Voltage drop for poer amplifier

[zesla]

Hello guys,

I am working on a power amplifier made by LM3875T power op-amp chip (actually I tried the typical design on the chip datasheet).
My power supply is a 2x18V AC rms transformer, so the said transformer will give me almost 48 volts DC output.

The power supply output voltage when the amplifier is connected to it and works (i.e. when the load is connected) drops to almost 38V and when I connect a 33nF cap between the input resistor of the LM3875T and the ground it drops to almost 40V.

I have 3 questions so.

1: What is your idea bout that drops? Is not It too much of dropping?
2: What is the function of that cap really (theatrically it should bypass the Input AC signal to the ground!)
3: Why connecting it or not connecting it makes difference to the supply voltage?

Thanks

 

[Nigel Goodwin]

What size electrolytics capacitors are you using in the PSU?.

As for your other capacitor 'problem', it sounds like the amplifier is unstable and oscillating - did you follow the exact recommended PCB layout.

 

[zesla]

Well, I am using two 6700uF caps on the PSU (one between negative pin ad ground and the other between the positive pin and ground).
I use two 1000uF caps as near as possible to the chip itself.

How much of voltage drop is acceptable so?

The 33nF cap acts as a low pass filter in the circuit (an integrator), for instance it converts an square wave input to a triangle wave, I have much lower distortion when I use the said 33nF in the circuit.

 

[zesla]

I have made the amplifierit on the veroboard.

 

[Nigel Goodwin]

I have made the amplifierit on the veroboard.

It may well be unstable then, many of these amplifier chips are critical as regards layout. Do you have a scope so you can check if it's oscillating?, you might also post a picture here of how you've built it (click on 'Go Advanced' then select 'Manage Attachments').

 

[zesla]

What If I put the 1000uF as the smoothing cap and the 6700uF caps to the amplifier chip? Which method is correct??

 

[Nigel Goodwin]

What If I put the 1000uF as the smoothing cap and the 6700uF caps to the amplifier chip? Which method is correct??

The larger one should be the smoothing capacitor, there's no need for anything as large as 1000uF near the chip.

 

[Martel]

I just looked at the PDF...

In the first schematic, there is no oscillation snubber but later in the PDF, they add one (0.7 µH coil shunted with a 10 Ω resistor). Did you use one in your prototype ?