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1970s Design Indulgence

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Sylvain View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 2021 at 11:16am

My ears also tell me that by making adjustments to resistor values, there is increased audible distortion.
.......I think I must therefore take the transient plot with a pinch of salt....

I am helpless, BUT read some HIFI mag report gloryfying the Venerable Amp product, subtly but deceitfully as part of a Marketing Plan ......and here We are fortunate to a good Yorkshire man of Values. My search for the ''elusive transient'' in the musical mix i have formed the view that it may be an organise in the circuit components for a  ''perception '' of '' transients effect '' rather than the ' real natural phenomenon '' in the music.....

Appreciate your passion for  TRUTH.

 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 2021 at 9:49pm
As I now have the "Amplimo" transformer (made in Chesterfield! Just over the South Yorkshire border), I may ring the changes at the same time, which means going back over my tracks a little.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jun 2021 at 10:32pm
No matter what method is chosen for the a.c. global negative feedback, it always results in a negative voltage near maximum power. A transient might be sufficient to tip the sum negative. In all cases the NFB is asymmetrical.

Thinking about this, we are interested in a.c. NFB, but what if we include the DC too?

This needs some Kirchhoff study to plot the outcome, but I have a feeling it might remove the negative term, and as such, place it inside the class-A envelope of the IPS emitter circuit.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 2021 at 7:52am
NFB: We've been here before, and for some silly reason, we're here again.

For the benefit of any new readers, I am trying to find the sore thumbs of this ancient amplifier configuration, to find fixes, which might make it sound great, long term. After curing most of its ills, the sound and stereo illusion are, IMO, excellent, but leave it on and it eventually unravels. That now lasts 2-3 weeks from switch-on.

Designers of yore paid scant attention to the negative feedback summing junction. It worked with valves, so it should work with transistors. But did it actually work with valves? I think not. You wouldn't have left a valve amplifier on for 2-3 weeks to find out!

Why would you leave any amplifier on? I will answer in a round about way. How many calculations does SPICE make during simulation? Quite a lot, but we can never give SPICE the complete information, and neither does it answer us in the ways we really ought to know.

Electrical mathematical models are based on the factors we know about components. They are models and they're not exact in every way, but are considered close-enough "for rock'n'roll!"

Anyone who has done successive approximation (iteration) might comprehend the number of calculations involved, but what if the factors we "plug in" were to vary? Because, they do!

The issue here, is the NFB summing node, and I'm trying to find a way to make it "linear," because, it isn't.

What use is negative feedback if the negative feedback is faulty? The single element input stage (valve or transistor) has this fault, and is class-C in nature (on hitting transients). Class-B would be acceptable, but a class-A node would be better.

I will leave you with this Kirchhoff type drawing to chew it over.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 2021 at 10:59am
Here's an illustration of what I mean by asymmetrical/class-C, class-B, and class-A waveforms at the summing node.

NFB summing node


The correct expression for asymmetrical/class-C is really the inverse: 1/class-C


Edited by Graham Slee - 11 Jun 2021 at 11:07am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 2021 at 2:13pm
the issue is perhaps about '' distortion'' Power output efficiency, about 'transients'' about Mid and high frequencies  ..........and the transformer function..........the stuff that expensive Hifi magazine and their elaborate Review would miss.
An impressive Amplifier UK design and manufacture would seek Class A output for it's purity of sound but lest efficient to drive smaller box speakers and the ''transients' performance would be attenuated for lack of better efficiency in the mix Amp and Speaker matching .....so AB class had seen prominance with bigger and bigger ' C' core and other transformers. I want to believe that your extended effort will allow better Speaker and Amp match but may be totally wrong.  


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jun 2021 at 5:38pm
Originally posted by Sylvain Sylvain wrote:

I want to believe that your extended effort will allow better Speaker and Amp match but may be totally wrong.  



I hope you're not Smile
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