Smoothing Cap for LM3875T

[zesla]

Hi All,

Well I am trying to make a Stereo Amplifier out of two LM3875T .
The power supply unit will be something like the below pic so that reduce the crosstalk to as minimum as possible. Is there any formula so that I could calculate the values of C1 & C2 and hence C3 & C3?
The transformer's secondary AC voltage is 2x18 (so the final DC voltage after smoothing would be almost 2x24V). The frequency of the transformer is 50Hz.
Well I will use two 2200uF 50V capacitors between the ground and pins 1 & 4 of the Amplifier chip (as close to chip as possible). Of course those pins are the supply pins of the chip.

So can you guys help me out to find the values for those smoothing caps too?

Thanks.

 

[audioguru]

With 4400uF of filtering on each polarity you will hear ripple (hum) only when the anplifier is clipping.
The output power of each amp into 8 ohms at clipping is 26W. The heating is 15W each. So the power supply must deliver a max of 82W to the stereo amp.

 

[zesla]

So?

What about using 4700uF caps for C1 and C2 and C3 and C4? then using two 2200uF for the chip itself (ground & pins 1 & 4)?

 

[audioguru]

You need only a single rectifier bridge and only two filter capacitors for the stereo amplifier.
You will not hear any crosstalk between the channels.

4400uF or 4700uF will filter almost the same since their tolerence is so wide that their actual values could be from 3500uF to 7000uF.

 

[zesla]

.
The output power of each amp into 8 ohms at clipping is 26W. The heating is 15W each. So the power supply must deliver a max of 82W to the stereo amp.

How could you calculte it? Specially the 15W heating?

Thanks

 

[zesla]

You need only a single rectifier bridge and only two filter capacitors for the stereo amplifier.
You will not hear any crosstalk between the channels.

4400uF or 4700uF will filter almost the same since their tolerence is so wide that their actual values could be from 3500uF to 7000uF.

But I worry abut Crosstalk by using just one bridge.

do you think that 4700uF cap is just enough for me?

Thanks

 

[audioguru]

An audio power amplifier has very good power supply noise rejection. The crosstalk will be so low that you will not hear it and it will be difficult to measure.

The power supply main filter capacitor powers the amplifier between the peaks of the supply voltage. If the output power of the amplifier is high then it might be modulated by poor filtering of the supply by the filter capacitor if its value too low.

You should calculate the value of the main filter capacitor, not just guess.