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Old 3rd June 2011, 10:04 PM   #1
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Default cascoded transisitors?

What are reasons for not using a cascoded topology whenever possible. It would seem to offer benefits in all circumstances with the exception of a minimalistic design. I think I remember Pass saying that some think it minimzes dynamics, is this true or just bias towards uber simplicity. I don't have a problem with simplicity, its just that cascoded designs offer so much in terms of controlling conditions that could improve an amp in terms of performance. Is this where the art of design comes into play.
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Old 4th June 2011, 02:02 AM   #2
flg is offline flg  United States
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One of the disadvantages is that the cascode voltage eats into your supply voltage limiting output voltage swing of the stage. In a pre this may not be any big deal you just use a hiher voltage supply. In an output stage it is much more of a compromise with power dissapation and efficiency.
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Old 4th June 2011, 02:11 AM   #3
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Aren't there advantages to running higher voltages as well. In most of the Pass articles, increased voltage tended to yield better results, although I think examples were SE (DeLite,SOZ). Perhaps the higher voltages across the fets also help with capacitance and this is the reason for the improvement.
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Old 4th June 2011, 07:15 AM   #4
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by buzzforb View Post
I think I remember Pass saying that some think it minimzes dynamics, is this true or just bias towards uber simplicity.
OTOH Nelson has a patent for an amp with everything (including the output devices) cascoded, which he claims improves dynamics. See here: http://www.passlabs.com/pdfs/articles/cascode.pdf
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Old 4th June 2011, 04:05 PM   #5
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Quote:
Originally Posted by godfrey View Post
OTOH Nelson has a patent for an amp with everything (including the output devices) cascoded, which he claims improves dynamics. See here: http://www.passlabs.com/pdfs/articles/cascode.pdf
That is simply an article published in Audio, not a patent. It's an old article,
and since then I have seen examples going either way. As a concept,
cascoding should increase dynamics, but it is not always perceived so by
listeners.

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Old 4th June 2011, 05:16 PM   #6
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I had read the cascoded amp article, but thought I had read elsewhere where you made this same statement about a perception of lack of dynamics. Perception is a tricky thing. One last question Nelson, based on your DiyOpamp article, cascoding a differential pair seems to increase performance significantly, do you choose not to use this in your FW stuff for simplicity sake. I guess there is the chance that you have listened to and preferred the non cascaded version, or perhaps, these amps are an introduction to a topology and have not been developed completely in terms of performance. Is this where Pass Labs begins and on the building blocks of FW? Thanks for info so far.
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Old 5th June 2011, 02:52 AM   #7
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The decision to cascode the Jfets on some of the amps is sometimes made
based on voltage and dissipation limitations of the input devices. This is not
a problem with the FW amplifiers. After that, the question is simply whether
it measures or sounds better with cascoding. It almost always measures
better, but sometimes the improvement is not worth the additional
complexity, and often the sonics are a wash.

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Old 5th June 2011, 03:28 AM   #8
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Thanks for the answer.
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