# Mild buzzing sound from power amp (unit itself)??



## dusdau (Feb 9, 2013)

I have an Adcom GFA-7500 power amp, and noticed yesterday there is a mild, buzzing sound coming from the unit (not through the speakers). Any thoughts on what it could be??

I unplugged the power cord from power strip, and plugged directly into the wall. There was no buzz at first, but then after a few seconds, it would start up. I then moved it back to the power strip, and it buzzed right away when I turned it on. I turned off unit, then back on, and it was silent for a few seconds, and then started buzzing again. So, in other words, doesn't seem to be a correlation...just randomly buzzes right away, or after a few seconds.

I also rearranged cords just in case, and same random results. It's only audible from about 6 feet in or so, so I'm not worried about it if it's not some huge issue, or an indication that it getting ready to 'blow up'. Seems to work fine, and sounds great otherwise. Anything I can do to discern the problem if I need to?

Did some searches already, but result all over the place...transformer, caps, etc. Thanks for any advice.

Dustin


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## Peter Loeser (Aug 11, 2012)

My initial guess would be transformer hum. Any vibration from the transformer (due to age or initial quality) will resonate through the chassis of the amp, making it audible. I don't think you need to worry about it blowing up 

Others with more expertise can add more insight.


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## Stereojeff (Apr 23, 2012)

Hum that fluctuates--goes away and comes back--or changes character is almost always from DC on the AC line.

Jeff


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## dusdau (Feb 9, 2013)

Peter Loeser said:


> My initial guess would be transformer hum. Any vibration from the transformer (due to age or initial quality) will resonate through the chassis of the amp, making it audible. I don't think you need to worry about it blowing up
> 
> Others with more expertise can add more insight.


Thanks, kind of what I'm leaning towards. It was at end of the evening, so I didn't spend a lot of time looking at it all. Check it out more tonight. It's a little long in the tooth, so that would make sense.


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## chashint (Jan 12, 2011)

Stereojeff said:


> Hum that fluctuates--goes away and comes back--or changes character is almost always from DC on the AC line.
> 
> Jeff


Well this is certainly interesting. 
You are going to have to go into some detail on this and I am anxiously awaiting your answer regarding how DC gets onto the AC power line.


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## Glen B (Jun 11, 2013)

chashint said:


> Well this is certainly interesting.
> You are going to have to go into some detail on this and I am anxiously awaiting your answer regarding how DC gets onto the AC power line.


Electrical devices that load the AC powerline asymmetrically (on ½ of the sinewave) can create DC offset on the line. Common culprits are halogen lights, fan speed controls, light dimmers, hair dryer/curling iron heat level controls, etc. These devices often use a thyristor or (voltage drop across) a single diode as the speed or heat controller. Toroidal transformers are notoriously susceptible to DC offset. The DC causes the transformer to go into saturation, and in an attempt to recover, it buzzes audibly. 

It takes only a few hundred millivolts of DC to cause this saturation noise. The solution is either to eliminate the source of DC as a first step, and if that cannot be accomplished, employ a DC blocking circuit. A number of high end amplifier manufacturers include DC blocking circuitry in their products, including Classé and Bryston. Even designers like Nelson Pass and Ralph Kartsen of Atma-Sphere Music Systems recommend DC blocking as a solution. 

Here are links to some discussion on Audiogon Forums. I post there as "Gbart".

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1247693407&openflup&23&4#23

http://forum.audiogon.com/cgi-bin/fr.pl?htech&1302916061&openflup&32&4#32


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## chashint (Jan 12, 2011)

I am still not buying into the DC on the AC line voltage, but from the trail of bread crumbs you left this is the basic circuit I have found that is referenced as a DC blocker or a DC mains.
http://www.diyaudio.com/forums/atta...s-what-right-main-dc-filter-10-variations.pdf 
It took a little more digging but I found this schematic posted which is actually labeled.
http://www.diyaudio.com/forums/atta...ry-special-thule-audio-inrush-current-lim.pdf 
I see exactly how this would work as a in rush current limiter, but you are going to have to walk me through the DC blocking function.


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## Glen B (Jun 11, 2013)

If you're not buying, why should I or anyone else waste their time selling ? :sarcastic:


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