Background Hum - Worsens w/ volume + present without input.

[Acclaim]

Freshly built Crack w/ Speedball (thanks for the help!) has a background hum that was present on building the standard Crack and persists with the addition of the Speedball upgrade.

The hum is inaudible at 0% volume all the way until 50% where it continually worsens as volume approaches 100%. The hum is present without input and when trying a few outlets on different breakers within the house.

I read a suggestion by PB to try and tighten up the transformer which I did using hand tools (I'm assuming he wasn't alluding to the use of power tools) to no avail.

Thoughts? Is this to be expected?

Sincerely,

Self-Admitted Paranoid Builder

[Paul Birkeland]

Are all your voltages correct? 

Did you paint the bottom of the chassis plate?

Is everything on the amp stock?

[Acclaim]

All voltages are within an acceptable ~10% discrepancy.
Stock without paint/stain.

So, this is abnormal then? Should I look at resoldering any areas in particular?

(And thanks for updating the guide with the 19/20/21 voltages, I think it may help the more OCD builders out there.)

[Paul Birkeland]

There may not be any issue here - can you connect a source to the Crack and retest? 

Also, what headphones are you using?  Some incredibly efficient, low impedance headphones will reveal some background noise that is otherwise not present. 

[Acclaim]

Still present.

HD650 - I thought all Cracks came with these in the box? Haha.

I can re-check my voltages as I didn't write anything down, but I don't recall any alarming values.

What is it about increasing the current/voltage that would cause an audible hum?

Tubes? Loose bolts?

[Paul Birkeland]

Eh, voltages that are off can point to problems. 

Another incredibly common issue that I've seen in Crack repairs lately will be that one of the 220uF caps isn't soldered in all the way.  This will lead to perfect DC voltages, but at the same time the amp will hum.

To check for this, grab each cap and gently rock it from side to side (about as hard as you would need to pull on taffy to get it to stretch).  There should be no movement.  If one cap moves, you've found your issue.

-PB

[Kris]

Did you (by any chance) solder resistors to RCA jacks to decrease the gain? See position 3 in Crack FAQ.
Asking, because adding those resistors caused exactly the same hum as you described in my crack.

[Acclaim]

5.  When I turn my Crack all the way up, I hear some noise, is this normal?

Generally yes, there will be some noise with the volume control at its maximum level.  The amount of noise will depend greatly on the impedance of the headphones, but the important thing to remember is that this is well beyond a comfortable listening level.

Honestly, I'd say you describe it well here. Although I wouldn't say it is well beyond a comfortable listening level. I've found a comfortable listening level to be at a quarter turn, the background hum shows up at a half turn which is still at a bearable level. My Crack may just have a lower threshold for this.

I'll trying wiggling the taffy.. err caps when I get home.

Did you (by any chance) solder resistors to RCA jacks to decrease the gain? See position 3 in Crack FAQ.
Asking, because adding those resistors caused exactly the same hum as you described in my crack.

Nope, but good thought! I would consider doing this mod to set my comfortable listening level to a half turn to maximize volume control but not unless I can get rid of the background hum at a half turn. We'll see! Did you end up removing the hum with the resistors or did you ditch that idea?

[Doc B.]

If the hum is not present with the volume turned down and gets louder as the volume is turned up it is coming in ahead of the volume control. The only parts in the Crack ahead of the volume control are the inputs.

You say hum is present without input. Does that mean that nothing is connected to the inputs? In that case the amp will pick up hum from the environment. Try shorting the inputs and see if the hum goes away. If it does, your source or your cables are introducing the hum.

[Kris]

I removed the resistors, so that my crack is dead silent again, even if I turn the pot all the way up.
Well, anyway good luck on tracing the hum and don't forget to post back when you find the culprit.

P.S. Just in case, can you post the picture(s) of your crack wiring.

[Acclaim]

If the hum is not present with the volume turned down and gets louder as the volume is turned up it is coming in ahead of the volume control. The only parts in the Crack ahead of the volume control are the inputs.

You say hum is present without input. Does that mean that nothing is connected to the inputs? In that case the amp will pick up hum from the environment. Try shorting the inputs and see if the hum goes away. If it does, your source or your cables are introducing the hum.

Apologies for my lack of knowledge (this has been my first real dabble in a DIY electronics project), but what would be the best way to short the inputs?

I removed the resistors, so that my crack is dead silent again, even if I turn the pot all the way up.
Well, anyway good luck on tracing the hum and don't forget to post back when you find the culprit.

P.S. Just in case, can you post the picture(s) of your crack wiring.

I will try to have a picture up by tomorrow.

[Paul Birkeland]

Apologies for my lack of knowledge (this has been my first real dabble in a DIY electronics project), but what would be the best way to short the inputs?

You can use a clip lead from center to tab on each RCA jack, or just snap off a length of solder with your hand and wrap it around the jack to short these connections. 

[Acclaim]

So, I ended up using two pairs of clip leads and shorting the inputs from center to ground tab and... The Crack became dead silent. This is with only the power and headphones connected; nothing was plugged into the input RCA jacks.

Does this mean I have a bad joint somewhere between the input and volume pot?

[Paul Birkeland]

No, it means that your Crack is working perfectly. 

Have you tried this Crack with a source and headphones? 

Which headphones are you using?

It's entirely normal that when you turn a Crack up all the way, there will be some background noise.  If you play music at this level, it will likely blow the drivers clean out of your headphones, so this noise is meaningless in terms of the operation of the amplifier itself.

[Acclaim]

http://imgur.com/ldfEu18
http://imgur.com/f80dNtg
http://imgur.com/Ouztjhi

Above are pictures of the full wiring, input joints and volume pot joints respectively.

I am using 300Ω HD650s PB.

The hum appears at a half-turn and it would still be desirable for me to listen at that level, it is far from driver-blowing in my chain.

Kris reports dead silence even if the pot is turned all the way up. Would like to have similar results if possible!

To be clear PB, there's nothing a resolder would help with?

Would something like this help? http://passionforsound.files.wordpress.com/2012/11/pb191598.jpg