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Old 20th June 2013, 06:24 PM   #1001
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Originally Posted by michaelkiwanuka View Post
The Lin topology hasn't got an LTP.
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 .

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Old 20th June 2013, 06:28 PM   #1002
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Originally Posted by nigel pearson View Post
LTP version is sometimes called modified Lin.

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.
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Old 20th June 2013, 06:33 PM   #1003
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I use VAS in my books just because it colloquially implies what the stage does better than TIS does.
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.
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Old 20th June 2013, 06:51 PM   #1004
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Click the image to open in full size.

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 .
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Old 20th June 2013, 07:17 PM   #1005
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The Gogny . Sorry about quality .
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Old 20th June 2013, 07:30 PM   #1006
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Default Slew

From The F-word by Bruno Putzeys

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Now I remember why I wanted GBW in there! Not only is slew induced distortion proportional to the third powers of frequency and signal level, it is also inversely proportional to the third power of GBW. Improving GBW by a factor of two, causes slew induced distortion to drop by a factor of eight.

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Old 20th June 2013, 08:15 PM   #1007
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Originally Posted by nigel pearson View Post
The last Leak designs grafted on a LTP . Hence modified Lin .
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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Old 20th June 2013, 10:04 PM   #1008
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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 .
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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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Old 20th June 2013, 10:08 PM   #1009
Bigun is offline Bigun  Canada
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Originally Posted by michaelkiwanuka View Post
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.
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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Old 20th June 2013, 10:28 PM   #1010
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Originally Posted by Bigun View Post
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.
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 .
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