let me hum a few bars for you....

[earwaxxer]

Question about hum - now this relates to the Quickie, and the strange thing is that when it is connected to my MSB Link DAC I get hum. I tried everything to cure it, including switching the 'floating ground' switch on the MSB. Swap tubes, same hum. Move things around, even extend the choke wires about a foot to move the chokes away from the kit. Actually the hum seemed to get worse when I did that. When I switch back to the Transporter the hum COMPLETELY goes away. I guess what I'm trying to understand is it related to impedance mismatch between the MSB and the Quickie, or is it a grounding issue. Not sure why the MSB would have a grounding issue. Any ideas would be helpful because I would like to avoid any situations like that in the future if I upgrade DAC's.

cheers - Eric

 

[fullheadofnothing]

Most of my hum problems have stemmed from ground loops, and I have cleared up 99% of my problems by using shielded twisted pair interconnects with the shield connected to the RCA shell on one end. Generally speaking, you want the shield to be connected on the side with the lowest ground potential, but try it both ways to see which is best. RadioShack cable and RCAs are cheap to experiment with.

 

[earwaxxer]

That makes sense - the RCA's do seem to respond with hum when I touch or move them.

 

[Grainger49]

That sounds like the grounds on the RCAs are not solid.  You can touch up the solder joints on the RCA plugs.

 

[2wo]

You can try lifting the ground from one channel or the other. In other words connect one interconnect normally, the other channel, just the center connection. You may be able to insert the RCA jack part way in, so just the center pin makes connection to experiment...John   

 

[Paul Birkeland]

You might also try running a ground wire from the ground buss on the Quickie to the chassis of the MSB.  The easiest way to experiment with this would be to use a clip lead from the screw that pokes through the terminal strip by the tubes and attaching the other end to the MSB chassis.

Also, through our experimentation, we have determined that this application is the text book case where simple coax is far and away the best noise performer.

 

[Paul Joppa]

What are called "ground loops" are usually just AC current in the ground lines, originating in capacitive coupling of the power transformer primary winding to the signal ground of the device.

In the simplest system, you would have a source (your DAC) then the Quickie, then a power amp. Both the source and the power amp have power transformers, so there will be some AC current flowing between them, on the ground lines of the interconnects and through the ground connections of all three devices. The voltage drop across the non-zero resistance of this ground path appears to the various input stages as if it were signal, so you hear hum and its harmonics. Since the coupling is capacitive, the harmonics come through more strongly than the fundamental, giving it something of a buzz character.

If more devices are connected, there are more sources of this ground hum/buzz and it gets harder to isolate the culprit - for example, unless the DAC is connected optically, the input to the DAC will likely be another source.

Think about how many pieces are involved in that ground path - each RCA jack and plug has a pyysical contact surface and a solder connection, plus there is the ground wire in the interconnect and the ground wires in the device (including Quickie). A lot of opportunities for a tiny bit of resistance to creep in!

Balanced lines greatly reduce this problem, which is why they are used in large, complex systems and those with long lines, such as recording studios and landline telephones.

 

[Grainger49]

I don't remember if my MSB had a phased plug.  If not, swap the power plug first.  That is, take it out, rotate 180 degrees and plug back in.

If it has a phased plug buy a "Cheater."  Pictured in the Pictures Of Tools thread:

http://www.bottlehead.com/smf/index.php/topic,3145.msg26803.html#msg26803

I got this from an old comparison of CD players.  The orientation that ends up with the lowest AC voltage to ground on the chassis is the way to go.  It helps eliminate circulating AC through the common of interconnects.

 

[earwaxxer]

Thanks so much guys! Lots of good info - I will have to read it several times to get the gist.

 

[earwaxxer]

PB gets the prize on this one. I ran a wire from the ground rail on the Quickie to my DAC. The funny thing was that the first time I tried it I must not have got a good contact due to paint etc. Trying again getting to bare metal on the chassie did the trick. - Thanks again!

 

[Paul Birkeland]

Yay!!!  Glad to hear it worked out for you. 

 

[Grainger49]

Eric,

I just reread this and shouldn't your interconnects give a ground path from the chassis of the MSB to the ground buss in the Quickie?

 

[earwaxxer]

Hey Granger - actually, I ran the wire to the Transporter - the MSB (now the Gungnir) is the outboard DAC that drives the Quickie. Although that shouldnt matter, they are all electrically connected (coax digital from Transporter to Gungnir).

You are right though. The Quickie should find ground through the RCA. There may still be issues there.

 

[Grainger49]

Eric,

Check the interconnects.  Something is off here.

 

[Paul Birkeland]

There is equipment where the chassis may float, or the shells of the RCA jacks aren't grounded, etc.  Therefore, the interconnects are merely coupling a floating output to the input of the Quickie (the manufacturer of the source is assuming the connection will be on the other end, or doesn't have the opportunity provide that connection).

Clipping the wire between the Quickie and the source chassis solves that problem, though it may be equivalent to just grounding the RCA jack shells to the source chassis.