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Old 15th March 2014, 10:01 PM   #1
JdAo is offline JdAo  Portugal
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Default Help with volume independent humming...

I few months ago without much time and some health issues I decided to buy a premaded headphone preamp and ended up getting a FidelityAudio (UK based company) HPA-100. I only had the chance to test it more seriously a few weeks ago and it's showing a slight humming that varies a bit with the pair of headphones used (but is always present). The worse case scenario is a very, very, and I mean VERY old pair of Onkyo headphones, they sound terrible per se and are circumaural.

Also to my surprise the humming is completely independed of the volume control and if I connect the headphones to other equipment's headphone outputs (preamp / power amp, even an old boombox!) this type of humming is not present with any headphones.

RCA sockets are connected.

I'm out of ideas and don't want to spend a fortune on shipping the unit around to them and back....
Can anyone help ?

Thank you!
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Old 15th March 2014, 10:03 PM   #2
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Got a pic? I'd look for grounding issues.
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Old 15th March 2014, 10:35 PM   #3
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Looks like the amp has an IEC power socket and may have signal ground connected to safety earth. Try some source that is known to have a floating ground - like most any ordinary CD player or a mobile player or cellphone. (Or don't connect anything at all, or better short the input.) In short, anything but your computer.

If the hum does not occur with these other sources, the problem is a ground loop and needs to be addressed accordingly.

If the hum remains present whatever you do, it's probably inherent - either a power supply rejection problem or a defect. Nothing that should be present in a 350 amp, anyway. In this case I would suggest something like a Lake People G103-S.

Those old Onkyo headphones would not happen to be 8 ohm parts? They might just be the most sensitive of the bunch and hence show the issue most clearly.
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Old 16th March 2014, 12:06 PM   #4
JdAo is offline JdAo  Portugal
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I don't understand how this could be an inherent problem when this amp got a 5 star review on WhatHiFi ?!!
Thank you for your answers. I've tried with other sources that definitely don't have an earth connection on the mains socket, same result. Even without anything connected to the RCA sockets the humming is exactly the same.

One thing I remembered is that it has pass through RCA connections. Could the output connections be getting some signal internally.

So it's either the design or some bad part.

Here are some internal pics:
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Old 16th March 2014, 12:14 PM   #5
JdAo is offline JdAo  Portugal
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Regarding the Onkyos, they are model PH-727, but I couldn't find any info about them anywhere, they're probably almost 40 years old. But the problem appears on other headphones too, just not so pronounced..
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Old 17th March 2014, 07:52 PM   #6
Bibio is offline Bibio  Scotland
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drop fidelity audio an email and explain your problem and i'm sure they will be glad to help.

also what is plugged into the headamp as source?

have you tried unplugging the source and if so is the hum still there?
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Old 20th March 2014, 01:12 PM   #7
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Fairly straightfoward circuit. Nothing that should have appreciable hum when designed and working correctly.

I would prefer to see the transformer wires twisted. The solder joints on the +/-15V regulators also are a bit odd-looking.
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Old 25th March 2014, 02:52 PM   #8
JdAo is offline JdAo  Portugal
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Thanks. Since this uses a toroidal should it still matter to twist the wires ?
Yes it's weird because I've taken it to other places including an hifi shop to test with other equipment and the hum was there, although not as pronounced TBH.

What about changing the voltage regulators, apart from probably lowering hiss, should it have any effect ?
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Old 25th March 2014, 04:21 PM   #9
Bibio is offline Bibio  Scotland
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is the capacitor marked in red supposed to be discoloured like it is?

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Old 15th May 2014, 10:22 PM   #10
JdAo is offline JdAo  Portugal
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Well, I've sent it to them for repair and upgrade and the problem continues! The amp sounds even more amazing after the upgrade, but I'd like to get rid of this problem which, to be honest is only heard when there is no music playing. They said they couldn't reproduce the problem

Is it possible for this to happen only in my home / county /country or whatever... ?
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