New Crack build, Hum

[Mike Ross]

Hello! Build my Crack this weekend; first without Speedball then added Speedball. Without, I had a hum, otherwise all played well. Added Speedball as I thought it might take care of the hum. Nope. It hums when my RCA cable is connected, but without it's dead quiet. I've walked through the sticky on checking ground loops. All seems well.

I did update the RCA's, attenuator and output caps. I was a bit curious as to whether the RCA's should ground to chassis. Mine don't appear to. Any suggestions would be welcome.

Hum aside - love this amp! It pairs wonderfully with my T90's.

[Loquah]

Does the hum get louder when you turn the Crack's volume knob?

I had similar issues when I first built mine and solved it by doing 2 things (so I'm not sure which of the 2 worked or if it was both together that did the trick)

• I replaced the braided hook-up wire with a quality shielded coaxial from an RCA cable I had - needing this wasn't a reflection on the Crack's design so much as the quality of my braid
• I ran a dedicated earth lead from the RCA ground tabs over to the main earth point near the power input socket

[Grainger49]

The proper way to check the input is to short it.  Get some jumpers and go from the center conductor to the outer conductor.  If this cures the hum then the hum is coming from your source, it isn't in the Crack.

[jimiclow]

Don't ground the RCA's to the chassis.
You should not hear a hum with your headphones.
What attenuator and caps are you using?

[Paul Birkeland]

It hums when my RCA cable is connected, but without it's dead quiet.

There's a grounding issue here.  Try touching a wire between the Crack chassis plate and the chassis of your source.