Troubleshooting new hiss / crackling

[abourg]

Hey all -

So I put together my crack more or less a month ago, it passed all of the checks, and until recently the sound was crystal clear with zero audible noise. Quick equipment overview:

• Crack w/o speedball (waiting to install speedball until it had some time to break in for reasons just like this)
• Schiit Magni 2 Uber DAC (also the Modi amp but crack is what I'm using now)
• Sennheiser HD650

Yesterday, I was playing a PC game with friends and noticed that when the game started up, there was distinct crackling in the right ear. I figured it was some kind of interference from the graphics card that the crack was more sensitive to, so I switched over to my Modi and all was well. I had noticed this before to a lesser extent with other games that put my computer at load, so I didn't think too much of it - figured I'd do some research on how to mitigate that interference. Today I wanted to figure out what was wrong exactly, so I switched back to the crack and now there is a distinct white noise hiss in both ears, perhaps a little louder in the left - it's quiet but absolutely audible, and is not affected by the position of the potentiometer. However, when I turn the potentiometer all the way clockwise, 80%+, there exists a loud hum like when you plug a guitar lead halfway into an amp that's powered on (I would never actually run the amp that loud, but I don't remember it doing that when I was testing it right after building). All of these sounds persist even with no input source attached and in a room with no large electronics, except the crackling which only happens from interference it seems.

I'm kind of at a loss for what to do right now - I absolutely love this amp and I'm pretty upset that it's started acting up when I've barely had time to use it. Really hoping I can pin down the reason and get it working right again.

[Paul Birkeland]

there is a distinct white noise hiss in both ears, perhaps a little louder in the left - it's quiet but absolutely audible, and is not affected by the position of the potentiometer.

This is indeed a little unusual.  With your headphones, the noise floor should be inaudible.

However, when I turn the potentiometer all the way clockwise, 80%+, there exists a loud hum like when you plug a guitar lead halfway into an amp that's powered on (I would never actually run the amp that loud

That's not super meaningful.  Without shorting plugs in the jack, you're just amplifying whatever noise the Crack's inputs pick up.

All of these sounds persist even with no input source attached and in a room with no large electronics, except the crackling which only happens from interference it seems.

Just to check, does the noise persist with nothing plugged into the inputs of the Crack and the volume control down all the way?

Most of the time, issues like these can be resolved by reheating your solder joints.

-PB

[abourg]

Correct, the noise persists with no cables even plugged into the input jacks and the volume control all the way down. To be clear, your recommendation is just to go through each of the joints and reheat the solder, maybe add a hair more to make sure all the connections are good? Out of curiosity as somebody with minimal electronics experience, what's the issue with the joints that reheating would solve?

[Paul Joppa]

A large majority of kit problems are due to solder joints that look good but have not made a solid connection to all the wires. It takes much less time to re-heat every joint than it takes to figure out which joint(s) might have failed. It's a good idea to get a magnifying glass  and carefully inspect each one as you re-heat it. If there are many wires, the bad one may be buried under others and difficult or impossible to see. I've been building hobby electronics since the third grade (I recently turned 71) and I still make the occasional bad joint.

Incidentally, the main reason to add a bit of solder is just to get a bit of flux into the joint. Flux helps cut through traces of corrosion which may be the reason for the failed joint.

Also, often (especially if you have been careful to make a solid mechanical connection) a marginal joint will work for a while, maybe even a long while, before some mechanical shock reveals the failure to "wet" every wire.

[abourg]

Gotcha, makes sense. I actually think I may have bumped one of the capacitors around the time this started happening, so that explanation makes perfect sense. I ordered some soldering equipment so I can actually deal with this kind of thing myself (originally built with supervision from a friend who's more of an electronics guy and has his own equipment), will update after I take a crack at redoing the joints. Thanks so much for the answers!

[abourg]

I went through all of the accessible joints today (there are a few that are hidden under capacitors that aren't really accessible), reheated the solder, and added a little more to each joint, especially ones that seemed to be potentially problematic. The amp still passes all of the resistance / voltage checks, though the ones that range up to 115V are reading 119V on my meter. Any other thoughts on what I can try? I'm considering just biting the bullet and sending it in for repair.

[Paul Birkeland]

You can always try a different set of tubes as well.  Without having your amp in front of me, it's hard to say if what you're hearing could be tube related, but it's an easy thing to rule out.

[abourg]

Since it's unaffected by the volume position, would that mean that it could only be the input tube, or could it theoretically be either the input or output tube?

[Paul Birkeland]

That doesn't point to either tube specifically.

[abourg]

Gotcha. If that fixes it, then it's fixed, otherwise I have a spare set of tubes for when it is. Will give it a try.

[abourg]

Good news! I just received new tubes (12AU7G input + 6AS7G output) and we are back to dead silence. I had a few people who could hear it before check it with me, and we all think it's completely gone - so excited to get back to listening! Thanks so much for all the help!