Quiet Hum?

[bmkuter]

I just finished building the base SEX and I hear quiet noise that increases with volume. When the volume is at 0, its still barely there.

Aside from that it sounds great, but is the hum normal?

[Paul Birkeland]

Hum that increases with the position of the volume pot tends to be external to the amp and may change depending on where you have it. 

If you have nothing plugged into the inputs when you're listening to this hum, then that is making things quite a bit worse. 

Having made those two points, it's also possible to have hum problems if the layout isn't reasonably carefully followed. 

-PB

[bmkuter]

The hum is still present when I unplug the RCA inputs. It sounds like general noise.

I'll try and attach a video:

Video is garbage quality, but best I could do with a phone. The noise is from SEX @ 100% volume through Focal Elegia with the mic ~1.5 inches away.

[Paul Birkeland]

What is the noise like with the volume all the way down?

When you have nothing plugged into the SEX amp and you turn the volume all the way up, you're just amplifying whatever ambient noise is available for the input of the amp to receive.  This isn't a meaningful measurement. 

[bmkuter]

With the sound all the way down, I hear a very faint hum but very faint. When I turn the volume up, static seems stack on top of the hum starting after a quarter turn.

The hum all the way down sounds like a generator through a wall.

[Paul Birkeland]

You could try tightening the hardware on the power transformer. 

[bmkuter]

Thanks! That helped a bit!

Anything look misplaced here?

EDIT: Weird, can't upload photos right now.

[Paul Birkeland]

There's a file size restriction on the photos.  You can just knock them down by about 50% and they should post OK.

[bmkuter]

Thanks!

[Paul Birkeland]

Nothing is jumping out at me there, everything looks to be about where it should be.

-PB

[bmkuter]

Should the amp be silent when not playing and volume all the way down?

I re-soldered some connections and converted the transformers from 8 ohms to 4 ohms, which helped the hum a little bit.

[Deke609]

Something easy to try: tighten all the mounting bolts/screws of the terminal strips.  Many of them are used as contacts to chassis ground and so a loose one could make for an iffy ground connection.  I noticed that the terminal strip near the headphone jack has black domed nuts.  I'd make sure that the nut closest to the jack that is securing a ground reference is tight.  I'd also take a peek at the ground tab closest to the IEC inlet - the one with the bare ground wire running through to the ground lug on the IEC. From the pic, it looks like it could use a bit of solder (but maybe not -- might just be the angle of the pic).

cheers and good luck, Derek

[Paul Birkeland]

Should the amp be silent when not playing and volume all the way down?

Yes. 

Having spent a lot of time capturing noise measurements of the SEX amp, sometimes it comes down to the environment too.  Another issue I have seen happen is that the 8 lug strip in front of the power transformer can touch the nuts securing the power transformer, and if this happens you can get some ground loop hum.  All you have to do to fix that is nudge the strip away from the transformer nuts and brush in a little nail polish or clear lacquer to keep them spaced apart.

[bmkuter]

Any ideas on how to check if its environmental? I checked everything upstream again, including some USB extension I forgot I was running. Something up there is causing the noise, since after resoldering and converting to 4 ohms, the noise is gone when RCAs are disconnected.

The hum sounds exactly like a generator we have near my apartment building, which supplies all the power. Any connection?

[Paul Birkeland]

Any ideas on how to check if its environmental? I checked everything upstream again, including some USB extension I forgot I was running. Something up there is causing the noise, since after resoldering and converting to 4 ohms, the noise is gone when RCAs are disconnected.

Hey, that's great news!

I would recommend trying this mod since it's inexpensive:
https://forum.bottlehead.com/index.php?topic=11676.0

The hum sounds exactly like a generator we have near my apartment building, which supplies all the power. Any connection?

If your powerline was a problem, then I would expect the noise to just be there, and for it to be independent of the level of the volume pot.