Avoiding amplifier input buzz when jack is disconnected

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eduardosm

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I have built amplifiers with different ICs. With all of them, when I have the input jack disconnected, I can hear a buzz if I touched it (I know that it comes from the AC wiring). It also hear it very loud it while I am connecting the amplifier to my computer.

I have seen that many battery-powered (so, they have a built-in amplifier) portable speakers do not produce any noise. These speakers are not screwdriver-friendly, so before buying one to scarify it, I would be grateful know if anyone here knew how they avoid the buzz.
 

They avoid it because the buzz is created by the potential difference between the AC power ground on the amplifier and the ground potential of the devcice you are connecting to or your body. On battery powered equipment the whole amplifier assumes the potential of the device you connect to so apart from a crackle as the voltages equalize there is no buzz signal to amplify.

The buzz is normally at twice your local AC mains frequency so it will be predominantly 100Hz or 120Hz depending on where in the World you are. The only sure way to avoid it is to prevent the voltages meeting so you have to provide an isolation barrier to that lets the signal you want to amplify through without any direct connection. You can do this with a signal isolating transformer (using magnetic field coupling) or with an opto-isolator (light beam coupling). Both are used commercially and cost about the same to construct.

Brian.
 


Thanks for you reply.
I tried powering my amplifiers with a battery, but I still got audible buzz (but lower). The portable speakers y tried could be charged using a wall adapter (USB). They did not play any buzz at all even when they connected to the wall adapter. I wonder if they use an isolating transformer. I have looked for isolating transformers and are all quite big to fit two in a small case.
 

betwixt said:
I use audio isolating transformers that are 15mm cubes, I think you are looking at AC power isolating transformers which are much bigger.

Brian.
Click to expand...

There are tons of audio transformers available. Have you any recommendation for line input? Thanks.
 

betwixt said:
This is the sort I mean: https://uk.farnell.com/neutrik/ntl1/transformer-1-1/dp/8314659 although that price is very high. It might be cheaper to use an opto-isolator instead.

Brian.
Click to expand...

Thanks. They are quite expensive, but they a much better frequency response than cheaper alternatives I have seen. Is it an opto-isolator suitable for audio taking into account that it is an signal that goes from positive to negative? They also may not get enough power directly from the audio signal.
 

There are different situations where "buzz" or hum can be expected:
- ground loops that are created between two devices involved in an audio transmission which have both protective earth connection, e.g. a PC (always PE connected) and a TV with antenna connection
- long audio cables with single ended signal connection
- amplifiers without sufficient shielding
- amplifier power supplies (either built-in or wall adapters) with poor filtering

It's not clear to me which problem we face in the present case. As far as described, I don't see a point where audio transformers would solve the problem.

"buzz when jack is disconnected" would primarly suggest insufficient shielding
 

I think the OP is referring to a buzz when touching the 'live' input pin on a jack plug and the louder buzz when inserting a plug and the input has connected but not the ground. As it only happens on AC powered amplifiers it must be capacitive coupling (body as an antenna) or a ground loop when the plug is being inserted.

I have used linear optocouplers in both audio and video applications with great success. You do need a power source for the LED but I use isolated DC-DC converters for that. I use them where a PC audio/video source is connected to broadcast TV studios where SMPS noise and high voltage isolation is necessary.

Brian.
 

I think the OP is referring to a buzz when touching the 'live' input pin on a jack plug and the louder buzz when inserting a plug and the input has connected but not the ground.
Click to expand...
Does he? I don't think that this problem would be worth to start a thread at edaboard. The usual solution is to insert the plug last at the amplifier side.
 



Most amplifiers have the buzz when the jack is disconnected if you touch it. What most concerns me is the loud noise I get when I connect my amplifiers to the computer.
If I have my laptop without the AC adapter, I do not get almost any noise during the connection. If I have the AC adapter, I get the noise. It louder if I use an AC-DC supply to power the amplifier, but it is still quite loud if I use batteries to power it. With an amplifier that didn't build, I get noise if I touch the jack, but not while I connect it.

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FvM said:
Does he? I don't think that this problem would be worth to start a thread at edaboard. The usual solution is to insert the plug last at the amplifier side.
Click to expand...

I have the same result connecting the cable first to the computer and then to the amplifier.
 

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