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Old 5th January 2014, 12:18 PM   #1
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Lightbulb XHS - Xtra High Speed Design

Just sketched a high-speed design using common-used transistors.

Minimal HF correction, high gain and linearity with open NFB. Low distortion at high frequencies (even at 50kHz not that bad).

At around 50W and 1kHz get 0.000 THD measurement (less than Multisim can measure)

Probably will try to prototype and see what it sounds like...
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File Type: jpg XHS-01.jpg (842.1 KB, 712 views)
File Type: jpg XHS-02.jpg (843.5 KB, 658 views)
File Type: jpg XHS-03.jpg (839.2 KB, 536 views)
File Type: jpg XHS-04.jpg (840.4 KB, 491 views)
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Old 5th January 2014, 12:42 PM   #2
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Exclamation Zobel network

Just noticed, I forgot to remove Zobel network from the model after some experiments in my previous post, so don't pay attention to some gain decrease at around 50 kHz - in fact it is flat up to 1MHz at least.
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Old 5th January 2014, 03:28 PM   #3
DUG is offline DUG  Canada
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Schematic is too small to read.

Have you built it yet?

It would be interesting to see what happens to the 1MHz when it hits reality.
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Old 5th January 2014, 08:48 PM   #4
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If you open schemstic and then hit "Full Screen" sign (bottom left corner) - it will be shown full size.

I have not built it yet, but I will do. Also very curious, how far it will go. Design is reletively high-current for maintaining speed, however this approach may compromise signal/noise ratio to some extent. Also, real b-c capacitances may bring some surprises...

We'll see
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Old 5th January 2014, 09:26 PM   #5
RNMarsh is offline RNMarsh  United States
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The circuit looks very good. It should do well in real life tests and listening. Looking forward to further development/build.


Thx-RNMarsh
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Old 5th January 2014, 09:47 PM   #6
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Complimentary "blameless" driving a beta enhanced Hawksford.
Interesting.

Drop the cascodes - simplify it ! led /2Q ccs + basic linn = fast.
It's still a VFA , more touchy compensation and it CAN'T come even
close in speed (cfa) .... It might be the fastest VFA, althrough.

PS- it might not have the PSRR of a standard "blameless".

OS
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Old 5th January 2014, 10:11 PM   #7
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I'm not enthusiastic about operating BC550 transistors, rated for 50 volts, at (70/2) = 35 volts of Vce.

I'd suggest you consider operating the current source devices (such as Q5) at a Vce around 10 volts, and the cascode devices (such as Q24) at a Vce around 60 volts. I'd suggest you employ higher Vceo rated transistors (KSC1845? 2N5551? MPSA42?) in the cascode positions.
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Old 5th January 2014, 10:35 PM   #8
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Default Stability

LTP + cascodes + Current Mirror + Emitter Follower+VAS + Hawksford cascodes => Instability will be a concern.
You will need to check inner loop carefully. Does Multisim do Tian probe or Middlebrook 2006 ("Final Solution") probe?

Best wishes
David
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Old 6th January 2014, 03:39 AM   #9
DUG is offline DUG  Canada
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Run sim without AC feedback. (open loop gain/phase)
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Old 6th January 2014, 07:31 AM   #10
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Arrow Open loop

Quote:
Originally Posted by DUG View Post
Run sim without AC feedback. (open loop gain/phase)
Actually, I've tried that already - looks not bad. Conditions are the same as for simulation at the first picture in the initial post, but the input signal is -40db relative to that.

Many thanks to all of you for the comments - inspired some good ideas for moving further. Already preparing the live prototype. Also got an interesting idea for improving the pre-out stage - let me test it first
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File Type: jpg XHS-11.jpg (818.5 KB, 128 views)
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