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Old 15th February 2008, 02:32 PM   #21
diyAudio Member
 
Join Date: Jun 2005
Location: Dallas, Texas
Marcel,

I started a thread last night that shows my grounding
arrangement. I don't know how to link it... but you'll see it
right away if you look for it. It's in the "Pass Labs" section of
the forums. I have no hum whatsoever now except out of one
speaker only - quite low level - when my preamp is at full
volume.

The grounding issue seems to cause a bit of consternation.
The problem is - when I do what the experts claim is the
only way that works, I have hum that is far far louder than the
signal in my amp chain. The papers I read also ignore the fact
that when you do what they say, the amps form huge ground
loops between the AC line ground, the chassis, and the cable
shields. I have to look more into it - I feel it's possible I'm doing
something incorrect, but I've yet to figure out what it is.

Congrats on your amp.... they are really nice aren't they....

W.
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Old 1st March 2008, 08:17 PM   #22
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Join Date: Jun 2005
Location: Dallas, Texas
OK so here's the latest in the saga.

The MC amp was a nice noise generator and the MM amp was
running fine, although the bias was not set right due to LED
having too high a Vf the last time I wrote.

So I changed out the resistors to get the current to the correct
levels. I also swapped the 22 ohm resistors on the MC front
end to 26.7 ohms.

I also changed around the grounding scheme to look like what
the various industry experts say to do:
1) pin 1 of XLR connected to chassis gound right at connector
2) chassis grounded
3) amps isolated from chassis ground and connected at a single
point through a thermistor a la the Pass products.

I also swapped out the twisted wires I had connecting my
input connectors to the amp boards for some shielded cables.

Now the amp is quiet and no hum on the MM input.

On the MC input, it's dead quiet IF I short out one of the 499
ohm resistors on the cascode - ie use the -4dB shorting plug.
When the plug is not in, there is a lot of high frequency screech
generated, and it goes away if I touch the shield of one of the
input RCA plugs.

What I have now is an amp that is simply unbelievable. The
limiting factor is by far the record itself. There is no sound or hum
or hash at all when the catridge is not tracing a groove. When
there is a record being played - you hear EVERYTHING. It is
an amazing experience to hear it the first time. THere are lots
of little details I've never heard before. The bass is very tight
and powerful. Everything else just seems to be very right.
There is lots of separation and the soundstage is as wide as my
room.

So I still have a bit of chasing down that noise at the front end,
but right now I'm going to just enjoy listening because it's been
a lot of work and the last little bit was not enjoyable while I was
thinking that there was an inherent problem with the amp and
I was not going to get it to work.
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