crack hums around 1/2 volume and up [solved]

[Kestenater]

I recently bought a used/pre-built crack and have been enjoying it. The only problem I have is that there is a background hum. It is there with or without the inputs plugged in and begins at right about listening volume. I figured because the hum was affected by the volume pot, it was probably before it, so I tried re-braiding the wires and re flowing the solder on the inputs but it is still there. I also notice that having the inputs plugged in introduces more hum. How can I find the issue and fix the crack, also would replacing the rca cables I'm using as inputs get rid of the noise they produce?

[Grainger49]

The first test is to short your inputs and see if the hum is still there.  If you don't have jumpers to short the outside to inside of the input RCA jacks you need to buy them.  They are a great help on all kinds of things.  This eliminates the source and interconnects.

Then if this doesn't help look at the sticky in the Crack folder and measure the resistance of all your grounds.

[Kestenater]

I tried shorting the inputs but the hum remained. I'll check the resistance and go to the sticky like you said. Thank you for your help.

[Paul Birkeland]

Which headphones are you using with the Crack? (May be relevant, maybe not)

[Kestenater]

I am using the hd600s. I also checked the grounding of the inputs and the volume pot on the main earth and on the chassis without finding any significant resistance.

[Doc B.]

If it goes up with volume, you are right that it is ahead of the volume control. Is the Crack sitting on or near another power transformer?

[Kestenater]

it's seated about a yard from my computer. There's a small audio interface next to it that gets power through a USB and the computer's monitor is a couple inches away.

[Doc B.]

I'll suggest moving the audio interface and the Crack farther away from each other and see if the noise drops any.

[Kestenater]

I tried both unplugging and moving the interface but the hum remained. I figured because nothing seemed to work I probably did something wrong earlier, so I retried shorting the inputs. My inputs have two 270k resistors before the wire so I figured when I tried shorting the input I did it wrong by putting it before the resistors. I took my alligator clips and clipped them to the chassis and/or the rca ground and tried clipping them after the resistors. One of the clips was not working, but with the other I found that the side I was connecting became dead quiet. Considering this, how do I go about fixing this problem?

[Kestenater]

I connected the clip to where either red or white cables connect to the volume pot and it went silent. In order to fix this, should I go back over the rca solder joints and see if that fixes it? When you say it may be the rca jacks, do you mean I would have to replace the rca jacks? Thank you guys very much for your help.

[Paul Birkeland]

My inputs have two 270k resistors before the wire

That's probably where your issue is.  Try the 33K/75K resistor mod mentioned in the FAQ.

-PB

[Kestenater]

I changed rca jacks and the buzzing seemed to be less and higher pitched. The buzz that came from me touching the rca jacks also dissappeared. Because there was still a little bit of buzz I switched 275k resistor with the 75k & 33k resistors. It now gets louder sooner on the knob, but the hum seems to be even more quiet at listening volume. The hum still seems to appear about halfway up the volume knob, but now that point is louder so I don't have as much a reason to hit that point. Also, I may be placebo-ing it, but does the higher resistance increase bass/suppress the highs? Thank you guys very much for your help, it's nice to start reaching solutions.

[Paul Joppa]

Check your connections on the 33K resistors; you should have slightly less gain than the 275K rather than the increased gain you observed.

[Kestenater]

I had attached the 33k resistor wrong, but I fixed it now. There is no noise, even at full volume; however, at full volume the output is only a loud listening volume. Is there a change in resistors I can make to increase the output a bit? Something a little higher gain than the 275k resistors as the listing volume for those was about 65%.

EDIT - Actually, the lower gain is nice, but I do want a bit more headroom. would flipping the 33k & 75k resistors possibly give me a gain between the 275k and the setup I have now?

[Paul Birkeland]

I had attached the 33k resistor wrong, but I fixed it now. There is no noise, even at full volume; however, at full volume the output is only a loud listening volume. Is there a change in resistors I can make to increase the output a bit? Something a little higher gain than the 275k resistors as the listing volume for those was about 65%.

Try removing the 33K resistors but leaving the other pair in place.

EDIT - Actually, the lower gain is nice, but I do want a bit more headroom. would flipping the 33k & 75k resistors possibly give me a gain between the 275k and the setup I have now?

That will have marginally lower gain than stock.