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Old 30th November 2003, 07:35 PM   #1
timH is offline timH  United Kingdom
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Join Date: Mar 2003
Location: Guildford, Surrey, England
Default Cascode input

I'm using a Naim Audio system and have recently built some clones of their 135 monoblocs using circuits from Neil McBride's website - they sound great especially with separate VA stage regulators which have been suggested by a couple of contributors to the Pink Fish Media site.

These two guys also suggested a mod to the amp circuit involving a cascode input which I've added with some success

However I have no idea what this all means - I'm a lecturer in Marketing and have limited theoretical knowledge but can wield a soldering iron

Anyway there's a reference to a Cascode mod to the Naim phono stage circuit at the London Live site here

I've asked the two pfm guys and they think this may not be an optimal configuration but they only use CD so no experience of the phono stage.

Can any of you knowledgeable people have a look at this and give me any advice please?
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Old 30th November 2003, 08:16 PM   #2
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
quote :
CASCODED INPUT STAGE : Q4 provides a cascode to the input
devices reducing distortion and improving bandwidth. The
cascode acts by stabilising the voltage at the collector of the
input devices and minimising the change in Vcb so the input
devices operate more linearly.
The effect of capacitance between the base and collector is
reduced as there is less voltage change, improving the
bandwidth.


Well hard to argue about without the two circuits, an oscillosope,
a distortion meter and a few other bits and pieces, though I'd
suggest the bandwidth gain is minor.

quote : may not be an optimal configuration

Well what is ?

All I'd suggest is that you go for the fully modified circuit. In this
particular case it may be that your not getting the full benefits of
cascode operation but its certainly not worse. But the same thing
can be said about cascoding the input of power amplifiers.

/sreten.
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