♫♪ My little cheap Circlophone© ♫♪

[danielwritesbac]

Okay. Howabout this?
3MHz Phase Linear
Different compensation
Restore sensor RC
Restore LTP comp
Remove sticking
0.002106% THD

[danielwritesbac]

Apparently, that buffer really is an upgrade. For comparison, here it is without the buffer. This time it is at 2.5MHz and 0.002215% THD. Not shown is the harmonic effects of the output device currents being fairly jagged on this version as the sensor is not nearly as well behaved on this un-buffered version. There's not actually any sticking, but it is just not quite as level. Even so, it looked worthy of checking out the differences between the two versions (with and without the buffer). The compensation differences are especially interesting. It seems that either you have a buffer or you have double the compensation efforts (see attachment).

[keantoken]

If you uncomment the .step command above the load resistor you'll see two lines, one with loading and one without.

[danielwritesbac]

To get both of those lines sensible looking, I think we'd have to ride the brakes. Does that explain why Elvee's LTP comp has 1.5n and why the 27p feedback cap as well?

[danielwritesbac]

0.000033%THD@1kHz
0.000077%THD@10kHz
0.000712%THD@50kHz (shown)
Top speed on that one is 1.5MHz if you want strictly phase linear. It was simulated with the load engaged for more realistic results (for example if I took off the buffer, then the passive component values will look very much like post 1). The fantastic THD figures are because of unusual manipulation of the sensor values (see attachment), Q16 which cuts THD in half anyway, and the 2x better tolerances (easier to comp) seen when using the buffer. I was attempting higher efficiency, but this looks unusual and I do not know how to interpret the results. Help?

[danielwritesbac]

Ah, practicality issues.
1
I read only one of the THD graps. The other says 0.0022%.
2
And R17 could go to 10 ohms or even a bit more if necessary for practical concerns of having that resistor large enough to assure that C2+R17 doesn't act like a bad cap but instead really does perform as an RC for consistent behavior. The entire goal of the latest exercise has been to break free of too small R17 values, and finally we did.

[danielwritesbac]

The simulator regards this circuit as 0.0013%THD. Although I show it without the buffer, adding the buffer is probably vital if one wants to support a much wider range of components. Without further ado, see the altered cascode.

[keantoken]

Bias at 700mA will get you low distortion, and this amp seems to like class A. However notice the bias point changes between loaded and non-loaded. For some reason loading pumps up the bias. This may have to do with your changes to the servo.

Also, dunno if you know this, in LTSpice, Tools->write bitmap to clipboard and then paste into MSPaint will give you an instant schematic with no need for cropping window borders. Too bad it doesn't work in Linux.

[Elvee]

Originally Posted by danielwritesbac

The simulator regards this circuit as 0.0013%THD. Although I show it without the buffer, adding the buffer is probably vital if one wants to support a much wider range of components. Without further ado, see the altered cascode.

I don't know what Q14 brings/improve, but it is unusable outside simulations: it will cause premature clipping of the output.
Cascoding Ken's way (on the other leg) does bring benefits, but here I only see additional problems.

[keantoken]

D9 has to be somewhere around 66V to prevent premature clipping. So it's a bit weird, and too much rail sag could cause VAS saturation. We could fix this and use fewer components by just cascoding Both halves of the VAS. For some reason that option never seemed attractive I take it, but now it looks like the best way. It would also allow us to use smaller VAS devices with better Vcesat characteristics; I'll bet we could get away with the BC550C or the BC327-40 there.