Balanced Load Design

audiobot · 30th April 2005, 06:33 PM

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

audiobot · 30th April 2005, 06:50 PM

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

Michael

EC8010 · 30th April 2005, 10:05 PM

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.

SY · 30th April 2005, 10:17 PM

More than that, the driver distortion rises in proportion to the positive feedback achieved by the bootstrap.

EC8010 · 30th April 2005, 10:23 PM

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...

audiobot · 30th April 2005, 11:09 PM

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.

Thanks,
Michael

smoking-amp · 30th April 2005, 11:39 PM

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

audiobot · 30th April 2005, 11:50 PM

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

Dave Cigna · 30th April 2005, 11:55 PM

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

SY · 1st May 2005, 12:01 AM

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.

30th April 2005, 10:05 PM	#3
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

30th April 2005, 10:17 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	More than that, the driver distortion rises in proportion to the positive feedback achieved by the bootstrap. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

30th April 2005, 10:23 PM	#5
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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... __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

30th April 2005, 11:09 PM	#6
audiobot diyAudio Member Join Date: Sep 2003 Location: iowa	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. Thanks, Michael

30th April 2005, 11:39 PM	#7
smoking-amp diyAudio Member Join Date: Dec 2001 Location: Hickory, NC	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 __________________ Ohms Law V = I R

30th April 2005, 11:55 PM	#9
Dave Cigna diyAudio Member Join Date: Oct 2003 Location: Finger Lakes, NY	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

1st May 2005, 12:01 AM	#10
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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