Tuesday, March 13, 2012

Classic Amp -- Plagiarism

Some of you, looking at my schematic, might have noticed similarities to this, this, or especially this. The last one has some marked similarities, right down to my R11. I rarely see emitter degeneration in the VA. In my final design, I might make it vanishingly small, or jumper it out altogether, but I wanted to experiment with it, and see what kind of effect it has on distortion, gain and bandwidth. I actually didn't see that schematic until a couple of days ago, when it turned up in a Google search for Sziklai parasitic oscillation.

Classic Amp - My Schematic - Click to Enlarge

Although I have the ESP site bookmarked, along with Douglas Self's and Marshall Leach's sites, I promise you that I was working independently when I did the schematic capture.

Of course, there are only a few ways to configure a "classic" amplifier and still have it be, well -- classic. So you should expect some similarities. As I said in my original post, I wanted to include a number of enhanced features, without over designing. There is only one way to make a differential amplifier that has a current source and a current mirror. There are only two typical output configurations: Darlington and Sziklai. The rest is all calculating values, parts selection, testing performance and optimization.

The area that lends itself to originality is the PCB layout. Although there's really nothing original about ground and power planes, I have never seen a power amp use them. I'm using Eagle Light for this project, so I can only do two layer boards. Otherwise, I would have done a four layer board, with power and ground on the two interior layers (V+ and V- planes could almost meet in the middle), and signal traces on the two outer layers.

But I think the other thing that makes this board unique is the ability to make the temperature compensation track by fastening together Q8 & Q10 and Q9 & Q11.

Classic Amp - My PCB - Click to Enlarge
I think that the power and ground planes might make this amplifier more stable, requiring less tweaking (fewer caps, or lower values) to control oscillation. Stay tuned. I believe this will be an unique amplifier.

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