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Old 12th March 2009, 02:57 AM   #11
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The Alephs are no longer made, except for a few remaining F3's.
The ones that were have a separate DC and AC input so that you can
easily set an arbitrary value for the DC current source (constant value)
vs the percentage of contribution to the AC output current.

The alternatives work fine also, but keep in mind that the power
resistor divider version will not typically be optimal with equal values
due to the limited transconductance of the device you use for the
current source.

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Old 14th March 2009, 05:26 PM   #12
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From LTSpice of schematic shown in Post#1.
These are the currents in the two resistors.

Aleph, with its bipolar reference, has more
than plenty of Amps/Volt (gM) gain to work
with even the direct coupled two resistor
divider. Where both resistors are equal.

What you say is true enough for case of
an unassisted depletion mosfet. The top
resistor must be smaller to balance.
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Old 14th March 2009, 06:14 PM   #13
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StOOpid ways to Abuse Mu. The other Zen.
Here, triode becomes the feedback network.

Yeah, for those who think of a triode only as
an obsolete JFET, I know this stage must look
like a slightly less than unity gain follower.
But its more than that! Plate is an input too.
A real triode is always in the triode region.

The "4th circuit" as OldEurope calls it.
Plate in, Cathode out...

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

This sims sweet. 3rd and 5th harmonics are
quiet, way down below the 2nd and 4th!

But I screwed up thinking how I might try
to stabilize DC bias with a low pass...
But too much phase shift round this loop.

Without 47K loading down the input, this
sort of servo almost wants to motorboat.
I'm still not entirely comfortable with it.

Shoulda just left the grid bias at some
fixed DC value, and tweak R9 to center
up the output wherever VGS works best.
No diff than regular Zen in this regard.

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

Certainly solves prob of low input Z.
But it aggrivates the prob of not being
able to pull the output below VGS...

With normal Zen, you got some DC
stored on the input cap that might
ride you through an output that dips
briefly below VGS.

But there is no storage here. Once
the plate drops to equal the cathode,
it shuts off. Just like any other diode.

Triodes aren't typically abused with
so little voltage on the plate, but they
still follow the same rule of Mu, and
thats all Zen needs for this circuit.
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Old 14th March 2009, 06:18 PM   #14
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And the .ASC file for those who LTSpice
Credit always to Duncan for dmtriodep.inc
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Old 16th March 2009, 04:57 PM   #15
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Here, I think I've debugged most of the problems in the previous
version. Almost entirely caused by low plate voltage. Making G1
leak far too much, even before it gets forward biased. More than
2nd harmonic the reason DC center wanders all over the place...
Thanks to Duncan's model, or I'd never have spotted it.

A low value of grid leak resistor helps, but only higher plate V
stops the problem sufficiently. I thought about different ways
to bootstrap that feedback without ruining the direct coupled
nature of Zen and/or Mu.

I folded Zen back toward the upper rail, and tapped into 50%
ultralinear style. The triode is no longer operated near cutoff.
Now most of the soft clipping you see is the MOSFET and Zen.
Lower Mu and higher Gm triode was also a biasing necessity.

The MOSFET borrows the Triode's G1 for higher input impedance,
and borrows the Triode's Plate almost as-if a Pentode's Screen.
The feedback is not entirely Zen or Mu, but something between.

We can now swing the output within a gnat's eyelash of either
rail without any difficulty. Linearity is up, what little distortion
may remain is still dominated by triode-like even orders.

If I had the servo all figured out, it'd be worthy of a build.
The voltage at C2 can be tweaked anywhere from 25 to
50VDC. High enough not to make the triode grid too leaky,
reflected in about 1V adjustability for VGS bias as seen at
the cathode on the bottom end. R8 is where I would most
likely target for making that adjustment.

But I am about to go off on another tangent entirely....
Nelson may just flip his wig what I'm about to post next.

Current mirrors, folded cascodes, Alephs that appear out
of thin air, etc... etc... All his favorite stuff, only different.
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Old 16th March 2009, 05:01 PM   #16
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Not quite yet though: I forgot to post the ASC first.
Extract the triode models from my earlier zip
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Old 16th March 2009, 05:09 PM   #17
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Full explaination will follow, but not right away.
Let him chew on it for a while. Lessee what the
old man makes of this, before I spring my next
surprise.
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Old 16th March 2009, 08:41 PM   #18
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Why would anyone in their right mind do this???
Who said I was in my right mind? Or left for that mater.

All that matters (for both minds): Direct currents are
forced to be equal and in phase. Alternating currents
forced to be equal and out of phase... Nuff said.

More on this later.
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Old 16th March 2009, 09:34 PM   #19
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From Zen-V7 we learn that Nelson is no stranger to
transformer coupling. But the bane of transformer
coupling (and loudspeakers) is DC imbalance.
I'll assume anyone reading this post has read or
written that document first.

And I'm going to throw Nelson a further curveball.
I'll only Zen one side (represented by the Triode).
Aleph here not merely represented by a MOSFET,
it literally IS a MOSFET. Nothing else. Pure magic.

Both gates can float with the same reference
voltage, or different... Our current mirrors will
automagically serve up the correct autobias.

If this were a balanced circuit, the bridge cap
would have a very hard time. Source impedance
for a plain Jane MOSFET is extremely low. And
for Aleph here, it is.

By the very same trickery that gives Zen a low
drain impedance, it may also aquire a higher
than usual source impedance. High enough for
reasonable caps to bridge.

Again, I said this was not a balanced circuit.
Even if identical MOSFETS are used for Zen and
Aleph, they don't behave the same in context.

What you got here is a Folded Cascode. Aleph
holds the bridge voltage at some virtual AC
ground. Zen (with the higher source impedance)
dictates the (equal and opposite) AC current
for both halves.

And of course drain voltages are equal and
opposite, because they are transformer coupled.
Both devices deliver equal power into the load.

Now, whats funny to me. You can drive Zen's
gate and ground Aleph. You can drive Aleph's
gate and ground Zen. Or both in differential.
Its all the same, single ended Zenness....

Though I will say: that Aleph in this context
(no feedback network) has the higher gate
impedance, may be the preferable target.
"Wag the dog" I say.

On teh other side of the curtain, we call this
topology "Brute bridged Anti-Triode". Yesss.

I'll workup a full circuit for you in a day or two.
Got to get some real work done meanwhile.
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Old 18th March 2009, 08:35 PM   #20
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We see Aleph does an excellent job already
at matching mismatched MOSFETS. We see in
the first two examples: Nearly half a volt
difference in VGS between IRF510 and IRF530.
5.972V for the gate of M1, 5.433V for M2.
But Aleph holds the currents to within 5mA.

Aleph also does a pretty decent job with
deliberately mismatched bipolars... But
75mA is still a pretty significant error.
Compared with how well it had just dealt
with a really bad VGS situation.

Threw the pointless cap at M4 for laughs.
Nothing proved here. Except the loss of
.7V headroom in all circuits to follow.

Now we double the local feedback and
subtract half from afar... This is
Alan Dower Blumlein's "Garter Bias."

For the mismatching MOSFET, we see an
improvement from 5mA to 2mA matching.
Aleph was already doing most of the
hard work. Probably not worth .7V bias
cost if that was all it was good for.

For the mismatching Bipolars, we see
an improvement from 75mA to just 24mA.
Now thats pretty significant! A factor
of three improvent in match.

Don't think anyone would choose to use
such deliberately mismatched components.
Only trying to show the concept works.

We are gonna explore some transformer
coupling next. And for that application,
matching DC might be significant.

Say for example: Abusing a dual voice
coil subwoofer as an autotransformer
for a full range loudspeaker...
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