50Hz hum when switching on pre amp

[Sean de Silva]

A couple of days after cyclone tauktae had wreaked havoc in Mumbai, I switched on the system only to hear a weird disturbance on the 3050i that I’ve never heard before. I usually switch on the power amp followed by pre amp. With only the power amp turned on there was no hum. As soon as the pre amp was turned on the disturbance appeared on both speakers. Sources not turned on. Turning up the volume dial did not increase the amplitude of the disturbance so that was a relief. Trying to figure out what this could be I then realized I was hearing the ac supply 50Hz hum! Exactly the same sound as this yt vid

My first assumption was this hum has something to do with Mr Murthy’s power cables and/or the Mogami ICs shielding/soldering. So I immediately got in touch with him and he asked if power cords and ICs were in close proximity of or touching each other which is the case. He then advised troubleshooting steps to pinpoint the source of this annoying hum. Last evening I switched on power amp followed by pre amp and lo and behold the hum on the speakers has mysteriously and miraculously disappeared! All that I hear when I press my ear to both tweeters is a very soft hiss which has always been there. When I told Mr Murthy what had just happened and that I didn’t fiddle around with or swap any cable he congratulated me on the disappearance and said that this hum could have been due to faulty ac supply. It goes without saying that if this reoccurs I will have to swap power cords and ICs as he has advised.

Hence the perplexity.. has anyone experienced something like this happen in your system where this ac hum suddenly appears and disappears?

 

[ssf]

Should not the pre amp be turned on first then the power amp ?

Likewise, the power amp should be turned off first then the pre amp.

This of course has no relevance to the hum but it is the recommended procedure.

 

[RajithKumar]

Should not the pre amp be turned on first then the power amp ?

Likewise, the power amp should be turned off first then the pre amp.

This of course has no relevance to the hum but it is the recommended procedure.

Yes this is procedure I heard and I'm following.

 

[RRR]

Any reason for you to switch on the power amp first ?
The following sequence needs to be followed for switching on and reverse for switching off :
Source~Pre~Power.
Since the Source is normally very low powered it does not affect much.
Whenever i switch on Pre after Power, i always hear a thump or buzz.
It is always advisable to switch on the Pre first. This should take care of other things like power cords being close to each other.

 

[Sean de Silva]

Any reason for you to switch on the power amp first ?
The following sequence needs to be followed for switching on and reverse for switching off :
Source~Pre~Power.
Since the Source is normally very low powered it does not affect much.
Whenever i switch on Pre after Power, i always hear a thump or buzz.
It is always advisable to switch on the Pre first. This should take care of other things like power cords being close to each other.

I'm aware of the "conventional" sequence for switching amps on and off alongside other gear. Still doesn't explain the mysterious appearance and disappearance of ac hum.

 

[Deleted member 15865]

Probably the unplugging and plugging back of the interconnects rectified a loose connection? It’s happened with me (though I don’t know the Hz of the buzz/hum I experienced).

 

[Sean de Silva]

Probably the unplugging and plugging back of the interconnects rectified a loose connection? It’s happened with me (though I don’t know the Hz of the buzz/hum I experienced).

I haven't touched any ic or power or gnd wire at all. Hence mystified! Hopefully it doesn't reoccur but if it does then it will involve step by step troubleshooting to locate the culprit.

 

[jenson]

with your eg. One more avenue to test opens up which is, test the system at a different plug point in house..especially if the house has two phase connection

 

[Sean de Silva]

with your eg. One more avenue to test opens up which is, test the system at a different plug point in house..especially if the house has two phase connection

We have single phase only

 

[DB1989]

with your eg. One more avenue to test opens up which is, test the system at a different plug point in house..especially if the house has two phase connection

We have single phase only

Hey Sean,

It seems it Does not necessarily have to be a dual phase connection. But as Jenson mentioned, sometimes selecting a different plug point helps. I had a very similar experience as yours when i was using multiple plug points to drive the various components in my chain. One fine day, out of the blue without any warning, i started noticing an audibly loud hum without any change in the manner in which i was running my gear. Then started the arduous task of reconnecting all the components and nothing seemed to work. As a measure of last resort, i unhooked all my sources which weren't even switched on, specifically my Xbox One, and that is when the hum diappeared. To clarify, only the switch at the power outlet was flipped on but the Xbox was powered down. Turns out, whether i have the xbox or any other gadget connected to that power outlet or not, simply flipping the switch on results in the hum.

Why? I have no clue. But that particular outlet is kept switched off at all times now. Maybe somebody else who has a better handle on things can explain.

 

[jenson]

One other thing about different plug points is, earthing... In an older setup, like clockwork music use to pan to left, had to toggle the volume at higher levels to get it back.. It was a mess, I thought it was the pre... Switched it to a different plug point at home and voila..problem solved

 

[ssf]

I had a similar problem recently. When I was turning on my Sub, it was giving a thud. After reading @DB1989 's post above, I thought I will try a different point. No thud at the new point. Measured the old point. Earthing was faulty.

 

[Sean de Silva]

One other thing about different plug points is, earthing... In an older setup, like clockwork music use to pan to left, had to toggle the volume at higher levels to get it back.. It was a mess, I thought it was the pre... Switched it to a different plug point at home and voila..problem solved

I have 9 sockets in the vicinity of the hifi unit. All earthed and tested fine!

 

[Sean de Silva]

Should not the pre amp be turned on first then the power amp ?

Likewise, the power amp should be turned off first then the pre amp.

This of course has no relevance to the hum but it is the recommended procedure.

I plugged the pre amp into a different socket board in the vicinity and have now gone back to the standard way of switching i.e. pre amp on before power amp and power amp off before pre amp. Just when I had reconciled to the painful task of troubleshooting to find the culprit, there was no hum at all while listening to Al Jarreau yesterday. To say that this is a blessing is an understatement. Will be a pain if it suddenly reappears. I suppose the tv was the source of the hum as the fiio d03k was usb powered from the tv. Either way I have disconnected the tv-dac-pre due to random network issues. Am only too thrilled to revert to good ol' physical discs

 

[Sean de Silva]

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I have tried setting this to 100 but it doesn't work. The default stays at 0 and I have to manually increase on android spotify app as mentioned earlier which is not a big deal. Anyways I haven't used the Boss 2 player after the MoOde upgrade as another annoying issue had suddenly popped up and I haven't got down to troubleshooting yet.

The return of that dreadful annoying 50Hz hum. This time when switching on pre / power amps and Allo Boss 2. I had recently installed new GM switch box modules. The amps were connected to the left box while the Boss was connected to the right box. While both switch boxes are on the same line, I guessed that this hum might be caused by a classic ground loop. So I experimented by connecting the Boss 2 to the left box i.e. same box along with the amps and lo and behold... the hum has gone away and hopefully for good ! I am relieved and back to music land.

 

[Hari Iyer]

Tangent POV. Could be a dry solder on the filter capacitor of the PSU. Swapping connectors don't give a 50 Hz hum but an RF kind of buzz.

 

[Subbu68]

The return of that dreadful annoying 50Hz hum. This time when switching on pre / power amps and Allo Boss 2. I had recently installed new GM switch box modules. The amps were connected to the left box while the Boss was connected to the right box. While both switch boxes are on the same line, I guessed that this hum might be caused by a classic ground loop. So I experimented by connecting the Boss 2 to the left box i.e. same box along with the amps and lo and behold... the hum has gone away and hopefully for good ! I am relieved and back to music land.

View attachment 68863

If you don't have a single point grounding, ground loops can cause this hum. No need of music signal. A single earth should return to the main socket of your house wiring. All my gear is on one earth bus of a power strip.

And a ground for the sub-woofer plate amp was necessary from this bus since the sub is connected to the speaker terminal of the amp and it has a two wire power chord

Also isolated the TV input to preamp. It was rewired with it's optical out and a cheap DAC. The DAC when powered by TV USB I had this hum. An old Apple USB charger now powers the DAC. That resolved the issue for me.

 

[Subbu68]

I had a similar problem recently. When I was turning on my Sub, it was giving a thud. After reading @DB1989 's post above, I thought I will try a different point. No thud at the new point. Measured the old point. Earthing was faulty.

My sub takes its signal from the amp. Switching off the sub and amp together used to give me this thud. Using a IFTTT dual switch and a sequential switching now sub is off and then amp after 10 secs. no thuds.

 

[ssf]

My sub takes its signal from the amp. Switching off the sub and amp together used to give me this thud. Using a IFTTT dual switch and a sequential switching now sub is off and then amp after 10 secs. no thuds.

My sub had a fault in its plate amp. It stopped working altogether sometime back. I am now using my Crown xls 2502 to drive it.