Occasional hum

[CageyH]

Thanks Jim.
It is a 100Hz and 200Hz noise, so for me, it is mains based.
I don’t use a fixed phone anymore, so the base stations have been unplugged a long time ago.
The internet router has been turned off, and it made no difference.
I tried all of this when the family was out, and also turned off the mobile phones.

The noise was still there.
It is intermittent, and low level. My money is it being my air source heat pump, so I need a way of filtering this noise.

[CageyH]

I have been reading up about this, and it seems that this is due to the full inverter in my heart pump, injecting harmonics back into my mains, and it is suggested that a line reactor may fix it.

I thought lune reactors were a three phase thing? Any thoughts on this?

[Barry]

I assume "heart pump" is a misspelling of heat pump.

A mains conditioner will be of little help as they have to pass the mains components of up to a few hundred Hz to allow rapid transient switch-on rise times.

It would seem that your heat pump uses a variable speed motor (VSD) and use an inverter, which as you have found out will cause switching spikes at harmonics of the nominal mains frequency. I think you will need to consult a qualified electrician for advice, as anything designed for audio use will be inadequate.

[CageyH]

Heart pump is indeed an autocorrect.

I am looking at solving the issue, rather than curing the effect, so I think you are correct. A qualified electrician will be required

[CageyH]

I recently addded a common mode noise filter to the BB3, as I had a PCB gathering dust.
It is now silent, but the heat pump only heats the hot water now, so is not on for so many hours a day.

The original case is now pretty full, so I am going to rehouse the BB3 in a bigger case that will give me better separation between components in there.

[CageyH]

https://www.diyaudio.com/community/t...-noise.412589/

An interesting article.

[Barry]

https://www.diyaudio.com/community/t...-noise.412589/

An interesting article.

Agreed - but nothing in it that should come as a surprise. It also 'criticises' Shaffner filters which is unfair if they are being used outside their specification.

Regarding Schaffner's CM and DM performance figures as a function of frequency, being only measured for terminating impedances of 50 Ohm, however if you delve further you can find similar measurements for a source impedance of 0 Ohm and an open circuit load. The IEC assumes the mains source impedance is 0.4 + j0.25 Ohm (IEC 725:1981).

The author hints that at high frequencies (that is above 50Hz) the mains distribution system ought to be treated as an RF transmission line. Well asuuming the dielectric insulation is PVC (Dielectric constant = 3.2) then the wavelength/2*pi at 20KHz is 33km, so assuming the distance to the nearest substation is no more than 1km, then this is 0.03 of a guide wavelenth/2*pi, so transmission line theory does not apply.

But the author is correct in that filters should not be cascaded - they will interact, and quite often with reduced overall performance.

I have an Olsen 6-way distribution strip that is fitted with a Schaffner 15A filter, which I used with my audio system without any problem.

[CageyH]

The new case arrived today.

PXL_20240729_082234678~2.jpg

It is bigger than the last one, as I am going for greater separation between the power and phono PCB this time around.

It is a full width case, and a little deeper than the one I used originally. This should make the build easier, and hopefully neater.

[CageyH]

Progress:

PXL_20240729_182410069~2.jpg

Power switch and RCA socket holes drilled out by hand.

[CageyH]

New case layout options:





Or





Which do you think will be best?