Fixed Hum : Dynaco PAS 3 using shielded cable to the RCA outputs

I've had some small hum on both channels that was constantly present. It was not linear with using the volume potentiometer, just a constant 60hz hum that was really distracting. I searched and found lot's of forums talking about ground loops and nothing really actionable on fixing the issue.

I finally fixed the hum today and wantd to share the good news. I replaced the wires to the output RCA jacks by using shielded cable from eyelets 14 and 7 on PC-5 to the right and left outputs, respectively. Also, the RCA jacks were grounded to the chassis and to eyelet 8. I removed the ground from eyelet 8 too.

Now, the hum is gone and sounds wonderful without that distracting hum.

You did, indeed, have a ground loop. That ground wire erroneously twisted around the output signal wiring was groundlooped due to the common ground at the linestage board. There is not supposed to be that twisted white ground wire around the output signal wiring; any hum induced in that undesired ground loop would have radiated directly into the output wiring and that's apparently exactly what was happening in your PAS.

gp49-unless i misread curcio's selector switch instructions-twisted pair left and right to preamp eyelets 7 and 14 respectively -grounds to 8-i have hum -never had it in a pas--along with i used roy mottrams i/o board and the resisitors on the i/o are omitted-with that combination there is no ground at all to the i/o board-incredible hum--with the 10 ohm and 510k in place-still hum -i jumpered from i/o ground to chassis and all was quiet-i see now why--my point is conflicting instructions-i think what joe and roy have in mind works with their total upgrades-but not if you are keeping original style boards-sooo!! put the 510k and 10 ohm in place unwrap the ground wires from 7 and 14 and run ground to eyelet 1 from i/o board--per original dynaco instructions howie

Hi, just wanted to mention that I found this thread quite helpful when i was connecting my stock line stage after other mods, it got rid of most of the hum that I had when fist connected it, I initially connected it like the after photo, but when that didn't help enough, what I ending up doing to completely eliminate audible hum was connect it like the before photo, when I did that , the ground loop disappeared, shielded cable made no difference in my case, as I far as I can tell, the output cable wasn't picking up hum, the circuit was, so I put aluminum plate shielding between the power supply and the line stage which got rid of the residual hum in the right channel,(which is nearest the PS),not sure why the opposite worked for me, maybe because in NZ we run on 230v 50Hz?

hum is a mysterious thing, you can quote me on that !

I'm running some long leads from the VTA board to input jacks located on the back panel of a custom build, and was planning to use shielded cable with just one end of the shield connected to ground. I remember reading somewhere that's supposed to bleed off any rf the shield encounters before it hits the center conductor?

I'm also planning to have TWO sets of input jacks that will act as a built in bridge for a tape loop hookup to my quad receiver without having to use an external "Y" adapter. I figure to add an aluminum box over the jacks to isolate the connections at the jacks too.

Unless somebody has a better idea ... (nudge nudge)

I was using shielded cable with just one end of the shield connected to ground too, but I was getting too much hum, the hum did not change up until half volume, where it disappeared, and after half volume it increased the higher the volume went, this problem was not fixed until I went to a twisted pair with ground connected at both ends!? you might have a different experience,as for the shield, it seems to mostly need to go around the transformer and rectifier tube as in my preamp that is where it made most difference, but I had some scrap that was big enough to go right around the entire power supply board. I still have a small amount of hum in the speakers but it is so quiet that I have to put my head right next to the driver to hear it. All you can do is try until it works, i can post a pic of mine if anyone wants to see it, now to find a nice shiny piece of copper!

Here's my layout ...



Quite a long run, and the jacks are right behind the power transformer, not to mention there's four of them. That's the reason I plan to do a box around them. I also plan to run the wires around the perimeter of the case. That puts them right tight to the chokes, but I don't expect any problem there.

Wire I got is maybe the best of both worlds then. It's twisted pair ... AND shielded! ...



http://www.performanceaudio.com/item/Mogami-W2549/5604/

Buck a foot ... must be good, right?

sKiZo wrote:Here's my layout ...



Quite a long run, and the jacks are right behind the power transformer, not to mention there's four of them. That's the reason I plan to do a box around them. I also plan to run the wires around the perimeter of the case. That puts them right tight to the chokes, but I don't expect any problem there.

Wire I got is maybe the best of both worlds then. It's twisted pair ... AND shielded! ...



http://www.performanceaudio.com/item/Mogami-W2549/5604/

Buck a foot ... must be good, right?

You could have ( in hindsight everything is better :-) placed one speaker outlet where the inlets are,
then the inlets could be at the outer side. Moving the choke to a middle position would make one side
almost free of hum-sources..

If you are in the process of altering the backside you could install a IEC-jack with fuses, thus freeing up
the inlet fuse and at the same time make all 110V ( or 220 ) cables as short as possible. There is also
IEC-jacks with power switches if you want.

Who's this Hindsight guy anyway, and why wasn't he there to stop me???  

Here's a pic of the back panel that would maybe explain why I'm not into shuffling jacks about ...



Good point on the choke though ... be real easy to put both of them on one side and run the audio lines down the other.

Well, the next amp You build will be better :-)

About the current amp, you _could_ skip the woodwork on the backside, it is not visible anyway.
Then mounting the rca's in a small iron box ( iron protects from magnetic fields ) mounted at the
extreme side, connections maybe upwards ? sideways ? This will create a "quiet" side for the inputs.

But i like the woodwork, it's beautiful!

Thanx for the flowers ... I like it too. That African Mokare is my new favorite wood to work with.

And I'll know it's there. So it stays !!  

Worse comes to worse, I can always add a breakout box after the fact. In the meantime, I'll just hope for the best.