Darius Loftin White explained

[oldeurope]

Splitted from this thread.

Darius Loftin White

Hi Sheldon,
thanks for your interest. Please let us make it step by step.
First, imagine that C2 has infinity capacitance no losses and it is charged.
In this case there will be no signal voltage drop at the cathode resistor.
Thus there is no signal feedback caused by "Chb".
In this fundamental schematic
the green line shows the signal current, the red line shows the signal path.
The left circuit is no Loftin White, it is simply direct coupling.
The right circuit is my triode Loftin White topology.

Kind regards,
Darius

[kenpeter]

LW cycles through only one cap, instead of down to ground and back
up though a bypass cap in series. Thus the one cap may be smaller?
Or did I miss the whole point?

And if I may imagine a "blue highlighted path" between stages,
might loop up from cathode to plate of the input triode down
through grid to cathode of the output triode, and back through
your CHB path, what is that, "Cathode High Frequency Bypass"?

Not sure at 28K+, why would CHB be significant compared to
the path 4.7K to GND and back up the 560ohm... Again, I may
have missed the point.

[oldeurope]

Hi,
what does LW cycles mean and what is a one cap?
This amp is an audio frequency amp not radio frequency.
What does your "blue path" mean? An additional current loop?
There is no current going from the grid to the cathode.
The grid is negative biased against the cathode.
Since no signal drops at the cathode of the output triode
no signal is on the Chb path. Thus Chb has no influence in
the signal.

Kind regards,
Darius

[Sheldon]

Originally posted by oldeurope
Darius Loftin White
the green line shows the signal current, the red line shows the signal path.
The left circuit is no Loftin White, it is simply direct coupling.
The right circuit is my triode Loftin White topology.

Ah, OK. The green AC current path means that the signal current should not be seen by the cathode resistor, assuming a perfect capacitor for C2. So, the AC connection to the input grid is only correcting for an imperfect C2 capacitor? Thanks for your patience.


Sheldon

Edit: I notice in your 300b design, that you have added a noise cancellation circuit - nice. Have you ever checked out this site?: http://members.aol.com/sbench101 He also likes to explore unusual topologies.

[oldeurope]

Originally posted by Sheldon


Ah, OK. The green AC current path means that the signal current should not be seen by the cathode resistor, assuming a perfect capacitor for C2. So, the AC connection to the input grid is only correcting for an imperfect C2 capacitor? Thanks for your patience.


Sheldon

...

Yes.

Originally posted by Sheldon

...

Sheldon

Edit: I notice in your 300b design, that you have added a noise cancellation circuit - nice. Have you ever checked out this site?: http://members.aol.com/sbench101 He also likes to explore unusual topologies.

Please note: The underlined words in the blog are links.

Kind regards,
Darius

[Sheldon]

Originally posted by oldeurope
Please note: The underlined words in the blog are links.

Ah, yes. I didn't see the English version, and I didn't spot those. I have a version of Steve's 801 resistor coupled amp. Sweet little amp. If I get ambitious, I may wind up some output transformers and try it as a L/W amp.

Sheldon

[kenpeter]

Originally posted by oldeurope
Hi,
what does LW cycles mean and what is a one cap?
This amp is an audio frequency amp not radio frequency.
What does your "blue path" mean? An additional current loop?
There is no current going from the grid to the cathode.
The grid is negative biased against the cathode.
Since no signal drops at the cathode of the output triode
no signal is on the Chb path. Thus Chb has no influence in
the signal.

Kind regards,
Darius

LW was supposed to read "Loftin White"

One cap, C2 - 24uF.

Cycle meaning a loop, like the green one you drew.

Audio, not Radio? Would never have figured that out.

Imaginary Blue Path as I fully described it, not highlighted
in your original drawing. Relevant only to asking a question.
Intending to describe a loop of interstage signal voltage
drops, summing back to unity as it returns to the orgin.

No current from Grid to Cathode? No, of course not.
I did not assume current when you draw a Red line
from Grid to Plate, Grid to Plate, then across an OPT
into the Speaker. If current loop makes no sense, why
assume I am asking about a current loop?

CHB Path... Does nothing? OK, whats it there for then?
I think it must do something, but what???

I see 220K/560, voltage gain of x393 or Mu of the input
Triode, whichever of the two gain factors is the lower...
Most likely Mu is setting input gain, not the resistors.
Relevant to my next statement/question.

The Output Cathode may follow "without signal drop?"
But it is following interstage signal of at least Mu Gain.
And coupled thru CHB path back to the input cathode
follower of Unity Gain or less. These two cathodes are
not of the same AC signal level, and of inverted phase!

Again, what exactly does CHB do if not conduct an
alternating current in proportion to the AC voltage
drop across it.? (Vin*(Mu+1))/Zchb)

[agent.5]

Can you tell us more about how you do the hum cancellation? I see that you are doing something different than what Steve Bench is doing.

[kenpeter]

I am guessing that all signals around the green output loop are
common mode to the power rail noise, including the cathode!!!
As are the rLoad and Plate of the input stage. Power hum can
have no effect in the output Triode circuit when all points are
common mode with it.

But the input plate is not common mode to the input signal, nor
to ground. And triodes will respond to plate voltage hum as an
input that modulates current gain. It cannot be ignored, and is
probably corrected in this circuit somehow, somewhere...

Perhaps CHB is for cancellation of hum in the input stage???
I see no great effect it has upon the output stage... At least
not directly...

[oldeurope]

Hello kenpeter,
I am sorry but I think I am not able to explain you this thing.
Absolutely no chance, I am very sorry.

Hello agent.5
In this blog you can see a conventional SE and the triode Loftin White.
The conventional SE is made sensitive for hum at the supply rail.
In this conventional SE the hum at the supply rail and the hum
caused by AC heating of the 300B are compensating each other.
(Please note that the component values are specially selected
this way that phase and level are ok for a hum cancellation.)

The Loftin White topology has an excellent supply voltage riple
rejection because riples at the positive supply rail are coupled to the cathode
of the power tube by C2. Chb makes the output of the driver stage
floating on the AC component at the cathode of the power tube.
It would be possible to generate a hum by miss adjusting P and
if phase and level are ok there would be a compensation of the hum
caused by the the double mains frequency hum caused by the low thermal
inertia. In this case the points of hum cancellation and
compensating an imperfect C2 capacitor would be different.
This is why I generated a hum signal in the power supply
and feed it to the cathode of the driver stage to compensate the hum
caused by AC heating of the 300B. Hum cancellation and compensating
an imperfect C2 are independent this way.
Have a look at the Oscilloscope screen shots.

Kind regards,
Darius