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Old 30th April 2005, 06:33 PM   #1
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Default Balanced Load Design

Hi all,

Seeking comments on this circuit design. Any takers?
Column one here rest to follow. Sorry about format of post, but I can't remember how to get my image to link from the web.

Michael
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Old 30th April 2005, 06:50 PM   #2
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Default Part 2

Hope it's readable. Again sorry for the format.

Michael
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Old 30th April 2005, 10:05 PM   #3
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It's the split-load design that was popularised by McIntosh. I believe Audio Research are still keen on it. What the diagram doesn't show is that you need an awful lot of swing from the driver stage. A common way of solving that problem is to use the output anodes as bootstrapped HT supplies for the drivers stage. It works, but it makes stability very much harder to achieve.
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Old 30th April 2005, 10:17 PM   #4
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More than that, the driver distortion rises in proportion to the positive feedback achieved by the bootstrap.
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Old 30th April 2005, 10:23 PM   #5
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Good point. As the HT voltage rises, the current through the anode load resistor rises, so it appears to the valve as a reduced value of anode load resistor, causing a steeper loadline and increased distortion. There's just no free lunch...
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Old 30th April 2005, 11:09 PM   #6
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Default Somewhat different than McIntosh

While I am not able to argue your assertion in the electrical sense, this article is from the Proceedings of the IRE July 1954 while the McIntosh circuit was published as least as early as 1949. This would seem to make it unlikely they are the same. Though I have difficulty deciphering the schematics in the McIntosh paper I do not see the capacitive coupling of the cathode windings with the plate windings as shown in the scheme I posted. True? Are there other differences? Enlighten me.

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Michael
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Old 30th April 2005, 11:39 PM   #7
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The capacitors allow the transformer to be made without bifilar winding as in the McIntosh design. The caps provide the close coupling between sections to overcome xfmr leakage reactance. The text refers to this and that it would be cheaper to build.

Don
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Old 30th April 2005, 11:50 PM   #8
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Default Aye, Cap'n

From the other side, as found in the McIntosh article. If bifilar winding does away with the need to sectionalize the primary where is the disadvantage? The need for thicker insulation?

Another question, is this output stage necessarily unity gain?

Michael
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Old 30th April 2005, 11:55 PM   #9
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Hi Michael,

It's exactly the McIntosh circuit except for the 'bridging' caps. Both McIntosh and Amemiya were interested in reducing crossover distortion in class B amps by AC coupling the cathode winding of one tube to the plate winding of the other. Mac did it by specifying bifilar windings and Amemiya used a cap and a presumably cheaper transformer.

In any case, it's not clear what the tight coupling buys you in class A. In other words, a class A amp without bifilar windings might work equally well with and without Amemiya's cap.

-- Dave Cigna
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Old 1st May 2005, 12:01 AM   #10
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The insulation requirements of the Mac transformer windings are indeed nontrivial.

The output stage does not necessarily need to be unity gain.

Caps are not the only way to couple the windings to make up for difficulties in the transformer- one could use MOSFET source followers.
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