Rush Cascode inputstage on JLH 10W - diyAudio
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Old 25th January 2010, 04:47 PM   #1
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Default Rush Cascode inputstage on JLH 10W

Inspired by keantokens headphone tests I just had to try out the Rush cascode/NTP on the JLH input.

In this application it isnīt that funny as it sims worse than the original?!

More current through the Rush?
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Old 25th January 2010, 06:08 PM   #2
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You might oughta Taylor around that Rush to keep both comparator emitters
current more or less at constant. That way voltage drop won't curve so much...
Its never gonna be constant, or you wouldn't get any error voltage swing at
the collector resistor. But you can approach that ideal.

No, wait... Your JLH IS a Taylor! Holding R10 (and R2 on the right) at two
emitter drops. I just didn't see at first. But then again, those are output
emitters under dynamic conditions for a constant voltage reference...???
Perhaps this makes emitter drops in the Rush to copy a similar dance?

Two emitter drops (in the Rush) may simply double the magnitude of this
non-linearity. Which is really the non-linearity of the output emitters.

Last edited by kenpeter; 25th January 2010 at 06:11 PM.
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Old 25th January 2010, 06:22 PM   #3
Bigun is offline Bigun  Canada
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I thought there was perhaps a benefit from having the complimentary emitters back to back like that in terms of distortion cancellation ?
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Old 25th January 2010, 06:26 PM   #4
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by kenpeter View Post
No, wait... Your JLH IS a Taylor! Holding R10 (and R2 on the right) at two
emitter drops. I just didn't see at first. But then again,
he he he, you will see from the thread I started for my JLH design my claim that All amplifier topologies are separated by '6 degress of freedom' - there are no new circuits left to find, they are all out there. I've started from a dozen different novel places and a few twists later I pop out at one of the other circuits.

Transistors only have 3 terminals, there's only so many ways of putting them together. Over the years we've enshrined all sorts of circuit pieces with names, usually after the person who published it somewhere, but there really aren't that many ways of doing this.

The tunnels and passages that connect all the different topologies together are a lot of fun to explore and I think it's a big part of the hobby for some.
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Old 25th January 2010, 08:53 PM   #5
AKSA is offline AKSA  Australia
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Gareth,

The RC is good, but so is the singleton...... bear with it, it's worth a good look too.

I suspect you need to run your RC at around 1mA minimum, 1.3/8.2mA (160uA) is not enough. I don't believe you need R9 as you are not using the RC as a phase splitter, though the idea does have merit here.

Hope this helps,

Hugh
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Old 26th January 2010, 05:40 AM   #6
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Try adding 100ohm resistor between emitters of rush cascode, it will help in linearising the response and will provide you with much flater bandwidth and HF response.
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Old 26th January 2010, 06:02 AM   #7
jcx is offline jcx  United States
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then you can "fold" the rush cascode input, changing one Q polarity and get the diff pair which has the even order distortion cancellation, input offset cancellation and high input Z on both inputs all in one
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Old 26th January 2010, 06:08 AM   #8
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Quote:
then you can "fold" the rush cascode input, changing one Q polarity and get the diff pair which has the even order distortion cancellation, input offset cancellation and high input Z on both inputs all in one
Have done that a longtime ago using both BJTs and FETs. Works great and gives a good harmonic stucture.

This one is just to find if there is some magic hidden in the Rush.

Quote:
Two emitter drops (in the Rush) may simply double the magnitude of this
non-linearity.
Higher current helps marginally but THD is still doubled with no better harmonic structure. kenpeter you might be on the right way there!.

Adding 100ohm doubled it one more time.

Will add the LTP version for comparision.

Last edited by revintage; 26th January 2010 at 06:17 AM.
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Old 27th January 2010, 12:48 AM   #9
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Not exactly... You got Rush + Taylor + JLH.

If Taylor was working with a good reference, it would be holding both emitters
of the Rush at constant working current and voltage. Linearity would be moot.

But your Taylor's voltage ref for comparison and correction of Rush collector
current is the JLH drive, and one output transistor. Not exactly what you call
a steady voltage reference, once the music starts...

This voltage non-linearity is copied to Rush collector current by Taylor circuit.
Then Rush modulates the problem twice over a single emitter. But not its own
fault, I gotta blame abusing output and drive transistors as voltage reference.
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Old 27th January 2010, 07:23 AM   #10
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Hey Ken,
You must show me the Taylor? This as the circuit is exactly a JLH 10W with the added Rush in the input/feedback stage. Note the 0.1ohm is just for monitoring Iq, should maybe be made smaller.

To me a Taylor, simply put, is closely related to a WF with transistor instead of capacitor in the "feedback" from the top of the upper device(mut be a resistor on top of the drain/coollector to generate a signal) driving the gate/base of the lower.

Last edited by revintage; 27th January 2010 at 07:25 AM.
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