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What Is Ultra-Linear?
24 November 2013 - by Graham Slee

Originally 'Ultra-Linear' described a
linearising technique used in the output stages of valve amplifiers - it
was to do with the degree of conduction of the 'push-pull' output valves
in relation with the output transformer. Biased to a particular current
setting the valve amplifier would behave more linear prior to applying
negative feedback.

Most solid-state circuit designers look at
closed loop performance but I started looking at how my circuits behave
open loop (without negative feedback). This is where I found how I could
improve the sonic abilities of my circuits.

Originally it was
about open-loop frequency response and how I could make my circuits 'flat'
across a wider audio bandwidth, and the Solo Ultra-Linear is one of my
products that makes use of that feature.

But its not only
open-loop frequency response but a whole host of other things too. Each
item has a bearing on the sound, for example 'slew rate' - how fast an
amplifier responds to an input stimulus.

Many might think the
higher the slew rate the better, and yes it matters to a degree, but
there's more to it than just speed.

Some techniques result in a
forced slew rate using 'sharpening techniques'.

Some high slew
rates just don't go with the available current.

Those two examples
have a negative effect on the sound where instead a smaller slew rate
would have worked much better.

There is a lot more to
'ultra-linear' than I can do justice to in a small article like this one,
and if I told all I would obviously be giving away my hard earned
knowledge for others to copy.

When I look back to older designs
such as the Era Gold V phono stage I can now see that I'd stumbled upon
one of those ultra-linear technigues before I knew it. However, being able
to understand what's behind it and make use of the many factors involved
has enabled my designs to be highly competitive in their sonic abilities.

And as that pleases my customers that's what I shall always do.

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1 Comments

BAK - 24 November 2013 14:40

'Ultra-Linear' was also nick-named the 'Sweet
Spot' in the transconductance curves. Not all valves (tubes) had one or
were to shakey.

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