Amp emits deafening hum when I switch the kitchen light off! - diyAudio
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Old 6th September 2008, 06:23 PM   #1
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Default Amp emits deafening hum when I switch the kitchen light off!

Sorry to start another hum thread but I think this hum may be a bit out of the ordinary.

I've got two Cyrus Two amps, both bridged to mono, one driving the left channel and one driving the right. Each is connected to its own Cyrus PSX power supply.

The problem is, on the one channel, I sometimes get an extremely loud, frighteningly loud hum when something in the near vicinity is switched off or on. Examples: when I switch the other amp off or on, when I turn the CD player on or off, when I plug my record cleaning machine into the same 6-gang extension and once, according to the missus, when she switched the nearby kitchen light off.

The hum is so terribly loud I've always rushed to switch off the faulty amp as soon as it starts so I haven't yet discovered whether it varies with the volume control or the mute switch, but I suspect it doesn't. It always seems to be at the same volume, regardless of anything else.

Also, the amp makes a very loud click when I turn it on. Actually, when I say turn it on, I mean turn the PSX on, because the power comes from the PSX and it's the PSX you turn on and off.

I'd be very grateful indeed if anyone could suggest what's at fault.
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Old 6th September 2008, 06:39 PM   #2
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Very strange one, sounds like some kind of oscillation being triggered. Maybe a dry joint on the amp board or faulty component.

A bit random but try running the suspect amp without the PSX, just to eliminate that.
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Old 7th September 2008, 07:59 AM   #3
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Thanks, I will try that, or else swap the PSXs around to see if it's the amp or the PSX at fault.
Meanwhile it's getting even more evil. Last night the hum happened when I switched off the faulty amp itself: in the following few seconds it used all the rapidly fading power at its disposal to emit the same horrific hum. This time I had turned down the volume to zero and switched the input source to mute so I can know say the hum is independent of both.
Anyone have any ideas?
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Old 7th September 2008, 08:24 AM   #4
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When it's on and not making a noise ...

Give the amp and the PSU a big thump !!

That should rule out any bad connection / dry joint


Andy
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Old 7th September 2008, 08:58 AM   #5
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Default The decay of oscilating frequencies of your ligthing bulb (electronic?)


can generate magnetic field into your home wiring..... your amplifier can
be producing huge amplification into this signal, and this amplification can be so huge that can drain your supplie's electrolitic condensers charge...then the supply without good filtering will send mains frequency (hum) into the amplifier sections that will send them to the speaker.... reason why the short timing hum you may be listening.

so.... for small moments, your amplifier may be producing full power into this non audible frequency.... and this may drain the supply filter condenser charge... having not those condensers..you have something alike pulse of mains frequency or double the mains frequency beeing sent to your audio circuits... because powering your amplifier circuits.... audio circuit will amplify this, or, at least, will let this signal goes to the speaker.. as speaker is able to reproduce, then you have sound.

This is possible .... now a days, transistors are able to work into 100 or 150 or 200 Megahertz and they are applied into circuits that will work from 20 Hertz to 20 Kilohertz, (Bureaucratic audio range).. and this is the result...some of them turns into Broadcasting amplifiers, broadcasting receivers of Frequency modulated stations... they are able to work into higher operational frequency than the ones you can listen.

Thanks to transistor factories... they are producing high frequency components, also because market is asking for that...thanks for the designers that wants "more linearity" into 20 kilohertz using parts that are able to work (oscilate) into 200 Kilohertz.... and this pushes the factories to follow the market needs..to offer high frequency transistors.

When you switch "off" electronic ligth bulbs, there are a fast drop of voltage... because they operate from Dc rectified from the mains and charged into a condenser...switching off the mains...this Dc drops fast into the zero volts direction....and those lamps works oscilating into high frequency, this induces higher voltage needed to produce the plasma into the lamp tubes.... but have enormous harmonic contents together the main frequency generated by the lamp... when the power goes off, the frequency comes into a frequency decay... a sweep down of frequencies and harmonics goes sweeping down with harmonic covering a big bandwidth... those lamps has onle "leg" (one wire connected) into the mains wiring... this "leg" of wire turns an aerial (behave as transmitting antenna) as it is in contact with the oscilating circuit, and this irradiates electro magnetic energies.... goes into the air and goes travelling into your home wiring.

This is a possibility, of course your problem can have other sources...other reasons can cause your problem, for sure..but this one is also possible as i had in my home this signal, that was detected into a HF (less than 30 Megahertz) transceiver as signal S plus 30 (very strong).

If this problem is found... and you cannot destroy your lamp bulbs, then have to build a mains filter, with inductors using ferrite core... twisting AC cable around and inner a toroidal ferrite core...have to install high voltage condensers draining some AC energy and capacitors into your audio sensitive circuits too.... also your DC supply may need some condensers and inductors.

But the best solution is to accept this noise and to be happy with it.

Is it really one channel or this "one" channel is just louder than the other seeming to be the single one to operate?

regards,

Carlos
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Old 7th September 2008, 10:22 AM   #6
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Thank you very much for those very interesting ideas Destroyer X but as you point out, if it is only in one channel, this does seem to suggest that the problem is internal.

I have now tried swapping over the PSX power supplies and I think this narrows it down to the amp, not the PSX. I haven't had a repeat of the hum yet but the other symptom is when you switch on the power supply there is a very loud initial click followed by a whump as the amp powers up. This is still coming from the same amp regardless of the fact that the PSXs have been switched over.

When the hum comes, by the way, it is at quite a low frequency. I'm guessing it's mains frequency.

Poynton - your suggestion certainly has the virtue of simplicity . Although, the amp has recently been on a journey no doubt involving repeated thumps which seem to have made the problem slightly worse rather than better.
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Old 7th September 2008, 12:03 PM   #7
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Default I see... all i can is to wish you the best possible luck debugging it


I hope you will find the problem... a really big one as i could see.

regards,

Carlos
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Old 7th September 2008, 03:04 PM   #8
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Thank you Carlos. Poynton, I just got the hum again and gave the top of the amp casing a sharp tap with my knuckles, whereupon the humming stopped! I'm not sure whether this is good news or bad news - does this mean I have to re-solder every single joint in the amp?
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Old 7th September 2008, 03:59 PM   #9
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It's a start anyway...

Take the case off .. after unplugging of course.

Look for an obvious bad joint.

If it is not obvious, reconnect (taking care not to short stuff out )and gently tap components one by one until the noise starts or stops. You can narrow down the search area this way. I would guess that it is near the input.

Then solder..

Andy
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