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"Current" control is a big issue using ClassD ?

audioguru

Well-Known Member
Most Helpful Member
He found one that is made but it is extremely expensive so without knowing anything about the circuit he wants to make some and sell them and get rich!
But how many people, if anyone, needs one or will buy it from him instead of from a company who properly makes them?
Why is it extremely expensive? Because almost nobody needs one.
 

kubeek

Well-Known Member
Most Helpful Member
He found one that is made but it is extremely expensive so without knowing anything about the circuit he wants to make some and sell them and get rich!
But how many people, if anyone, needs one or will buy it from him instead of from a company who properly makes them?
Why is it extremely expensive? Because almost nobody needs one.
Thank you for your very helpful heap of assumptions.
 
He found one that is made but it is extremely expensive so without knowing anything about the circuit he wants to make some and sell them and get rich!But how many people, if anyone, needs one or will buy it from him instead of from a company who properly makes them?
Why is it extremely expensive? Because almost nobody needs one.
I have showed you a "TOY" that has mouth and legs but cant'nt run or unable to speak.
The money that make sense to you is nothing but a bundle of papers, not more then a bundle of roses those gives fragnnance.
The way you see this earth may different than others.
 

unclejed613

Well-Known Member
Most Helpful Member
with class AB or B amplifiers, there have been commercially made amplifiers using both voltage feedback and current feedback in order to have some control over the output impedance. the Yamaha M-4 (or was it B-4?), had a variable output impedance control, and i think the range of the control was from -2 ohms to +8 ohms. the trick is to get the class D to behave in a similar manner. there's one catch, very few class D amps have adequate feedback for such "feedback magic". there's really not much open loop gain to work with. an analog amp can have a lot of open loop gain, usually somewhere between 60 and 90db, and when the loop is closed, and the gain is fixed at a voltage gain of 30 (for a 100 watt amplifier) and the "excess gain" (called gain margin) is what has the effect of eliminating distortion as well as lowering the output impedance. with a class D amp there is no gain margin because the gain depends on the amplitude of the triangle wave going into the comparator, and the output transistors' power supply voltage. you can get some gain margin if you use an op amp to feed the comparator with audio, and tap the feedback for the op amp from the speaker side of the filter.
 
with class AB or B amplifiers, there have been commercially made amplifiers using both voltage feedback and current feedback in order to have some control over the output impedance. the Yamaha M-4 (or was it B-4?), had a variable output impedance control, and i think the range of the control was from -2 ohms to +8 ohms. the trick is to get the class D to behave in a similar manner. there's one catch, very few class D amps have adequate feedback for such "feedback magic". there's really not much open loop gain to work with. an analog amp can have a lot of open loop gain, usually somewhere between 60 and 90db, and when the loop is closed, and the gain is fixed at a voltage gain of 30 (for a 100 watt amplifier) and the "excess gain" (called gain margin) is what has the effect of eliminating distortion as well as lowering the output impedance. with a class D amp there is no gain margin because the gain depends on the amplitude of the triangle wave going into the comparator, and the output transistors' power supply voltage. you can get some gain margin if you use an op amp to feed the comparator with audio, and tap the feedback for the op amp from the speaker side of the filter.
If I can save my life from corona, I will for sure disscuss it again.
 

audioguru

Well-Known Member
Most Helpful Member
I looked at the datasheet of the Texas Instruments TPA3116D2 class-D amplifier IC functional diagram. It shows negative feedback directly from each output back to each inverting input. Each input has two opamps in series so the gain margin must be very high which reduces the distortion to 0.1% at half max output power and reduces the output impedance for damping of speaker resonances.
The class-D switching occurs at a frequency as high as 1.2MHz and the input opamps have lowpass filters, so the amplifier has plenty of audio negative feedback.
 
I looked at the datasheet of the Texas Instruments TPA3116D2 class-D amplifier IC functional diagram. It shows negative feedback directly from each output back to each inverting input. Each input has two opamps in series so the gain margin must be very high which reduces the distortion to 0.1% at half max output power and reduces the output impedance for damping of speaker resonances.
The class-D switching occurs at a frequency as high as 1.2MHz and the input opamps have lowpass filters, so the amplifier has plenty of audio negative feedback.
I did see it before while trying to find one. TPA3116 does not have facilities what I was looking for!
 

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