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-   -   Pass X250.5 right channel hum problem (http://www.diyaudio.com/forums/pass-labs/82425-pass-x250-5-right-channel-hum-problem.html)

georgehifi 1st July 2006 09:10 AM

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

Nelson Pass 3rd July 2006 07:06 PM

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.

VHF man 25th July 2014 08:04 AM

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.


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