Old_School_Brad
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Instead of assuming they have come up with a method of getting rid of crossover distortion, we should ask if it ever was an audible issue to begin with?Is 'feedforward' the same as positive feedback (regeneration)? I could see using that as a way to fill in the crossover notch inherent in push-pull class B operation.
Who here understands the concept of class B operation?
View attachment 392733
Class B operation was employed in the output stages of radio transmitters and giant PA amplifiers back in the day. The idea is to have the opposite polarity output devices at low or zero current draw with no signal, but start to draw current when signal is applied. You get maximum efficiency that way, but there's a gap at the zero crossing of each sine wave where both devices switch off, causing the notch in the output sine wave that you see in the above illustration.
My question is what magic trick did the Topping engineers employ to get rid of that crossover notch? The classical electronics texts state that simply applying great gobs of negative feedback won't do the trick, because both output devices are momentarily shut off, therefore there is no gain, therefore there is no signal to drive the negative feedback loop. So how did Topping do it?
One problem is that an amplifier with great gobs of negative feedback will need to be carefully optimized so that it doesn't ring or oscillate on signal transients. The classic way to test that was to apply a fairly high frequency square wave (like 10kHz) to the input of the amplifier and look at the amp's output on an oscilloscope. If the amp is ringing, the corners of the square wave will look exaggerated, with a spike coming up from the corner of the wave, instead of a perfect 90 degree angle. Something like this:
View attachment 392738
That's why I asked Amir to show us what the output looks like with a 10kHz square wave input, if possible. It should like as close as possible to this:
View attachment 392739
A lot of pro audio PA amplifiers are class B in practice but don't have audible distortion.
Has anyone ever heard crossover distortion? -I haven't. And I've had a lot of pro and consumer gear during the years.