Low level hum after Speedball upgrade

[kxanthop]

I have just finished upgrading my Crack and it sounds great! I have noticed though, that there is a low level hum from both channels. The hum is perceptible only when no music is playing. Another odd thing is that when I power up the amp and the headphones are plugged in, some distortion can be heard through the left channel, which soon disappears. Both phenomena appeared after the upgrade; the Crack was dead silent before the upgrade. All the voltage reads were within specs, with the exception of the headphone jack which topped at 22V (ergo the distortion?), but quickly went down to 0.
Is the hum normal? Is there anything I could check/do?
Thank you in advance!

 

[Paul Birkeland]

Is there any hum with nothing plugged into your Crack?

 

[kxanthop]

Thanks for th reply!
I have just tried the crack with nothing plugged in and the humming is identical. Rather than humming, it is more like very low level noise.

 

[Paul Birkeland]

Some distortion in the left channel could be an indicator of a flaky solder joint.

The next test that I would do is to short the inputs of the amplifier, then listen for noise.  This can be done with clip leads or shorting plugs, whichever is most handy.

-PB

 

[kxanthop]

I am afraid I don't have either. Would these shorting plugs work
http://www.takefiveaudio.com/mall/shopcontent.asp?type=RCA_Input_shorting_plugs
I guess I would have to plug one in each input, right?
Many thanks!

 

[Paul Birkeland]

Yes, though you can substitute a piece of wire for the 470 Ohm resistors.  Generally, any local electronics shop will have some inexpensive phono plugs that you can use for this purpose.  If such a retailer evades you, you can take a pair of really cheap interconnects, hack off all but 1" of the wire on one end, strip it back, and solder everything together to create the short. 

-PB

 

[kxanthop]

Only today did I manage to prepare the shorting plugs and listen for noise. There was no difference between the DAC and the shorting plugs. However when the was nothing plugged in the amplifier and I turned the volume all the way up I could hear the hum. Does any of that make any sense?
Many thanks!

 

[Paul Birkeland]

However when the was nothing plugged in the amplifier and I turned the volume all the way up I could hear the hum.

Is all the way up a normal listening level for your setup?  If not, find the volume level where you listen, turn off the music, then listen for hum.

 

[John EH]

I've had the tiniest him in one of my Crack's at near full to full volume for some time.  I thought it was my ungrounded Japanese home.  While I was reading this thread I did a search and found a post on Crack hum with a loose connection on the power supply caps.

Sure enough I turned my Crack upside down and gently twisted the three black power supply caps and sure enough one leg on one was loose. Re-soldered it and it's dead black quiet now.

Check your power supply caps too.

 

[Jimb0]

This is an interesting post. I heard low level hum noise with my Crack plugged into my desktop computer. No noise when it's plugged into my iPhone or Laptop. But since you still hear noise without it plugged into anything, could it be the tubes that you're using?

 

[kxanthop]

Thanks for the suggestions. The power supply capacitors seem to be properly connected. The tubes, I too had thought about them, but I don't have a replacement set to check their effect.
To be honest, with the exception of the startup noise, I am not sure I can actually hear any hums (the other day I was thinking I could hear the hum and then realized the headphones weren't plugged in) so maybe I will stop trying to listen for hums and maybe concentrate on the music.

 

[jabbr]

I fixed this after my speedball upgrade by swapping out the 12au7.