Pass X250.5 right channel hum problem - diyAudio
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Old 1st July 2006, 09:10 AM   #1
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Default Pass X250.5 right channel hum problem

Nelson this is a strange problem, without any inputs attached the amp is dead silent out of both channels, as soon as one of the many pre's and a passive we tried, be they balanced or single ended, there is an audible annoying hum from the right channel, we tried to disconect the earth from the pre's with no help.
The amp sounds fine either in balanced or single ended, but the hum has me stumped, and it's just loud enough to be heard from the listening chair during quiet passages, Nelson have you a simple fix for this?

Cheers George
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Old 3rd July 2006, 07:06 PM   #2
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I have also heard from the customer on this. The 250.5 has
slightly more hum on the right channel, about 250 uV as
opposed to the left channel at around 200 uV. This is due to
its closer proximity to the AC line input, and is most apparent
with RCA cables. In practice we find that the issue is minimized
by twisting the RCA cables helically between the amp and
preamp, and of course balanced lines takes care of it well.

As I understand it from the customer, this problem showed up
after the amplifier was modified by inserting more resistance
in series with the input. I has been my experience that doing
so will not increase the noise, so I wonder if something else
has crept into the equation, but I don't have any better advice.
Ordinarily at this point we like to see the amplifier for ourselves.
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Old 25th July 2014, 08:04 AM   #3
VHF man is offline VHF man  Australia
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I purchased a X250.5 last year and I noticed the same problem.

However, it's only a problem when using the unbalanced input, and since I use balanced it's not a problem for me.

On thing I've noticed is that the right channel uses an earth lift resistor, whereas the left channel does not.

I've measured to confirm this, and visually there is indeed a 10 ohm resistor in series with the earth, but only on the right channel.

Note: When I short out the 10 ohm lift resistor the hum disappears.

Last edited by VHF man; 25th July 2014 at 08:11 AM. Reason: update to problem2
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Old 1st October 2014, 01:10 AM   #4
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Hi VHF,
I think i have the same issue as well, but the hum is mainly from both speakers whenever the amp is hooked up with Pre amp via RCA ports.
I did notice there's a little resistor soldered at the rear left RCA, not quite sure whether it's the same thing you described.

Click the image to open in full size.

Hope to hear from you soon.
I have PM you on this as well.
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Old 1st October 2014, 04:24 AM   #5
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That is not stock, and it might be the source of the difficulty.

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Old 14th May 2015, 06:50 PM   #6
rubli is offline rubli  Mexico
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Default X250.5 hum annoying... HELP

Help !!
I am kind of desperate here...

I have a X250.5, with the HUM claim.

I have the jumpers on the XLR's. without connection on the RCA's, everything is fine, as soon as I connect it to the preamp, lots of hum in both channels.

but wait, I'll take the preamp out of the equation, I put a straight RCA cable between left and right of the amp, and there is the noise !!!!!

If I put a jumper only on the grounds of both inputs, no noise, whenever the hot inputs are somehow close, this annoying hum appears...

please help me out !!
thanx
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Old 14th May 2015, 09:31 PM   #7
Zen Mod is offline Zen Mod  Serbia
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just one question - is everything stock inside ?
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Old 14th May 2015, 09:51 PM   #8
rubli is offline rubli  Mexico
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yeap !!!

maybe this is interesing too:

at the beggining I thought it is a ground issue, as if I shorted the grounds (at each RCA's) the hum appeared, so I bypassed the 10 ohm resistor from ground to the connectors ground, and put a 10 ohm resistor at the other channel, between ground and the rca's ground, and the noise dissapeared.

but for my surprise the noise is much louder as I connect both lives togheter.

(it does not matter the resistors to ground, if I bypass them or not)

I am really annoyed

thanx for replying

regards

Alexander
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Old 14th May 2015, 10:41 PM   #9
wayne is offline wayne  United States
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The X250.5 is built a bit different since it has two boards VS one for our other amps. In some situations it can have some hum on it usually on the RCA inputs. I have a really good fix for this but it does involve pulling the power supply board out and some soldering but so far always works. Best done at the factory
but this is DIY.
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Old 14th May 2015, 11:42 PM   #10
rubli is offline rubli  Mexico
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Quote:
Originally Posted by wayne View Post
I have a really good fix for this but it does involve pulling the power supply board out and some soldering but so far always works. Best done at the factory
but this is DIY.
By all means, please share your knowledge , PLEASE...

I was about to put a thick wire between the ground connections, the ones that come from the PS, and strap both PS ground cables on the middle of it, but I will await for your comentaries..

muchas gracias !!!!!
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