Amplifier problems, Hum, preamp and filter

[FilipSebik]

Hey, I just built my amplifier which is 3x TDA2005 3x20W. I am powering it using SMPS, yes it has input frequency filters.
The Hum problem:
I have a stereo line transformer https://www.monacor.com/products/pa-technology/signal-processing/signal-optimisation/fga-40mf/
And I don't know if to use it on audio input or amplifier output (to speakers)

Preamp:
I have 2 stereo inputs for audio, that can be connected together to make 1 stereo input, but the problem is that I need preamp that could give off 2 outputs, 1 for speakers and 1 for subwoofer (20-150 or 200Hz). I need a scheme for that or I could use a LM386 as low gain preamp.

Filter:
I tried to use low pass filter using 1K resistor and 1uF capacitor, which should cut off frequency higher than 150Hz for the subwoofer but it didn't work or I connected it bad way (I connected it at amp output)


AMP scheme (The bridge doesn't work, it will always burn out the amplifier IC, so I use the OUT + and GNDz to connect a speaker)

Speakers: 4x 6ohm 8W
Subwoofer: 1x 8ohm 30W

 

[Audioguru]

The audio transformer will not work and might burn out if you connect it to the output of an amplifier. IF you need isolation then it connects to the amplifier input.

Your schematic does not show the extremely important power supply voltage. Its maximum is about 14.4V when its two amplifiers are bridged like in your circuit and drives one 4 ohm speaker. Your subwoofer speaker is 8 ohms so the power output to it will be only about 9W before clipping.

Maybe your amplifiers do not have shielded audio cables at their inputs? Ordinary unshielded wires act like antennas that pick up hum.

The 1k and 1uF filter should be used at the INPUT of the subwoofer amplifier. It is a very simple filter so it will not reduce high frequencies very much.

The bridge amplifier shuts down without being damaged if the supply voltage is higher than 14.4V and it does not have a pretty big heatsink.

The bridge amplifier has DC on each output so a speaker should NEVER be connected to one output and ground without a series capacitor.

 

[FilipSebik]

The audio transformer will not work and might burn out if you connect it to the output of an amplifier. IF you need isolation then it connects to the amplifier input.

Your schematic does not show the extremely important power supply voltage. Its maximum is about 14.4V when its two amplifiers are bridged like in your circuit and drives one 4 ohm speaker. Your subwoofer speaker is 8 ohms so the power output to it will be only about 9W before clipping.

Maybe your amplifiers do not have shielded audio cables at their inputs? Ordinary unshielded wires act like antennas that pick up hum.

The 1k and 1uF filter should be used at the INPUT of the subwoofer amplifier. It is a very simple filter so it will not reduce high frequencies very much.

The bridge amplifier shuts down without being damaged if the supply voltage is higher than 14.4V and it does not have a pretty big heatsink.

The bridge amplifier has DC on each output so a speaker should NEVER be connected to one output and ground without a series capacitor.

So connect the isolation transformer to input

The max voltage the amplifier can handle is not 14.4V, its 18V

No they don't have shielded cables, because shielded did the same thing

What is better filter then?

This is pretty much BS that you said here. No it doesn't shutdown at 14.4V and it is stone cold with heatsinks and fan blowing on them

I forgot to say, that I have DC blocking caps at outputs. They are 1000uF electrolytic

 

[Audioguru]

The datasheet of the obsolete (it is not made anymore and is not available) TDA2005 amplifier shows that its maximum allowed output current is 3.5A which is produced when it is bridged with a 4 ohm speaker and a power supply that is about +16V. When its speaker is 8 ohms then it can use an 18V power supply.

Its output power is rated when its supply is 14.4V and its speaker is 4 ohms and when its supply is 13.2V and its speaker is 3.2 ohms.

ALL audio circuits must use shielded audio cables at their inputs because the hum from electricity will be picked up with ordinary unshielded wires. Maybe you built the circuit on a solderless breadboard and the many strips of contacts and messy wires all over the place pick up hum?

Most crossover networks use active lowpass and active highpass filters using opamps each filter with 2 or 3 RCs.

A class-AB amplifier is stone cold when its output power is low. It produces almost as much heat as its output power when its output power is high.

The bridged amplifier does not need output capacitors because the DC output from each output is the same.