Interference picked up by amplifiers

[ropie]

I had this problem with my DIY chip amps in the area I live in now - this manifested itself as an electronic-sounding chattering noise coming through both speakers.

I swapped out my DIY amp and have been using a fairly nice 90s Technics integrated amp for the past few months instead. However, I noticed it is also subject to some kind of interference - however for this amp it manifests itself as a mechanical buzzing sound coming from inside the amp itself. As this was easier to live with than before, I have kept it in place.

The thing is, today I was cleaning behind the amp (the Technics) and noticed that if I rotate the amp, the buzzing noise stops Only a small rotation is required, maybe 10 degrees, and the noise almost totally disappears (a 95% improvement). If I rotate it back, the buzzing comes back. I know it's not caused by a nearby piece of equipment because I have ended up rotating the entire hifi stand and the problem has gone.

What's going on? It must be an airborne signal of some kind (ie: not coming directly through cables) but one that's enough to cause internal components to buzz?

[stratokaster83]

Hi Piero

Realistically, the only two things that can mechanically buzz in a solid state amplifier are either the transformer or the main filter capacitors, I cannot possibly comment on why rotating the amplifier reduced the buzz, I would just say it is very strange indeed!

Regarding DIY amps, I have noticed that most people don't pay any attention to RFI suppression in their DIY designs. Given that we live in a world full of cellphones, Wi-Fi routers and LED lights, every piece of audio equipment should have an RFI suppression network at the input. The chattering noise you describe is probably the sound of GSM/LTE envelope detection. It can be significantly reduced by adding several passive components to your chipamps.

[ropie]

Pavel, thanks. I took the lid off and tried to discern where the noise is coming from. Poking around with my finger I couldn't detect any vibrations on or near the transformer (it's a large R-core type, maybe these are more prone to vibrations) or large capacitors. Listening closely it was hard to say if the noise was actually coming from one part of the amp or another - I imagine whatever it is is being amplified by the casework. Lifting the unit at the front also reduces the hum, so there's definitely some interplay there.

The chattering noise you describe is probably the sound of GSM/LTE envelope detection. It can be significantly reduced by adding several passive components to your chipamps.

Would that be a zobel network? Until now I never had any major problems with interference on my chipamps in over 20 years (unless a nearby mobile phone starts ringing of course!) so always felt it best to leave such things off. I could be tempted to build a small external 'dongle' to add before the amp if that would work...

[stratokaster83]

Pavel, thanks. I took the lid off and tried to discern where the noise is coming from. Poking around with my finger I couldn't detect any vibrations on or near the transformer (it's a large R-core type, maybe these are more prone to vibrations) or large capacitors. Listening closely it was hard to say if the noise was actually coming from one part of the amp or another - I imagine whatever it is is being amplified by the casework. Lifting the unit at the front also reduces the hum, so there's definitely some interplay there.

If it's indeed the transformer, you could try tightening the screws that are holding it to the chassis. Also adding something pliable (like pieces of an old mousepad) between the chassis and the cover might reduce the audibility of vibrations.

Would that be a zobel network? Until now I never had any major problems with interference on my chipamps in over 20 years (unless a nearby mobile phone starts ringing of course!) so always felt it best to leave such things off. I could be tempted to build a small external 'dongle' to add before the amp if that would work...

No, Zobel should be used on the output to isolate the amplifier from capacitive loading and ensure its stability. I am talking about an RC low-pass filter at the input. This, for example, is from a line level preamplifier that I have designed recently:



C101 is a DC blocking capacitor, but R102 and C102 form a low-pass filter for RFI suppression. Pay no attention to the diodes, they are included merely to protect the opamp in case voltage at the input exceeds the supply voltages.

[wenig watt]

I had a similar problem that could not be solved. Sometimes it buzzed, sometimes not. Until one day I noticed that when it got dark in the yard the buzzing disappeared. It was a faulty neon lamp in the music school below me that had been emitting interferences into the system through the ceiling. Despite mains filters, metal housings and so on.
Your system behaves like a "tracking frame". Some transmitter couples into the components with optimum alignment.
Perhaps a powerful military or civilian transmitter in the vicinity?

Best regards

Arndt

[ropie]

Thanks Pavel, it looks simple enough to be worth trying. I'll take a look in my parts box to see what I have... 

Perhaps a powerful military or civilian transmitter in the vicinity?

Yes, could be

[stratokaster83]

I would add that this kind of RFI suppression only works for RFI picked up by the input cabling, it assumes that your amplifier has a nicely grounded metal case. If the amp is in a plastic or wooden box, it likely wouldn’t be effective since in that case the interference is picked up by the internal components of the amp.