Finished Crack - Intermintent, soft, low frequency, scratchy noise on both chann

[thatsamess]

Hi all. I'm a newbie at building but finished my stock Crack and simply amazing to me it works; a testament to BH robust design and excellent instructions . I don't have golden ears (that's an understatement), but I love the sound. However, I am getting a intermittent, rather soft, low frequency scratchy noise from both channels. I only hear it during very quiet music passages. I haven't been able to correlate it to anything as yet. Tried wiggling Head Phone and Input cables but that has no affect. The other thing that I would like to ask about is the volume level I should be getting. I'm using an FiiO X3 as input with it set to line out and volume set all the way. Headphones are DT800 600 Ohm. With this setup I have to turn the Crack volume up to about 80% to reach the volume level I prefer.  Is this to be expected with 600 Ohm cans? Also, could the volume turned up this high cause the noise I'm hearing?

Regards,
MB

 

[Jameson]

What are you feeding into the FiiO? Do you have a mobile phone or anything with a wireless signal near the amp? These could create the interference you're describing. There's a stickied thread in the Crack section of the forum where Paul suggests a modification that could also help with power line noise.

 

[Jameson]

This is the thread I was referring to.

 

[Paul Birkeland]

Scratching in both channels could be external interference.  I have a powerline ethernet kit that causes that exact noise in anything I use near it unless I install this mod:
https://forum.bottlehead.com/index.php?topic=11676.0

The X3 looks like it puts out about 2V of signal, which should allow for lots of range on the level control.  Perhaps you could try setting it to headphone out?  The specifications for this product are not particularly ample, so you may need to do some experimentation.  Having said that, there's nothing wrong or harmful with having the pot turned up that far, though you will  probably notice some noise with the pot turned up that far and nothing plugged into the amp.

 

[thatsamess]

Thanks for the replies and suggestions. I've been doing some more testing/listening and discovered something. After the Crack had been powered up for an hour or so the noise (I'll call it static like) subsided. That led me to think that perhaps something underneath the chassis was getting warm, maybe an iffy solder joint or something and at a warmer temp was conducting better. While I was thinking about heat buildup under the chassis (and hear comes the confession), I remembered the rubber feet that I was suppose to attach to the wood base. Rubber feet are now attached and the thing is dead quiet. I guess they provide electrical isolation as well as helping with cooling. Anyway, the noise problem seems to be solved.
Volume issue - I tried using my cd player as a source (rather than the FiiO X3 and volume level was much improved. It would seem the FiiO has a weak output whether set to headphone or line output. I did find a high gain setting on the X3 that helped quite a lot. Does anyone run a preamp in front of the Crack?

 

[Paul Birkeland]

In the process of putting the feet on, you likely jostled the connection that has been giving you these issues.  Lots and lots of heat under the chassis isn't going to manifest the noise you were getting, especially if that noise was present within the first three or four hours of using the amp.  The feet do not provide any electrical isolation that the wood base doesn't already provide.