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Old 1st October 2011, 10:51 PM   #1
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Default Shunt Regulated JLH

Actual current source here is obviously the bootstrap.
But we can regulate this by shunting away excess bias.

One of the things that has always bugged me about JLH
current steering into 2N3055 , was that it takes a LOT of
bias to fully turn one of those dinosaurs. Way more than
needed for linear class A through the middle.

You can see what normally happens with JLH in the 1st
graph "Enriched A". Any shunt regulation here is disabled,
and an old fashioned constant current is steered in the
traditional JLH way. These outputs are running silly hot!

SRPP alone could be used as a phase splitter, but this is
not whats happening here today. It is merely regulating
the sum of output currents. Watch the currents, and you
can see JLH steering in time with the negative going peak.
SRPP shunting in time with the zero crossings at twice
the fundamental frequency.

Whats really going on here is TWO feedback loops acting
in quadrature. Global negative feedback we are already
familiar with, controlling the output stage voltage. Then
we have this new feedback, controlling and shaping the
output stage currents. Voltage and Current feedbacks
cooperating at 90 degree angles of intended effect.

Note: the sim won't run properly untill you connect one
or more output stage shunts. Select an Output Class..
Attached Images
File Type: gif SRJLH.gif (29.7 KB, 1025 views)
File Type: gif SRJLH_EnrichedA.gif (33.1 KB, 979 views)
File Type: gif SRJLH_LinearA.gif (33.3 KB, 922 views)
File Type: gif SRJLH_HyperA.gif (32.3 KB, 886 views)
Attached Files
File Type: txt SRJLH.asc.txt (6.8 KB, 27 views)

Last edited by kenpeter; 1st October 2011 at 10:53 PM.
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Old 2nd October 2011, 06:53 PM   #2
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One other trick of note: Stiffened dynamic resistance
of the tail by injecting 1/11th of the bootstrap wiggle.
just slightly less than global negative feedback.

---------------

We got six transistors here, vs bootstrap'd JHL four.
But same count as JLH with typical constant current.
Q4 upgrades the bootstrap to smart shunted source.
Q2 is merely correcting offset that most JLH suffer.

Q2 and Q4 can easily be put to sleep with shunts
J1, J2, ans J8. Reverts to the original JLH topolgy.
More or less, as Q2 is still helping to correct offset.

Regardless which flavor of output is selected, I
think it works better with Q2 dumbed down to a
diode. LTP loses half open gain to no advantage.

------------

Also intersting that all three modes are class A.
No transistor ever turns off, and all three have
similar current maxima and minima. Yet there is
a huge difference quiescent current depending
on which curve is selecetd. 3A, 2A, 1A.

We are talking 50W heat+ per step backward
toward using constant current source for bias.

Last edited by kenpeter; 2nd October 2011 at 07:15 PM.
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Old 2nd October 2011, 07:31 PM   #3
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Delete R1, only needed if C2 isn't used.
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Old 2nd October 2011, 08:26 PM   #4
Elvee is offline Elvee  Belgium
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Interesting concepts, but I fail to see what the main feature of the circuit (Q4, I think) brings objectively.
OK, it shapes a certain crossing profile, but that's essentially a matter of taste, it contributes to thermal stability, which is good but not essential for class A, but other than that, it seems to degrade the objective, quantitative results from a purely numerical point of view.

This is a first level assessment, I am probably missing a thing or two.
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Old 2nd October 2011, 11:33 PM   #5
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The best objective result would probably be the 2nd case, linear class A.
Where we hold the voltage span across two sensing resistors a constant.

Yet I can see the currents of the 2nd graph are not quite perfectly linear.
Still a heckuvalot more linear than JLH defaults. Steering a constant sum
of drive currents into a pair of imperfect and possibly mismatched legacy
power devices does not result in constant sum of output currents!!! JLH
current nonlinearity takes almost the same shape as the non-switching
Hyper-A case. Merely invert Hyper-A curves, make far less smooth, and
100W hotter, with no protection from runaway, you have traditional JLH.

We show this behavior is manageable, and quite easily too. I would steer
those interested in a JLH project towards Linear A at 50W power savings.
Shunt regulation gives us output device automatching within reason, and
some runaway protection. Linear A also gives us the most constant load
upon the power supply.

Last edited by kenpeter; 2nd October 2011 at 11:43 PM.
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Old 3rd October 2011, 12:16 AM   #6
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Whoa... Just spotted that I had simmed with 2n3019 output devices...
Was supposed to be 2n3055. Might obtain completely different result?
Where was my mind when I posted with the wrong transistor???

--------

No biggie, 2n3055's, adjust bootstraps to 39R and R12 to 3.9R.
JLH curves no longer worse than Hyper-A, merely inverted, and hot...

Linear and Hyper A work best with 33R in the boot and R12=3.3R.
Throw the shunt a few extra mA to steal, so it won't shut off..
Beware JLH mode runs absurdly hot with 33R bootstraps.
Attached Files
File Type: txt SRJLH.asc.txt (6.8 KB, 31 views)

Last edited by kenpeter; 3rd October 2011 at 12:44 AM.
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Old 6th October 2011, 03:10 PM   #7
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I simplified a few things for clarity.
Six transistors, no problems...

Includes shunt regulated current
sources for both drive and offset.
Attached Images
File Type: gif SRJLH_Simple.gif (42.2 KB, 784 views)

Last edited by kenpeter; 6th October 2011 at 03:17 PM.
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Old 6th October 2011, 04:55 PM   #8
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Quote:
Originally Posted by kenpeter View Post
I simplified a few things for clarity.
Six transistors, no problems...
Kenpeter
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Old 8th October 2011, 12:22 PM   #9
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Lets talk a few minutes about precedents found within JLH's original work.

In Fig 10 from 1969, we see a primitive form of shunt regulation for bias.
The only real issue holding this one back is high part count. And perhaps
assumptions that output currents were already linear compliments, rather
than force them to be so. This circuit can't sense what the lower output
transistor is doing, and makes assumptions on the basis of linear class A
rules, that unwatched beta drooping output transistors almost certainly
do not follow.

If this were allowed to reduce quiescent to meet Linear Class A rules,
there would not be enough drive current for either output transistor to
reach the rail under Enriched Class A rules (way JLH actually behaves).
The long time constant of 100u4V will still be restricting base current
more than necessary during peaks. Such restriction was only required
(and remembered on that cap) during the quiescent state...

Totally fixable, and with 5 less parts, no big deal...
Where you sense the current can make all the difference.

-----------------------------

Then we can look toward Fig2 from 1996. We see again transistor
dumbed down to a constant current source by placing a long time
constant cap across VBE, and the forward VBE drop abused to
make that CCS regulate. Same sensing location and assumptions
as 1969, still suffers all the same issues previously mentioned.

I just move his circuit to the feedback path, and abuse it to fix
offset instead. In that role, it works much better.
Attached Images
File Type: gif Fig10_1969.gif (16.9 KB, 371 views)
File Type: gif Fig2_1996.gif (21.1 KB, 191 views)

Last edited by kenpeter; 8th October 2011 at 12:29 PM.
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Old 8th October 2011, 12:58 PM   #10
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Let me show you the cooperative relationship between
JLH current steering vs SRPP current thieving.

You can see JLH splitting phase in time with the music.
And SRPP shunting away excess bias in time with zero
crossings. No phase splitting happens within this SRPP,
its function is pure shunt regulated push pull.

These two transistors are working completely different
tasks, and don't have to match. They are not a "pair".
If I sometimes draw them as-if a pair, only for art...

----------

JLH regulates the DIFFERENCE of output currents
to create a specified voltage. 11 times input. Yes,
this amp goes to 11, somehow that's important...

SRPP regulates the SUM of output currents to
create a specified current profile. In this case, we
have specified that profile will be Linear Class A.

Since my chart is B/W, know the big wave is JLH.
The smaller wiggle is pure SRPP, no phase split.

Ok, there's maybe 10mA phase split in the SRPP.
A drop in the bucket compared to JLH's 240mA,
clearly the dominant phase splitter here.
Attached Images
File Type: gif Steering_vs_Thieving.gif (37.0 KB, 186 views)
Attached Files
File Type: txt Thieving2.asc.txt (4.5 KB, 12 views)

Last edited by kenpeter; 8th October 2011 at 01:26 PM.
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