Audio Circuit Ground Rail

I build amplifiers at times for fun but I always have a problem with them, I get small hums and a higher pitch sound from tweeters, at times I realize when I touch like pots the humming gets higher and if I touch like a speaker wire the humming goes down or stops, I normally wonder if its grounding issues as I don't think I know how to ground my circuits when I build them. IS there something I am missing or is it that the amp and preamp circuits just isnt clean enough to stay silent.

I built a couple tda7293 basic amplifier circuit that I got from the datasheet of the chip and I normally add a tl072 preamp to it but it hums.

Enzy said:

I build amplifiers at times for fun but I always have a problem with them, I get small hums and a higher pitch sound from tweeters, at times I realize when I touch like pots the humming gets higher and if I touch like a speaker wire the humming goes down or stops, I normally wonder if its grounding issues as I don't think I know how to ground my circuits when I build them. IS there something I am missing or is it that the amp and preamp circuits just isnt clean enough to stay silent.

I built a couple tda7293 basic amplifier circuit that I got from the datasheet of the chip and I normally add a tl072 preamp to it but it hums.

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In audio amplifiers , hum and stray signals can be coupled into the desired signal from various sources. The most vulnerable are DC power lines and sources, front-end circuits and also magnetic field coupled from close AC transformers and power lines. Some glass-envelope diodes sense light from fluorescent tubes, etc.

It is a good idea to power the low-level circuits from a battery (the DC lines do need capacitors to block noise, too), before you connect these circuits to the main DC power lines. Then you can determine the hum/noise source, and use shields or blocking filters to get rid of them.
Modern operational amplifiers are quite wideband and can oscillate high above audio spectrum,, as well as receive AM signals from the air. Be ready to a hard work.

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Also, as you mention the pickup from touching small metal components, the new amplifier technology uses high-impedance circuits that can amplify such small voltages easier than the low-impedance circuits used earlier.
The front-end ciruits like electret microphone amplifiers require a complete grounded enclosure to avoid such pickup, in the following circuit I would recommend to use lower-impedance circuits and a good grounding (one-point is preferred).

Recently, the main filter capacitor in my 51 years old stereo receiver failed open circuit and the speakers produced lots of hum all the time. I replaced the capacitor and my 'scope shows some 120Hz hum on it with no signal but the "power supply rejection ratio" of the amplifier circuit is good enough so that there is no hum at the outputs.

The inputs to any audio amplifier should use shielded audio cables where the shield blocks hum pickup. To test an input cable when the output of your amplifier produces hum, unplug the cable at its source. If the hum goes away then the hum is from the source or is a "ground loop". If the cable with no source is unplugged at the amplifier input and the hum goes away then the cable is unshielded. If the amplifier produces hum with no input cable then the hum is caused by the enclosure of the amplifier making a poor shield or the amplifier wiring causes a ground loop.

Hi,

If "touching" changes anything, then I'd say it rater is a design problem than a device problem.

Grounding, ground plane, shielding, (missing) capacitors on supplies, (missing) capacitors at feedback.

Klaus

Ok so not having enough capacitors at the supply can cause that? if so then that might be the issue because I dont actually know how to determine how much capacitors I need for the power-supply stage of the amplifiers I build.

Hi,

Bigger capacitors are good for low frequency / high power applications.

But yor problem seems to be high frequency.
--> You need to add high speed ceramic capacitor at critical points if your circuit.
But they don't help if ground plane is not solid.

Klaus

It seems to me that you need to use some metal screening around your amplifier. if you short out the input then any sound you hear must be from the amplifier itself. If it is hum its down to a lack of supply line decoupling. if it is a hiss then it is most likely due to the internal noise of the components being amplified. To cure this then better quality components or a better design is called for. If the sound is not either of these two then the most likely problem is that the amplifier is oscillating at some high frequency, as said extra low value capacitors are required from the supply rails to earth, say .1 or 1MF .
Touching the metal case of pots means that you are acting as an aerial, picking up noise from the local mains supply and capacitively coupling the noise to the circuit via the metal case to track capacitance. Because this will be small, no "hum" gets in this way only high frequency components. To cure earth the cases.
Frank