry he listed suppliers who might help . It was for cathode feedback using a third winding some call Quad UL. I was sure it was out of patent . He said if serious he would help me clone the valve designs . I was given Whiteley's number for the transformers . They were known for speaker units . They were surprised and delighted . When I said why them they said because Quad were friends .
I met Peter Walker a few times . I never had the courage to speak . Donald Aldous also . I had bangers and mash with John Borwick in Marlborough . They don't come more down to earth than John . He was Percy Wilson's follow on at Gramophone .
I think history is important . My German friend told me off for always referring to a inventor ( Cockcroft Walton set him off ) .
I met Peter Walker a few times . I never had the courage to speak . Donald Aldous also . I had bangers and mash with John Borwick in Marlborough . They don't come more down to earth than John . He was Percy Wilson's follow on at Gramophone .
I think history is important . My German friend told me off for always referring to a inventor ( Cockcroft Walton set him off ) .
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I was in contact with Peter Baxandall's daughter when I was trying to find a (c) holder for the Baxandall letter. She was absolutely delighted that after so many years anybody was still interested in his work. These designers did not at all consider themselves extraordinary. She freely provided literally dozens of personal family pictures for me to use one for the cover.
Guru-hood seems very much something of the current day, not just in audio but in many walks of life. People who are pretty good at something are, often very much to their own surprise, elevated to guru-status. Some also have trouble to handle it, some remain modest and down-to-earth.
What you also see is that if you have some money and a good idea, guru's, like presidents, can be 'made'.
jan
Guru-hood seems very much something of the current day, not just in audio but in many walks of life. People who are pretty good at something are, often very much to their own surprise, elevated to guru-status. Some also have trouble to handle it, some remain modest and down-to-earth.
What you also see is that if you have some money and a good idea, guru's, like presidents, can be 'made'.
jan
Sorry folks I must learn how to use a computer !
Thanks for tolerating the history bit . I learned recently the inventor of the London Underground was Charles Pearson . He might be a family member ? We have the oldest bike shop in the world the family says . This Charles Pearson was not a name I ever heard . I think he changed the world ?
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Making factually incorrect averments is not an aid to "communication".
There's nothing "difinitive" about that "treatment".
And which "pseudo gurus and pedants", precisely, have made "misguided, misinformed and downright fallacious statements"?
And what, precisely, are those "misguided, misinformed and downright fallacious statements"?
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