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In Reply to: RE: No, I'm saying we don't know what is really happening here due to the use of innuendo posted by PaulF70 on May 30, 2025 at 09:12:56
Where is Ralph wrong, in particular? I'm listening.
Follow Ups:
He is wrong in saying that Class AB and Class B amps don't have zero crossing distortion as Benchmark rightly claims. Only Class A amps are truly free from this distortion. Benchmark claims they found another way to eliminate it with feed forward...- this I cannot say whether it works or not.
I meant in the specifics of the critique of that Benchmark article.
It seemed to me he had a point regarding the sine wave images.
There has been some discussion recently on this forum regarding bias classes and cross-over distortion. I have written before that solid-state follower power amp classes are really a continuum that deserve more consideration of what is happening than effort expended in classification.
And I've written that cross-over distortion exists in all follower stages except class-A. A good resource is Douglass Self's Power Amplifier design book and, to help draw a line under this topic, I have captured some of the figures to reiterate these points (colored notations are mine).
Below is the simulated small signal gain of a BJT complimentary follower stage. Ideally the voltage gain would be unity but with loses it is less than unity. The x-axis is the output voltage, going from negative on the left side (the PNP pulling the load down) to positive on the right (the NPN pulling the load up). Various bias settings are shown and note that none have a constant gain as the output voltage traverses the range.
Self gives a reference that proves it is impossible for complimentary BJTs to have constant gain across the hand-over and the simulation shows it. Any wiggle in a transfer curve equates to distortion and the sharper the wiggle the higher the harmonics that will be generated (think of fitting a power series to a line, the sharper the curves the higher the order of the fitting polynomial). The green circle is Self's 'optimal class-B bias point' chosen for the least wiggling (NOTE, as I post this I think the green circle should be on the curve above). If the bias is reduced then the small signal gain takes a precipitous dive downwards and once the bias is low enough to reach a gain of zero that would represent class-B with the devices turned off. Why anyone would even consider this, I do not know. Going the other way, applying more bias moves us into class-AB but that does not make the overall transfer function any smoother. It is more continuous at zero but now we're developing more serious wiggles where the transistors do turn off around +/-5V for this case. Extending the bias into class-A gives the plot below:
Here we see smoothness across the zero crossing with gain droop at the maximum voltages where a device is starting the turn off at the limit of class-A current. The continuity from under-biased class-B through 'optimal class-B' to class-AB to class-A is now apparent.
Let's now have a look at the cross-over distortion created by the transfer curve wiggling. Below is a distortion residual from one of Self's amps that has most distortion mechanisms minimized leaving cross-over distortion apparent in the residual. Note this measurement was made with a lot of averaging and the amount of sharpness captured will depend on the resolution bandwidth of the oscilloscope. Note also that this distortion residual is 0.0004% of 25W into 8ohms. Compare this to the distortion residuals of tube amps shown in Stereophile measurements - there the THD is in the 0.X% range and the residuals are usually a fairly clean third harmonic swamping out any cross-over.
Here is an under-biased case as we head down that gain null creating a larger, sharper wiggle in the transfer curve. The zero crossing spikes are now much larger (though there is no note about their amplitude compared to the previous graph or the corresponding THD)
And, lastly, the distortion residual for a class-AB case
Note here that sharp changes in the residual are not aligned with zero crossing but now there is one one each side of the zero crossing for each transistor turn-off one side of the zero crossing
In summary, we should expect cross-over distortion in anything but class-A even if we have to work hard to capture it as it might be small. Though these pictures are for solid-state follower amplifiers, I can't image a push-pull tube design can be immune to cross-over distortion, though it might be buried by large low-order harmonics.
As for the Benchmark material, looking beyond the sales & marketing hurrahs I don't see anything obviously fallacious there. The distortion residuals in their plots are amplified by 60dB compared to the output sinusoids. For the competitor's amp the residual is of similar size so it must at least -60dB or 0.1%, roughly (they don't quote the competitor's 1W THD). The AHB2 quoted 1W distortion is 0.0004% so that is 250x smaller. It seems not unreasonable to me that if you shrank the competitor residual by 250x it would look like the AHB2 straight line on the same scale.
If the device does not go into cutoff its class AB of some sort.
If it cuts off prior to reaching zero, by definition its class C.
Class B is really theoretical; I don't think anyone has ever made one. The Self amp appears to be class AB, with the outputs biased very low resulting in poor linearity near zero crossing. So there is a bit of distortion generated as a result.
To be clear: class C is defined as 'less than 180 degrees of the waveform'. A class C push pull amplifier is possible; the output devices might conduct for only 179.5 degrees, cutting off just before zero.
Put another way, so-called 'class B' amps are actually either class AB or class C, unless one chooses to ignore the definition of what class B really is.
No disagreement from me with anything you put in this post really. It is what I have been saying across several threads on this forum while getting shouted down by Ralph that I don't know what I am talking about.
I told him clearly that ONLY Class A will not have crossover distortion and that Class B and even a lot of Class AB will have visible zero crossing distortion. He then tried to deflect and say that those must be Class C, which is nonsense.
We can debate the audibility of this until the cows come home. Suffice to say that I think it is nearly always audible and serves to destroy the realism in push pull amplifiers (both SS and Tube) that are not true Class A.
The ONLY PP amps that I have heard (tube or SS) that I find not objectionable after some time are Class A biased amps, like my friend's Plinius SA103, which has both A and AB modes. In A it sounds pretty nice, although that odd order dominated spectrum still renders it a bit grey sounding compared to the best amps. PP triodes in Class A can sound very good. For whatever reason, the cancellation of even order harmonics is often not as complete for tubes as it is for transistors, leaving some psychoacoustic benefit I think.
This for me leaves only single ended amps (tube, transistor or a mix of technologies) as having the most natural sound...they may not be ideal for all speakers...but there are now sufficient numbers of easy to drive speakers that I need not worry about speaker choice.
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