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Old 16th April 2012, 03:58 PM   #1
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Default Hiraga's Improved Concertina Phase Splitter

I found this circuit in an old French "L'Audiophile" book describing a PP amp designed by Jean Hiraga. The circuit looks conventional, except for the phase splitter which (the author claims) is an "improved" version of the classic Concertina (aka split load or cathodyne) circuit. The "improvement" consists in the addition of a single resistor connected between plate and cathode of the splitter tube, which looks like positive feedback to me. I've never seen this circuit used anywhere and am wondering what are the real advantages (if any) of this circuit in comparison with the classic Concertina phase splitter. I breadboarded this circuit and confirmed it is stable and works quite well, though I didn't measured all the electrical spec's yet.
Any thoughts ?
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Old 16th April 2012, 04:09 PM   #2
Merlinb is online now Merlinb  United Kingdom
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Since it's directly in parallel with the triode it effectively becomes a portion of ra, so I suppose you could argue that it has a linearising effect on ra. But it also reduces the effective gm at the same time, so I would question its usefulness. Maybe its meant to improve recovery time from blocking distortion?
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Old 16th April 2012, 04:47 PM   #3
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Here's what the author says regarding the improvements (translated from original french text):

... we can use three resistors of the same value, in this case it looks like the input voltage is increased by 33% and the loads multiplied by 66% from their initial value. Consequently, this will increase the gain (which is between 0,7 to 0,9 for a classic Concertina) to a value close to 2 (when µ raise), as well as an increased bandwidth...

I understand the increase in gain (a side effect of any P.F) but the positive effects on the frequency bandwidth is less clear to me.
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Old 16th April 2012, 04:56 PM   #4
SY is offline SY  United States
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Quote:
Originally Posted by Merlinb View Post
Maybe its meant to improve recovery time from blocking distortion?
If you get blocking between the cathodyne and the following voltage amp, the design is severely flawed. The circuit already has 100% feedback, so this won't really linearize it-in fact, since it effectively makes the loadine more vertical, I would expect distortion to be higher. (Remember, the outputs at plate and cathode are equal and opposite, so the center of that resistor is at ground) I am doubtful that the gain increases to 2.

I wonder if this is one more case of trying to fix a non-existent problem...
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Old 16th April 2012, 05:50 PM   #5
DF96 is offline DF96  England
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Think what a gain of 2 will mean. You raise the grid by 10V, and the cathode goes up 20V. The grid-cathode voltage has shifted by 10V. Do you really want your phase splitter staggering between cutoff and grid current? This is a daft idea. Why am I not surprised?

And when did positive feedback increase bandwidth and reduce distortion?
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Old 16th April 2012, 05:57 PM   #6
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Maybe he means that the gain from cathode to plate is 2? Either way, this is not positive feedback, it's just an extra load to drive.
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Old 16th April 2012, 06:13 PM   #7
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Quote:
Originally Posted by DF96 View Post
Think what a gain of 2 will mean. You raise the grid by 10V, and the cathode goes up 20V. The grid-cathode voltage has shifted by 10V. Do you really want your phase splitter staggering between cutoff and grid current?
Seems logical to me too.

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And when did positive feedback increase bandwidth and reduce distortion?

Never. PF usually has a detrimental effect both on distorsion and bandwidth.
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Old 16th April 2012, 06:17 PM   #8
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Originally Posted by SY View Post
it's just an extra load to drive.
That looks like it. Since it's connected to the two opposing phases before the caps and input resistors to ground, will this have any effect in canceling distortion, like some crisscross circuits a la early Audio Research? Or it reminds me of an input transformer with no center tap secondary and shunting it a resistor and then a pair to ground which I saw that in some circuits. I have no idea, just speculating. It is interesting though.
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Old 16th April 2012, 06:18 PM   #9
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It's not even that. It's an extra load, no more, no less.

Pretend the 39k resistor is two 19k5 in series. At their junction, the voltage is zero. So the effect is the same as paralleling the plate and cathode loads with 19k5 each.
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Old 16th April 2012, 06:21 PM   #10
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Quote:
Originally Posted by SY View Post
Maybe he means that the gain from cathode to plate is 2? Either way, this is not positive feedback, it's just an extra load to drive.
Nevertheless, JH wrote (p.14):

... (la résistance) introduisant de la sorte une réaction positive de tension,...

which means:

... (the resistor) introducing in this way a positive voltage reaction...
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