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Old 21st February 2013, 01:57 PM   #1
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Default Please help with hum problem..

Hoping someone would be able to offer some advice to a beginner. I expect most people on this forum are quite advanced. I have an audioromy fu29 which I think sounds fantastic. I've recently moved my speakers around and am no sat much closer to them and there's a fairly noticeable hum from both (~50hz - in the UK). I think it's been there since I bought it but it didn't bother me as I was further away.

I don't believe it's a ground loop as the amp is plugged into the same mains plug as everything else. Also, whether the source is connected or not makes no difference. Changing the volume control makes a small difference but it's clearly audible on minimum. I've also noticed the hum is loudest when the amp is turned on but settles after ~30s.

Is there an easy solution to this? I have a multimeter but not very experienced and a bit wary about poking around in a valve amp. I presume a mains conditioner is only to help?

Many thanks for any advice.
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Old 21st February 2013, 03:21 PM   #2
DF96 is offline DF96  England
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Hum which gets better as the amp warms up is probably 100Hz supply rail ripple. The push-pull output stage will partially cancel this, but the cancellation can be poorer if one side of the output warms its cathode slightly faster than the other.

It could be a sign that the power supply capacitors are drying up, or that the output valves are wearing out and becoming unbalanced. How long have you had the amp?
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Old 21st February 2013, 03:38 PM   #3
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Thanks for the reply. Yes, it could well be 100Hz. I should try and measure it. I said 50Hz in my description but I was just assuming that as it sounds like the typical mains hum I get from other devices, but wanted to make it clear I wasn't talking about buzz which I believe is something different.

I've had the amp 6 months with fairly standard usage. I wouldn't be surprised if valves need replacing or tweaking - in fact I had to replace on of the small 6N1 tubes at one stage as there was a very loud buzz which suddenly developed and I did this on the advice of the seller - this solved the problem. The hum though I'm fairly sure was there when I bought the amp. However I accept as these amps are relatively cheap, you accept they might not be as reliable and they might not be optimally set up in the factory.

The valves I would be able to replace easily assuming they wouldn't need to be rebiased. Replacing the caps I'd be a bit wary of.

Looking on forums some people mention the transformer as a possible source of hum, but you don't think that's likely right?
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Old 21st February 2013, 05:56 PM   #4
DF96 is offline DF96  England
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The mains transformer may be a source of mechanical hum, but from your description that is not what you have.
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Old 21st February 2013, 06:18 PM   #5
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So the best thing for me to do would be to just replace the valves? Anything I can do first to test it before shelling out for new ones? We are just talking about the FU29 output valves I assume.

Thanks
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Old 21st February 2013, 08:34 PM   #6
DF96 is offline DF96  England
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To test it you need test equipment and the knowledge to use it safely.

Is the hum the same on both channels? If not, you could try swapping valves and see if the hum moves or stays the same.
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Old 21st February 2013, 08:43 PM   #7
smbrown is offline smbrown  United States
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The hum started after you moved your speakers, maybe you're now sitting in a place where the room reinforces the perception of the hum (bass does funny things based on room shape, furniture placement, etc.). Maybe try readjsusting your speakers?
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Old 21st February 2013, 08:53 PM   #8
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Quote:
To test it you need test equipment and the knowledge to use it safely.

Is the hum the same on both channels? If not, you could try swapping valves and see if the hum moves or stays the same.
It's the same on both channels I'd say. I might try swapping them around anyway just to see.

Quote:
The hum started after you moved your speakers, maybe you're now sitting in a place where the room reinforces the perception of the hum (bass does funny things based on room shape, furniture placement, etc.). Maybe try readjsusting your speakers?
I'm sure that is the reason I'm noticing it more now, I'm much closer to the speakers and in a different room, which unfortunately I have to stay in.. The hum was present before but less noticeable because of positioning. However, there was absolutely no hum when I used a different amp (a tripath chip amp) to compare, so it is definitely specific to the valve amp. I would use the chip amp but it causes another problem with my subwoofer when that's also connected... So for now I'm going to try and stick with the valves...

Thanks for both replies.
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Old 4th March 2013, 03:33 PM   #9
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Unfortunately getting new valves (GU29 to replace to FU29 power tubes, I think they're equivalent) made no difference. Neither did using a mains "power conditioner" (as expected). Any ideas what to try next?
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Old 4th March 2013, 04:01 PM   #10
DF96 is offline DF96  England
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It might be helpful before going any further to know whether we are fault-tracing on a broken amp, or critiquing the design of an inadequate amp. Does anyone else have one of these, and do they get hum?
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