[guf]

After years of having my First Watt Sit2 I now have a hum through my speakers. I mostly leave it on and turn off the other components. When the pre and dac are off I have a hum coming from both speakers. When my dac and pre are powered up there is no hum. Why after years of no problem now I have one?  Any thoughts or suggestions? Is this one of those ground loops? I am on a dedicated circuit.

[Speedskater]

Ground loops don't come and go. Is it a pure 50/60 Hz hum or a low frequency buzz? Most likely a bad connection.

[poseidonsvoice]

Ground loops don't come and go. Is it a pure 50/60 Hz hum or a low frequency buzz? Most likely a bad connection.

Exactly. Time to clean the contacts, remove interconnects, power cords and reconnect them.

Best,
Anand.

[guf]

Ground loops don't come and go. Is it a pure 50/60 Hz hum or a low frequency buzz? Most likely a bad connection.
[/quote

I knew that was going to be asked. I don't know the difference in hums....it doesn't seem too low.  Thanks I'll try the suggestions! thanks!

[guf]

ok. followed suggestions. hum is still there.
Interesting thing is that my amp has a jumper that is connected to an XLR that allows to switching from 100k to 10k impedance. When I touch that with my finger the hum gets noticeably louder.

[poseidonsvoice]

ok. followed suggestions. hum is still there.
Interesting thing is that my amp and a jumper that connected to an XLR  that allows to switching from 100k to 10k impedance. When I touch that with my finger the hum gets noticeably louder.

Here is the manual with a simplified schematic as well as an explanation for the jumpered positions on the XLR jack:

https://www.manualslib.com/manual/831925/Firstwatt-Sit-2.html#manual

May be useful for others reading your thread.

Best,
Anand.

[richidoo]

Try a different song, maybe it doesn't know the words, so it hums instead   

Since it gets quiet when turned on, I think your system is OK, but the house wiring is suspect.

A loose neutral or earth in the shared main circuit house wiring will increase resistance and cause hum as a loop between earth and neutral. Loose wiring is very common on newer houses. Movement from using the outlet can loosen the wires if not tightened enough, temperature variations in exterior walls cause expenasion and contraction to loosen the screw, corrosion from humidity or water intrusion increases resistance. A loose connection has higher resistance.

Hum is caused by imbalanced resistance in the grounds. If one ground has higher resistance than the others it causes more current to flow which is the source of the hum. All grounds power and signal should have minimal resistance.

Check all the outlets, switches, ceiling fans and lamps, wirenuts in the entire shared circuit. Tighten everything as tight as you can with a large screwdriver. Move backstabbers on outlets to screw terminals. Don't open the service panel, you have to trust that one is OK, the licensed electrician did that one, but his apprentices did the rest of the house. It is very common for house wiring to come loose especially in newer construction.

[Speedskater]

An interesting use of a XLR jack.

[Speedskater]

About AC power.
Is everything plugged into the same wall outlet?
Are there any internet, cable TV or phone wires connected to the hi-fi system?

[guf]

About AC power.
Is everything plugged into the same wall outlet?
Are there any internet, cable TV or phone wires connected to the hi-fi system?

Only audio stuff plugged into that outlet and that outlet goes directly to the panel.
I spoke with Pass Labs and they suggested turning the amp off when I'm not using it if I want the hum to go away But they would be happy to look at it if I wanted to send it in.
I pulled to cover off and nothing looked out of place. I'm suspecting something wrong in the XLR jumper area, but will follow the suggestion of tightening up the connections where I can.

[Elizabeth]

I would look at that jumper more.
Particularly if touching it make the hum louder.

[Speedskater]

If everything is on one outlet, then it's most probably not an AC power system problem.

[WGH]

The most common source of the sudden appearance of a 60 Hz hum is a bad filter cap, especially if you leave the component on all the time. I have no idea why it disappears when other electronics are turned on.

[*Scotty*]

When the preamp is turned on, the power amp sees a low impedance load terminating its input stage.
When the preamp is turned off its circuitry no longer presents the input stage of the power amp with a low impedance load with respect to ground. The higher the terminating impedance is, the more prone the input stage of the power amp is to picking up hum or RF from the air. If you are going to turn the preamp off and expect the power amp not to hum, the shielding on the ICs connecting the preamp to the power amp must be very good . If there is a bad ground connection at either end of the ICs then you will probably pick up a hum when the preamp is off. Unless there are concerns about how much electrical power is consumed by power amp if it were to be left on 24/7, I would never turn it off, thus lessening the likelihood of the problem cropping up.
 In my primarily solid state system I run unshielded, unbalanced connections between all components and never turn anything off. No hum problems. The sole exception being the tube FM tuner which is turned off when not in use.
Scotty

[guf]

Here is an interesting update. I replaced my preamp and added new ic cables. This is hard to understand how this would effect the hum because the hum was happening when only the amp was on and speakers were connected.  Does this mean its not a problem with the amp and one of the things I removed? Or I wonder if my new cables are better shielded as Scotty is talking about?

I guess I could change it back and see....instead of bothering you with my first would problems