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rhythmsandy 20th July 2021 05:02 PM

Unity gain output stage Class D driver
Im thinking of using a Class D unity gain buffer ( which is meant purely for current gain basis ) and not using too much of feedback. Is there any way that I can do that in SI8244?

newvirus2008 21st July 2021 05:44 AM

Dear rhythmsandy,

To be honest, having minimal gain and not using much feedback sounds counter-intuitive to me. If I were you, I would use a lot of feedback, especially considering that it's a class-D amplifier that has several mechanisms contributing to pulse-width error. Besides, without much feedback, even power supply fluctuations would get through to the output, almost unopposed.

I would suggest adding enough feedback (with enough compensation) to get the best out of what you have. If you fear poor stability margins and instability, you may try self-oscillating designs such as UcD or the hysteresis control.

All the best.

analog_sa 21st July 2021 07:28 AM

What if he just hates nfb sound?

Ah, yes, class D may then be the wrong amp for him :)

steveu 21st July 2021 07:46 AM

You can parallel more FETs to an existing class-D amplifier and use a bigger or multiple output filters.

But if you want to buffer an analog amplifier, you just have to attenuate it to compensate for the class-D amp gain because the PWM modulation is done at low signal levels and not at the output level. Some class-D amplifiers may be unity gain stable and some may not but it is unlikely the input common mode range is anywhere close to rail-rail. So it's pointless to drive a class-D amp with an analog amp. Just use the class-D amp.

rhythmsandy 21st July 2021 06:00 PM

ok then how devialet is using voltage gain in the front end calling AHD?

rhythmsandy 21st July 2021 06:01 PM


Originally Posted by analog_sa (
What if he just hates nfb sound?

Ah, yes, class D may then be the wrong amp for him :)

Just thinking why not reduce the feedback in the class D.

newvirus2008 22nd July 2021 05:46 AM

If you do, you would lose error control / linearisation. Usually, smaller gains use larger amounts of feedback.

ubergeeknz 22nd July 2021 06:08 AM

That's not what ADH does.

Based on the marketing video, ADH seems to be an adaptation of "Current Dumping" or "Feed Forward" amplification (a very old concept, but only used successfully in HiFi since about the mid 70's) but using Class D instead of a Class C / B / AB "Dumping" stage.

ubergeeknz 22nd July 2021 06:10 AM

Also the waveforms shown in the ADH video are nonsense. This is not what Class D amplifier output looks like at all. It actually looks like a square wave, the amplitude is the total Rail-to-Rail voltage, with a varying duty cycle. After it is filtered, it just looks like a regular sinusoidal audio waveform. The basic operation of a Class D amplifier is the same as a switch mode power supply, only that the output is varied dependent on the input, rather than set to a fixed voltage.

steveu 22nd July 2021 10:39 AM

It is difficult to say exactly what Devialet are doing but I suspect it is similar to the Yamaha hybrid amplifier where the power supply rails are switch-mode controlled so that the analog output devices operate with a maximum of about 4 Volts drop and therefore very little heat is wasted. In any case, the class-D outputs are not tacked on the output of an analog amplifier, but perhaps the other way around so that the analog amplifier dictates the final result.

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