My class D amp

Hi Tekko,

The heart of a Class-D amplifier(all type) filter is a L-C low-pass filter. The corner frequency of the filter is chosen so that the filter will have minimal effect on the desired output frequency range while attenuating the switching noise as much as possible.

The optimum value for the filter inductor is

L = RL/2pfC

where fC is the desired corner frequency of the filter and RL is the load (speaker) resistance. Note that the inductor value is dependent on both the desired corner frequency and the speaker impedance so the inductor value will changes if the speaker impedance changes.

Practical designs require the use of standard component values so small adjustments usually have to be made to the ideal inductor and capacitor values. Rather than calculating the inductor and capacitor values independently and then adjusting their values, it's better to calculate the inductor value, select the closest standard inductance value, and then calculate the required capacitance using the selected inductor.

C = 1/((2pfC)2 • L)
The quality factor (Q) of a filter is the ratio of the center frequency to the filter bandwidth.

Q = RLv(C/2L)
A high Q produces an underdamped curve and a low Q produces an overdamped curve. The Q of the filter should be in the range 0.6 > Q > 0.8 to avoid underdamped or overdamped behavior. If you use the equations above the filter should have a Q of about 0.7, which provides good performance and allows for impedance variation in the speakers. Note that the Q of the filter will change if the speaker impedance is changed without adjusting the filter component values, which can result in an underdamped or overdamped response.


Not only is it important to choose the correct L-C filter values, it is also important to choose the correct types of components for the class-D amplifier in order to avoid losses and minimize harmonic distortion.

The DC current rating of filter inductors must be greater than or equal to the maximum current that it will see. The change in inductance versus load current should not be more than 10%. The core material can affect the amplifier's harmonic distortion and should have very low hysteresis losses.
 
I switched between open circuit and 0.8 ohm load on my ucd class d in BTL mode yesterday and the switching freq dident even flinge, it stayed on ~360kHz and did not move.

The problem with pre filter feedback self osc as several have outlined is that if the amp is made for 4 ohms, then at 8 ohms the freq response will increase as a spike at higher frequencies, whilst it may not be audible to most its still unacceptable as it means the amp is unstable.

Likewise a amp made for 8 ohms will start rolling off at early as 5kHz with a 4 ohm load.

While on UcD (post filter fb selfosc) the frq response barely changes at all from 0.5 ohms(some car subwoofers) to unloaded, it never starts spiking/peaking at higher freq like pre filter fb does.

What's the big deal here. I have 8 ohm speakers for years now and will probably have them for another 20 years . My ( pre ) filter is designed for 8 ohms , so fine and flat response here, no roll off in the highs .If I ever buy new 4 ohm or whatever impedance speakers to replace my current ones with the same quality , I have to spend at least a couple of thousand dollars . Replacing my output filter with high quality inductors and caps to match the new speakers impedance will set me back at least 12 dollars per channel ;)

Cheers ,

Rens
 
pls anybody tell me is this core suitable for class d amps,
outer dm-2 inch
inner dm-1.25 inch
 

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hi all pictures of the smps :) ;)

i added the primary turns so as to cool the outputs . i pushed it a bit hard without a heatsink both for the outputs ,the 7815,and the diodes rbv5006 and the mur1560 except a small heatsink to the mje350
 

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